/*
 *  mpi.c
 *
 *  Arbitrary precision integer arithmetic library
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "mpi-priv.h"
#include "mplogic.h"

#include <assert.h>

#if defined(__arm__) && \
    ((defined(__thumb__) && !defined(__thumb2__)) || defined(__ARM_ARCH_3__))
/* 16-bit thumb or ARM v3 doesn't work inlined assember version */
#undef MP_ASSEMBLY_MULTIPLY
#undef MP_ASSEMBLY_SQUARE
#endif

#if MP_LOGTAB
/*
  A table of the logs of 2 for various bases (the 0 and 1 entries of
  this table are meaningless and should not be referenced).

  This table is used to compute output lengths for the mp_toradix()
  function.  Since a number n in radix r takes up about log_r(n)
  digits, we estimate the output size by taking the least integer
  greater than log_r(n), where:

  log_r(n) = log_2(n) * log_r(2)

  This table, therefore, is a table of log_r(2) for 2 <= r <= 36,
  which are the output bases supported.
 */
#include "logtab.h"
#endif

#ifdef CT_VERIF
#include <valgrind/memcheck.h>
#endif

/* {{{ Constant strings */

/* Constant strings returned by mp_strerror() */
static const char *mp_err_string[] = {
    "unknown result code",     /* say what?            */
    "boolean true",            /* MP_OKAY, MP_YES      */
    "boolean false",           /* MP_NO                */
    "out of memory",           /* MP_MEM               */
    "argument out of range",   /* MP_RANGE             */
    "invalid input parameter", /* MP_BADARG            */
    "result is undefined"      /* MP_UNDEF             */
};

/* Value to digit maps for radix conversion   */

/* s_dmap_1 - standard digits and letters */
static const char *s_dmap_1 =
    "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz+/";

/* }}} */

/* {{{ Default precision manipulation */

/* Default precision for newly created mp_int's      */
static mp_size s_mp_defprec = MP_DEFPREC;

mp_size
mp_get_prec(void)
{
    return s_mp_defprec;

} /* end mp_get_prec() */

void
mp_set_prec(mp_size prec)
{
    if (prec == 0)
        s_mp_defprec = MP_DEFPREC;
    else
        s_mp_defprec = prec;

} /* end mp_set_prec() */

/* }}} */

#ifdef CT_VERIF
void
mp_taint(mp_int *mp)
{
    size_t i;
    for (i = 0; i < mp->used; ++i) {
        VALGRIND_MAKE_MEM_UNDEFINED(&(mp->dp[i]), sizeof(mp_digit));
    }
}

void
mp_untaint(mp_int *mp)
{
    size_t i;
    for (i = 0; i < mp->used; ++i) {
        VALGRIND_MAKE_MEM_DEFINED(&(mp->dp[i]), sizeof(mp_digit));
    }
}
#endif

/*------------------------------------------------------------------------*/
/* {{{ mp_init(mp) */

/*
  mp_init(mp)

  Initialize a new zero-valued mp_int.  Returns MP_OKAY if successful,
  MP_MEM if memory could not be allocated for the structure.
 */

mp_err
mp_init(mp_int *mp)
{
    return mp_init_size(mp, s_mp_defprec);

} /* end mp_init() */

/* }}} */

/* {{{ mp_init_size(mp, prec) */

/*
  mp_init_size(mp, prec)

  Initialize a new zero-valued mp_int with at least the given
  precision; returns MP_OKAY if successful, or MP_MEM if memory could
  not be allocated for the structure.
 */

mp_err
mp_init_size(mp_int *mp, mp_size prec)
{
    ARGCHK(mp != NULL && prec > 0, MP_BADARG);

    prec = MP_ROUNDUP(prec, s_mp_defprec);
    if ((DIGITS(mp) = s_mp_alloc(prec, sizeof(mp_digit))) == NULL)
        return MP_MEM;

    SIGN(mp) = ZPOS;
    USED(mp) = 1;
    ALLOC(mp) = prec;

    return MP_OKAY;

} /* end mp_init_size() */

/* }}} */

/* {{{ mp_init_copy(mp, from) */

/*
  mp_init_copy(mp, from)

  Initialize mp as an exact copy of from.  Returns MP_OKAY if
  successful, MP_MEM if memory could not be allocated for the new
  structure.
 */

mp_err
mp_init_copy(mp_int *mp, const mp_int *from)
{
    ARGCHK(mp != NULL && from != NULL, MP_BADARG);

    if (mp == from)
        return MP_OKAY;

    if ((DIGITS(mp) = s_mp_alloc(ALLOC(from), sizeof(mp_digit))) == NULL)
        return MP_MEM;

    s_mp_copy(DIGITS(from), DIGITS(mp), USED(from));
    USED(mp) = USED(from);
    ALLOC(mp) = ALLOC(from);
    SIGN(mp) = SIGN(from);

    return MP_OKAY;

} /* end mp_init_copy() */

/* }}} */

/* {{{ mp_copy(from, to) */

/*
  mp_copy(from, to)

  Copies the mp_int 'from' to the mp_int 'to'.  It is presumed that
  'to' has already been initialized (if not, use mp_init_copy()
  instead). If 'from' and 'to' are identical, nothing happens.
 */

mp_err
mp_copy(const mp_int *from, mp_int *to)
{
    ARGCHK(from != NULL && to != NULL, MP_BADARG);

    if (from == to)
        return MP_OKAY;

    { /* copy */
        mp_digit *tmp;

        /*
          If the allocated buffer in 'to' already has enough space to hold
          all the used digits of 'from', we'll re-use it to avoid hitting
          the memory allocater more than necessary; otherwise, we'd have
          to grow anyway, so we just allocate a hunk and make the copy as
          usual
         */
        if (ALLOC(to) >= USED(from)) {
            s_mp_setz(DIGITS(to) + USED(from), ALLOC(to) - USED(from));
            s_mp_copy(DIGITS(from), DIGITS(to), USED(from));

        } else {
            if ((tmp = s_mp_alloc(ALLOC(from), sizeof(mp_digit))) == NULL)
                return MP_MEM;

            s_mp_copy(DIGITS(from), tmp, USED(from));

            if (DIGITS(to) != NULL) {
                s_mp_setz(DIGITS(to), ALLOC(to));
                s_mp_free(DIGITS(to));
            }

            DIGITS(to) = tmp;
            ALLOC(to) = ALLOC(from);
        }

        /* Copy the precision and sign from the original */
        USED(to) = USED(from);
        SIGN(to) = SIGN(from);
    } /* end copy */

    return MP_OKAY;

} /* end mp_copy() */

/* }}} */

/* {{{ mp_exch(mp1, mp2) */

/*
  mp_exch(mp1, mp2)

  Exchange mp1 and mp2 without allocating any intermediate memory
  (well, unless you count the stack space needed for this call and the
  locals it creates...).  This cannot fail.
 */

void
mp_exch(mp_int *mp1, mp_int *mp2)
{
#if MP_ARGCHK == 2
    assert(mp1 != NULL && mp2 != NULL);
#else
    if (mp1 == NULL || mp2 == NULL)
        return;
#endif

    s_mp_exch(mp1, mp2);

} /* end mp_exch() */

/* }}} */

/* {{{ mp_clear(mp) */

/*
  mp_clear(mp)

  Release the storage used by an mp_int, and void its fields so that
  if someone calls mp_clear() again for the same int later, we won't
  get tollchocked.
 */

void
mp_clear(mp_int *mp)
{
    if (mp == NULL)
        return;

    if (DIGITS(mp) != NULL) {
        s_mp_setz(DIGITS(mp), ALLOC(mp));
        s_mp_free(DIGITS(mp));
        DIGITS(mp) = NULL;
    }

    USED(mp) = 0;
    ALLOC(mp) = 0;

} /* end mp_clear() */

/* }}} */

/* {{{ mp_zero(mp) */

/*
  mp_zero(mp)

  Set mp to zero.  Does not change the allocated size of the structure,
  and therefore cannot fail (except on a bad argument, which we ignore)
 */
void
mp_zero(mp_int *mp)
{
    if (mp == NULL)
        return;

    s_mp_setz(DIGITS(mp), ALLOC(mp));
    USED(mp) = 1;
    SIGN(mp) = ZPOS;

} /* end mp_zero() */

/* }}} */

/* {{{ mp_set(mp, d) */

void
mp_set(mp_int *mp, mp_digit d)
{
    if (mp == NULL)
        return;

    mp_zero(mp);
    DIGIT(mp, 0) = d;

} /* end mp_set() */

/* }}} */

/* {{{ mp_set_int(mp, z) */

mp_err
mp_set_int(mp_int *mp, long z)
{
    unsigned long v = labs(z);
    mp_err res;

    ARGCHK(mp != NULL, MP_BADARG);

    /* https://bugzilla.mozilla.org/show_bug.cgi?id=1509432 */
    if ((res = mp_set_ulong(mp, v)) != MP_OKAY) { /* avoids duplicated code */
        return res;
    }

    if (z < 0) {
        SIGN(mp) = NEG;
    }

    return MP_OKAY;
} /* end mp_set_int() */

/* }}} */

/* {{{ mp_set_ulong(mp, z) */

mp_err
mp_set_ulong(mp_int *mp, unsigned long z)
{
    int ix;
    mp_err res;

    ARGCHK(mp != NULL, MP_BADARG);

    mp_zero(mp);
    if (z == 0)
        return MP_OKAY; /* shortcut for zero */

    if (sizeof z <= sizeof(mp_digit)) {
        DIGIT(mp, 0) = z;
    } else {
        for (ix = sizeof(long) - 1; ix >= 0; ix--) {
            if ((res = s_mp_mul_d(mp, (UCHAR_MAX + 1))) != MP_OKAY)
                return res;

            res = s_mp_add_d(mp, (mp_digit)((z >> (ix * CHAR_BIT)) & UCHAR_MAX));
            if (res != MP_OKAY)
                return res;
        }
    }
    return MP_OKAY;
} /* end mp_set_ulong() */

/* }}} */

/*------------------------------------------------------------------------*/
/* {{{ Digit arithmetic */

/* {{{ mp_add_d(a, d, b) */

/*
  mp_add_d(a, d, b)

  Compute the sum b = a + d, for a single digit d.  Respects the sign of
  its primary addend (single digits are unsigned anyway).
 */

mp_err
mp_add_d(const mp_int *a, mp_digit d, mp_int *b)
{
    mp_int tmp;
    mp_err res;

    ARGCHK(a != NULL && b != NULL, MP_BADARG);

    if ((res = mp_init_copy(&tmp, a)) != MP_OKAY)
        return res;

    if (SIGN(&tmp) == ZPOS) {
        if ((res = s_mp_add_d(&tmp, d)) != MP_OKAY)
            goto CLEANUP;
    } else if (s_mp_cmp_d(&tmp, d) >= 0) {
        if ((res = s_mp_sub_d(&tmp, d)) != MP_OKAY)
            goto CLEANUP;
    } else {
        mp_neg(&tmp, &tmp);

        DIGIT(&tmp, 0) = d - DIGIT(&tmp, 0);
    }

    if (s_mp_cmp_d(&tmp, 0) == 0)
        SIGN(&tmp) = ZPOS;

    s_mp_exch(&tmp, b);

CLEANUP:
    mp_clear(&tmp);
    return res;

} /* end mp_add_d() */

/* }}} */

/* {{{ mp_sub_d(a, d, b) */

/*
  mp_sub_d(a, d, b)

  Compute the difference b = a - d, for a single digit d.  Respects the
  sign of its subtrahend (single digits are unsigned anyway).
 */

mp_err
mp_sub_d(const mp_int *a, mp_digit d, mp_int *b)
{
    mp_int tmp;
    mp_err res;

    ARGCHK(a != NULL && b != NULL, MP_BADARG);

    if ((res = mp_init_copy(&tmp, a)) != MP_OKAY)
        return res;

    if (SIGN(&tmp) == NEG) {
        if ((res = s_mp_add_d(&tmp, d)) != MP_OKAY)
            goto CLEANUP;
    } else if (s_mp_cmp_d(&tmp, d) >= 0) {
        if ((res = s_mp_sub_d(&tmp, d)) != MP_OKAY)
            goto CLEANUP;
    } else {
        mp_neg(&tmp, &tmp);

        DIGIT(&tmp, 0) = d - DIGIT(&tmp, 0);
        SIGN(&tmp) = NEG;
    }

    if (s_mp_cmp_d(&tmp, 0) == 0)
        SIGN(&tmp) = ZPOS;

    s_mp_exch(&tmp, b);

CLEANUP:
    mp_clear(&tmp);
    return res;

} /* end mp_sub_d() */

/* }}} */

/* {{{ mp_mul_d(a, d, b) */

/*
  mp_mul_d(a, d, b)

  Compute the product b = a * d, for a single digit d.  Respects the sign
  of its multiplicand (single digits are unsigned anyway)
 */

mp_err
mp_mul_d(const mp_int *a, mp_digit d, mp_int *b)
{
    mp_err res;

    ARGCHK(a != NULL && b != NULL, MP_BADARG);

    if (d == 0) {
        mp_zero(b);
        return MP_OKAY;
    }

    if ((res = mp_copy(a, b)) != MP_OKAY)
        return res;

    res = s_mp_mul_d(b, d);

    return res;

} /* end mp_mul_d() */

/* }}} */

/* {{{ mp_mul_2(a, c) */

mp_err
mp_mul_2(const mp_int *a, mp_int *c)
{
    mp_err res;

    ARGCHK(a != NULL && c != NULL, MP_BADARG);

    if ((res = mp_copy(a, c)) != MP_OKAY)
        return res;

    return s_mp_mul_2(c);

} /* end mp_mul_2() */

/* }}} */

/* {{{ mp_div_d(a, d, q, r) */

/*
  mp_div_d(a, d, q, r)

  Compute the quotient q = a / d and remainder r = a mod d, for a
  single digit d.  Respects the sign of its divisor (single digits are
  unsigned anyway).
 */

mp_err
mp_div_d(const mp_int *a, mp_digit d, mp_int *q, mp_digit *r)
{
    mp_err res;
    mp_int qp;
    mp_digit rem = 0;
    int pow;

    ARGCHK(a != NULL, MP_BADARG);

    if (d == 0)
        return MP_RANGE;

    /* Shortcut for powers of two ... */
    if ((pow = s_mp_ispow2d(d)) >= 0) {
        mp_digit mask;

        mask = ((mp_digit)1 << pow) - 1;
        rem = DIGIT(a, 0) & mask;

        if (q) {
            if ((res = mp_copy(a, q)) != MP_OKAY) {
                return res;
            }
            s_mp_div_2d(q, pow);
        }

        if (r)
            *r = rem;

        return MP_OKAY;
    }

    if ((res = mp_init_copy(&qp, a)) != MP_OKAY)
        return res;

    res = s_mp_div_d(&qp, d, &rem);

    if (s_mp_cmp_d(&qp, 0) == 0)
        SIGN(&qp) = ZPOS;

    if (r) {
        *r = rem;
    }

    if (q)
        s_mp_exch(&qp, q);

    mp_clear(&qp);
    return res;

} /* end mp_div_d() */

/* }}} */

/* {{{ mp_div_2(a, c) */

/*
  mp_div_2(a, c)

  Compute c = a / 2, disregarding the remainder.
 */

mp_err
mp_div_2(const mp_int *a, mp_int *c)
{
    mp_err res;

    ARGCHK(a != NULL && c != NULL, MP_BADARG);

    if ((res = mp_copy(a, c)) != MP_OKAY)
        return res;

    s_mp_div_2(c);

    return MP_OKAY;

} /* end mp_div_2() */

/* }}} */

/* {{{ mp_expt_d(a, d, b) */

mp_err
mp_expt_d(const mp_int *a, mp_digit d, mp_int *c)
{
    mp_int s, x;
    mp_err res;

    ARGCHK(a != NULL && c != NULL, MP_BADARG);

    if ((res = mp_init(&s)) != MP_OKAY)
        return res;
    if ((res = mp_init_copy(&x, a)) != MP_OKAY)
        goto X;

    DIGIT(&s, 0) = 1;

    while (d != 0) {
        if (d & 1) {
            if ((res = s_mp_mul(&s, &x)) != MP_OKAY)
                goto CLEANUP;
        }

        d /= 2;

        if ((res = s_mp_sqr(&x)) != MP_OKAY)
            goto CLEANUP;
    }

    s_mp_exch(&s, c);

CLEANUP:
    mp_clear(&x);
X:
    mp_clear(&s);

    return res;

} /* end mp_expt_d() */

/* }}} */

/* }}} */

/*------------------------------------------------------------------------*/
/* {{{ Full arithmetic */

/* {{{ mp_abs(a, b) */

/*
  mp_abs(a, b)

  Compute b = |a|.  'a' and 'b' may be identical.
 */

mp_err
mp_abs(const mp_int *a, mp_int *b)
{
    mp_err res;

    ARGCHK(a != NULL && b != NULL, MP_BADARG);

    if ((res = mp_copy(a, b)) != MP_OKAY)
        return res;

    SIGN(b) = ZPOS;

    return MP_OKAY;

} /* end mp_abs() */

/* }}} */

/* {{{ mp_neg(a, b) */

/*
  mp_neg(a, b)

  Compute b = -a.  'a' and 'b' may be identical.
 */

mp_err
mp_neg(const mp_int *a, mp_int *b)
{
    mp_err res;

    ARGCHK(a != NULL && b != NULL, MP_BADARG);

    if ((res = mp_copy(a, b)) != MP_OKAY)
        return res;

    if (s_mp_cmp_d(b, 0) == MP_EQ)
        SIGN(b) = ZPOS;
    else
        SIGN(b) = (SIGN(b) == NEG) ? ZPOS : NEG;

    return MP_OKAY;

} /* end mp_neg() */

/* }}} */

/* {{{ mp_add(a, b, c) */

/*
  mp_add(a, b, c)

  Compute c = a + b.  All parameters may be identical.
 */

mp_err
mp_add(const mp_int *a, const mp_int *b, mp_int *c)
{
    mp_err res;

    ARGCHK(a != NULL && b != NULL && c != NULL, MP_BADARG);

    if (SIGN(a) == SIGN(b)) { /* same sign:  add values, keep sign */
        MP_CHECKOK(s_mp_add_3arg(a, b, c));
    } else if (s_mp_cmp(a, b) >= 0) { /* different sign: |a| >= |b|   */
        MP_CHECKOK(s_mp_sub_3arg(a, b, c));
    } else { /* different sign: |a|  < |b|   */
        MP_CHECKOK(s_mp_sub_3arg(b, a, c));
    }

    if (s_mp_cmp_d(c, 0) == MP_EQ)
        SIGN(c) = ZPOS;

CLEANUP:
    return res;

} /* end mp_add() */

/* }}} */

/* {{{ mp_sub(a, b, c) */

/*
  mp_sub(a, b, c)

  Compute c = a - b.  All parameters may be identical.
 */

mp_err
mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
{
    mp_err res;
    int magDiff;

