The branch OpenSSL_1_0_2-stable has been updated
via b18162a7c9bbfb57112459a4d6631fa258fd8c0c (commit)
from 59b9c67fcaf1c1e2c0e30de6facca85910ac361a (commit)
- Log -----------------------------------------------------------------
commit b18162a7c9bbfb57112459a4d6631fa258fd8c0c
Author: Billy Brumley <bbrum...@gmail.com>
Date: Thu Nov 8 13:57:54 2018 +0200
CVE-2018-5407 fix: ECC ladder
Reviewed-by: Matt Caswell <m...@openssl.org>
Reviewed-by: Paul Dale <paul.d...@oracle.com>
Reviewed-by: Nicola Tuveri <nic....@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/7593)
-----------------------------------------------------------------------
Summary of changes:
CHANGES | 13 +++
crypto/bn/bn_lib.c | 32 +++++++
crypto/ec/ec_mult.c | 246 ++++++++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 291 insertions(+)
diff --git a/CHANGES b/CHANGES
index b574074..fde66b5 100644
--- a/CHANGES
+++ b/CHANGES
@@ -9,6 +9,19 @@
Changes between 1.0.2p and 1.0.2q [xx XXX xxxx]
+ *) Microarchitecture timing vulnerability in ECC scalar multiplication
+
+ OpenSSL ECC scalar multiplication, used in e.g. ECDSA and ECDH, has been
+ shown to be vulnerable to a microarchitecture timing side channel attack.
+ An attacker with sufficient access to mount local timing attacks during
+ ECDSA signature generation could recover the private key.
+
+ This issue was reported to OpenSSL on 26th October 2018 by Alejandro
+ Cabrera Aldaya, Billy Brumley, Sohaib ul Hassan, Cesar Pereida Garcia and
+ Nicola Tuveri.
+ (CVE-2018-5407)
+ [Billy Brumley]
+
*) Resolve a compatibility issue in EC_GROUP handling with the FIPS Object
Module, accidentally introduced while backporting security fixes from the
development branch and hindering the use of ECC in FIPS mode.
diff --git a/crypto/bn/bn_lib.c b/crypto/bn/bn_lib.c
index 8f1042b..9b95e5f 100644
--- a/crypto/bn/bn_lib.c
+++ b/crypto/bn/bn_lib.c
@@ -903,6 +903,38 @@ void BN_consttime_swap(BN_ULONG condition, BIGNUM *a,
BIGNUM *b, int nwords)
a->top ^= t;
b->top ^= t;
+ t = (a->neg ^ b->neg) & condition;
+ a->neg ^= t;
+ b->neg ^= t;
+
+ /*-
+ * BN_FLG_STATIC_DATA: indicates that data may not be written to. Intention
+ * is actually to treat it as it's read-only data, and some (if not most)
+ * of it does reside in read-only segment. In other words observation of
+ * BN_FLG_STATIC_DATA in BN_consttime_swap should be treated as fatal
+ * condition. It would either cause SEGV or effectively cause data
+ * corruption.
+ *
+ * BN_FLG_MALLOCED: refers to BN structure itself, and hence must be
+ * preserved.
+ *
+ * BN_FLG_SECURE: must be preserved, because it determines how x->d was
+ * allocated and hence how to free it.
+ *
+ * BN_FLG_CONSTTIME: sufficient to mask and swap
+ *
+ * BN_FLG_FIXED_TOP: indicates that we haven't called bn_correct_top() on
+ * the data, so the d array may be padded with additional 0 values (i.e.
+ * top could be greater than the minimal value that it could be). We should
+ * be swapping it
+ */
+
+#define BN_CONSTTIME_SWAP_FLAGS (BN_FLG_CONSTTIME | BN_FLG_FIXED_TOP)
+
+ t = ((a->flags ^ b->flags) & BN_CONSTTIME_SWAP_FLAGS) & condition;
+ a->flags ^= t;
+ b->flags ^= t;
+
#define BN_CONSTTIME_SWAP(ind) \
do { \
t = (a->d[ind] ^ b->d[ind]) & condition; \
diff --git a/crypto/ec/ec_mult.c b/crypto/ec/ec_mult.c
index 2231f99..c573d4b 100644
--- a/crypto/ec/ec_mult.c
+++ b/crypto/ec/ec_mult.c
@@ -310,6 +310,224 @@ static signed char *compute_wNAF(const BIGNUM *scalar,
int w, size_t *ret_len)
return r;
}
+#define EC_POINT_BN_set_flags(P, flags) do { \
+ BN_set_flags(&(P)->X, (flags)); \
+ BN_set_flags(&(P)->Y, (flags)); \
+ BN_set_flags(&(P)->Z, (flags)); \
+} while(0)
+
+/*-
+ * This functions computes (in constant time) a point multiplication over the
+ * EC group.
