diff options
author | Andy Polyakov <appro@openssl.org> | 2018-11-23 17:23:31 +0100 |
---|---|---|
committer | Matt Caswell <matt@openssl.org> | 2018-12-05 10:33:57 +0000 |
commit | 3a4a88f436ed1dd1165e0b59c1ca4a25e9e1d690 (patch) | |
tree | 2a457f25f6824e8417472ba2f0074035a0e75a29 /crypto/bn/bn_shift.c | |
parent | 3da2e9c4ee45989a426ff513dc6c6250d1e460de (diff) |
bn/bn_{div|shift}.c: introduce fixed-top interfaces.
Fixed-top interfaces tolerate zero-padded inputs and facilitate
constant-time-ness. bn_div_fixed_top tolerates zero-padded dividend,
but not divisor. It's argued that divisor's length is public even
when value is secret.
[extended tests]
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7589)
Diffstat (limited to 'crypto/bn/bn_shift.c')
-rw-r--r-- | crypto/bn/bn_shift.c | 130 |
1 files changed, 106 insertions, 24 deletions
diff --git a/crypto/bn/bn_shift.c b/crypto/bn/bn_shift.c index 15d4b321ba..b7a1e0ff9a 100644 --- a/crypto/bn/bn_shift.c +++ b/crypto/bn/bn_shift.c @@ -1,5 +1,5 @@ /* - * Copyright 1995-2016 The OpenSSL Project Authors. All Rights Reserved. + * Copyright 1995-2018 The OpenSSL Project Authors. All Rights Reserved. * * Licensed under the OpenSSL license (the "License"). You may not use * this file except in compliance with the License. You can obtain a copy @@ -7,6 +7,7 @@ * https://www.openssl.org/source/license.html */ +#include <assert.h> #include "internal/cryptlib.h" #include "bn_lcl.h" @@ -82,40 +83,70 @@ int BN_rshift1(BIGNUM *r, const BIGNUM *a) int BN_lshift(BIGNUM *r, const BIGNUM *a, int n) { - int i, nw, lb, rb; - BN_ULONG *t, *f; - BN_ULONG l; - - bn_check_top(r); - bn_check_top(a); + int ret; if (n < 0) { BNerr(BN_F_BN_LSHIFT, BN_R_INVALID_SHIFT); return 0; } + ret = bn_lshift_fixed_top(r, a, n); + + bn_correct_top(r); + bn_check_top(r); + + return ret; +} + +/* + * In respect to shift factor the execution time is invariant of + * |n % BN_BITS2|, but not |n / BN_BITS2|. Or in other words pre-condition + * for constant-time-ness is |n < BN_BITS2| or |n / BN_BITS2| being + * non-secret. + */ +int bn_lshift_fixed_top(BIGNUM *r, const BIGNUM *a, int n) +{ + int i, nw; + unsigned int lb, rb; + BN_ULONG *t, *f; + BN_ULONG l, m, rmask = 0; + + assert(n >= 0); + + bn_check_top(r); + bn_check_top(a); + nw = n / BN_BITS2; if (bn_wexpand(r, a->top + nw + 1) == NULL) return 0; - r->neg = a->neg; - lb = n % BN_BITS2; - rb = BN_BITS2 - lb; - f = a->d; - t = r->d; - t[a->top + nw] = 0; - if (lb == 0) - for (i = a->top - 1; i >= 0; i--) - t[nw + i] = f[i]; - else - for (i = a->top - 1; i >= 0; i--) { - l = f[i]; - t[nw + i + 1] |= (l >> rb) & BN_MASK2; - t[nw + i] = (l << lb) & BN_MASK2; + + if (a->top != 0) { + lb = (unsigned int)n % BN_BITS2; + rb = BN_BITS2 - lb; + rb %= BN_BITS2; /* say no to undefined behaviour */ + rmask = (BN_ULONG)0 - rb; /* rmask = 0 - (rb != 0) */ + rmask |= rmask >> 8; + f = &(a->d[0]); + t = &(r->d[nw]); + l = f[a->top - 1]; + t[a->top] = (l >> rb) & rmask; + for (i = a->top - 1; i > 0; i--) { + m = l << lb; + l = f[i - 1]; + t[i] = (m | ((l >> rb) & rmask)) & BN_MASK2; } - memset(t, 0, sizeof(*t) * nw); + t[0] = (l << lb) & BN_MASK2; + } else { + /* shouldn't happen, but formally required */ + r->d[nw] = 0; + } + if (nw != 0) + memset(r->d, 0, sizeof(*t) * nw); + + r->neg = a->neg; r->top = a->top + nw + 1; - bn_correct_top(r); - bn_check_top(r); + r->flags |= BN_FLG_FIXED_TOP; + return 1; } @@ -173,3 +204,54 @@ int BN_rshift(BIGNUM *r, const BIGNUM *a, int n) bn_check_top(r); return 1; } + +/* + * In respect to shift factor the execution time is invariant of + * |n % BN_BITS2|, but not |n / BN_BITS2|. Or in other words pre-condition + * for constant-time-ness for sufficiently[!] zero-padded inputs is + * |n < BN_BITS2| or |n / BN_BITS2| being non-secret. + */ +int bn_rshift_fixed_top(BIGNUM *r, const BIGNUM *a, int n) +{ + int i, top, nw; + unsigned int lb, rb; + BN_ULONG *t, *f; + BN_ULONG l, m, mask; + + bn_check_top(r); + bn_check_top(a); + + assert(n >= 0); + + nw = n / BN_BITS2; + if (nw >= a->top) { + /* shouldn't happen, but formally required */ + BN_zero(r); + return 1; + } + + rb = (unsigned int)n % BN_BITS2; + lb = BN_BITS2 - rb; + lb %= BN_BITS2; /* say no to undefined behaviour */ + mask = (BN_ULONG)0 - lb; /* mask = 0 - (lb != 0) */ + mask |= mask >> 8; + top = a->top - nw; + if (r != a && bn_wexpand(r, top) == NULL) + return 0; + + t = &(r->d[0]); + f = &(a->d[nw]); + l = f[0]; + for (i = 0; i < top - 1; i++) { + m = f[i + 1]; + t[i] = (l >> rb) | ((m << lb) & mask); + l = m; + } + t[i] = l >> rb; + + r->neg = a->neg; + r->top = top; + r->flags |= BN_FLG_FIXED_TOP; + + return 1; +} |