    ARGCHK(a != NULL && b != NULL && c != NULL, MP_BADARG);

    if (a == b) {
        mp_zero(c);
        return MP_OKAY;
    }

    if (MP_SIGN(a) != MP_SIGN(b)) {
        MP_CHECKOK(s_mp_add_3arg(a, b, c));
    } else if (!(magDiff = s_mp_cmp(a, b))) {
        mp_zero(c);
        res = MP_OKAY;
    } else if (magDiff > 0) {
        MP_CHECKOK(s_mp_sub_3arg(a, b, c));
    } else {
        MP_CHECKOK(s_mp_sub_3arg(b, a, c));
        MP_SIGN(c) = !MP_SIGN(a);
    }

    if (s_mp_cmp_d(c, 0) == MP_EQ)
        MP_SIGN(c) = MP_ZPOS;

CLEANUP:
    return res;

} /* end mp_sub() */

/* }}} */

/* {{{ s_mp_mulg(a, b, c) */

/*
  s_mp_mulg(a, b, c)

  Compute c = a * b.  All parameters may be identical. if constantTime is set,
  then the operations are done in constant time. The original is mostly
  constant time as long as s_mpv_mul_d_add() is constant time. This is true
  of the x86 assembler, as well as the current c code.
 */
mp_err
s_mp_mulg(const mp_int *a, const mp_int *b, mp_int *c, int constantTime)
{
    mp_digit *pb;
    mp_int tmp;
    mp_err res;
    mp_size ib;
    mp_size useda, usedb;

    ARGCHK(a != NULL && b != NULL && c != NULL, MP_BADARG);

    if (a == c) {
        if ((res = mp_init_copy(&tmp, a)) != MP_OKAY)
            return res;
        if (a == b)
            b = &tmp;
        a = &tmp;
    } else if (b == c) {
        if ((res = mp_init_copy(&tmp, b)) != MP_OKAY)
            return res;
        b = &tmp;
    } else {
        MP_DIGITS(&tmp) = 0;
    }

    if (MP_USED(a) < MP_USED(b)) {
        const mp_int *xch = b; /* switch a and b, to do fewer outer loops */
        b = a;
        a = xch;
    }

    MP_USED(c) = 1;
    MP_DIGIT(c, 0) = 0;
    if ((res = s_mp_pad(c, USED(a) + USED(b))) != MP_OKAY)
        goto CLEANUP;

#ifdef NSS_USE_COMBA
    /* comba isn't constant time because it clamps! If we cared
     * (we needed a constant time version of multiply that was 'faster'
     * we could easily pass constantTime down to the comba code and
     * get it to skip the clamp... but here are assembler versions
     * which add comba to platforms that can't compile the normal
     * comba's imbedded assembler which would also need to change, so
     * for now we just skip comba when we are running constant time. */
    if (!constantTime && (MP_USED(a) == MP_USED(b)) && IS_POWER_OF_2(MP_USED(b))) {
        if (MP_USED(a) == 4) {
            s_mp_mul_comba_4(a, b, c);
            goto CLEANUP;
        }
        if (MP_USED(a) == 8) {
            s_mp_mul_comba_8(a, b, c);
            goto CLEANUP;
        }
        if (MP_USED(a) == 16) {
            s_mp_mul_comba_16(a, b, c);
            goto CLEANUP;
        }
        if (MP_USED(a) == 32) {
            s_mp_mul_comba_32(a, b, c);
            goto CLEANUP;
        }
    }
#endif

    pb = MP_DIGITS(b);
    s_mpv_mul_d(MP_DIGITS(a), MP_USED(a), *pb++, MP_DIGITS(c));

    /* Outer loop:  Digits of b */
    useda = MP_USED(a);
    usedb = MP_USED(b);
    for (ib = 1; ib < usedb; ib++) {
        mp_digit b_i = *pb++;

        /* Inner product:  Digits of a */
        if (constantTime || b_i)
            s_mpv_mul_d_add(MP_DIGITS(a), useda, b_i, MP_DIGITS(c) + ib);
        else
            MP_DIGIT(c, ib + useda) = b_i;
    }

    if (!constantTime) {
        s_mp_clamp(c);
    }

    if (SIGN(a) == SIGN(b) || s_mp_cmp_d(c, 0) == MP_EQ)
        SIGN(c) = ZPOS;
    else
        SIGN(c) = NEG;

CLEANUP:
    mp_clear(&tmp);
    return res;
} /* end smp_mulg() */

/* }}} */

/* {{{ mp_mul(a, b, c) */

/*
  mp_mul(a, b, c)

  Compute c = a * b.  All parameters may be identical.
 */

mp_err
mp_mul(const mp_int *a, const mp_int *b, mp_int *c)
{
    return s_mp_mulg(a, b, c, 0);
} /* end mp_mul() */

/* }}} */

/* {{{ mp_mulCT(a, b, c) */

/*
  mp_mulCT(a, b, c)

  Compute c = a * b. In constant time. Parameters may not be identical.
  NOTE: a and b may be modified.
 */

mp_err
mp_mulCT(mp_int *a, mp_int *b, mp_int *c, mp_size setSize)
{
    mp_err res;

    /* make the multiply values fixed length so multiply
     * doesn't leak the length. at this point all the
     * values are blinded, but once we finish we want the
     * output size to be hidden (so no clamping the out put) */
    MP_CHECKOK(s_mp_pad(a, setSize));
    MP_CHECKOK(s_mp_pad(b, setSize));
    MP_CHECKOK(s_mp_pad(c, 2 * setSize));
    MP_CHECKOK(s_mp_mulg(a, b, c, 1));
CLEANUP:
    return res;
} /* end mp_mulCT() */

/* }}} */

/* {{{ mp_sqr(a, sqr) */

#if MP_SQUARE
/*
  Computes the square of a.  This can be done more
  efficiently than a general multiplication, because many of the
  computation steps are redundant when squaring.  The inner product
  step is a bit more complicated, but we save a fair number of
  iterations of the multiplication loop.
 */

/* sqr = a^2;   Caller provides both a and tmp; */
mp_err
mp_sqr(const mp_int *a, mp_int *sqr)
{
    mp_digit *pa;
    mp_digit d;
    mp_err res;
    mp_size ix;
    mp_int tmp;
    int count;

    ARGCHK(a != NULL && sqr != NULL, MP_BADARG);

    if (a == sqr) {
        if ((res = mp_init_copy(&tmp, a)) != MP_OKAY)
            return res;
        a = &tmp;
    } else {
        DIGITS(&tmp) = 0;
        res = MP_OKAY;
    }

    ix = 2 * MP_USED(a);
    if (ix > MP_ALLOC(sqr)) {
        MP_USED(sqr) = 1;
        MP_CHECKOK(s_mp_grow(sqr, ix));
    }
    MP_USED(sqr) = ix;
    MP_DIGIT(sqr, 0) = 0;

#ifdef NSS_USE_COMBA
    if (IS_POWER_OF_2(MP_USED(a))) {
        if (MP_USED(a) == 4) {
            s_mp_sqr_comba_4(a, sqr);
            goto CLEANUP;
        }
        if (MP_USED(a) == 8) {
            s_mp_sqr_comba_8(a, sqr);
            goto CLEANUP;
        }
        if (MP_USED(a) == 16) {
            s_mp_sqr_comba_16(a, sqr);
            goto CLEANUP;
        }
        if (MP_USED(a) == 32) {
            s_mp_sqr_comba_32(a, sqr);
            goto CLEANUP;
        }
    }
#endif

    pa = MP_DIGITS(a);
    count = MP_USED(a) - 1;
    if (count > 0) {
        d = *pa++;
        s_mpv_mul_d(pa, count, d, MP_DIGITS(sqr) + 1);
        for (ix = 3; --count > 0; ix += 2) {
            d = *pa++;
            s_mpv_mul_d_add(pa, count, d, MP_DIGITS(sqr) + ix);
        }                                    /* for(ix ...) */
        MP_DIGIT(sqr, MP_USED(sqr) - 1) = 0; /* above loop stopped short of this. */

        /* now sqr *= 2 */
        s_mp_mul_2(sqr);
    } else {
        MP_DIGIT(sqr, 1) = 0;
    }

    /* now add the squares of the digits of a to sqr. */
    s_mpv_sqr_add_prop(MP_DIGITS(a), MP_USED(a), MP_DIGITS(sqr));

    SIGN(sqr) = ZPOS;
    s_mp_clamp(sqr);

CLEANUP:
    mp_clear(&tmp);
    return res;

} /* end mp_sqr() */
#endif

/* }}} */

/* {{{ mp_div(a, b, q, r) */

/*
  mp_div(a, b, q, r)

  Compute q = a / b and r = a mod b.  Input parameters may be re-used
  as output parameters.  If q or r is NULL, that portion of the
  computation will be discarded (although it will still be computed)
 */
mp_err
mp_div(const mp_int *a, const mp_int *b, mp_int *q, mp_int *r)
{
    mp_err res;
    mp_int *pQ, *pR;
    mp_int qtmp, rtmp, btmp;
    int cmp;
    mp_sign signA;
    mp_sign signB;

    ARGCHK(a != NULL && b != NULL, MP_BADARG);

    signA = MP_SIGN(a);
    signB = MP_SIGN(b);

    if (mp_cmp_z(b) == MP_EQ)
        return MP_RANGE;

    DIGITS(&qtmp) = 0;
    DIGITS(&rtmp) = 0;
    DIGITS(&btmp) = 0;

    /* Set up some temporaries... */
    if (!r || r == a || r == b) {
        MP_CHECKOK(mp_init_copy(&rtmp, a));
        pR = &rtmp;
    } else {
        MP_CHECKOK(mp_copy(a, r));
        pR = r;
    }

    if (!q || q == a || q == b) {
        MP_CHECKOK(mp_init_size(&qtmp, MP_USED(a)));
        pQ = &qtmp;
    } else {
        MP_CHECKOK(s_mp_pad(q, MP_USED(a)));
        pQ = q;
        mp_zero(pQ);
    }

    /*
      If |a| <= |b|, we can compute the solution without division;
      otherwise, we actually do the work required.
     */
    if ((cmp = s_mp_cmp(a, b)) <= 0) {
        if (cmp) {
            /* r was set to a above. */
            mp_zero(pQ);
        } else {
            mp_set(pQ, 1);
            mp_zero(pR);
        }
    } else {
        MP_CHECKOK(mp_init_copy(&btmp, b));
        MP_CHECKOK(s_mp_div(pR, &btmp, pQ));
    }

    /* Compute the signs for the output  */
    MP_SIGN(pR) = signA;        /* Sr = Sa              */
    /* Sq = ZPOS if Sa == Sb */ /* Sq = NEG if Sa != Sb */
    MP_SIGN(pQ) = (signA == signB) ? ZPOS : NEG;

    if (s_mp_cmp_d(pQ, 0) == MP_EQ)
        SIGN(pQ) = ZPOS;
    if (s_mp_cmp_d(pR, 0) == MP_EQ)
        SIGN(pR) = ZPOS;

    /* Copy output, if it is needed      */
    if (q && q != pQ)
        s_mp_exch(pQ, q);

    if (r && r != pR)
        s_mp_exch(pR, r);

CLEANUP:
    mp_clear(&btmp);
    mp_clear(&rtmp);
    mp_clear(&qtmp);

    return res;

} /* end mp_div() */

/* }}} */

/* {{{ mp_div_2d(a, d, q, r) */

mp_err
mp_div_2d(const mp_int *a, mp_digit d, mp_int *q, mp_int *r)
{
    mp_err res;

    ARGCHK(a != NULL, MP_BADARG);

    if (q) {
        if ((res = mp_copy(a, q)) != MP_OKAY)
            return res;
    }
    if (r) {
        if ((res = mp_copy(a, r)) != MP_OKAY)
            return res;
    }
    if (q) {
        s_mp_div_2d(q, d);
    }
    if (r) {
        s_mp_mod_2d(r, d);
    }

    return MP_OKAY;

} /* end mp_div_2d() */

/* }}} */

/* {{{ mp_expt(a, b, c) */

/*
  mp_expt(a, b, c)

  Compute c = a ** b, that is, raise a to the b power.  Uses a
  standard iterative square-and-multiply technique.
 */

mp_err
mp_expt(mp_int *a, mp_int *b, mp_int *c)
{
    mp_int s, x;
    mp_err res;
    mp_digit d;
    unsigned int dig, bit;

    ARGCHK(a != NULL && b != NULL && c != NULL, MP_BADARG);

    if (mp_cmp_z(b) < 0)
        return MP_RANGE;

    if ((res = mp_init(&s)) != MP_OKAY)
        return res;

    mp_set(&s, 1);

    if ((res = mp_init_copy(&x, a)) != MP_OKAY)
        goto X;

    /* Loop over low-order digits in ascending order */
    for (dig = 0; dig < (USED(b) - 1); dig++) {
        d = DIGIT(b, dig);

        /* Loop over bits of each non-maximal digit */
        for (bit = 0; bit < DIGIT_BIT; bit++) {
            if (d & 1) {
                if ((res = s_mp_mul(&s, &x)) != MP_OKAY)
                    goto CLEANUP;
            }

            d >>= 1;

            if ((res = s_mp_sqr(&x)) != MP_OKAY)
                goto CLEANUP;
        }
    }

    /* Consider now the last digit... */
    d = DIGIT(b, dig);

    while (d) {
        if (d & 1) {
            if ((res = s_mp_mul(&s, &x)) != MP_OKAY)
                goto CLEANUP;
        }

        d >>= 1;

        if ((res = s_mp_sqr(&x)) != MP_OKAY)
            goto CLEANUP;
    }

    if (mp_iseven(b))
        SIGN(&s) = SIGN(a);

    res = mp_copy(&s, c);

CLEANUP:
    mp_clear(&x);
X:
    mp_clear(&s);

    return res;

} /* end mp_expt() */

/* }}} */

/* {{{ mp_2expt(a, k) */

/* Compute a = 2^k */

mp_err
mp_2expt(mp_int *a, mp_digit k)
{
    ARGCHK(a != NULL, MP_BADARG);

    return s_mp_2expt(a, k);

} /* end mp_2expt() */

/* }}} */

/* {{{ mp_mod(a, m, c) */

/*
  mp_mod(a, m, c)

  Compute c = a (mod m).  Result will always be 0 <= c < m.
 */

mp_err
mp_mod(const mp_int *a, const mp_int *m, mp_int *c)
{
    mp_err res;
    int mag;

    ARGCHK(a != NULL && m != NULL && c != NULL, MP_BADARG);

    if (SIGN(m) == NEG)
        return MP_RANGE;

    /*
     If |a| > m, we need to divide to get the remainder and take the
     absolute value.

     If |a| < m, we don't need to do any division, just copy and adjust
     the sign (if a is negative).

     If |a| == m, we can simply set the result to zero.

     This order is intended to minimize the average path length of the
     comparison chain on common workloads -- the most frequent cases are
     that |a| != m, so we do those first.
     */
    if ((mag = s_mp_cmp(a, m)) > 0) {
        if ((res = mp_div(a, m, NULL, c)) != MP_OKAY)
            return res;

        if (SIGN(c) == NEG) {
            if ((res = mp_add(c, m, c)) != MP_OKAY)
                return res;
        }

    } else if (mag < 0) {
        if ((res = mp_copy(a, c)) != MP_OKAY)
            return res;

        if (mp_cmp_z(a) < 0) {
            if ((res = mp_add(c, m, c)) != MP_OKAY)
                return res;
        }

    } else {
        mp_zero(c);
    }

    return MP_OKAY;

} /* end mp_mod() */

/* }}} */

/* {{{ s_mp_subCT_d(a, b, borrow, c) */

/*
  s_mp_subCT_d(a, b, borrow, c)

  Compute c = (a -b) - subtract in constant time. returns borrow
 */
mp_digit
s_mp_subCT_d(mp_digit a, mp_digit b, mp_digit borrow, mp_digit *ret)
{
    *ret = a - b - borrow;
    return MP_CT_LTU(a, *ret) | (MP_CT_EQ(a, *ret) & borrow);
} /*  s_mp_subCT_d() */

/* }}} */

/* {{{ mp_subCT(a, b, ret, borrow) */

/* return ret= a - b and borrow in borrow. done in constant time.
 * b could be modified.
 */
mp_err
mp_subCT(const mp_int *a, mp_int *b, mp_int *ret, mp_digit *borrow)
{
    mp_size used_a = MP_USED(a);
    mp_size i;
    mp_err res;

    MP_CHECKOK(s_mp_pad(b, used_a));
    MP_CHECKOK(s_mp_pad(ret, used_a));
    *borrow = 0;
    for (i = 0; i < used_a; i++) {
        *borrow = s_mp_subCT_d(MP_DIGIT(a, i), MP_DIGIT(b, i), *borrow,
                               &MP_DIGIT(ret, i));
    }

    res = MP_OKAY;
CLEANUP:
    return res;
} /*  end mp_subCT() */

/* }}} */

/* {{{ mp_selectCT(cond, a, b, ret) */

/*
 * return ret= cond ? a : b; cond should be either 0 or 1
 */
mp_err
mp_selectCT(mp_digit cond, const mp_int *a, const mp_int *b, mp_int *ret)
{
    mp_size used_a = MP_USED(a);
    mp_err res;
    mp_size i;

    cond *= MP_DIGIT_MAX;

    /* we currently require these to be equal on input,
     * we could use pad to extend one of them, but that might
     * leak data as it wouldn't be constant time */
    if (used_a != MP_USED(b)) {
        return MP_BADARG;
    }

    MP_CHECKOK(s_mp_pad(ret, used_a));
    for (i = 0; i < used_a; i++) {
        MP_DIGIT(ret, i) = MP_CT_SEL_DIGIT(cond, MP_DIGIT(a, i), MP_DIGIT(b, i));
    }
    res = MP_OKAY;
CLEANUP:
    return res;
} /* end mp_selectCT() */

/* {{{ mp_reduceCT(a, m, c) */

/*
  mp_reduceCT(a, m, c)

  Compute c = aR^-1 (mod m) in constant time.
   input should be in montgomery form. If input is the
   result of a montgomery multiply then out put will be
   in mongomery form.
   Result will be reduced to MP_USED(m), but not be
   clamped.
 */

mp_err
mp_reduceCT(const mp_int *a, const mp_int *m, mp_digit n0i, mp_int *c)
{
    mp_size used_m = MP_USED(m);
    mp_size used_c = used_m * 2 + 1;
    mp_digit *m_digits, *c_digits;
    mp_size i;
    mp_digit borrow, carry;
    mp_err res;
    mp_int sub;

