From 37b07c68ef55058cdb990e8cf81650ae58dbd3ee Mon Sep 17 00:00:00 2001 From: Billy Brumley Date: Mon, 12 Nov 2018 15:47:54 +0200 Subject: [PATCH] Clean up BN_consttime_swap. Updated "condition" logic lifted from Theo Buehler's LibreSSL commit https://github.com/libressl-portable/openbsd/commit/517358603b4be76d48a50007a0d414c2072697dd Reviewed-by: Matt Caswell Reviewed-by: Nicola Tuveri (Merged from https://github.com/openssl/openssl/pull/7619) (cherry picked from commit 900fd8f375ca758d182e894bc1556509b231dbc8) --- crypto/bn/bn_lib.c | 61 +++++++++++++--------------------------------- 1 file changed, 17 insertions(+), 44 deletions(-) diff --git a/crypto/bn/bn_lib.c b/crypto/bn/bn_lib.c index 80f910c807..040c4cd9b3 100644 --- a/crypto/bn/bn_lib.c +++ b/crypto/bn/bn_lib.c @@ -737,26 +737,25 @@ int bn_cmp_part_words(const BN_ULONG *a, const BN_ULONG *b, int cl, int dl) 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 + * a and b are swapped if condition is not 0. + * nwords is the number of words to swap. + * Assumes that at least nwords are allocated in both a and b. + * Assumes that no more than nwords are used by either a or b. */ void BN_consttime_swap(BN_ULONG condition, BIGNUM *a, BIGNUM *b, int nwords) { BN_ULONG t; int i; + if (a == b) + return; + 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; + condition = ((~condition & ((condition - 1))) >> (BN_BITS2 - 1)) - 1; t = (a->top ^ b->top) & condition; a->top ^= t; @@ -794,42 +793,16 @@ void BN_consttime_swap(BN_ULONG condition, BIGNUM *a, BIGNUM *b, int nwords) a->flags ^= t; b->flags ^= 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 + /* conditionally swap the data */ + for (i = 0; i < nwords; i++) { + t = (a->d[i] ^ b->d[i]) & condition; + a->d[i] ^= t; + b->d[i] ^= t; + } } +#undef BN_CONSTTIME_SWAP_FLAGS + /* Bits of security, see SP800-57 */ int BN_security_bits(int L, int N) -- 2.25.1