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Don't bother retrying in bn_blinding_create_param.

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The probability of stumbling on a non-invertible b->A is negligible;
it's equivalent to accidentally factoring the RSA key. Relatedly,
document the slight caveat in BN_mod_inverse_blinded.

Change-Id: I308d17d12f5d6a12c444dda8c8fcc175ef2f5d45
Reviewed-on: https://boringssl-review.googlesource.com/26344
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>
This commit is contained in:
David Benjamin 2018-03-02 19:17:23 -05:00 committed by CQ bot account: commit-bot@chromium.org
parent f8058d4114
commit 929a9d7d42
2 changed files with 20 additions and 40 deletions
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54
crypto/fipsmodule/rsa/blinding.c
View File

@ -215,46 +215,22 @@ int BN_BLINDING_invert(BIGNUM *n, const BN_BLINDING *b, BN_MONT_CTX *mont,
static int bn_blinding_create_param(BN_BLINDING *b, const BIGNUM *e,
const BN_MONT_CTX *mont, BN_CTX *ctx) {
int retry_counter = 32;
do {
if (!BN_rand_range_ex(b->A, 1, &mont->N)) {
OPENSSL_PUT_ERROR(RSA, ERR_R_INTERNAL_ERROR);
return 0;
}
// |BN_from_montgomery| + |BN_mod_inverse_blinded| is equivalent to, but
// more efficient than, |BN_mod_inverse_blinded| + |BN_to_montgomery|.
if (!BN_from_montgomery(b->Ai, b->A, mont, ctx)) {
OPENSSL_PUT_ERROR(RSA, ERR_R_INTERNAL_ERROR);
return 0;
}
int no_inverse;
if (BN_mod_inverse_blinded(b->Ai, &no_inverse, b->Ai, mont, ctx)) {
break;
}
if (!no_inverse) {
OPENSSL_PUT_ERROR(RSA, ERR_R_INTERNAL_ERROR);
return 0;
}
// For reasonably-sized RSA keys, it should almost never be the case that a
// random value doesn't have an inverse.
if (retry_counter-- == 0) {
OPENSSL_PUT_ERROR(RSA, RSA_R_TOO_MANY_ITERATIONS);
return 0;
}
ERR_clear_error();
} while (1);
if (!BN_mod_exp_mont(b->A, b->A, e, &mont->N, ctx, mont)) {
OPENSSL_PUT_ERROR(RSA, ERR_R_INTERNAL_ERROR);
return 0;
}
if (!BN_to_montgomery(b->A, b->A, mont, ctx)) {
if (!BN_rand_range_ex(b->A, 1, &mont->N) ||
// Compute |b->A|^-1 in Montgomery form. Note |BN_from_montgomery| +
// |BN_mod_inverse_blinded| is equivalent to, but more efficient than,
// |BN_mod_inverse_blinded| + |BN_to_montgomery|.
//
// We do not retry if |b->A| has no inverse. Finding a non-invertible
// value of |b->A| is equivalent to factoring |mont->N|. There is
// negligible probability of stumbling on one at random.
!BN_from_montgomery(b->Ai, b->A, mont, ctx) ||
!BN_mod_inverse_blinded(b->Ai, &no_inverse, b->Ai, mont, ctx) ||
// TODO(davidben): |BN_mod_exp_mont| internally computes the result in
// Montgomery form. Save a pair of Montgomery reductions and a
// multiplication by returning that value directly.
!BN_mod_exp_mont(b->A, b->A, e, &mont->N, ctx, mont) ||
!BN_to_montgomery(b->A, b->A, mont, ctx)) {
OPENSSL_PUT_ERROR(RSA, ERR_R_INTERNAL_ERROR);
return 0;
}

4
include/openssl/bn.h
View File

@ -773,6 +773,10 @@ OPENSSL_EXPORT BIGNUM *BN_mod_inverse(BIGNUM *out, const BIGNUM *a,
// value) to protect it against side-channel attacks. On failure, if the failure
// was caused by |a| having no inverse mod |n| then |*out_no_inverse| will be
// set to one; otherwise it will be set to zero.
//
// Note this function may incorrectly report |a| has no inverse if the random
// blinding value has no inverse. It should only be used when |n| has few
// non-invertible elements, such as an RSA modulus.
int BN_mod_inverse_blinded(BIGNUM *out, int *out_no_inverse, const BIGNUM *a,
const BN_MONT_CTX *mont, BN_CTX *ctx);