    MP_DIGITS(&sub) = 0;
    MP_CHECKOK(mp_init_size(&sub, used_m));

    if (a != c) {
        MP_CHECKOK(mp_copy(a, c));
    }
    MP_CHECKOK(s_mp_pad(c, used_c));
    m_digits = MP_DIGITS(m);
    c_digits = MP_DIGITS(c);
    for (i = 0; i < used_m; i++) {
        mp_digit m_i = MP_DIGIT(c, i) * n0i;
        s_mpv_mul_d_add_propCT(m_digits, used_m, m_i, c_digits++, used_c--);
    }
    s_mp_rshd(c, used_m);
    /* MP_USED(c) should be used_m+1 with the high word being any carry
     * from the previous multiply, save that carry and drop the high
     * word for the substraction below */
    carry = MP_DIGIT(c, used_m);
    MP_DIGIT(c, used_m) = 0;
    MP_USED(c) = used_m;
    /* mp_subCT wants c and m to be the same size, we've already
     * guarrenteed that in the previous statement, so mp_subCT won't actually
     * modify m, so it's safe to recast */
    MP_CHECKOK(mp_subCT(c, (mp_int *)m, &sub, &borrow));

    /* we return c-m if c >= m no borrow or there was a borrow and a carry */
    MP_CHECKOK(mp_selectCT(borrow ^ carry, c, &sub, c));
    res = MP_OKAY;
CLEANUP:
    mp_clear(&sub);
    return res;
} /* end mp_reduceCT() */

/* }}} */

/* {{{ mp_mod_d(a, d, c) */

/*
  mp_mod_d(a, d, c)

  Compute c = a (mod d).  Result will always be 0 <= c < d
 */
mp_err
mp_mod_d(const mp_int *a, mp_digit d, mp_digit *c)
{
    mp_err res;
    mp_digit rem;

    ARGCHK(a != NULL && c != NULL, MP_BADARG);

    if (s_mp_cmp_d(a, d) > 0) {
        if ((res = mp_div_d(a, d, NULL, &rem)) != MP_OKAY)
            return res;

    } else {
        if (SIGN(a) == NEG)
            rem = d - DIGIT(a, 0);
        else
            rem = DIGIT(a, 0);
    }

    if (c)
        *c = rem;

    return MP_OKAY;

} /* end mp_mod_d() */

/* }}} */

/* }}} */

/*------------------------------------------------------------------------*/
/* {{{ Modular arithmetic */

#if MP_MODARITH
/* {{{ mp_addmod(a, b, m, c) */

/*
  mp_addmod(a, b, m, c)

  Compute c = (a + b) mod m
 */

mp_err
mp_addmod(const mp_int *a, const mp_int *b, const mp_int *m, mp_int *c)
{
    mp_err res;

    ARGCHK(a != NULL && b != NULL && m != NULL && c != NULL, MP_BADARG);

    if ((res = mp_add(a, b, c)) != MP_OKAY)
        return res;
    if ((res = mp_mod(c, m, c)) != MP_OKAY)
        return res;

    return MP_OKAY;
}

/* }}} */

/* {{{ mp_submod(a, b, m, c) */

/*
  mp_submod(a, b, m, c)

  Compute c = (a - b) mod m
 */

mp_err
mp_submod(const mp_int *a, const mp_int *b, const mp_int *m, mp_int *c)
{
    mp_err res;

    ARGCHK(a != NULL && b != NULL && m != NULL && c != NULL, MP_BADARG);

    if ((res = mp_sub(a, b, c)) != MP_OKAY)
        return res;
    if ((res = mp_mod(c, m, c)) != MP_OKAY)
        return res;

    return MP_OKAY;
}

/* }}} */

/* {{{ mp_mulmod(a, b, m, c) */

/*
  mp_mulmod(a, b, m, c)

  Compute c = (a * b) mod m
 */

mp_err
mp_mulmod(const mp_int *a, const mp_int *b, const mp_int *m, mp_int *c)
{
    mp_err res;

    ARGCHK(a != NULL && b != NULL && m != NULL && c != NULL, MP_BADARG);

    if ((res = mp_mul(a, b, c)) != MP_OKAY)
        return res;
    if ((res = mp_mod(c, m, c)) != MP_OKAY)
        return res;

    return MP_OKAY;
}

/* }}} */

/* {{{ mp_mulmontmodCT(a, b, m, c) */

/*
  mp_mulmontmodCT(a, b, m, c)

  Compute c = (a * b) mod m in constant time wrt a and b. either a or b
  should be in montgomery form and the output is native. If both a and b
  are in montgomery form, then the output will also be in montgomery form
  and can be recovered with an mp_reduceCT call.
  NOTE: a and b may be modified.
 */

mp_err
mp_mulmontmodCT(mp_int *a, mp_int *b, const mp_int *m, mp_digit n0i,
                mp_int *c)
{
    mp_err res;

    ARGCHK(a != NULL && b != NULL && m != NULL && c != NULL, MP_BADARG);

    if ((res = mp_mulCT(a, b, c, MP_USED(m))) != MP_OKAY)
        return res;

    if ((res = mp_reduceCT(c, m, n0i, c)) != MP_OKAY)
        return res;

    return MP_OKAY;
}

/* }}} */

/* {{{ mp_sqrmod(a, m, c) */

#if MP_SQUARE
mp_err
mp_sqrmod(const mp_int *a, const mp_int *m, mp_int *c)
{
    mp_err res;

    ARGCHK(a != NULL && m != NULL && c != NULL, MP_BADARG);

    if ((res = mp_sqr(a, c)) != MP_OKAY)
        return res;
    if ((res = mp_mod(c, m, c)) != MP_OKAY)
        return res;

    return MP_OKAY;

} /* end mp_sqrmod() */
#endif

/* }}} */

/* {{{ s_mp_exptmod(a, b, m, c) */

/*
  s_mp_exptmod(a, b, m, c)

  Compute c = (a ** b) mod m.  Uses a standard square-and-multiply
  method with modular reductions at each step. (This is basically the
  same code as mp_expt(), except for the addition of the reductions)

  The modular reductions are done using Barrett's algorithm (see
  s_mp_reduce() below for details)
 */

mp_err
s_mp_exptmod(const mp_int *a, const mp_int *b, const mp_int *m, mp_int *c)
{
    mp_int s, x, mu;
    mp_err res;
    mp_digit d;
    unsigned int dig, bit;

    ARGCHK(a != NULL && b != NULL && c != NULL && m != NULL, MP_BADARG);

    if (mp_cmp_z(b) < 0 || mp_cmp_z(m) <= 0)
        return MP_RANGE;

    if ((res = mp_init(&s)) != MP_OKAY)
        return res;
    if ((res = mp_init_copy(&x, a)) != MP_OKAY ||
        (res = mp_mod(&x, m, &x)) != MP_OKAY)
        goto X;
    if ((res = mp_init(&mu)) != MP_OKAY)
        goto MU;

    mp_set(&s, 1);

    /* mu = b^2k / m */
    if ((res = s_mp_add_d(&mu, 1)) != MP_OKAY)
        goto CLEANUP;
    if ((res = s_mp_lshd(&mu, 2 * USED(m))) != MP_OKAY)
        goto CLEANUP;
    if ((res = mp_div(&mu, m, &mu, NULL)) != MP_OKAY)
        goto CLEANUP;

    /* Loop over digits of b in ascending order, except highest order */
    for (dig = 0; dig < (USED(b) - 1); dig++) {
        d = DIGIT(b, dig);

        /* Loop over the bits of the lower-order digits */
        for (bit = 0; bit < DIGIT_BIT; bit++) {
            if (d & 1) {
                if ((res = s_mp_mul(&s, &x)) != MP_OKAY)
                    goto CLEANUP;
                if ((res = s_mp_reduce(&s, m, &mu)) != MP_OKAY)
                    goto CLEANUP;
            }

            d >>= 1;

            if ((res = s_mp_sqr(&x)) != MP_OKAY)
                goto CLEANUP;
            if ((res = s_mp_reduce(&x, m, &mu)) != MP_OKAY)
                goto CLEANUP;
        }
    }

    /* Now do the last digit... */
    d = DIGIT(b, dig);

    while (d) {
        if (d & 1) {
            if ((res = s_mp_mul(&s, &x)) != MP_OKAY)
                goto CLEANUP;
            if ((res = s_mp_reduce(&s, m, &mu)) != MP_OKAY)
                goto CLEANUP;
        }

        d >>= 1;

        if ((res = s_mp_sqr(&x)) != MP_OKAY)
            goto CLEANUP;
        if ((res = s_mp_reduce(&x, m, &mu)) != MP_OKAY)
            goto CLEANUP;
    }

    s_mp_exch(&s, c);

CLEANUP:
    mp_clear(&mu);
MU:
    mp_clear(&x);
X:
    mp_clear(&s);

    return res;

} /* end s_mp_exptmod() */

/* }}} */

/* {{{ mp_exptmod_d(a, d, m, c) */

mp_err
mp_exptmod_d(const mp_int *a, mp_digit d, const mp_int *m, mp_int *c)
{
    mp_int s, x;
    mp_err res;

    ARGCHK(a != NULL && c != NULL && m != NULL, MP_BADARG);

    if ((res = mp_init(&s)) != MP_OKAY)
        return res;
    if ((res = mp_init_copy(&x, a)) != MP_OKAY)
        goto X;

    mp_set(&s, 1);

    while (d != 0) {
        if (d & 1) {
            if ((res = s_mp_mul(&s, &x)) != MP_OKAY ||
                (res = mp_mod(&s, m, &s)) != MP_OKAY)
                goto CLEANUP;
        }

        d /= 2;

        if ((res = s_mp_sqr(&x)) != MP_OKAY ||
            (res = mp_mod(&x, m, &x)) != MP_OKAY)
            goto CLEANUP;
    }

    s_mp_exch(&s, c);

CLEANUP:
    mp_clear(&x);
X:
    mp_clear(&s);

    return res;

} /* end mp_exptmod_d() */

/* }}} */
#endif /* if MP_MODARITH */

/* }}} */

/*------------------------------------------------------------------------*/
/* {{{ Comparison functions */

/* {{{ mp_cmp_z(a) */

/*
  mp_cmp_z(a)

  Compare a <=> 0.  Returns <0 if a<0, 0 if a=0, >0 if a>0.
 */

int
mp_cmp_z(const mp_int *a)
{
    ARGMPCHK(a != NULL);

    if (SIGN(a) == NEG)
        return MP_LT;
    else if (USED(a) == 1 && DIGIT(a, 0) == 0)
        return MP_EQ;
    else
        return MP_GT;

} /* end mp_cmp_z() */

/* }}} */

/* {{{ mp_cmp_d(a, d) */

/*
  mp_cmp_d(a, d)

  Compare a <=> d.  Returns <0 if a<d, 0 if a=d, >0 if a>d
 */

int
mp_cmp_d(const mp_int *a, mp_digit d)
{
    ARGCHK(a != NULL, MP_EQ);

    if (SIGN(a) == NEG)
        return MP_LT;

    return s_mp_cmp_d(a, d);

} /* end mp_cmp_d() */

/* }}} */

/* {{{ mp_cmp(a, b) */

int
mp_cmp(const mp_int *a, const mp_int *b)
{
    ARGCHK(a != NULL && b != NULL, MP_EQ);

    if (SIGN(a) == SIGN(b)) {
        int mag;

        if ((mag = s_mp_cmp(a, b)) == MP_EQ)
            return MP_EQ;

        if (SIGN(a) == ZPOS)
            return mag;
        else
            return -mag;

    } else if (SIGN(a) == ZPOS) {
        return MP_GT;
    } else {
        return MP_LT;
    }

} /* end mp_cmp() */

/* }}} */

/* {{{ mp_cmp_mag(a, b) */

/*
  mp_cmp_mag(a, b)

  Compares |a| <=> |b|, and returns an appropriate comparison result
 */

int
mp_cmp_mag(const mp_int *a, const mp_int *b)
{
    ARGCHK(a != NULL && b != NULL, MP_EQ);

    return s_mp_cmp(a, b);

} /* end mp_cmp_mag() */

/* }}} */

/* {{{ mp_isodd(a) */

/*
  mp_isodd(a)

  Returns a true (non-zero) value if a is odd, false (zero) otherwise.
 */
int
mp_isodd(const mp_int *a)
{
    ARGMPCHK(a != NULL);

    return (int)(DIGIT(a, 0) & 1);

} /* end mp_isodd() */

/* }}} */

/* {{{ mp_iseven(a) */

int
mp_iseven(const mp_int *a)
{
    return !mp_isodd(a);

} /* end mp_iseven() */

/* }}} */

/* }}} */

/*------------------------------------------------------------------------*/
/* {{{ Number theoretic functions */

/* {{{ mp_gcd(a, b, c) */

/*
  Computes the GCD using the constant-time algorithm
  by Bernstein and Yang (https://eprint.iacr.org/2019/266)
  "Fast constant-time gcd computation and modular inversion"
 */
mp_err
mp_gcd(mp_int *a, mp_int *b, mp_int *c)
{
    mp_err res;
    mp_digit cond = 0, mask = 0;
    mp_int g, temp, f;
    int i, j, m, bit = 1, delta = 1, shifts = 0, last = -1;
    mp_size top, flen, glen;
    mp_int *clear[3];

    ARGCHK(a != NULL && b != NULL && c != NULL, MP_BADARG);
    /*
    Early exit if either of the inputs is zero.
    Caller is responsible for the proper handling of inputs.
    */
    if (mp_cmp_z(a) == MP_EQ) {
        res = mp_copy(b, c);
        SIGN(c) = ZPOS;
        return res;
    } else if (mp_cmp_z(b) == MP_EQ) {
        res = mp_copy(a, c);
        SIGN(c) = ZPOS;
        return res;
    }

    MP_CHECKOK(mp_init(&temp));
    clear[++last] = &temp;
    MP_CHECKOK(mp_init_copy(&g, a));
    clear[++last] = &g;
    MP_CHECKOK(mp_init_copy(&f, b));
    clear[++last] = &f;

    /*
    For even case compute the number of
    shared powers of 2 in f and g.
    */
    for (i = 0; i < USED(&f) && i < USED(&g); i++) {
        mask = ~(DIGIT(&f, i) | DIGIT(&g, i));
        for (j = 0; j < MP_DIGIT_BIT; j++) {
            bit &= mask;
            shifts += bit;
            mask >>= 1;
        }
    }
    /* Reduce to the odd case by removing the powers of 2. */
    s_mp_div_2d(&f, shifts);
    s_mp_div_2d(&g, shifts);

    /* Allocate to the size of largest mp_int. */
    top = (mp_size)1 + ((USED(&f) >= USED(&g)) ? USED(&f) : USED(&g));
    MP_CHECKOK(s_mp_grow(&f, top));
    MP_CHECKOK(s_mp_grow(&g, top));
    MP_CHECKOK(s_mp_grow(&temp, top));

    /* Make sure f contains the odd value. */
    MP_CHECKOK(mp_cswap((~DIGIT(&f, 0) & 1), &f, &g, top));

    /* Upper bound for the total iterations. */
    flen = mpl_significant_bits(&f);
    glen = mpl_significant_bits(&g);
    m = 4 + 3 * ((flen >= glen) ? flen : glen);

#if defined(_MSC_VER)
#pragma warning(push)
#pragma warning(disable : 4146) // Thanks MSVC, we know what we're negating an unsigned mp_digit
#endif

    for (i = 0; i < m; i++) {
        /* Step 1: conditional swap. */
        /* Set cond if delta > 0 and g is odd. */
        cond = (-delta >> (8 * sizeof(delta) - 1)) & DIGIT(&g, 0) & 1;
        /* If cond is set replace (delta,f) with (-delta,-f). */
        delta = (-cond & -delta) | ((cond - 1) & delta);
        SIGN(&f) ^= cond;
        /* If cond is set swap f with g. */
        MP_CHECKOK(mp_cswap(cond, &f, &g, top));

        /* Step 2: elemination. */
        /* Update delta. */
        delta++;
        /* If g is odd, right shift (g+f) else right shift g. */
        MP_CHECKOK(mp_add(&g, &f, &temp));
        MP_CHECKOK(mp_cswap((DIGIT(&g, 0) & 1), &g, &temp, top));
        s_mp_div_2(&g);
    }

#if defined(_MSC_VER)
#pragma warning(pop)
#endif

    /* GCD is in f, take the absolute value. */
    SIGN(&f) = ZPOS;

    /* Add back the removed powers of 2. */
    MP_CHECKOK(s_mp_mul_2d(&f, shifts));

    MP_CHECKOK(mp_copy(&f, c));

CLEANUP:
    while (last >= 0)
        mp_clear(clear[last--]);
    return res;
} /* end mp_gcd() */

/* }}} */

/* {{{ mp_lcm(a, b, c) */

/* We compute the least common multiple using the rule:

   ab = [a, b](a, b)

   ... by computing the product, and dividing out the gcd.
 */

mp_err
mp_lcm(mp_int *a, mp_int *b, mp_int *c)
{
    mp_int gcd, prod;
    mp_err res;

    ARGCHK(a != NULL && b != NULL && c != NULL, MP_BADARG);

    /* Set up temporaries */
    if ((res = mp_init(&gcd)) != MP_OKAY)
        return res;
    if ((res = mp_init(&prod)) != MP_OKAY)
        goto GCD;

    if ((res = mp_mul(a, b, &prod)) != MP_OKAY)
        goto CLEANUP;
    if ((res = mp_gcd(a, b, &gcd)) != MP_OKAY)
        goto CLEANUP;

    res = mp_div(&prod, &gcd, c, NULL);

CLEANUP:
    mp_clear(&prod);
GCD:
    mp_clear(&gcd);

    return res;

} /* end mp_lcm() */

/* }}} */

/* {{{ mp_xgcd(a, b, g, x, y) */

/*
  mp_xgcd(a, b, g, x, y)

  Compute g = (a, b) and values x and y satisfying Bezout's identity
  (that is, ax + by = g).  This uses the binary extended GCD algorithm
  based on the Stein algorithm used for mp_gcd()
  See algorithm 14.61 in Handbook of Applied Cryptogrpahy.
 */

mp_err
mp_xgcd(const mp_int *a, const mp_int *b, mp_int *g, mp_int *x, mp_int *y)
{
    mp_int gx, xc, yc, u, v, A, B, C, D;
    mp_int *clean[9];
    mp_err res;
    int last = -1;

    if (mp_cmp_z(b) == 0)
        return MP_RANGE;

    /* Initialize all these variables we need */
    MP_CHECKOK(mp_init(&u));
    clean[++last] = &u;
    MP_CHECKOK(mp_init(&v));
    clean[++last] = &v;
    MP_CHECKOK(mp_init(&gx));
    clean[++last] = &gx;
    MP_CHECKOK(mp_init(&A));
    clean[++last] = &A;
    MP_CHECKOK(mp_init(&B));
    clean[++last] = &B;
    MP_CHECKOK(mp_init(&C));
    clean[++last] = &C;
    MP_CHECKOK(mp_init(&D));
    clean[++last] = &D;
    MP_CHECKOK(mp_init_copy(&xc, a));
    clean[++last] = &xc;
    mp_abs(&xc, &xc);
    MP_CHECKOK(mp_init_copy(&yc, b));
    clean[++last] = &yc;
    mp_abs(&yc, &yc);

    mp_set(&gx, 1);

