crypto/fipsmodule/bn/cmp.cc.inc

// Copyright 1995-2016 The OpenSSL Project Authors. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include <openssl/bn.h>

#include <assert.h>

#include <openssl/mem.h>

#include "internal.h"
#include "../../internal.h"


static int bn_cmp_words_consttime(const BN_ULONG *a, size_t a_len,
                                  const BN_ULONG *b, size_t b_len) {
  static_assert(sizeof(BN_ULONG) <= sizeof(crypto_word_t),
                "crypto_word_t is too small");
  int ret = 0;
  // Process the common words in little-endian order.
  size_t min = a_len < b_len ? a_len : b_len;
  for (size_t i = 0; i < min; i++) {
    crypto_word_t eq = constant_time_eq_w(a[i], b[i]);
    crypto_word_t lt = constant_time_lt_w(a[i], b[i]);
    ret =
        constant_time_select_int(eq, ret, constant_time_select_int(lt, -1, 1));
  }

  // If |a| or |b| has non-zero words beyond |min|, they take precedence.
  if (a_len < b_len) {
    crypto_word_t mask = 0;
    for (size_t i = a_len; i < b_len; i++) {
      mask |= b[i];
    }
    ret = constant_time_select_int(constant_time_is_zero_w(mask), ret, -1);
  } else if (b_len < a_len) {
    crypto_word_t mask = 0;
    for (size_t i = b_len; i < a_len; i++) {
      mask |= a[i];
    }
    ret = constant_time_select_int(constant_time_is_zero_w(mask), ret, 1);
  }

  return ret;
}

int BN_ucmp(const BIGNUM *a, const BIGNUM *b) {
  return bn_cmp_words_consttime(a->d, a->width, b->d, b->width);
}

int BN_cmp(const BIGNUM *a, const BIGNUM *b) {
  if ((a == nullptr) || (b == nullptr)) {
    if (a != nullptr) {
      return -1;
    } else if (b != nullptr) {
      return 1;
    } else {
      return 0;
    }
  }

  // We do not attempt to process the sign bit in constant time. Negative
  // |BIGNUM|s should never occur in crypto, only calculators.
  if (a->neg != b->neg) {
    if (a->neg) {
      return -1;
    }
    return 1;
  }

  int ret = BN_ucmp(a, b);
  return a->neg ? -ret : ret;
}

int bn_less_than_words(const BN_ULONG *a, const BN_ULONG *b, size_t len) {
  return bn_cmp_words_consttime(a, len, b, len) < 0;
}

int BN_abs_is_word(const BIGNUM *bn, BN_ULONG w) {
  if (bn->width == 0) {
    return w == 0;
  }
  BN_ULONG mask = bn->d[0] ^ w;
  for (int i = 1; i < bn->width; i++) {
    mask |= bn->d[i];
  }
  return mask == 0;
}

int BN_cmp_word(const BIGNUM *a, BN_ULONG b) {
  BIGNUM b_bn;
  BN_init(&b_bn);

  b_bn.d = &b;
  b_bn.width = b > 0;
  b_bn.dmax = 1;
  b_bn.flags = BN_FLG_STATIC_DATA;
  return BN_cmp(a, &b_bn);
}

int BN_is_zero(const BIGNUM *bn) {
  return bn_fits_in_words(bn, 0);
}

int BN_is_one(const BIGNUM *bn) {
  return bn->neg == 0 && BN_abs_is_word(bn, 1);
}

int BN_is_word(const BIGNUM *bn, BN_ULONG w) {
  return BN_abs_is_word(bn, w) && (w == 0 || bn->neg == 0);
}

int BN_is_odd(const BIGNUM *bn) {
  return bn->width > 0 && (bn->d[0] & 1) == 1;
}

int BN_is_pow2(const BIGNUM *bn) {
  int width = bn_minimal_width(bn);
  if (width == 0 || bn->neg) {
    return 0;
  }

  for (int i = 0; i < width - 1; i++) {
    if (bn->d[i] != 0) {
      return 0;
    }
  }

  return 0 == (bn->d[width-1] & (bn->d[width-1] - 1));
}

int BN_equal_consttime(const BIGNUM *a, const BIGNUM *b) {
  BN_ULONG mask = 0;
  // If |a| or |b| has more words than the other, all those words must be zero.
  for (int i = a->width; i < b->width; i++) {
    mask |= b->d[i];
  }
  for (int i = b->width; i < a->width; i++) {
    mask |= a->d[i];
  }
  // Common words must match.
  int min = a->width < b->width ? a->width : b->width;
  for (int i = 0; i < min; i++) {
    mask |= (a->d[i] ^ b->d[i]);
  }
  // The sign bit must match.
  mask |= (a->neg ^ b->neg);
  return mask == 0;
}