From 4b7a4ba29cafa432fc4266fe6e59e60bc1c96332 Mon Sep 17 00:00:00 2001 From: "Dr. Stephen Henson" Date: Wed, 12 Mar 2014 14:16:19 +0000 Subject: [PATCH] Fix for CVE-2014-0076 Fix for the attack described in the paper "Recovering OpenSSL ECDSA Nonces Using the FLUSH+RELOAD Cache Side-channel Attack" by Yuval Yarom and Naomi Benger. Details can be obtained from: http://eprint.iacr.org/2014/140 Thanks to Yuval Yarom and Naomi Benger for discovering this flaw and to Yuval Yarom for supplying a fix. (cherry picked from commit 2198be3483259de374f91e57d247d0fc667aef29) Conflicts: CHANGES --- CHANGES | 9 ++++++++ crypto/bn/bn.h | 11 ++++++++++ crypto/bn/bn_lib.c | 52 ++++++++++++++++++++++++++++++++++++++++++++ crypto/ec/ec2_mult.c | 27 +++++++++++++---------- 4 files changed, 88 insertions(+), 11 deletions(-) diff --git a/CHANGES b/CHANGES index 58ac884d80..99aeefb4c2 100644 --- a/CHANGES +++ b/CHANGES @@ -4,6 +4,15 @@ Changes between 1.0.1f and 1.0.1g [xx XXX xxxx] + *) Fix for the attack described in the paper "Recovering OpenSSL + ECDSA Nonces Using the FLUSH+RELOAD Cache Side-channel Attack" + by Yuval Yarom and Naomi Benger. Details can be obtained from: + http://eprint.iacr.org/2014/140 + + Thanks to Yuval Yarom and Naomi Benger for discovering this + flaw and to Yuval Yarom for supplying a fix (CVE-2014-0076) + [Yuval Yarom and Naomi Benger] + *) TLS pad extension: draft-agl-tls-padding-02 Workaround for the "TLS hang bug" (see FAQ and PR#2771): if the diff --git a/crypto/bn/bn.h b/crypto/bn/bn.h index f34248ec4f..21a1a3fe35 100644 --- a/crypto/bn/bn.h +++ b/crypto/bn/bn.h @@ -538,6 +538,8 @@ BIGNUM *BN_mod_inverse(BIGNUM *ret, BIGNUM *BN_mod_sqrt(BIGNUM *ret, const BIGNUM *a, const BIGNUM *n,BN_CTX *ctx); +void BN_consttime_swap(BN_ULONG swap, BIGNUM *a, BIGNUM *b, int nwords); + /* Deprecated versions */ #ifndef OPENSSL_NO_DEPRECATED BIGNUM *BN_generate_prime(BIGNUM *ret,int bits,int safe, @@ -774,11 +776,20 @@ int RAND_pseudo_bytes(unsigned char *buf,int num); #define bn_fix_top(a) bn_check_top(a) +#define bn_check_size(bn, bits) bn_wcheck_size(bn, ((bits+BN_BITS2-1))/BN_BITS2) +#define bn_wcheck_size(bn, words) \ + do { \ + const BIGNUM *_bnum2 = (bn); \ + assert(words <= (_bnum2)->dmax && words >= (_bnum2)->top); \ + } while(0) + #else /* !BN_DEBUG */ #define bn_pollute(a) #define bn_check_top(a) #define bn_fix_top(a) bn_correct_top(a) +#define bn_check_size(bn, bits) +#define bn_wcheck_size(bn, words) #endif diff --git a/crypto/bn/bn_lib.c b/crypto/bn/bn_lib.c index 7a5676de69..5461e6ee7d 100644 --- a/crypto/bn/bn_lib.c +++ b/crypto/bn/bn_lib.c @@ -824,3 +824,55 @@ int bn_cmp_part_words(const BN_ULONG *a, const BN_ULONG *b, } return bn_cmp_words(a,b,cl); } + +/* + * Constant-time conditional swap of a and b. + * a and b are swapped if condition is not 0. The code assumes that at most one bit of condition is set. + * nwords is the number of words to swap. The code assumes that at least nwords are allocated in both a and b, + * and that no more than nwords are used by either a or b. + * a and b cannot be the same number + */ +void BN_consttime_swap(BN_ULONG condition, BIGNUM *a, BIGNUM *b, int