    /* Divide by two until at least one of them is odd */
    while (mp_iseven(&xc) && mp_iseven(&yc)) {
        mp_size nx = mp_trailing_zeros(&xc);
        mp_size ny = mp_trailing_zeros(&yc);
        mp_size n = MP_MIN(nx, ny);
        s_mp_div_2d(&xc, n);
        s_mp_div_2d(&yc, n);
        MP_CHECKOK(s_mp_mul_2d(&gx, n));
    }

    MP_CHECKOK(mp_copy(&xc, &u));
    MP_CHECKOK(mp_copy(&yc, &v));
    mp_set(&A, 1);
    mp_set(&D, 1);

    /* Loop through binary GCD algorithm */
    do {
        while (mp_iseven(&u)) {
            s_mp_div_2(&u);

            if (mp_iseven(&A) && mp_iseven(&B)) {
                s_mp_div_2(&A);
                s_mp_div_2(&B);
            } else {
                MP_CHECKOK(mp_add(&A, &yc, &A));
                s_mp_div_2(&A);
                MP_CHECKOK(mp_sub(&B, &xc, &B));
                s_mp_div_2(&B);
            }
        }

        while (mp_iseven(&v)) {
            s_mp_div_2(&v);

            if (mp_iseven(&C) && mp_iseven(&D)) {
                s_mp_div_2(&C);
                s_mp_div_2(&D);
            } else {
                MP_CHECKOK(mp_add(&C, &yc, &C));
                s_mp_div_2(&C);
                MP_CHECKOK(mp_sub(&D, &xc, &D));
                s_mp_div_2(&D);
            }
        }

        if (mp_cmp(&u, &v) >= 0) {
            MP_CHECKOK(mp_sub(&u, &v, &u));
            MP_CHECKOK(mp_sub(&A, &C, &A));
            MP_CHECKOK(mp_sub(&B, &D, &B));
        } else {
            MP_CHECKOK(mp_sub(&v, &u, &v));
            MP_CHECKOK(mp_sub(&C, &A, &C));
            MP_CHECKOK(mp_sub(&D, &B, &D));
        }
    } while (mp_cmp_z(&u) != 0);

    /* copy results to output */
    if (x)
        MP_CHECKOK(mp_copy(&C, x));

    if (y)
        MP_CHECKOK(mp_copy(&D, y));

    if (g)
        MP_CHECKOK(mp_mul(&gx, &v, g));

CLEANUP:
    while (last >= 0)
        mp_clear(clean[last--]);

    return res;

} /* end mp_xgcd() */

/* }}} */

mp_size
mp_trailing_zeros(const mp_int *mp)
{
    mp_digit d;
    mp_size n = 0;
    unsigned int ix;

    if (!mp || !MP_DIGITS(mp) || !mp_cmp_z(mp))
        return n;

    for (ix = 0; !(d = MP_DIGIT(mp, ix)) && (ix < MP_USED(mp)); ++ix)
        n += MP_DIGIT_BIT;
    if (!d)
        return 0; /* shouldn't happen, but ... */
#if !defined(MP_USE_UINT_DIGIT)
    if (!(d & 0xffffffffU)) {
        d >>= 32;
        n += 32;
    }
#endif
    if (!(d & 0xffffU)) {
        d >>= 16;
        n += 16;
    }
    if (!(d & 0xffU)) {
        d >>= 8;
        n += 8;
    }
    if (!(d & 0xfU)) {
        d >>= 4;
        n += 4;
    }
    if (!(d & 0x3U)) {
        d >>= 2;
        n += 2;
    }
    if (!(d & 0x1U)) {
        d >>= 1;
        n += 1;
    }
#if MP_ARGCHK == 2
    assert(0 != (d & 1));
#endif
    return n;
}

/* Given a and prime p, computes c and k such that a*c == 2**k (mod p).
** Returns k (positive) or error (negative).
** This technique from the paper "Fast Modular Reciprocals" (unpublished)
** by Richard Schroeppel (a.k.a. Captain Nemo).
*/
mp_err
s_mp_almost_inverse(const mp_int *a, const mp_int *p, mp_int *c)
{
    mp_err res;
    mp_err k = 0;
    mp_int d, f, g;

    ARGCHK(a != NULL && p != NULL && c != NULL, MP_BADARG);

    MP_DIGITS(&d) = 0;
    MP_DIGITS(&f) = 0;
    MP_DIGITS(&g) = 0;
    MP_CHECKOK(mp_init(&d));
    MP_CHECKOK(mp_init_copy(&f, a)); /* f = a */
    MP_CHECKOK(mp_init_copy(&g, p)); /* g = p */

    mp_set(c, 1);
    mp_zero(&d);

    if (mp_cmp_z(&f) == 0) {
        res = MP_UNDEF;
    } else
        for (;;) {
            int diff_sign;
            while (mp_iseven(&f)) {
                mp_size n = mp_trailing_zeros(&f);
                if (!n) {
                    res = MP_UNDEF;
                    goto CLEANUP;
                }
                s_mp_div_2d(&f, n);
                MP_CHECKOK(s_mp_mul_2d(&d, n));
                k += n;
            }
            if (mp_cmp_d(&f, 1) == MP_EQ) { /* f == 1 */
                res = k;
                break;
            }
            diff_sign = mp_cmp(&f, &g);
            if (diff_sign < 0) { /* f < g */
                s_mp_exch(&f, &g);
                s_mp_exch(c, &d);
            } else if (diff_sign == 0) { /* f == g */
                res = MP_UNDEF;          /* a and p are not relatively prime */
                break;
            }
            if ((MP_DIGIT(&f, 0) % 4) == (MP_DIGIT(&g, 0) % 4)) {
                MP_CHECKOK(mp_sub(&f, &g, &f)); /* f = f - g */
                MP_CHECKOK(mp_sub(c, &d, c));   /* c = c - d */
            } else {
                MP_CHECKOK(mp_add(&f, &g, &f)); /* f = f + g */
                MP_CHECKOK(mp_add(c, &d, c));   /* c = c + d */
            }
        }
    if (res >= 0) {
        if (mp_cmp_mag(c, p) >= 0) {
            MP_CHECKOK(mp_div(c, p, NULL, c));
        }
        if (MP_SIGN(c) != MP_ZPOS) {
            MP_CHECKOK(mp_add(c, p, c));
        }
        res = k;
    }

CLEANUP:
    mp_clear(&d);
    mp_clear(&f);
    mp_clear(&g);
    return res;
}

/* Compute T = (P ** -1) mod MP_RADIX.  Also works for 16-bit mp_digits.
** This technique from the paper "Fast Modular Reciprocals" (unpublished)
** by Richard Schroeppel (a.k.a. Captain Nemo).
*/
mp_digit
s_mp_invmod_radix(mp_digit P)
{
    mp_digit T = P;
    T *= 2 - (P * T);
    T *= 2 - (P * T);
    T *= 2 - (P * T);
    T *= 2 - (P * T);
#if !defined(MP_USE_UINT_DIGIT)
    T *= 2 - (P * T);
    T *= 2 - (P * T);
#endif
    return T;
}

/* Given c, k, and prime p, where a*c == 2**k (mod p),
** Compute x = (a ** -1) mod p.  This is similar to Montgomery reduction.
** This technique from the paper "Fast Modular Reciprocals" (unpublished)
** by Richard Schroeppel (a.k.a. Captain Nemo).
*/
mp_err
s_mp_fixup_reciprocal(const mp_int *c, const mp_int *p, int k, mp_int *x)
{
    int k_orig = k;
    mp_digit r;
    mp_size ix;
    mp_err res;

    if (mp_cmp_z(c) < 0) {           /* c < 0 */
        MP_CHECKOK(mp_add(c, p, x)); /* x = c + p */
    } else {
        MP_CHECKOK(mp_copy(c, x)); /* x = c */
    }

    /* make sure x is large enough */
    ix = MP_HOWMANY(k, MP_DIGIT_BIT) + MP_USED(p) + 1;
    ix = MP_MAX(ix, MP_USED(x));
    MP_CHECKOK(s_mp_pad(x, ix));

    r = 0 - s_mp_invmod_radix(MP_DIGIT(p, 0));

    for (ix = 0; k > 0; ix++) {
        int j = MP_MIN(k, MP_DIGIT_BIT);
        mp_digit v = r * MP_DIGIT(x, ix);
        if (j < MP_DIGIT_BIT) {
            v &= ((mp_digit)1 << j) - 1; /* v = v mod (2 ** j) */
        }
        s_mp_mul_d_add_offset(p, v, x, ix); /* x += p * v * (RADIX ** ix) */
        k -= j;
    }
    s_mp_clamp(x);
    s_mp_div_2d(x, k_orig);
    res = MP_OKAY;

CLEANUP:
    return res;
}

/*
  Computes the modular inverse using the constant-time algorithm
  by Bernstein and Yang (https://eprint.iacr.org/2019/266)
  "Fast constant-time gcd computation and modular inversion"
 */
mp_err
s_mp_invmod_odd_m(const mp_int *a, const mp_int *m, mp_int *c)
{
    mp_err res;
    mp_digit cond = 0;
    mp_int g, f, v, r, temp;
    int i, its, delta = 1, last = -1;
    mp_size top, flen, glen;
    mp_int *clear[6];

    ARGCHK(a != NULL && m != NULL && c != NULL, MP_BADARG);
    /* Check for invalid inputs. */
    if (mp_cmp_z(a) == MP_EQ || mp_cmp_d(m, 2) == MP_LT)
        return MP_RANGE;

    if (a == m || mp_iseven(m))
        return MP_UNDEF;

    MP_CHECKOK(mp_init(&temp));
    clear[++last] = &temp;
    MP_CHECKOK(mp_init(&v));
    clear[++last] = &v;
    MP_CHECKOK(mp_init(&r));
    clear[++last] = &r;
    MP_CHECKOK(mp_init_copy(&g, a));
    clear[++last] = &g;
    MP_CHECKOK(mp_init_copy(&f, m));
    clear[++last] = &f;

    mp_set(&v, 0);
    mp_set(&r, 1);

    /* Allocate to the size of largest mp_int. */
    top = (mp_size)1 + ((USED(&f) >= USED(&g)) ? USED(&f) : USED(&g));
    MP_CHECKOK(s_mp_grow(&f, top));
    MP_CHECKOK(s_mp_grow(&g, top));
    MP_CHECKOK(s_mp_grow(&temp, top));
    MP_CHECKOK(s_mp_grow(&v, top));
    MP_CHECKOK(s_mp_grow(&r, top));

    /* Upper bound for the total iterations. */
    flen = mpl_significant_bits(&f);
    glen = mpl_significant_bits(&g);
    its = 4 + 3 * ((flen >= glen) ? flen : glen);

#if defined(_MSC_VER)
#pragma warning(push)
#pragma warning(disable : 4146) // Thanks MSVC, we know what we're negating an unsigned mp_digit
#endif

    for (i = 0; i < its; i++) {
        /* Step 1: conditional swap. */
        /* Set cond if delta > 0 and g is odd. */
        cond = (-delta >> (8 * sizeof(delta) - 1)) & DIGIT(&g, 0) & 1;
        /* If cond is set replace (delta,f,v) with (-delta,-f,-v). */
        delta = (-cond & -delta) | ((cond - 1) & delta);
        SIGN(&f) ^= cond;
        SIGN(&v) ^= cond;
        /* If cond is set swap (f,v) with (g,r). */
        MP_CHECKOK(mp_cswap(cond, &f, &g, top));
        MP_CHECKOK(mp_cswap(cond, &v, &r, top));

        /* Step 2: elemination. */
        /* Update delta */
        delta++;
        /* If g is odd replace r with (r+v). */
        MP_CHECKOK(mp_add(&r, &v, &temp));
        MP_CHECKOK(mp_cswap((DIGIT(&g, 0) & 1), &r, &temp, top));
        /* If g is odd, right shift (g+f) else right shift g. */
        MP_CHECKOK(mp_add(&g, &f, &temp));
        MP_CHECKOK(mp_cswap((DIGIT(&g, 0) & 1), &g, &temp, top));
        s_mp_div_2(&g);
        /*
        If r is even, right shift it.
        If r is odd, right shift (r+m) which is even because m is odd.
        We want the result modulo m so adding in multiples of m here vanish.
        */
        MP_CHECKOK(mp_add(&r, m, &temp));
        MP_CHECKOK(mp_cswap((DIGIT(&r, 0) & 1), &r, &temp, top));
        s_mp_div_2(&r);
    }

#if defined(_MSC_VER)
#pragma warning(pop)
#endif

    /* We have the inverse in v, propagate sign from f. */
    SIGN(&v) ^= SIGN(&f);
    /* GCD is in f, take the absolute value. */
    SIGN(&f) = ZPOS;

    /* If gcd != 1, not invertible. */
    if (mp_cmp_d(&f, 1) != MP_EQ) {
        res = MP_UNDEF;
        goto CLEANUP;
    }

    /* Return inverse modulo m. */
    MP_CHECKOK(mp_mod(&v, m, c));

CLEANUP:
    while (last >= 0)
        mp_clear(clear[last--]);
    return res;
}

/* Known good algorithm for computing modular inverse.  But slow. */
mp_err
mp_invmod_xgcd(const mp_int *a, const mp_int *m, mp_int *c)
{
    mp_int g, x;
    mp_err res;

    ARGCHK(a != NULL && m != NULL && c != NULL, MP_BADARG);

    if (mp_cmp_z(a) == 0 || mp_cmp_z(m) == 0)
        return MP_RANGE;

    MP_DIGITS(&g) = 0;
    MP_DIGITS(&x) = 0;
    MP_CHECKOK(mp_init(&x));
    MP_CHECKOK(mp_init(&g));

    MP_CHECKOK(mp_xgcd(a, m, &g, &x, NULL));

    if (mp_cmp_d(&g, 1) != MP_EQ) {
        res = MP_UNDEF;
        goto CLEANUP;
    }

    res = mp_mod(&x, m, c);
    SIGN(c) = SIGN(a);

CLEANUP:
    mp_clear(&x);
    mp_clear(&g);

    return res;
}

/* modular inverse where modulus is 2**k. */
/* c = a**-1 mod 2**k */
mp_err
s_mp_invmod_2d(const mp_int *a, mp_size k, mp_int *c)
{
    mp_err res;
    mp_size ix = k + 4;
    mp_int t0, t1, val, tmp, two2k;

    static const mp_digit d2 = 2;
    static const mp_int two = { MP_ZPOS, 1, 1, (mp_digit *)&d2 };

    if (mp_iseven(a))
        return MP_UNDEF;

#if defined(_MSC_VER)
#pragma warning(push)
#pragma warning(disable : 4146) // Thanks MSVC, we know what we're negating an unsigned mp_digit
#endif
    if (k <= MP_DIGIT_BIT) {
        mp_digit i = s_mp_invmod_radix(MP_DIGIT(a, 0));
        /* propagate the sign from mp_int */
        i = (i ^ -(mp_digit)SIGN(a)) + (mp_digit)SIGN(a);
        if (k < MP_DIGIT_BIT)
            i &= ((mp_digit)1 << k) - (mp_digit)1;
        mp_set(c, i);
        return MP_OKAY;
    }
#if defined(_MSC_VER)
#pragma warning(pop)
#endif

    MP_DIGITS(&t0) = 0;
    MP_DIGITS(&t1) = 0;
    MP_DIGITS(&val) = 0;
    MP_DIGITS(&tmp) = 0;
    MP_DIGITS(&two2k) = 0;
    MP_CHECKOK(mp_init_copy(&val, a));
    s_mp_mod_2d(&val, k);
    MP_CHECKOK(mp_init_copy(&t0, &val));
    MP_CHECKOK(mp_init_copy(&t1, &t0));
    MP_CHECKOK(mp_init(&tmp));
    MP_CHECKOK(mp_init(&two2k));
    MP_CHECKOK(s_mp_2expt(&two2k, k));
    do {
        MP_CHECKOK(mp_mul(&val, &t1, &tmp));
        MP_CHECKOK(mp_sub(&two, &tmp, &tmp));
        MP_CHECKOK(mp_mul(&t1, &tmp, &t1));
        s_mp_mod_2d(&t1, k);
        while (MP_SIGN(&t1) != MP_ZPOS) {
            MP_CHECKOK(mp_add(&t1, &two2k, &t1));
        }
        if (mp_cmp(&t1, &t0) == MP_EQ)
            break;
        MP_CHECKOK(mp_copy(&t1, &t0));
    } while (--ix > 0);
    if (!ix) {
        res = MP_UNDEF;
    } else {
        mp_exch(c, &t1);
    }

CLEANUP:
    mp_clear(&t0);
    mp_clear(&t1);
    mp_clear(&val);
    mp_clear(&tmp);
    mp_clear(&two2k);
    return res;
}

mp_err
s_mp_invmod_even_m(const mp_int *a, const mp_int *m, mp_int *c)
{
    mp_err res;
    mp_size k;
    mp_int oddFactor, evenFactor; /* factors of the modulus */
    mp_int oddPart, evenPart;     /* parts to combine via CRT. */
    mp_int C2, tmp1, tmp2;

    ARGCHK(a != NULL && m != NULL && c != NULL, MP_BADARG);

    /*static const mp_digit d1 = 1; */
    /*static const mp_int one = { MP_ZPOS, 1, 1, (mp_digit *)&d1 }; */

    if ((res = s_mp_ispow2(m)) >= 0) {
        k = res;
        return s_mp_invmod_2d(a, k, c);
    }
    MP_DIGITS(&oddFactor) = 0;
    MP_DIGITS(&evenFactor) = 0;
    MP_DIGITS(&oddPart) = 0;
    MP_DIGITS(&evenPart) = 0;
    MP_DIGITS(&C2) = 0;
    MP_DIGITS(&tmp1) = 0;
    MP_DIGITS(&tmp2) = 0;

    MP_CHECKOK(mp_init_copy(&oddFactor, m)); /* oddFactor = m */
    MP_CHECKOK(mp_init(&evenFactor));
    MP_CHECKOK(mp_init(&oddPart));
    MP_CHECKOK(mp_init(&evenPart));
    MP_CHECKOK(mp_init(&C2));
    MP_CHECKOK(mp_init(&tmp1));
    MP_CHECKOK(mp_init(&tmp2));

    k = mp_trailing_zeros(m);
    s_mp_div_2d(&oddFactor, k);
    MP_CHECKOK(s_mp_2expt(&evenFactor, k));

    /* compute a**-1 mod oddFactor. */
    MP_CHECKOK(s_mp_invmod_odd_m(a, &oddFactor, &oddPart));
    /* compute a**-1 mod evenFactor, where evenFactor == 2**k. */
    MP_CHECKOK(s_mp_invmod_2d(a, k, &evenPart));

    /* Use Chinese Remainer theorem to compute a**-1 mod m. */
    /* let m1 = oddFactor,  v1 = oddPart,
     * let m2 = evenFactor, v2 = evenPart.
     */