+ *
+ * At a high level, it is Montgomery ladder with conditional swaps.
+ *
+ * It performs either a fixed scalar point multiplication
+ * (scalar * generator)
+ * when point is NULL, or a generic scalar point multiplication
+ * (scalar * point)
+ * when point is not NULL.
+ *
+ * scalar should be in the range [0,n) otherwise all constant time bets are
off.
+ *
+ * NB: This says nothing about EC_POINT_add and EC_POINT_dbl,
+ * which of course are not constant time themselves.
+ *
+ * The product is stored in r.
+ *
+ * Returns 1 on success, 0 otherwise.
+ */
+static int ec_mul_consttime(const EC_GROUP *group, EC_POINT *r,
+ const BIGNUM *scalar, const EC_POINT *point,
+ BN_CTX *ctx)
+{
+ int i, cardinality_bits, group_top, kbit, pbit, Z_is_one;
+ EC_POINT *s = NULL;
+ BIGNUM *k = NULL;
+ BIGNUM *lambda = NULL;
+ BIGNUM *cardinality = NULL;
+ BN_CTX *new_ctx = NULL;
+ int ret = 0;
+
+ if (ctx == NULL && (ctx = new_ctx = BN_CTX_new()) == NULL)
+ return 0;
+
+ BN_CTX_start(ctx);
+
+ s = EC_POINT_new(group);
+ if (s == NULL)
+ goto err;
+
+ if (point == NULL) {
+ if (!EC_POINT_copy(s, group->generator))
+ goto err;
+ } else {
+ if (!EC_POINT_copy(s, point))
+ goto err;
+ }
+
+ EC_POINT_BN_set_flags(s, BN_FLG_CONSTTIME);
+
+ cardinality = BN_CTX_get(ctx);
+ lambda = BN_CTX_get(ctx);
+ k = BN_CTX_get(ctx);
+ if (k == NULL || !BN_mul(cardinality, &group->order, &group->cofactor,
ctx))
+ goto err;
+
+ /*
+ * Group cardinalities are often on a word boundary.
+ * So when we pad the scalar, some timing diff might
+ * pop if it needs to be expanded due to carries.
+ * So expand ahead of time.
+ */
+ cardinality_bits = BN_num_bits(cardinality);
+ group_top = cardinality->top;
+ if ((bn_wexpand(k, group_top + 2) == NULL)
+ || (bn_wexpand(lambda, group_top + 2) == NULL))
+ goto err;
+
+ if (!BN_copy(k, scalar))
+ goto err;
+
+ BN_set_flags(k, BN_FLG_CONSTTIME);
+
+ if ((BN_num_bits(k) > cardinality_bits) || (BN_is_negative(k))) {
+ /*-
+ * this is an unusual input, and we don't guarantee
+ * constant-timeness
+ */
+ if (!BN_nnmod(k, k, cardinality, ctx))
+ goto err;
+ }
+
+ if (!BN_add(lambda, k, cardinality))
+ goto err;
+ BN_set_flags(lambda, BN_FLG_CONSTTIME);
+ if (!BN_add(k, lambda, cardinality))
+ goto err;
+ /*
+ * lambda := scalar + cardinality
+ * k := scalar + 2*cardinality
+ */
+ kbit = BN_is_bit_set(lambda, cardinality_bits);
+ BN_consttime_swap(kbit, k, lambda, group_top + 2);
+
+ group_top = group->field.top;
+ if ((bn_wexpand(&s->X, group_top) == NULL)
+ || (bn_wexpand(&s->Y, group_top) == NULL)
+ || (bn_wexpand(&s->Z, group_top) == NULL)
+ || (bn_wexpand(&r->X, group_top) == NULL)
+ || (bn_wexpand(&r->Y, group_top) == NULL)
+ || (bn_wexpand(&r->Z, group_top) == NULL))
+ goto err;
+
+ /* top bit is a 1, in a fixed pos */
+ if (!EC_POINT_copy(r, s))
+ goto err;
+
+ EC_POINT_BN_set_flags(r, BN_FLG_CONSTTIME);
+
+ if (!EC_POINT_dbl(group, s, s, ctx))
+ goto err;
+
+ pbit = 0;
+
+#define EC_POINT_CSWAP(c, a, b, w, t) do { \
+ BN_consttime_swap(c, &(a)->X, &(b)->X, w); \
+ BN_consttime_swap(c, &(a)->Y, &(b)->Y, w); \
+ BN_consttime_swap(c, &(a)->Z, &(b)->Z, w); \
+ t = ((a)->Z_is_one ^ (b)->Z_is_one) & (c); \
+ (a)->Z_is_one ^= (t); \
+ (b)->Z_is_one ^= (t); \
+} while(0)
+
+ /*-
+ * The ladder step, with branches, is
+ *
+ * k[i] == 0: S = add(R, S), R = dbl(R)
+ * k[i] == 1: R = add(S, R), S = dbl(S)
+ *
+ * Swapping R, S conditionally on k[i] leaves you with state
+ *
+ * k[i] == 0: T, U = R, S
+ * k[i] == 1: T, U = S, R
+ *
+ * Then perform the ECC ops.
