From d6482a82bc2228327aa4ba98aeeecd9979542a31 Mon Sep 17 00:00:00 2001 From: Andy Polyakov Date: Mon, 25 Jan 2016 20:38:38 +0100 Subject: [PATCH] bn/bn_exp.c: constant-time MOD_EXP_CTIME_COPY_FROM_PREBUF. Performance penalty varies from platform to platform, and even key length. For rsa2048 sign it was observed to reach almost 10%. CVE-2016-0702 Reviewed-by: Richard Levitte Reviewed-by: Rich Salz --- crypto/bn/bn_exp.c | 75 +++++++++++++++++++++++++++++++++++----------- 1 file changed, 57 insertions(+), 18 deletions(-) diff --git a/crypto/bn/bn_exp.c b/crypto/bn/bn_exp.c index b625c3b42b..07f9288bb5 100644 --- a/crypto/bn/bn_exp.c +++ b/crypto/bn/bn_exp.c @@ -109,6 +109,7 @@ */ #include "internal/cryptlib.h" +#include "internal/constant_time_locl.h" #include "bn_lcl.h" #include @@ -605,15 +606,17 @@ static BN_ULONG bn_get_bits(const BIGNUM *a, int bitpos) static int MOD_EXP_CTIME_COPY_TO_PREBUF(const BIGNUM *b, int top, unsigned char *buf, int idx, - int width) + int window) { - size_t i, j; + int i, j; + int width = 1 << window; + BN_ULONG *table = (BN_ULONG *)buf; if (top > b->top) top = b->top; /* this works because 'buf' is explicitly * zeroed */ - for (i = 0, j = idx; i < top * sizeof b->d[0]; i++, j += width) { - buf[j] = ((unsigned char *)b->d)[i]; + for (i = 0, j = idx; i < top; i++, j += width) { + table[j] = b->d[i]; } return 1; @@ -621,15 +624,51 @@ static int MOD_EXP_CTIME_COPY_TO_PREBUF(const BIGNUM *b, int top, static int MOD_EXP_CTIME_COPY_FROM_PREBUF(BIGNUM *b, int top, unsigned char *buf, int idx, - int width) + int window) { - size_t i, j; + int i, j; + int width = 1 << window; + volatile BN_ULONG *table = (volatile BN_ULONG *)buf; if (bn_wexpand(b, top) == NULL) return 0; - for (i = 0, j = idx; i < top * sizeof b->d[0]; i++, j += width) { - ((unsigned char *)b->d)[i] = buf[j]; + if (window <= 3) { + for (i = 0; i < top; i++, table += width) { + BN_ULONG acc = 0; + + for (j = 0; j < width; j++) { + acc |= table[j] & + ((BN_ULONG)0 - (constant_time_eq_int(j,idx)&1)); + } + + b->d[i] = acc; + } + } else { + int xstride = 1 << (window - 2); + BN_ULONG y0, y1, y2, y3; + + i = idx >> (window - 2); /* equivalent of idx / xstride */ + idx &= xstride - 1; /* equivalent of idx % xstride */ + + y0 = (BN_ULONG)0 - (constant_time_eq_int(i,0)&1); + y1 = (BN_ULONG)0 - (constant_time_eq_int(i,1)&1); + y2 = (BN_ULONG)0 - (constant_time_eq_int(i,2)&1); + y3 = (BN_ULONG)0 - (constant_time_eq_int(i,3)&1); + + for (i = 0; i < top; i++, table += width) { + BN_ULONG acc = 0; + + for (j = 0; j < xstride; j++) { + acc |= ( (table[j + 0 * xstride] & y0) | + (table[j + 1 * xstride] & y1) | + (table[j + 2 * xstride] & y2) | + (table[j + 3 * xstride] & y3) ) + & ((BN_ULONG)0 - (constant_time_eq_int(j,idx)&1)); + } + + b->d[i] = acc; + } } b->top = top; @@ -1063,9 +1102,9 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, } else #endif { - if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, 0, numPowers)) + if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, 0, window)) goto err; - if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&am, top, powerbuf, 1, numPowers)) + if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&am, top, powerbuf, 1, window)) goto err; /* @@ -1077,15 +1116,15 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, if (window > 1) { if (!BN_mod_mul_montgomery(&tmp, &am, &am, mont, ctx)) goto err; - if (!MOD_EXP_CTIME_COPY_TO_PREBUF - (&tmp, top, powerbuf, 2, numPowers)) + if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, 2, + window)) goto err; for (i = 3; i < numPowers; i++) { /* Calculate a^i = a^(i-1) * a */ if (!BN_mod_mul_montgomery(&tmp, &am, &tmp, mont, ctx)) goto err; - if (!MOD_EXP_CTIME_COPY_TO_PREBUF - (&tmp, top, powerbuf, i, numPowers)) + if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, i, + window)) goto err; } } @@ -1093,8 +1132,8 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, bits--; for (wvalue = 0, i = bits % window; i >= 0; i--, bits--) wvalue = (wvalue << 1) + BN_is_bit_set(p, bits); - if (!MOD_EXP_CTIME_COPY_FROM_PREBUF - (&tmp, top, powerbuf, wvalue, numPowers)) + if (!MOD_EXP_CTIME_COPY_FROM_PREBUF(&tmp, top, powerbuf, wvalue, + window)) goto err; /* @@ -1114,8 +1153,8 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, /* * Fetch the appropriate pre-computed value from the pre-buf */ - if (!MOD_EXP_CTIME_COPY_FROM_PREBUF - (&am, top, powerbuf, wvalue, numPowers)) + if (!MOD_EXP_CTIME_COPY_FROM_PREBUF(&am, top, powerbuf, wvalue, + window)) goto err; /* Multiply the result into the intermediate result */ -- 2.34.1