    /* Compute C2 = m1**-1 mod m2. */
    MP_CHECKOK(s_mp_invmod_2d(&oddFactor, k, &C2));

    /* compute u = (v2 - v1)*C2 mod m2 */
    MP_CHECKOK(mp_sub(&evenPart, &oddPart, &tmp1));
    MP_CHECKOK(mp_mul(&tmp1, &C2, &tmp2));
    s_mp_mod_2d(&tmp2, k);
    while (MP_SIGN(&tmp2) != MP_ZPOS) {
        MP_CHECKOK(mp_add(&tmp2, &evenFactor, &tmp2));
    }

    /* compute answer = v1 + u*m1 */
    MP_CHECKOK(mp_mul(&tmp2, &oddFactor, c));
    MP_CHECKOK(mp_add(&oddPart, c, c));
    /* not sure this is necessary, but it's low cost if not. */
    MP_CHECKOK(mp_mod(c, m, c));

CLEANUP:
    mp_clear(&oddFactor);
    mp_clear(&evenFactor);
    mp_clear(&oddPart);
    mp_clear(&evenPart);
    mp_clear(&C2);
    mp_clear(&tmp1);
    mp_clear(&tmp2);
    return res;
}

/* {{{ mp_invmod(a, m, c) */

/*
  mp_invmod(a, m, c)

  Compute c = a^-1 (mod m), if there is an inverse for a (mod m).
  This is equivalent to the question of whether (a, m) = 1.  If not,
  MP_UNDEF is returned, and there is no inverse.
 */

mp_err
mp_invmod(const mp_int *a, const mp_int *m, mp_int *c)
{
    ARGCHK(a != NULL && m != NULL && c != NULL, MP_BADARG);

    if (mp_cmp_z(a) == 0 || mp_cmp_z(m) == 0)
        return MP_RANGE;

    if (mp_isodd(m)) {
        return s_mp_invmod_odd_m(a, m, c);
    }
    if (mp_iseven(a))
        return MP_UNDEF; /* not invertable */

    return s_mp_invmod_even_m(a, m, c);

} /* end mp_invmod() */

/* }}} */

/* }}} */

/*------------------------------------------------------------------------*/
/* {{{ mp_print(mp, ofp) */

#if MP_IOFUNC
/*
  mp_print(mp, ofp)

  Print a textual representation of the given mp_int on the output
  stream 'ofp'.  Output is generated using the internal radix.
 */

void
mp_print(mp_int *mp, FILE *ofp)
{
    int ix;

    if (mp == NULL || ofp == NULL)
        return;

    fputc((SIGN(mp) == NEG) ? '-' : '+', ofp);

    for (ix = USED(mp) - 1; ix >= 0; ix--) {
        fprintf(ofp, DIGIT_FMT, DIGIT(mp, ix));
    }

} /* end mp_print() */

#endif /* if MP_IOFUNC */

/* }}} */

/*------------------------------------------------------------------------*/
/* {{{ More I/O Functions */

/* {{{ mp_read_raw(mp, str, len) */

/*
   mp_read_raw(mp, str, len)

   Read in a raw value (base 256) into the given mp_int
 */

mp_err
mp_read_raw(mp_int *mp, char *str, int len)
{
    int ix;
    mp_err res;
    unsigned char *ustr = (unsigned char *)str;

    ARGCHK(mp != NULL && str != NULL && len > 0, MP_BADARG);

    mp_zero(mp);

    /* Read the rest of the digits */
    for (ix = 1; ix < len; ix++) {
        if ((res = mp_mul_d(mp, 256, mp)) != MP_OKAY)
            return res;
        if ((res = mp_add_d(mp, ustr[ix], mp)) != MP_OKAY)
            return res;
    }

    /* Get sign from first byte */
    if (ustr[0])
        SIGN(mp) = NEG;
    else
        SIGN(mp) = ZPOS;

    return MP_OKAY;

} /* end mp_read_raw() */

/* }}} */

/* {{{ mp_raw_size(mp) */

int
mp_raw_size(mp_int *mp)
{
    ARGCHK(mp != NULL, 0);

    return (USED(mp) * sizeof(mp_digit)) + 1;

} /* end mp_raw_size() */

/* }}} */

/* {{{ mp_toraw(mp, str) */

mp_err
mp_toraw(mp_int *mp, char *str)
{
    int ix, jx, pos = 1;

    ARGCHK(mp != NULL && str != NULL, MP_BADARG);

    str[0] = (char)SIGN(mp);

    /* Iterate over each digit... */
    for (ix = USED(mp) - 1; ix >= 0; ix--) {
        mp_digit d = DIGIT(mp, ix);

        /* Unpack digit bytes, high order first */
        for (jx = sizeof(mp_digit) - 1; jx >= 0; jx--) {
            str[pos++] = (char)(d >> (jx * CHAR_BIT));
        }
    }

    return MP_OKAY;

} /* end mp_toraw() */

/* }}} */

/* {{{ mp_read_radix(mp, str, radix) */

/*
  mp_read_radix(mp, str, radix)

  Read an integer from the given string, and set mp to the resulting
  value.  The input is presumed to be in base 10.  Leading non-digit
  characters are ignored, and the function reads until a non-digit
  character or the end of the string.
 */

mp_err
mp_read_radix(mp_int *mp, const char *str, int radix)
{
    int ix = 0, val = 0;
    mp_err res;
    mp_sign sig = ZPOS;

    ARGCHK(mp != NULL && str != NULL && radix >= 2 && radix <= MAX_RADIX,
           MP_BADARG);

    mp_zero(mp);

    /* Skip leading non-digit characters until a digit or '-' or '+' */
    while (str[ix] &&
           (s_mp_tovalue(str[ix], radix) < 0) &&
           str[ix] != '-' &&
           str[ix] != '+') {
        ++ix;
    }

    if (str[ix] == '-') {
        sig = NEG;
        ++ix;
    } else if (str[ix] == '+') {
        sig = ZPOS; /* this is the default anyway... */
        ++ix;
    }

    while ((val = s_mp_tovalue(str[ix], radix)) >= 0) {
        if ((res = s_mp_mul_d(mp, radix)) != MP_OKAY)
            return res;
        if ((res = s_mp_add_d(mp, val)) != MP_OKAY)
            return res;
        ++ix;
    }

    if (s_mp_cmp_d(mp, 0) == MP_EQ)
        SIGN(mp) = ZPOS;
    else
        SIGN(mp) = sig;

    return MP_OKAY;

} /* end mp_read_radix() */

mp_err
mp_read_variable_radix(mp_int *a, const char *str, int default_radix)
{
    int radix = default_radix;
    int cx;
    mp_sign sig = ZPOS;
    mp_err res;

    /* Skip leading non-digit characters until a digit or '-' or '+' */
    while ((cx = *str) != 0 &&
           (s_mp_tovalue(cx, radix) < 0) &&
           cx != '-' &&
           cx != '+') {
        ++str;
    }

    if (cx == '-') {
        sig = NEG;
        ++str;
    } else if (cx == '+') {
        sig = ZPOS; /* this is the default anyway... */
        ++str;
    }

    if (str[0] == '0') {
        if ((str[1] | 0x20) == 'x') {
            radix = 16;
            str += 2;
        } else {
            radix = 8;
            str++;
        }
    }
    res = mp_read_radix(a, str, radix);
    if (res == MP_OKAY) {
        MP_SIGN(a) = (s_mp_cmp_d(a, 0) == MP_EQ) ? ZPOS : sig;
    }
    return res;
}

/* }}} */

/* {{{ mp_radix_size(mp, radix) */

int
mp_radix_size(mp_int *mp, int radix)
{
    int bits;

    if (!mp || radix < 2 || radix > MAX_RADIX)
        return 0;

    bits = USED(mp) * DIGIT_BIT - 1;

    return SIGN(mp) + s_mp_outlen(bits, radix);

} /* end mp_radix_size() */

/* }}} */

/* {{{ mp_toradix(mp, str, radix) */

mp_err
mp_toradix(mp_int *mp, char *str, int radix)
{
    int ix, pos = 0;

    ARGCHK(mp != NULL && str != NULL, MP_BADARG);
    ARGCHK(radix > 1 && radix <= MAX_RADIX, MP_RANGE);

    if (mp_cmp_z(mp) == MP_EQ) {
        str[0] = '0';
        str[1] = '\0';
    } else {
        mp_err res;
        mp_int tmp;
        mp_sign sgn;
        mp_digit rem, rdx = (mp_digit)radix;
        char ch;

        if ((res = mp_init_copy(&tmp, mp)) != MP_OKAY)
            return res;

        /* Save sign for later, and take absolute value */
        sgn = SIGN(&tmp);
        SIGN(&tmp) = ZPOS;

        /* Generate output digits in reverse order      */
        while (mp_cmp_z(&tmp) != 0) {
            if ((res = mp_div_d(&tmp, rdx, &tmp, &rem)) != MP_OKAY) {
                mp_clear(&tmp);
                return res;
            }

            /* Generate digits, use capital letters */
            ch = s_mp_todigit(rem, radix, 0);

            str[pos++] = ch;
        }

        /* Add - sign if original value was negative */
        if (sgn == NEG)
            str[pos++] = '-';

        /* Add trailing NUL to end the string        */
        str[pos--] = '\0';

        /* Reverse the digits and sign indicator     */
        ix = 0;
        while (ix < pos) {
            char tmpc = str[ix];

            str[ix] = str[pos];
            str[pos] = tmpc;
            ++ix;
            --pos;
        }

        mp_clear(&tmp);
    }

    return MP_OKAY;

} /* end mp_toradix() */

/* }}} */

/* {{{ mp_tovalue(ch, r) */

int
mp_tovalue(char ch, int r)
{
    return s_mp_tovalue(ch, r);

} /* end mp_tovalue() */

/* }}} */

/* }}} */

/* {{{ mp_strerror(ec) */

/*
  mp_strerror(ec)

  Return a string describing the meaning of error code 'ec'.  The
  string returned is allocated in static memory, so the caller should
  not attempt to modify or free the memory associated with this
  string.
 */
const char *
mp_strerror(mp_err ec)
{
    int aec = (ec < 0) ? -ec : ec;

    /* Code values are negative, so the senses of these comparisons
     are accurate */
    if (ec < MP_LAST_CODE || ec > MP_OKAY) {
        return mp_err_string[0]; /* unknown error code */
    } else {
        return mp_err_string[aec + 1];
    }

} /* end mp_strerror() */

/* }}} */

/*========================================================================*/
/*------------------------------------------------------------------------*/
/* Static function definitions (internal use only)                        */

/* {{{ Memory management */

/* {{{ s_mp_grow(mp, min) */

/* Make sure there are at least 'min' digits allocated to mp              */
mp_err
s_mp_grow(mp_int *mp, mp_size min)
{
    ARGCHK(mp != NULL, MP_BADARG);

    if (min > ALLOC(mp)) {
        mp_digit *tmp;

        /* Set min to next nearest default precision block size */
        min = MP_ROUNDUP(min, s_mp_defprec);

        if ((tmp = s_mp_alloc(min, sizeof(mp_digit))) == NULL)
            return MP_MEM;

        s_mp_copy(DIGITS(mp), tmp, USED(mp));

        s_mp_setz(DIGITS(mp), ALLOC(mp));
        s_mp_free(DIGITS(mp));
        DIGITS(mp) = tmp;
        ALLOC(mp) = min;
    }

    return MP_OKAY;

} /* end s_mp_grow() */

/* }}} */

/* {{{ s_mp_pad(mp, min) */

/* Make sure the used size of mp is at least 'min', growing if needed     */
mp_err
s_mp_pad(mp_int *mp, mp_size min)
{
    ARGCHK(mp != NULL, MP_BADARG);

    if (min > USED(mp)) {
        mp_err res;

        /* Make sure there is room to increase precision  */
        if (min > ALLOC(mp)) {
            if ((res = s_mp_grow(mp, min)) != MP_OKAY)
                return res;
        } else {
            s_mp_setz(DIGITS(mp) + USED(mp), min - USED(mp));
        }

        /* Increase precision; should already be 0-filled */
        USED(mp) = min;
    }

    return MP_OKAY;

} /* end s_mp_pad() */

/* }}} */

/* {{{ s_mp_setz(dp, count) */

/* Set 'count' digits pointed to by dp to be zeroes                       */
void
s_mp_setz(mp_digit *dp, mp_size count)
{
    memset(dp, 0, count * sizeof(mp_digit));
} /* end s_mp_setz() */

/* }}} */

/* {{{ s_mp_copy(sp, dp, count) */

/* Copy 'count' digits from sp to dp                                      */
void
s_mp_copy(const mp_digit *sp, mp_digit *dp, mp_size count)
{
    memcpy(dp, sp, count * sizeof(mp_digit));
} /* end s_mp_copy() */

/* }}} */

/* {{{ s_mp_alloc(nb, ni) */

/* Allocate ni records of nb bytes each, and return a pointer to that     */
void *
s_mp_alloc(size_t nb, size_t ni)
{
    return calloc(nb, ni);

} /* end s_mp_alloc() */

/* }}} */

/* {{{ s_mp_free(ptr) */

/* Free the memory pointed to by ptr                                      */
void
s_mp_free(void *ptr)
{
    if (ptr) {
        free(ptr);
    }
} /* end s_mp_free() */

/* }}} */

/* {{{ s_mp_clamp(mp) */

/* Remove leading zeroes from the given value                             */
void
s_mp_clamp(mp_int *mp)
{
    mp_size used = MP_USED(mp);
    while (used > 1 && DIGIT(mp, used - 1) == 0)
        --used;
    MP_USED(mp) = used;
    if (used == 1 && DIGIT(mp, 0) == 0)
        MP_SIGN(mp) = ZPOS;
} /* end s_mp_clamp() */

/* }}} */

/* {{{ s_mp_exch(a, b) */

/* Exchange the data for a and b; (b, a) = (a, b)                         */
void
s_mp_exch(mp_int *a, mp_int *b)
{
    mp_int tmp;
    if (!a || !b) {
        return;
    }

    tmp = *a;
    *a = *b;
    *b = tmp;

} /* end s_mp_exch() */

/* }}} */

/* }}} */

/* {{{ Arithmetic helpers */

/* {{{ s_mp_lshd(mp, p) */

/*
   Shift mp leftward by p digits, growing if needed, and zero-filling
   the in-shifted digits at the right end.  This is a convenient
   alternative to multiplication by powers of the radix
 */

mp_err
s_mp_lshd(mp_int *mp, mp_size p)
{
    mp_err res;
    unsigned int ix;

    ARGCHK(mp != NULL, MP_BADARG);

    if (p == 0)
        return MP_OKAY;

    if (MP_USED(mp) == 1 && MP_DIGIT(mp, 0) == 0)
        return MP_OKAY;

    if ((res = s_mp_pad(mp, USED(mp) + p)) != MP_OKAY)
        return res;

    /* Shift all the significant figures over as needed */
    for (ix = USED(mp) - p; ix-- > 0;) {
        DIGIT(mp, ix + p) = DIGIT(mp, ix);
    }

    /* Fill the bottom digits with zeroes */
    for (ix = 0; (mp_size)ix < p; ix++)
        DIGIT(mp, ix) = 0;

    return MP_OKAY;

} /* end s_mp_lshd() */

/* }}} */

/* {{{ s_mp_mul_2d(mp, d) */

/*
  Multiply the integer by 2^d, where d is a number of bits.  This
  amounts to a bitwise shift of the value.
 */
mp_err
s_mp_mul_2d(mp_int *mp, mp_digit d)
{
    mp_err res;
    mp_digit dshift, rshift, mask, x, prev = 0;
    mp_digit *pa = NULL;
    int i;

    ARGCHK(mp != NULL, MP_BADARG);

    dshift = d / MP_DIGIT_BIT;
    d %= MP_DIGIT_BIT;
    /* mp_digit >> rshift is undefined behavior for rshift >= MP_DIGIT_BIT */
    /* mod and corresponding mask logic avoid that when d = 0 */
    rshift = MP_DIGIT_BIT - d;
    rshift %= MP_DIGIT_BIT;
    /* mask = (2**d - 1) * 2**(w-d) mod 2**w */
    mask = (DIGIT_MAX << rshift) + 1;
    mask &= DIGIT_MAX - 1;
    /* bits to be shifted out of the top word */
    x = MP_DIGIT(mp, MP_USED(mp) - 1) & mask;

    if (MP_OKAY != (res = s_mp_pad(mp, MP_USED(mp) + dshift + (x != 0))))
        return res;

    if (dshift && MP_OKAY != (res = s_mp_lshd(mp, dshift)))
        return res;

    pa = MP_DIGITS(mp) + dshift;

    for (i = MP_USED(mp) - dshift; i > 0; i--) {
        x = *pa;
        *pa++ = (x << d) | prev;
        prev = (x & mask) >> rshift;
    }

    s_mp_clamp(mp);
    return MP_OKAY;
} /* end s_mp_mul_2d() */

/* {{{ s_mp_rshd(mp, p) */

/*
   Shift mp rightward by p digits.  Maintains the invariant that
   digits above the precision are all zero.  Digits shifted off the
   end are lost.  Cannot fail.
 */

void
s_mp_rshd(mp_int *mp, mp_size p)
{
    mp_size ix;
    mp_digit *src, *dst;

    if (p == 0)
        return;

    /* Shortcut when all digits are to be shifted off */
    if (p >= USED(mp)) {
        s_mp_setz(DIGITS(mp), ALLOC(mp));
        USED(mp) = 1;
        SIGN(mp) = ZPOS;
        return;
    }

    /* Shift all the significant figures over as needed */
    dst = MP_DIGITS(mp);
    src = dst + p;
    for (ix = USED(mp) - p; ix > 0; ix--)
        *dst++ = *src++;

    MP_USED(mp) -= p;
    /* Fill the top digits with zeroes */
    while (p-- > 0)
        *dst++ = 0;

} /* end s_mp_rshd() */

/* }}} */

/* {{{ s_mp_div_2(mp) */

/* Divide by two -- take advantage of radix properties to do it fast      */
void
s_mp_div_2(mp_int *mp)
{
    s_mp_div_2d(mp, 1);

} /* end s_mp_div_2() */

/* }}} */

/* {{{ s_mp_mul_2(mp) */

mp_err
s_mp_mul_2(mp_int *mp)
{
    mp_digit *pd;
    unsigned int ix, used;
    mp_digit kin = 0;

    ARGCHK(mp != NULL, MP_BADARG);

    /* Shift digits leftward by 1 bit */
    used = MP_USED(mp);
    pd = MP_DIGITS(mp);
    for (ix = 0; ix < used; ix++) {
        mp_digit d = *pd;
        *pd++ = (d << 1) | kin;
        kin = (d >> (DIGIT_BIT - 1));
    }

    /* Deal with rollover from last digit */
    if (kin) {
        if (ix >= ALLOC(mp)) {
            mp_err res;
            if ((res = s_mp_grow(mp, ALLOC(mp) + 1)) != MP_OKAY)
                return res;
        }