nwords) + { + BN_ULONG t; + int i; + + bn_wcheck_size(a, nwords); + bn_wcheck_size(b, nwords); + + assert(a != b); + assert((condition & (condition - 1)) == 0); + assert(sizeof(BN_ULONG) >= sizeof(int)); + + condition = ((condition - 1) >> (BN_BITS2 - 1)) - 1; + + t = (a->top^b->top) & condition; + a->top ^= t; + b->top ^= t; + +#define BN_CONSTTIME_SWAP(ind) \ + do { \ + t = (a->d[ind] ^ b->d[ind]) & condition; \ + a->d[ind] ^= t; \ + b->d[ind] ^= t; \ + } while (0) + + + switch (nwords) { + default: + for (i = 10; i < nwords; i++) + BN_CONSTTIME_SWAP(i); + /* Fallthrough */ + case 10: BN_CONSTTIME_SWAP(9); /* Fallthrough */ + case 9: BN_CONSTTIME_SWAP(8); /* Fallthrough */ + case 8: BN_CONSTTIME_SWAP(7); /* Fallthrough */ + case 7: BN_CONSTTIME_SWAP(6); /* Fallthrough */ + case 6: BN_CONSTTIME_SWAP(5); /* Fallthrough */ + case 5: BN_CONSTTIME_SWAP(4); /* Fallthrough */ + case 4: BN_CONSTTIME_SWAP(3); /* Fallthrough */ + case 3: BN_CONSTTIME_SWAP(2); /* Fallthrough */ + case 2: BN_CONSTTIME_SWAP(1); /* Fallthrough */ + case 1: BN_CONSTTIME_SWAP(0); + } +#undef BN_CONSTTIME_SWAP +} diff --git a/crypto/ec/ec2_mult.c b/crypto/ec/ec2_mult.c index 26f4a783fc..1c575dc47a 100644 --- a/crypto/ec/ec2_mult.c +++ b/crypto/ec/ec2_mult.c @@ -208,11 +208,15 @@ static int gf2m_Mxy(const EC_GROUP *group, const BIGNUM *x, const BIGNUM *y, BIG return ret; } + /* Computes scalar*point and stores the result in r. * point can not equal r. - * Uses algorithm 2P of + * Uses a modified algorithm 2P of * Lopez, J. and Dahab, R. "Fast multiplication on elliptic curves over * GF(2^m) without precomputation" (CHES '99, LNCS 1717). + * + * To protect against side-channel attack the function uses constant time swap, + * avoiding conditional branches. */ static int ec_GF2m_montgomery_point_multiply(const EC_GROUP *group, EC_POINT *r, const BIGNUM *scalar, const EC_POINT *point, BN_CTX *ctx) @@ -246,6 +250,11 @@ static int ec_GF2m_montgomery_point_multiply(const EC_GROUP *group, EC_POINT *r, x2 = &r->X; z2 = &r->Y; + bn_wexpand(x1, group->field.top); + bn_wexpand(z1, group->field.top); + bn_wexpand(x2, group->field.top); + bn_wexpand(z2, group->field.top); + if (!BN_GF2m_mod_arr(x1, &point->X, group->poly)) goto err; /* x1 = x */ if (!BN_one(z1)) goto err; /* z1 = 1 */ if (!group->meth->field_sqr(group, z2, x1, ctx)) goto err; /* z2 = x1^2 = x^2 */ @@ -270,16 +279,12 @@ static int ec_GF2m_montgomery_point_multiply(const EC_GROUP *group, EC_POINT *r, word = scalar->d[i]; while (mask) { - if (word & mask) - { - if (!gf2m_Madd(group, &point->X, x1, z1, x2, z2, ctx)) goto err; - if (!gf2m_Mdouble(group, x2, z2, ctx)) goto err; - } - else - { - if (!gf2m_Madd(group, &point->X, x2, z2, x1, z1, ctx)) goto err; - if (!gf2m_Mdouble(group, x1, z1, ctx)) goto err; - } + BN_consttime_swap(word & mask, x1, x2, group->field.top); + BN_consttime_swap(word & mask, z1, z2, group->field.top); + if (!gf2m_Madd(group, &point->X, x2, z2, x1, z1, ctx)) goto err; + if (!gf2m_Mdouble(group, x1, z1, ctx)) goto err; + BN_consttime_swap(word & mask, x1, x2, group->field.top); + BN_consttime_swap(word & mask, z1, z2, group->field.top); mask >>= 1; } mask = BN_TBIT; -- 2.34.1