boringssl/crypto/bn/jacobi.c

/* ====================================================================
 * Copyright (c) 1998-2000 The OpenSSL Project.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
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 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
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 *    distribution.
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 * 3. All advertising materials mentioning features or use of this
 *    software must display the following acknowledgment:
 *    "This product includes software developed by the OpenSSL Project
 *    for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
 *
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 *    openssl-core@openssl.org.
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 *    acknowledgment:
 *    "This product includes software developed by the OpenSSL Project
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 *
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 * ====================================================================
 *
 * This product includes cryptographic software written by Eric Young
 * (eay@cryptsoft.com).  This product includes software written by Tim
 * Hudson (tjh@cryptsoft.com). */

#include <openssl/bn.h>

#include <openssl/err.h>

#include "internal.h"


/* least significant word */
#define BN_lsw(n) (((n)->top == 0) ? (BN_ULONG) 0 : (n)->d[0])

int bn_jacobi(const BIGNUM *a, const BIGNUM *b, BN_CTX *ctx) {
  /* In 'tab', only odd-indexed entries are relevant:
   * For any odd BIGNUM n,
   *     tab[BN_lsw(n) & 7]
   * is $(-1)^{(n^2-1)/8}$ (using TeX notation).
   * Note that the sign of n does not matter. */
  static const int tab[8] = {0, 1, 0, -1, 0, -1, 0, 1};

  /* The Jacobi symbol is only defined for odd modulus. */
  if (!BN_is_odd(b)) {
    OPENSSL_PUT_ERROR(BN, BN_R_CALLED_WITH_EVEN_MODULUS);
    return -2;
  }

  /* Require b be positive. */
  if (BN_is_negative(b)) {
    OPENSSL_PUT_ERROR(BN, BN_R_NEGATIVE_NUMBER);
    return -2;
  }

  int ret = -2;
  BN_CTX_start(ctx);
  BIGNUM *A = BN_CTX_get(ctx);
  BIGNUM *B = BN_CTX_get(ctx);
  if (B == NULL) {
    goto end;
  }

  if (!BN_copy(A, a) ||
      !BN_copy(B, b)) {
    goto end;
  }

  /* Adapted from logic to compute the Kronecker symbol, originally implemented
   * according to Henri Cohen, "A Course in Computational Algebraic Number
   * Theory" (algorithm 1.4.10). */

  ret = 1;

  while (1) {
    /* Cohen's step 3: */

    /* B is positive and odd */
    if (BN_is_zero(A)) {
      ret = BN_is_one(B) ? ret : 0;
      goto end;
    }

    /* now A is non-zero */
    int i = 0;
    while (!BN_is_bit_set(A, i)) {
      i++;
    }
    if (!BN_rshift(A, A, i)) {
      ret = -2;
      goto end;
    }
    if (i & 1) {
      /* i is odd */
      /* multiply 'ret' by  $(-1)^{(B^2-1)/8}$ */
      ret = ret * tab[BN_lsw(B) & 7];
    }

    /* Cohen's step 4: */
    /* multiply 'ret' by  $(-1)^{(A-1)(B-1)/4}$ */
    if ((A->neg ? ~BN_lsw(A) : BN_lsw(A)) & BN_lsw(B) & 2) {
      ret = -ret;
    }

    /* (A, B) := (B mod |A|, |A|) */
    if (!BN_nnmod(B, B, A, ctx)) {
      ret = -2;
      goto end;
    }
    BIGNUM *tmp = A;
    A = B;
    B = tmp;
    tmp->neg = 0;
  }

end:
  BN_CTX_end(ctx);
  return ret;
}