        DIGIT(mp, ix) = kin;
        USED(mp) += 1;
    }

    return MP_OKAY;

} /* end s_mp_mul_2() */

/* }}} */

/* {{{ s_mp_mod_2d(mp, d) */

/*
  Remainder the integer by 2^d, where d is a number of bits.  This
  amounts to a bitwise AND of the value, and does not require the full
  division code
 */
void
s_mp_mod_2d(mp_int *mp, mp_digit d)
{
    mp_size ndig = (d / DIGIT_BIT), nbit = (d % DIGIT_BIT);
    mp_size ix;
    mp_digit dmask;

    if (ndig >= USED(mp))
        return;

    /* Flush all the bits above 2^d in its digit */
    dmask = ((mp_digit)1 << nbit) - 1;
    DIGIT(mp, ndig) &= dmask;

    /* Flush all digits above the one with 2^d in it */
    for (ix = ndig + 1; ix < USED(mp); ix++)
        DIGIT(mp, ix) = 0;

    s_mp_clamp(mp);

} /* end s_mp_mod_2d() */

/* }}} */

/* {{{ s_mp_div_2d(mp, d) */

/*
  Divide the integer by 2^d, where d is a number of bits.  This
  amounts to a bitwise shift of the value, and does not require the
  full division code (used in Barrett reduction, see below)
 */
void
s_mp_div_2d(mp_int *mp, mp_digit d)
{
    int ix;
    mp_digit save, next, mask, lshift;

    s_mp_rshd(mp, d / DIGIT_BIT);
    d %= DIGIT_BIT;
    /* mp_digit << lshift is undefined behavior for lshift >= MP_DIGIT_BIT */
    /* mod and corresponding mask logic avoid that when d = 0 */
    lshift = DIGIT_BIT - d;
    lshift %= DIGIT_BIT;
    mask = ((mp_digit)1 << d) - 1;
    save = 0;
    for (ix = USED(mp) - 1; ix >= 0; ix--) {
        next = DIGIT(mp, ix) & mask;
        DIGIT(mp, ix) = (save << lshift) | (DIGIT(mp, ix) >> d);
        save = next;
    }
    s_mp_clamp(mp);

} /* end s_mp_div_2d() */

/* }}} */

/* {{{ s_mp_norm(a, b, *d) */

/*
  s_mp_norm(a, b, *d)

  Normalize a and b for division, where b is the divisor.  In order
  that we might make good guesses for quotient digits, we want the
  leading digit of b to be at least half the radix, which we
  accomplish by multiplying a and b by a power of 2.  The exponent
  (shift count) is placed in *pd, so that the remainder can be shifted
  back at the end of the division process.
 */

mp_err
s_mp_norm(mp_int *a, mp_int *b, mp_digit *pd)
{
    mp_digit d;
    mp_digit mask;
    mp_digit b_msd;
    mp_err res = MP_OKAY;

    ARGCHK(a != NULL && b != NULL && pd != NULL, MP_BADARG);

    d = 0;
    mask = DIGIT_MAX & ~(DIGIT_MAX >> 1); /* mask is msb of digit */
    b_msd = DIGIT(b, USED(b) - 1);
    while (!(b_msd & mask)) {
        b_msd <<= 1;
        ++d;
    }

    if (d) {
        MP_CHECKOK(s_mp_mul_2d(a, d));
        MP_CHECKOK(s_mp_mul_2d(b, d));
    }

    *pd = d;
CLEANUP:
    return res;

} /* end s_mp_norm() */

/* }}} */

/* }}} */

/* {{{ Primitive digit arithmetic */

/* {{{ s_mp_add_d(mp, d) */

/* Add d to |mp| in place                                                 */
mp_err
s_mp_add_d(mp_int *mp, mp_digit d) /* unsigned digit addition */
{
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_ADD_WORD)
    mp_word w, k = 0;
    mp_size ix = 1;

    w = (mp_word)DIGIT(mp, 0) + d;
    DIGIT(mp, 0) = ACCUM(w);
    k = CARRYOUT(w);

    while (ix < USED(mp) && k) {
        w = (mp_word)DIGIT(mp, ix) + k;
        DIGIT(mp, ix) = ACCUM(w);
        k = CARRYOUT(w);
        ++ix;
    }

    if (k != 0) {
        mp_err res;

        if ((res = s_mp_pad(mp, USED(mp) + 1)) != MP_OKAY)
            return res;

        DIGIT(mp, ix) = (mp_digit)k;
    }

    return MP_OKAY;
#else
    mp_digit *pmp = MP_DIGITS(mp);
    mp_digit sum, mp_i, carry = 0;
    mp_err res = MP_OKAY;
    int used = (int)MP_USED(mp);

    mp_i = *pmp;
    *pmp++ = sum = d + mp_i;
    carry = (sum < d);
    while (carry && --used > 0) {
        mp_i = *pmp;
        *pmp++ = sum = carry + mp_i;
        carry = !sum;
    }
    if (carry && !used) {
        /* mp is growing */
        used = MP_USED(mp);
        MP_CHECKOK(s_mp_pad(mp, used + 1));
        MP_DIGIT(mp, used) = carry;
    }
CLEANUP:
    return res;
#endif
} /* end s_mp_add_d() */

/* }}} */

/* {{{ s_mp_sub_d(mp, d) */

/* Subtract d from |mp| in place, assumes |mp| > d                        */
mp_err
s_mp_sub_d(mp_int *mp, mp_digit d) /* unsigned digit subtract */
{
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_SUB_WORD)
    mp_word w, b = 0;
    mp_size ix = 1;

    /* Compute initial subtraction    */
    w = (RADIX + (mp_word)DIGIT(mp, 0)) - d;
    b = CARRYOUT(w) ? 0 : 1;
    DIGIT(mp, 0) = ACCUM(w);

    /* Propagate borrows leftward     */
    while (b && ix < USED(mp)) {
        w = (RADIX + (mp_word)DIGIT(mp, ix)) - b;
        b = CARRYOUT(w) ? 0 : 1;
        DIGIT(mp, ix) = ACCUM(w);
        ++ix;
    }

    /* Remove leading zeroes          */
    s_mp_clamp(mp);

    /* If we have a borrow out, it's a violation of the input invariant */
    if (b)
        return MP_RANGE;
    else
        return MP_OKAY;
#else
    mp_digit *pmp = MP_DIGITS(mp);
    mp_digit mp_i, diff, borrow;
    mp_size used = MP_USED(mp);

    mp_i = *pmp;
    *pmp++ = diff = mp_i - d;
    borrow = (diff > mp_i);
    while (borrow && --used) {
        mp_i = *pmp;
        *pmp++ = diff = mp_i - borrow;
        borrow = (diff > mp_i);
    }
    s_mp_clamp(mp);
    return (borrow && !used) ? MP_RANGE : MP_OKAY;
#endif
} /* end s_mp_sub_d() */

/* }}} */

/* {{{ s_mp_mul_d(a, d) */

/* Compute a = a * d, single digit multiplication                         */
mp_err
s_mp_mul_d(mp_int *a, mp_digit d)
{
    mp_err res;
    mp_size used;
    int pow;

    if (!d) {
        mp_zero(a);
        return MP_OKAY;
    }
    if (d == 1)
        return MP_OKAY;
    if (0 <= (pow = s_mp_ispow2d(d))) {
        return s_mp_mul_2d(a, (mp_digit)pow);
    }

    used = MP_USED(a);
    MP_CHECKOK(s_mp_pad(a, used + 1));

    s_mpv_mul_d(MP_DIGITS(a), used, d, MP_DIGITS(a));

    s_mp_clamp(a);

CLEANUP:
    return res;

} /* end s_mp_mul_d() */

/* }}} */

/* {{{ s_mp_div_d(mp, d, r) */

/*
  s_mp_div_d(mp, d, r)

  Compute the quotient mp = mp / d and remainder r = mp mod d, for a
  single digit d.  If r is null, the remainder will be discarded.
 */

mp_err
s_mp_div_d(mp_int *mp, mp_digit d, mp_digit *r)
{
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_DIV_WORD)
    mp_word w = 0, q;
#else
    mp_digit w = 0, q;
#endif
    int ix;
    mp_err res;
    mp_int quot;
    mp_int rem;

    if (d == 0)
        return MP_RANGE;
    if (d == 1) {
        if (r)
            *r = 0;
        return MP_OKAY;
    }
    /* could check for power of 2 here, but mp_div_d does that. */
    if (MP_USED(mp) == 1) {
        mp_digit n = MP_DIGIT(mp, 0);
        mp_digit remdig;

        q = n / d;
        remdig = n % d;
        MP_DIGIT(mp, 0) = q;
        if (r) {
            *r = remdig;
        }
        return MP_OKAY;
    }

    MP_DIGITS(&rem) = 0;
    MP_DIGITS(&quot) = 0;
    /* Make room for the quotient */
    MP_CHECKOK(mp_init_size(&quot, USED(mp)));

#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_DIV_WORD)
    for (ix = USED(mp) - 1; ix >= 0; ix--) {
        w = (w << DIGIT_BIT) | DIGIT(mp, ix);

        if (w >= d) {
            q = w / d;
            w = w % d;
        } else {
            q = 0;
        }

        s_mp_lshd(&quot, 1);
        DIGIT(&quot, 0) = (mp_digit)q;
    }
#else
    {
        mp_digit p;
#if !defined(MP_ASSEMBLY_DIV_2DX1D)
        mp_digit norm;
#endif

        MP_CHECKOK(mp_init_copy(&rem, mp));

#if !defined(MP_ASSEMBLY_DIV_2DX1D)
        MP_DIGIT(&quot, 0) = d;
        MP_CHECKOK(s_mp_norm(&rem, &quot, &norm));
        if (norm)
            d <<= norm;
        MP_DIGIT(&quot, 0) = 0;
#endif

        p = 0;
        for (ix = USED(&rem) - 1; ix >= 0; ix--) {
            w = DIGIT(&rem, ix);

            if (p) {
                MP_CHECKOK(s_mpv_div_2dx1d(p, w, d, &q, &w));
            } else if (w >= d) {
                q = w / d;
                w = w % d;
            } else {
                q = 0;
            }

            MP_CHECKOK(s_mp_lshd(&quot, 1));
            DIGIT(&quot, 0) = q;
            p = w;
        }
#if !defined(MP_ASSEMBLY_DIV_2DX1D)
        if (norm)
            w >>= norm;
#endif
    }
#endif

    /* Deliver the remainder, if desired */
    if (r) {
        *r = (mp_digit)w;
    }

    s_mp_clamp(&quot);
    mp_exch(&quot, mp);
CLEANUP:
    mp_clear(&quot);
    mp_clear(&rem);

    return res;
} /* end s_mp_div_d() */

/* }}} */

/* }}} */

/* {{{ Primitive full arithmetic */

/* {{{ s_mp_add(a, b) */

/* Compute a = |a| + |b|                                                  */
mp_err
s_mp_add(mp_int *a, const mp_int *b) /* magnitude addition      */
{
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_ADD_WORD)
    mp_word w = 0;
#else
    mp_digit d, sum, carry = 0;
#endif
    mp_digit *pa, *pb;
    mp_size ix;
    mp_size used;
    mp_err res;

    /* Make sure a has enough precision for the output value */
    if ((USED(b) > USED(a)) && (res = s_mp_pad(a, USED(b))) != MP_OKAY)
        return res;

    /*
      Add up all digits up to the precision of b.  If b had initially
      the same precision as a, or greater, we took care of it by the
      padding step above, so there is no problem.  If b had initially
      less precision, we'll have to make sure the carry out is duly
      propagated upward among the higher-order digits of the sum.
     */
    pa = MP_DIGITS(a);
    pb = MP_DIGITS(b);
    used = MP_USED(b);
    for (ix = 0; ix < used; ix++) {
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_ADD_WORD)
        w = w + *pa + *pb++;
        *pa++ = ACCUM(w);
        w = CARRYOUT(w);
#else
        d = *pa;
        sum = d + *pb++;
        d = (sum < d); /* detect overflow */
        *pa++ = sum += carry;
        carry = d + (sum < carry); /* detect overflow */
#endif
    }

    /* If we run out of 'b' digits before we're actually done, make
       sure the carries get propagated upward...
     */
    used = MP_USED(a);
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_ADD_WORD)
    while (w && ix < used) {
        w = w + *pa;
        *pa++ = ACCUM(w);
        w = CARRYOUT(w);
        ++ix;
    }
#else
    while (carry && ix < used) {
        sum = carry + *pa;
        *pa++ = sum;
        carry = !sum;
        ++ix;
    }
#endif

/* If there's an overall carry out, increase precision and include
     it.  We could have done this initially, but why touch the memory
     allocator unless we're sure we have to?
   */
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_ADD_WORD)
    if (w) {
        if ((res = s_mp_pad(a, used + 1)) != MP_OKAY)
            return res;

        DIGIT(a, ix) = (mp_digit)w;
    }
#else
    if (carry) {
        if ((res = s_mp_pad(a, used + 1)) != MP_OKAY)
            return res;

        DIGIT(a, used) = carry;
    }
#endif

    return MP_OKAY;
} /* end s_mp_add() */

/* }}} */

/* Compute c = |a| + |b|         */ /* magnitude addition      */
mp_err
s_mp_add_3arg(const mp_int *a, const mp_int *b, mp_int *c)
{
    mp_digit *pa, *pb, *pc;
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_ADD_WORD)
    mp_word w = 0;
#else
    mp_digit sum, carry = 0, d;
#endif
    mp_size ix;
    mp_size used;
    mp_err res;

    MP_SIGN(c) = MP_SIGN(a);
    if (MP_USED(a) < MP_USED(b)) {
        const mp_int *xch = a;
        a = b;
        b = xch;
    }

    /* Make sure a has enough precision for the output value */
    if (MP_OKAY != (res = s_mp_pad(c, MP_USED(a))))
        return res;

    /*
     Add up all digits up to the precision of b.  If b had initially
     the same precision as a, or greater, we took care of it by the
     exchange step above, so there is no problem.  If b had initially
     less precision, we'll have to make sure the carry out is duly
     propagated upward among the higher-order digits of the sum.
    */
    pa = MP_DIGITS(a);
    pb = MP_DIGITS(b);
    pc = MP_DIGITS(c);
    used = MP_USED(b);
    for (ix = 0; ix < used; ix++) {
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_ADD_WORD)
        w = w + *pa++ + *pb++;
        *pc++ = ACCUM(w);
        w = CARRYOUT(w);
#else
        d = *pa++;
        sum = d + *pb++;
        d = (sum < d); /* detect overflow */
        *pc++ = sum += carry;
        carry = d + (sum < carry); /* detect overflow */
#endif
    }

    /* If we run out of 'b' digits before we're actually done, make
     sure the carries get propagated upward...
   */
    for (used = MP_USED(a); ix < used; ++ix) {
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_ADD_WORD)
        w = w + *pa++;
        *pc++ = ACCUM(w);
        w = CARRYOUT(w);
#else
        *pc++ = sum = carry + *pa++;
        carry = (sum < carry);
#endif
    }

/* If there's an overall carry out, increase precision and include
     it.  We could have done this initially, but why touch the memory
     allocator unless we're sure we have to?
   */
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_ADD_WORD)
    if (w) {
        if ((res = s_mp_pad(c, used + 1)) != MP_OKAY)
            return res;

        DIGIT(c, used) = (mp_digit)w;
        ++used;
    }
#else
    if (carry) {
        if ((res = s_mp_pad(c, used + 1)) != MP_OKAY)
            return res;

        DIGIT(c, used) = carry;
        ++used;
    }
#endif
    MP_USED(c) = used;
    return MP_OKAY;
}
/* {{{ s_mp_add_offset(a, b, offset) */

/* Compute a = |a| + ( |b| * (RADIX ** offset) )             */
mp_err
s_mp_add_offset(mp_int *a, mp_int *b, mp_size offset)
{
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_ADD_WORD)
    mp_word w, k = 0;
#else
    mp_digit d, sum, carry = 0;
#endif
    mp_size ib;
    mp_size ia;
    mp_size lim;
    mp_err res;

    /* Make sure a has enough precision for the output value */
    lim = MP_USED(b) + offset;
    if ((lim > USED(a)) && (res = s_mp_pad(a, lim)) != MP_OKAY)
        return res;

    /*
    Add up all digits up to the precision of b.  If b had initially
    the same precision as a, or greater, we took care of it by the
    padding step above, so there is no problem.  If b had initially
    less precision, we'll have to make sure the carry out is duly
    propagated upward among the higher-order digits of the sum.
   */
    lim = USED(b);
    for (ib = 0, ia = offset; ib < lim; ib++, ia++) {
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_ADD_WORD)
        w = (mp_word)DIGIT(a, ia) + DIGIT(b, ib) + k;
        DIGIT(a, ia) = ACCUM(w);
        k = CARRYOUT(w);
#else
        d = MP_DIGIT(a, ia);
        sum = d + MP_DIGIT(b, ib);
        d = (sum < d);
        MP_DIGIT(a, ia) = sum += carry;
        carry = d + (sum < carry);
#endif
    }

/* If we run out of 'b' digits before we're actually done, make
     sure the carries get propagated upward...
   */
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_ADD_WORD)
    for (lim = MP_USED(a); k && (ia < lim); ++ia) {
        w = (mp_word)DIGIT(a, ia) + k;
        DIGIT(a, ia) = ACCUM(w);
        k = CARRYOUT(w);
    }
#else
    for (lim = MP_USED(a); carry && (ia < lim); ++ia) {
        d = MP_DIGIT(a, ia);
        MP_DIGIT(a, ia) = sum = d + carry;
        carry = (sum < d);
    }
#endif

/* If there's an overall carry out, increase precision and include
     it.  We could have done this initially, but why touch the memory
     allocator unless we're sure we have to?
   */
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_ADD_WORD)
    if (k) {
        if ((res = s_mp_pad(a, USED(a) + 1)) != MP_OKAY)
            return res;

        DIGIT(a, ia) = (mp_digit)k;
    }
#else
    if (carry) {
        if ((res = s_mp_pad(a, lim + 1)) != MP_OKAY)
            return res;

        DIGIT(a, lim) = carry;
    }
#endif
    s_mp_clamp(a);

    return MP_OKAY;

} /* end s_mp_add_offset() */

/* }}} */

/* {{{ s_mp_sub(a, b) */

/* Compute a = |a| - |b|, assumes |a| >= |b|                              */
mp_err
s_mp_sub(mp_int *a, const mp_int *b) /* magnitude subtract      */
{
    mp_digit *pa, *pb, *limit;
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_SUB_WORD)
    mp_sword w = 0;
#else
    mp_digit d, diff, borrow = 0;
#endif

    /*
    Subtract and propagate borrow.  Up to the precision of b, this
    accounts for the digits of b; after that, we just make sure the
    carries get to the right place.  This saves having to pad b out to
    the precision of a just to make the loops work right...
   */
    pa = MP_DIGITS(a);
    pb = MP_DIGITS(b);
    limit = pb + MP_USED(b);
    while (pb < limit) {
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_SUB_WORD)
        w = w + *pa - *pb++;
        *pa++ = ACCUM(w);
        w >>= MP_DIGIT_BIT;
#else
        d = *pa;
        diff = d - *pb++;
        d = (diff > d); /* detect borrow */
        if (borrow && --diff == MP_DIGIT_MAX)
            ++d;
        *pa++ = diff;
        borrow = d;
#endif
    }
    limit = MP_DIGITS(a) + MP_USED(a);
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_SUB_WORD)
    while (w && pa < limit) {
        w = w + *pa;
        *pa++ = ACCUM(w);
        w >>= MP_DIGIT_BIT;
    }
#else
    while (borrow && pa < limit) {
        d = *pa;
        *pa++ = diff = d - borrow;
        borrow = (diff > d);
    }
#endif

    /* Clobber any leading zeroes we created    */
    s_mp_clamp(a);

/*
     If there was a borrow out, then |b| > |a| in violation
     of our input invariant.  We've already done the work,
     but we'll at least complain about it...
   */
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_SUB_WORD)
    return w ? MP_RANGE : MP_OKAY;
#else
    return borrow ? MP_RANGE : MP_OKAY;
#endif
} /* end s_mp_sub() */

/* }}} */

/* Compute c = |a| - |b|, assumes |a| >= |b| */ /* magnitude subtract      */
mp_err
s_mp_sub_3arg(const mp_int *a, const mp_int *b, mp_int *c)
{
    mp_digit *pa, *pb, *pc;
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_SUB_WORD)
    mp_sword w = 0;
#else
    mp_digit d, diff, borrow = 0;
#endif
    int ix, limit;
    mp_err res;

    MP_SIGN(c) = MP_SIGN(a);

    /* Make sure a has enough precision for the output value */
    if (MP_OKAY != (res = s_mp_pad(c, MP_USED(a))))
        return res;

    /*
    Subtract and propagate borrow.  Up to the precision of b, this
    accounts for the digits of b; after that, we just make sure the
    carries get to the right place.  This saves having to pad b out to
    the precision of a just to make the loops work right...
   */
    pa = MP_DIGITS(a);
    pb = MP_DIGITS(b);
    pc = MP_DIGITS(c);
    limit = MP_USED(b);
    for (ix = 0; ix < limit; ++ix) {
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_SUB_WORD)
        w = w + *pa++ - *pb++;
        *pc++ = ACCUM(w);
        w >>= MP_DIGIT_BIT;
#else
        d = *pa++;
        diff = d - *pb++;
        d = (diff > d);
        if (borrow && --diff == MP_DIGIT_MAX)
            ++d;
        *pc++ = diff;
        borrow = d;
#endif
    }
    for (limit = MP_USED(a); ix < limit; ++ix) {
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_SUB_WORD)
        w = w + *pa++;
        *pc++ = ACCUM(w);
        w >>= MP_DIGIT_BIT;
#else
        d = *pa++;
        *pc++ = diff = d - borrow;
        borrow = (diff > d);
#endif
    }

    /* Clobber any leading zeroes we created    */
    MP_USED(c) = ix;
    s_mp_clamp(c);

/*
     If there was a borrow out, then |b| > |a| in violation
     of our input invariant.  We've already done the work,
     but we'll at least complain about it...
   */
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_SUB_WORD)
    return w ? MP_RANGE : MP_OKAY;
#else
    return borrow ? MP_RANGE : MP_OKAY;
#endif
}
/* {{{ s_mp_mul(a, b) */

/* Compute a = |a| * |b|                                                  */
mp_err
s_mp_mul(mp_int *a, const mp_int *b)
{
    return mp_mul(a, b, a);
} /* end s_mp_mul() */

/* }}} */

#if defined(MP_USE_UINT_DIGIT) && defined(MP_USE_LONG_LONG_MULTIPLY)
/* This trick works on Sparc V8 CPUs with the Workshop compilers. */
#define MP_MUL_DxD(a, b, Phi, Plo)                              \
    {                                                           \
        unsigned long long product = (unsigned long long)a * b; \
        Plo = (mp_digit)product;                                \
        Phi = (mp_digit)(product >> MP_DIGIT_BIT);              \
    }
#else
#define MP_MUL_DxD(a, b, Phi, Plo)                                 \
    {                                                              \
        mp_digit a0b1, a1b0;                                       \
        Plo = (a & MP_HALF_DIGIT_MAX) * (b & MP_HALF_DIGIT_MAX);   \
        Phi = (a >> MP_HALF_DIGIT_BIT) * (b >> MP_HALF_DIGIT_BIT); \
        a0b1 = (a & MP_HALF_DIGIT_MAX) * (b >> MP_HALF_DIGIT_BIT); \
        a1b0 = (a >> MP_HALF_DIGIT_BIT) * (b & MP_HALF_DIGIT_MAX); \
        a1b0 += a0b1;                                              \
        Phi += a1b0 >> MP_HALF_DIGIT_BIT;                          \
        Phi += (MP_CT_LTU(a1b0, a0b1)) << MP_HALF_DIGIT_BIT;       \
        a1b0 <<= MP_HALF_DIGIT_BIT;                                \
        Plo += a1b0;                                               \
        Phi += MP_CT_LTU(Plo, a1b0);                               \
    }
#endif

/* Constant time version of s_mpv_mul_d_add_prop.
 * Presently, this is only used by the Constant time Montgomery arithmetic code. */
/* c += a * b */
void
s_mpv_mul_d_add_propCT(const mp_digit *a, mp_size a_len, mp_digit b,
                       mp_digit *c, mp_size c_len)
{
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_MUL_WORD)
    mp_digit d = 0;

    c_len -= a_len;
    /* Inner product:  Digits of a */
    while (a_len--) {
        mp_word w = ((mp_word)b * *a++) + *c + d;
        *c++ = ACCUM(w);
        d = CARRYOUT(w);
    }

    /* propagate the carry to the end, even if carry is zero */
    while (c_len--) {
        mp_word w = (mp_word)*c + d;
        *c++ = ACCUM(w);
        d = CARRYOUT(w);
    }
#else
    mp_digit carry = 0;
    c_len -= a_len;
    while (a_len--) {
        mp_digit a_i = *a++;
        mp_digit a0b0, a1b1;
        MP_MUL_DxD(a_i, b, a1b1, a0b0);

        a0b0 += carry;
        a1b1 += MP_CT_LTU(a0b0, carry);
        a0b0 += a_i = *c;
        a1b1 += MP_CT_LTU(a0b0, a_i);

        *c++ = a0b0;
        carry = a1b1;
    }
    /* propagate the carry to the end, even if carry is zero */
    while (c_len--) {
        mp_digit c_i = *c;
        carry += c_i;
        *c++ = carry;
        carry = MP_CT_LTU(carry, c_i);
    }
#endif
}

#if !defined(MP_ASSEMBLY_MULTIPLY)
/* c = a * b */
void
s_mpv_mul_d(const mp_digit *a, mp_size a_len, mp_digit b, mp_digit *c)
{
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_MUL_WORD)
    mp_digit d = 0;

    /* Inner product:  Digits of a */
    while (a_len--) {
        mp_word w = ((mp_word)b * *a++) + d;
        *c++ = ACCUM(w);
        d = CARRYOUT(w);
    }
    *c = d;
#else
    mp_digit carry = 0;
    while (a_len--) {
        mp_digit a_i = *a++;
        mp_digit a0b0, a1b1;

        MP_MUL_DxD(a_i, b, a1b1, a0b0);

        a0b0 += carry;
        a1b1 += MP_CT_LTU(a0b0, carry);
        *c++ = a0b0;
        carry = a1b1;
    }
    *c = carry;
#endif
}

/* c += a * b */
void
s_mpv_mul_d_add(const mp_digit *a, mp_size a_len, mp_digit b,
                mp_digit *c)
{
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_MUL_WORD)
    mp_digit d = 0;

    /* Inner product:  Digits of a */
    while (a_len--) {
        mp_word w = ((mp_word)b * *a++) + *c + d;
        *c++ = ACCUM(w);
        d = CARRYOUT(w);
    }
    *c = d;
#else
    mp_digit carry = 0;
    while (a_len--) {
        mp_digit a_i = *a++;
        mp_digit a0b0, a1b1;

        MP_MUL_DxD(a_i, b, a1b1, a0b0);

        a0b0 += carry;
        a1b1 += MP_CT_LTU(a0b0, carry);
        a0b0 += a_i = *c;
        a1b1 += MP_CT_LTU(a0b0, a_i);
        *c++ = a0b0;
        carry = a1b1;
    }
    *c = carry;
#endif
}

/* Presently, this is only used by the Montgomery arithmetic code. */
/* c += a * b */
void
s_mpv_mul_d_add_prop(const mp_digit *a, mp_size a_len, mp_digit b, mp_digit *c)
{
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_MUL_WORD)
    mp_digit d = 0;

    /* Inner product:  Digits of a */
    while (a_len--) {
        mp_word w = ((mp_word)b * *a++) + *c + d;
        *c++ = ACCUM(w);
        d = CARRYOUT(w);
    }

    while (d) {
        mp_word w = (mp_word)*c + d;
        *c++ = ACCUM(w);
        d = CARRYOUT(w);
    }
#else
    mp_digit carry = 0;
    while (a_len--) {
        mp_digit a_i = *a++;
        mp_digit a0b0, a1b1;

        MP_MUL_DxD(a_i, b, a1b1, a0b0);

        a0b0 += carry;
        if (a0b0 < carry)
            ++a1b1;

        a0b0 += a_i = *c;
        if (a0b0 < a_i)
            ++a1b1;

        *c++ = a0b0;
        carry = a1b1;
    }
    while (carry) {
        mp_digit c_i = *c;
        carry += c_i;
        *c++ = carry;
        carry = carry < c_i;
    }
#endif
}
#endif

#if defined(MP_USE_UINT_DIGIT) && defined(MP_USE_LONG_LONG_MULTIPLY)
/* This trick works on Sparc V8 CPUs with the Workshop compilers. */
#define MP_SQR_D(a, Phi, Plo)                                  \
    {                                                          \
        unsigned long long square = (unsigned long long)a * a; \
        Plo = (mp_digit)square;                                \
        Phi = (mp_digit)(square >> MP_DIGIT_BIT);              \
    }
#else
#define MP_SQR_D(a, Phi, Plo)                                      \
    {                                                              \
        mp_digit Pmid;                                             \
        Plo = (a & MP_HALF_DIGIT_MAX) * (a & MP_HALF_DIGIT_MAX);   \
        Phi = (a >> MP_HALF_DIGIT_BIT) * (a >> MP_HALF_DIGIT_BIT); \
        Pmid = (a & MP_HALF_DIGIT_MAX) * (a >> MP_HALF_DIGIT_BIT); \
        Phi += Pmid >> (MP_HALF_DIGIT_BIT - 1);                    \
        Pmid <<= (MP_HALF_DIGIT_BIT + 1);                          \
        Plo += Pmid;                                               \
        if (Plo < Pmid)                                            \
            ++Phi;                                                 \
    }
#endif

#if !defined(MP_ASSEMBLY_SQUARE)
/* Add the squares of the digits of a to the digits of b. */
void
s_mpv_sqr_add_prop(const mp_digit *pa, mp_size a_len, mp_digit *ps)
{
#if !defined(MP_NO_MP_WORD) && !defined(MP_NO_MUL_WORD)
    mp_word w;
    mp_digit d;
    mp_size ix;

    w = 0;
#define ADD_SQUARE(n)                     \
    d = pa[n];                            \
    w += (d * (mp_word)d) + ps[2 * n];    \
    ps[2 * n] = ACCUM(w);                 \
    w = (w >> DIGIT_BIT) + ps[2 * n + 1]; \
    ps[2 * n + 1] = ACCUM(w);             \
    w = (w >> DIGIT_BIT)

    for (ix = a_len; ix >= 4; ix -= 4) {
        ADD_SQUARE(0);
        ADD_SQUARE(1);
        ADD_SQUARE(2);
        ADD_SQUARE(3);
        pa += 4;
        ps += 8;
    }
    if (ix) {
        ps += 2 * ix;
        pa += ix;
        switch (ix) {
            case 3:
                ADD_SQUARE(-3); /* FALLTHRU */
            case 2:
                ADD_SQUARE(-2); /* FALLTHRU */
            case 1:
                ADD_SQUARE(-1); /* FALLTHRU */
            case 0:
                break;
        }
    }
    while (w) {
        w += *ps;
        *ps++ = ACCUM(w);
        w = (w >> DIGIT_BIT);
    }
#else
    mp_digit carry = 0;
    while (a_len--) {
        mp_digit a_i = *pa++;
        mp_digit a0a0, a1a1;

        MP_SQR_D(a_i, a1a1, a0a0);

        /* here a1a1 and a0a0 constitute a_i ** 2 */
        a0a0 += carry;
        if (a0a0 < carry)
            ++a1a1;

        /* now add to ps */
        a0a0 += a_i = *ps;
        if (a0a0 < a_i)
            ++a1a1;
        *ps++ = a0a0;
        a1a1 += a_i = *ps;
        carry = (a1a1 < a_i);
        *ps++ = a1a1;
    }
    while (carry) {
        mp_digit s_i = *ps;
        carry += s_i;
        *ps++ = carry;
        carry = carry < s_i;
    }
#endif
}
#endif

#if !defined(MP_ASSEMBLY_DIV_2DX1D)
/*
** Divide 64-bit (Nhi,Nlo) by 32-bit divisor, which must be normalized
** so its high bit is 1.   This code is from NSPR.
*/
mp_err
s_mpv_div_2dx1d(mp_digit Nhi, mp_digit Nlo, mp_digit divisor,
                mp_digit *qp, mp_digit *rp)
{
    mp_digit d1, d0, q1, q0;
    mp_digit r1, r0, m;

    d1 = divisor >> MP_HALF_DIGIT_BIT;
    d0 = divisor & MP_HALF_DIGIT_MAX;
    r1 = Nhi % d1;
    q1 = Nhi / d1;
    m = q1 * d0;
    r1 = (r1 << MP_HALF_DIGIT_BIT) | (Nlo >> MP_HALF_DIGIT_BIT);
    if (r1 < m) {
        q1--, r1 += divisor;
        if (r1 >= divisor && r1 < m) {
            q1--, r1 += divisor;
        }
    }
    r1 -= m;
    r0 = r1 % d1;
    q0 = r1 / d1;
    m = q0 * d0;
    r0 = (r0 << MP_HALF_DIGIT_BIT) | (Nlo & MP_HALF_DIGIT_MAX);
    if (r0 < m) {
        q0--, r0 += divisor;
        if (r0 >= divisor && r0 < m) {
            q0--, r0 += divisor;
        }
    }
    if (qp)
        *qp = (q1 << MP_HALF_DIGIT_BIT) | q0;
    if (rp)
        *rp = r0 - m;
    return MP_OKAY;
}
#endif

#if MP_SQUARE
/* {{{ s_mp_sqr(a) */

mp_err
s_mp_sqr(mp_int *a)
{
    mp_err res;
    mp_int tmp;

    if ((res = mp_init_size(&tmp, 2 * USED(a))) != MP_OKAY)
        return res;
    res = mp_sqr(a, &tmp);
    if (res == MP_OKAY) {
        s_mp_exch(&tmp, a);
    }
    mp_clear(&tmp);
    return res;
}

/* }}} */
#endif

/* {{{ s_mp_div(a, b) */

/*
  s_mp_div(a, b)

  Compute a = a / b and b = a mod b.  Assumes b > a.
 */

mp_err
s_mp_div(mp_int *rem,  /* i: dividend, o: remainder */
         mp_int *div,  /* i: divisor                */
         mp_int *quot) /* i: 0;        o: quotient  */
{
    mp_int part, t;
    mp_digit q_msd;
    mp_err res;
    mp_digit d;
    mp_digit div_msd;
    int ix;

    if (mp_cmp_z(div) == 0)
        return MP_RANGE;

    DIGITS(&t) = 0;
    /* Shortcut if divisor is power of two */
    if ((ix = s_mp_ispow2(div)) >= 0) {
        MP_CHECKOK(mp_copy(rem, quot));
        s_mp_div_2d(quot, (mp_digit)ix);
        s_mp_mod_2d(rem, (mp_digit)ix);

        return MP_OKAY;
    }

    MP_SIGN(rem) = ZPOS;
    MP_SIGN(div) = ZPOS;
    MP_SIGN(&part) = ZPOS;

    /* A working temporary for division     */
    MP_CHECKOK(mp_init_size(&t, MP_ALLOC(rem)));

    /* Normalize to optimize guessing       */
    MP_CHECKOK(s_mp_norm(rem, div, &d));

    /* Perform the division itself...woo!   */
    MP_USED(quot) = MP_ALLOC(quot);

    /* Find a partial substring of rem which is at least div */
    /* If we didn't find one, we're finished dividing    */
    while (MP_USED(rem) > MP_USED(div) || s_mp_cmp(rem, div) >= 0) {
        int i;
        int unusedRem;
        int partExtended = 0; /* set to true if we need to extend part */

        unusedRem = MP_USED(rem) - MP_USED(div);
        MP_DIGITS(&part) = MP_DIGITS(rem) + unusedRem;
        MP_ALLOC(&part) = MP_ALLOC(rem) - unusedRem;
        MP_USED(&part) = MP_USED(div);

        /* We have now truncated the part of the remainder to the same length as
         * the divisor. If part is smaller than div, extend part by one digit. */
        if (s_mp_cmp(&part, div) < 0) {
            --unusedRem;
#if MP_ARGCHK == 2
            assert(unusedRem >= 0);
#endif
            --MP_DIGITS(&part);
            ++MP_USED(&part);
            ++MP_ALLOC(&part);
            partExtended = 1;
        }

        /* Compute a guess for the next quotient digit       */
        q_msd = MP_DIGIT(&part, MP_USED(&part) - 1);
        div_msd = MP_DIGIT(div, MP_USED(div) - 1);
        if (!partExtended) {
            /* In this case, q_msd /= div_msd is always 1. First, since div_msd is
             * normalized to have the high bit set, 2*div_msd > MP_DIGIT_MAX. Since
             * we didn't extend part, q_msd >= div_msd. Therefore we know that
             * div_msd <= q_msd <= MP_DIGIT_MAX < 2*div_msd. Dividing by div_msd we
             * get 1 <= q_msd/div_msd < 2. So q_msd /= div_msd must be 1. */
            q_msd = 1;
        } else {
            if (q_msd == div_msd) {
                q_msd = MP_DIGIT_MAX;
            } else {
                mp_digit r;
                MP_CHECKOK(s_mpv_div_2dx1d(q_msd, MP_DIGIT(&part, MP_USED(&part) - 2),
                                           div_msd, &q_msd, &r));
            }
        }
#if MP_ARGCHK == 2
        assert(q_msd > 0); /* This case should never occur any more. */
#endif
        if (q_msd <= 0)
            break;

        /* See what that multiplies out to                   */
        mp_copy(div, &t);
        MP_CHECKOK(s_mp_mul_d(&t, q_msd));

        /*
           If it's too big, back it off.  We should not have to do this
           more than once, or, in rare cases, twice.  Knuth describes a
           method by which this could be reduced to a maximum of once, but
           I didn't implement that here.
           When using s_mpv_div_2dx1d, we may have to do this 3 times.
         */
        for (i = 4; s_mp_cmp(&t, &part) > 0 && i > 0; --i) {
            --q_msd;
            MP_CHECKOK(s_mp_sub(&t, div)); /* t -= div */
        }
        if (i < 0) {
            res = MP_RANGE;
            goto CLEANUP;
        }

        /* At this point, q_msd should be the right next digit   */
        MP_CHECKOK(s_mp_sub(&part, &t)); /* part -= t */
        s_mp_clamp(rem);

        /*
          Include the digit in the quotient.  We allocated enough memory
          for any quotient we could ever possibly get, so we should not
          have to check for failures here
         */
        MP_DIGIT(quot, unusedRem) = q_msd;
    }

    /* Denormalize remainder                */
    if (d) {
        s_mp_div_2d(rem, d);
    }

    s_mp_clamp(quot);

CLEANUP:
    mp_clear(&t);

    return res;

} /* end s_mp_div() */

/* }}} */

/* {{{ s_mp_2expt(a, k) */

mp_err
s_mp_2expt(mp_int *a, mp_digit k)
{
    mp_err res;
    mp_size dig, bit;

    dig = k / DIGIT_BIT;
    bit = k % DIGIT_BIT;

    mp_zero(a);
    if ((res = s_mp_pad(a, dig + 1)) != MP_OKAY)
        return res;

    DIGIT(a, dig) |= ((mp_digit)1 << bit);

    return MP_OKAY;

} /* end s_mp_2expt() */

/* }}} */

/* {{{ s_mp_reduce(x, m, mu) */

/*
  Compute Barrett reduction, x (mod m), given a precomputed value for
  mu = b^2k / m, where b = RADIX and k = #digits(m).  This should be
  faster than straight division, when many reductions by the same
  value of m are required (such as in modular exponentiation).  This
  can nearly halve the time required to do modular exponentiation,
  as compared to using the full integer divide to reduce.

  This algorithm was derived from the _Handbook of Applied
  Cryptography_ by Menezes, Oorschot and VanStone, Ch. 14,
  pp. 603-604.
 */

mp_err
s_mp_reduce(mp_int *x, const mp_int *m, const mp_int *mu)
{
    mp_int q;
    mp_err res;

    if ((res = mp_init_copy(&q, x)) != MP_OKAY)
        return res;

    s_mp_rshd(&q, USED(m) - 1); /* q1 = x / b^(k-1)  */
    s_mp_mul(&q, mu);           /* q2 = q1 * mu      */
    s_mp_rshd(&q, USED(m) + 1); /* q3 = q2 / b^(k+1) */

    /* x = x mod b^(k+1), quick (no division) */
    s_mp_mod_2d(x, DIGIT_BIT * (USED(m) + 1));

    /* q = q * m mod b^(k+1), quick (no division) */
    s_mp_mul(&q, m);
    s_mp_mod_2d(&q, DIGIT_BIT * (USED(m) + 1));

    /* x = x - q */
    if ((res = mp_sub(x, &q, x)) != MP_OKAY)
        goto CLEANUP;

    /* If x < 0, add b^(k+1) to it */
    if (mp_cmp_z(x) < 0) {
        mp_set(&q, 1);
        if ((res = s_mp_lshd(&q, USED(m) + 1)) != MP_OKAY)
            goto CLEANUP;
        if ((res = mp_add(x, &q, x)) != MP_OKAY)
            goto CLEANUP;
    }

    /* Back off if it's too big */
    while (mp_cmp(x, m) >= 0) {
        if ((res = s_mp_sub(x, m)) != MP_OKAY)
            break;
    }

CLEANUP:
    mp_clear(&q);

    return res;

} /* end s_mp_reduce() */

/* }}} */

/* }}} */

/* {{{ Primitive comparisons */

/* {{{ s_mp_cmp(a, b) */

/* Compare |a| <=> |b|, return 0 if equal, <0 if a<b, >0 if a>b           */
int
s_mp_cmp(const mp_int *a, const mp_int *b)
{
    ARGMPCHK(a != NULL && b != NULL);

    mp_size used_a = MP_USED(a);
    {
        mp_size used_b = MP_USED(b);

        if (used_a > used_b)
            goto IS_GT;
        if (used_a < used_b)
            goto IS_LT;
    }
    {
        mp_digit *pa, *pb;
        mp_digit da = 0, db = 0;

#define CMP_AB(n)                     \
    if ((da = pa[n]) != (db = pb[n])) \
    goto done

        pa = MP_DIGITS(a) + used_a;
        pb = MP_DIGITS(b) + used_a;
        while (used_a >= 4) {
            pa -= 4;
            pb -= 4;
            used_a -= 4;
            CMP_AB(3);
            CMP_AB(2);
            CMP_AB(1);
            CMP_AB(0);
        }
        while (used_a-- > 0 && ((da = *--pa) == (db = *--pb)))
            /* do nothing */;
    done:
        if (da > db)
            goto IS_GT;
        if (da < db)
            goto IS_LT;
    }
    return MP_EQ;
IS_LT:
    return MP_LT;
IS_GT:
    return MP_GT;
} /* end s_mp_cmp() */

/* }}} */

/* {{{ s_mp_cmp_d(a, d) */

/* Compare |a| <=> d, return 0 if equal, <0 if a<d, >0 if a>d             */
int
s_mp_cmp_d(const mp_int *a, mp_digit d)
{
    ARGMPCHK(a != NULL);

    if (USED(a) > 1)
        return MP_GT;

    if (DIGIT(a, 0) < d)
        return MP_LT;
    else if (DIGIT(a, 0) > d)
        return MP_GT;
    else
        return MP_EQ;

} /* end s_mp_cmp_d() */

/* }}} */

/* {{{ s_mp_ispow2(v) */

/*
  Returns -1 if the value is not a power of two; otherwise, it returns
  k such that v = 2^k, i.e. lg(v).
 */
int
s_mp_ispow2(const mp_int *v)
{
    mp_digit d;
    int extra = 0, ix;

    ARGMPCHK(v != NULL);

    ix = MP_USED(v) - 1;
    d = MP_DIGIT(v, ix); /* most significant digit of v */

    extra = s_mp_ispow2d(d);
    if (extra < 0 || ix == 0)
        return extra;

    while (--ix >= 0) {
        if (DIGIT(v, ix) != 0)
            return -1; /* not a power of two */
        extra += MP_DIGIT_BIT;
    }

    return extra;

} /* end s_mp_ispow2() */

/* }}} */

/* {{{ s_mp_ispow2d(d) */

int
s_mp_ispow2d(mp_digit d)
{
    if ((d != 0) && ((d & (d - 1)) == 0)) { /* d is a power of 2 */
        int pow = 0;
#if defined(MP_USE_UINT_DIGIT)
        if (d & 0xffff0000U)
            pow += 16;
        if (d & 0xff00ff00U)
            pow += 8;
        if (d & 0xf0f0f0f0U)
            pow += 4;
        if (d & 0xccccccccU)
            pow += 2;
        if (d & 0xaaaaaaaaU)
            pow += 1;
#elif defined(MP_USE_LONG_LONG_DIGIT)
        if (d & 0xffffffff00000000ULL)
            pow += 32;
        if (d & 0xffff0000ffff0000ULL)
            pow += 16;
        if (d & 0xff00ff00ff00ff00ULL)
            pow += 8;
        if (d & 0xf0f0f0f0f0f0f0f0ULL)
            pow += 4;
        if (d & 0xccccccccccccccccULL)
            pow += 2;
        if (d & 0xaaaaaaaaaaaaaaaaULL)
            pow += 1;
#elif defined(MP_USE_LONG_DIGIT)
        if (d & 0xffffffff00000000UL)
            pow += 32;
        if (d & 0xffff0000ffff0000UL)
            pow += 16;
        if (d & 0xff00ff00ff00ff00UL)
            pow += 8;
        if (d & 0xf0f0f0f0f0f0f0f0UL)
            pow += 4;
        if (d & 0xccccccccccccccccUL)
            pow += 2;
        if (d & 0xaaaaaaaaaaaaaaaaUL)
            pow += 1;
#else
#error "unknown type for mp_digit"
#endif
        return pow;
    }
    return -1;

} /* end s_mp_ispow2d() */

/* }}} */

/* }}} */

/* {{{ Primitive I/O helpers */

/* {{{ s_mp_tovalue(ch, r) */

/*
  Convert the given character to its digit value, in the given radix.
  If the given character is not understood in the given radix, -1 is
  returned.  Otherwise the digit's numeric value is returned.

  The results will be odd if you use a radix < 2 or > 62, you are
  expected to know what you're up to.
 */
int
s_mp_tovalue(char ch, int r)
{
    int val, xch;

    if (r > 36)
        xch = ch;
    else
        xch = toupper((unsigned char)ch);

    if (isdigit((unsigned char)xch))
        val = xch - '0';
    else if (isupper((unsigned char)xch))
        val = xch - 'A' + 10;
    else if (islower((unsigned char)xch))
        val = xch - 'a' + 36;
    else if (xch == '+')
        val = 62;
    else if (xch == '/')
        val = 63;
    else
        return -1;

    if (val < 0 || val >= r)
        return -1;

    return val;

} /* end s_mp_tovalue() */

/* }}} */

/* {{{ s_mp_todigit(val, r, low) */

/*
  Convert val to a radix-r digit, if possible.  If val is out of range
  for r, returns zero.  Otherwise, returns an ASCII character denoting
  the value in the given radix.

  The results may be odd if you use a radix < 2 or > 64, you are
  expected to know what you're doing.
 */

char
s_mp_todigit(mp_digit val, int r, int low)
{
    char ch;

    if (val >= r)
        return 0;

    ch = s_dmap_1[val];

    if (r <= 36 && low)
        ch = tolower((unsigned char)ch);

    return ch;

} /* end s_mp_todigit() */

/* }}} */

/* {{{ s_mp_outlen(bits, radix) */

/*
   Return an estimate for how long a string is needed to hold a radix
   r representation of a number with 'bits' significant bits, plus an
   extra for a zero terminator (assuming C style strings here)
 */
int
s_mp_outlen(int bits, int r)
{
    return (int)((double)bits * LOG_V_2(r) + 1.5) + 1;

} /* end s_mp_outlen() */

/* }}} */

/* }}} */

/* {{{ mp_read_unsigned_octets(mp, str, len) */
/* mp_read_unsigned_octets(mp, str, len)
   Read in a raw value (base 256) into the given mp_int
   No sign bit, number is positive.  Leading zeros ignored.
 */

mp_err
mp_read_unsigned_octets(mp_int *mp, const unsigned char *str, mp_size len)
{
    int count;
    mp_err res;
    mp_digit d;

    ARGCHK(mp != NULL && str != NULL && len > 0, MP_BADARG);

    mp_zero(mp);

    count = len % sizeof(mp_digit);
    if (count) {
        for (d = 0; count-- > 0; --len) {
            d = (d << 8) | *str++;
        }
        MP_DIGIT(mp, 0) = d;
    }

    /* Read the rest of the digits */
    for (; len > 0; len -= sizeof(mp_digit)) {
        for (d = 0, count = sizeof(mp_digit); count > 0; --count) {
            d = (d << 8) | *str++;
        }
        if (MP_EQ == mp_cmp_z(mp)) {
            if (!d)
                continue;
        } else {
            if ((res = s_mp_lshd(mp, 1)) != MP_OKAY)
                return res;
        }
        MP_DIGIT(mp, 0) = d;
    }
    return MP_OKAY;
} /* end mp_read_unsigned_octets() */
/* }}} */

/* {{{ mp_unsigned_octet_size(mp) */
unsigned int
mp_unsigned_octet_size(const mp_int *mp)
{
    unsigned int bytes;
    int ix;
    mp_digit d = 0;

    ARGCHK(mp != NULL, MP_BADARG);
    ARGCHK(MP_ZPOS == SIGN(mp), MP_BADARG);

    bytes = (USED(mp) * sizeof(mp_digit));

    /* subtract leading zeros. */
    /* Iterate over each digit... */
    for (ix = USED(mp) - 1; ix >= 0; ix--) {
        d = DIGIT(mp, ix);
        if (d)
            break;
        bytes -= sizeof(d);
    }
    if (!bytes)
        return 1;

    /* Have MSD, check digit bytes, high order first */
    for (ix = sizeof(mp_digit) - 1; ix >= 0; ix--) {
        unsigned char x = (unsigned char)(d >> (ix * CHAR_BIT));
        if (x)
            break;
        --bytes;
    }
    return bytes;
} /* end mp_unsigned_octet_size() */
/* }}} */

/* {{{ mp_to_unsigned_octets(mp, str) */
/* output a buffer of big endian octets no longer than specified. */
mp_err
mp_to_unsigned_octets(const mp_int *mp, unsigned char *str, mp_size maxlen)
{
    int ix, pos = 0;
    unsigned int bytes;

    ARGCHK(mp != NULL && str != NULL && !SIGN(mp), MP_BADARG);

    bytes = mp_unsigned_octet_size(mp);
    ARGCHK(bytes <= maxlen, MP_BADARG);

    /* Iterate over each digit... */
    for (ix = USED(mp) - 1; ix >= 0; ix--) {
        mp_digit d = DIGIT(mp, ix);
        int jx;

        /* Unpack digit bytes, high order first */
        for (jx = sizeof(mp_digit) - 1; jx >= 0; jx--) {
            unsigned char x = (unsigned char)(d >> (jx * CHAR_BIT));
            if (!pos && !x) /* suppress leading zeros */
                continue;
            str[pos++] = x;
        }
    }
    if (!pos)
        str[pos++] = 0;
    return pos;
} /* end mp_to_unsigned_octets() */
/* }}} */

/* {{{ mp_to_signed_octets(mp, str) */
/* output a buffer of big endian octets no longer than specified. */
mp_err
mp_to_signed_octets(const mp_int *mp, unsigned char *str, mp_size maxlen)
{
    int ix, pos = 0;
    unsigned int bytes;

    ARGCHK(mp != NULL && str != NULL && !SIGN(mp), MP_BADARG);

    bytes = mp_unsigned_octet_size(mp);
    ARGCHK(bytes <= maxlen, MP_BADARG);

    /* Iterate over each digit... */
    for (ix = USED(mp) - 1; ix >= 0; ix--) {
        mp_digit d = DIGIT(mp, ix);
        int jx;

        /* Unpack digit bytes, high order first */
        for (jx = sizeof(mp_digit) - 1; jx >= 0; jx--) {
            unsigned char x = (unsigned char)(d >> (jx * CHAR_BIT));
            if (!pos) {
                if (!x) /* suppress leading zeros */
                    continue;
                if (x & 0x80) { /* add one leading zero to make output positive.  */
                    ARGCHK(bytes + 1 <= maxlen, MP_BADARG);
                    if (bytes + 1 > maxlen)
                        return MP_BADARG;
                    str[pos++] = 0;
                }
            }
            str[pos++] = x;
        }
    }
    if (!pos)
        str[pos++] = 0;
    return pos;
} /* end mp_to_signed_octets() */
/* }}} */

/* {{{ mp_to_fixlen_octets(mp, str) */
/* output a buffer of big endian octets exactly as long as requested.
   constant time on the value of mp. */
mp_err
mp_to_fixlen_octets(const mp_int *mp, unsigned char *str, mp_size length)
{
    int ix, jx;
    unsigned int bytes;

    ARGCHK(mp != NULL && str != NULL && !SIGN(mp) && length > 0, MP_BADARG);

    /* Constant time on the value of mp.  Don't use mp_unsigned_octet_size. */
    bytes = USED(mp) * MP_DIGIT_SIZE;

    /* If the output is shorter than the native size of mp, then check that any
     * bytes not written have zero values.  This check isn't constant time on
     * the assumption that timing-sensitive callers can guarantee that mp fits
     * in the allocated space. */
    ix = USED(mp) - 1;
    if (bytes > length) {
        unsigned int zeros = bytes - length;

        while (zeros >= MP_DIGIT_SIZE) {
            ARGCHK(DIGIT(mp, ix) == 0, MP_BADARG);
            zeros -= MP_DIGIT_SIZE;
            ix--;
        }

        if (zeros > 0) {
            mp_digit d = DIGIT(mp, ix);
            mp_digit m = ~0ULL << ((MP_DIGIT_SIZE - zeros) * CHAR_BIT);
            ARGCHK((d & m) == 0, MP_BADARG);
            for (jx = MP_DIGIT_SIZE - zeros - 1; jx >= 0; jx--) {
                *str++ = d >> (jx * CHAR_BIT);
            }
            ix--;
        }
    } else if (bytes < length) {
        /* Place any needed leading zeros. */
        unsigned int zeros = length - bytes;
        memset(str, 0, zeros);
        str += zeros;
    }

    /* Iterate over each whole digit... */
    for (; ix >= 0; ix--) {
        mp_digit d = DIGIT(mp, ix);

        /* Unpack digit bytes, high order first */
        for (jx = MP_DIGIT_SIZE - 1; jx >= 0; jx--) {
            *str++ = d >> (jx * CHAR_BIT);
        }
    }
    return MP_OKAY;
} /* end mp_to_fixlen_octets() */
/* }}} */

/* {{{ mp_cswap(condition, a, b, numdigits) */
/* performs a conditional swap between mp_int. */
mp_err
mp_cswap(mp_digit condition, mp_int *a, mp_int *b, mp_size numdigits)
{
    mp_digit x;
    unsigned int i;
    mp_err res = 0;

    /* if pointers are equal return */
    if (a == b)
        return res;

    if (MP_ALLOC(a) < numdigits || MP_ALLOC(b) < numdigits) {
        MP_CHECKOK(s_mp_grow(a, numdigits));
        MP_CHECKOK(s_mp_grow(b, numdigits));
    }

    condition = ((~condition & ((condition - 1))) >> (MP_DIGIT_BIT - 1)) - 1;

    x = (USED(a) ^ USED(b)) & condition;
    USED(a) ^= x;
    USED(b) ^= x;

    x = (SIGN(a) ^ SIGN(b)) & condition;
    SIGN(a) ^= x;
    SIGN(b) ^= x;

    for (i = 0; i < numdigits; i++) {
        x = (DIGIT(a, i) ^ DIGIT(b, i)) & condition;
        DIGIT(a, i) ^= x;
        DIGIT(b, i) ^= x;
    }

CLEANUP:
    return res;
} /* end mp_cswap() */
/* }}} */

/*------------------------------------------------------------------------*/
/* HERE THERE BE DRAGONS                                                  */