+ *
+ * U = add(T, U)
+ * T = dbl(T)
+ *
+ * Which leaves you with state
+ *
+ * k[i] == 0: U = add(R, S), T = dbl(R)
+ * k[i] == 1: U = add(S, R), T = dbl(S)
+ *
+ * Swapping T, U conditionally on k[i] leaves you with state
+ *
+ * k[i] == 0: R, S = T, U
+ * k[i] == 1: R, S = U, T
+ *
+ * Which leaves you with state
+ *
+ * k[i] == 0: S = add(R, S), R = dbl(R)
+ * k[i] == 1: R = add(S, R), S = dbl(S)
+ *
+ * So we get the same logic, but instead of a branch it's a
+ * conditional swap, followed by ECC ops, then another conditional swap.
+ *
+ * Optimization: The end of iteration i and start of i-1 looks like
+ *
+ * ...
+ * CSWAP(k[i], R, S)
+ * ECC
+ * CSWAP(k[i], R, S)
+ * (next iteration)
+ * CSWAP(k[i-1], R, S)
+ * ECC
+ * CSWAP(k[i-1], R, S)
+ * ...
+ *
+ * So instead of two contiguous swaps, you can merge the condition
+ * bits and do a single swap.
+ *
+ * k[i] k[i-1] Outcome
+ * 0 0 No Swap
+ * 0 1 Swap
+ * 1 0 Swap
+ * 1 1 No Swap
+ *
+ * This is XOR. pbit tracks the previous bit of k.
+ */
+
+ for (i = cardinality_bits - 1; i >= 0; i--) {
+ kbit = BN_is_bit_set(k, i) ^ pbit;
+ EC_POINT_CSWAP(kbit, r, s, group_top, Z_is_one);
+ if (!EC_POINT_add(group, s, r, s, ctx))
+ goto err;
+ if (!EC_POINT_dbl(group, r, r, ctx))
+ goto err;
+ /*
+ * pbit logic merges this cswap with that of the
+ * next iteration
+ */
+ pbit ^= kbit;
+ }
+ /* one final cswap to move the right value into r */
+ EC_POINT_CSWAP(pbit, r, s, group_top, Z_is_one);
+#undef EC_POINT_CSWAP
+
+ ret = 1;
+
+ err:
+ EC_POINT_free(s);
+ BN_CTX_end(ctx);
+ BN_CTX_free(new_ctx);
+
+ return ret;
+}
+
+#undef EC_POINT_BN_set_flags
+
/*
* TODO: table should be optimised for the wNAF-based implementation,
* sometimes smaller windows will give better performance (thus the
@@ -369,6 +587,34 @@ int ec_wNAF_mul(const EC_GROUP *group, EC_POINT *r, const
BIGNUM *scalar,
return EC_POINT_set_to_infinity(group, r);
}
+ if (!BN_is_zero(&group->order) && !BN_is_zero(&group->cofactor)) {
+ /*-
+ * Handle the common cases where the scalar is secret, enforcing a
constant
+ * time scalar multiplication algorithm.
+ */
+ if ((scalar != NULL) && (num == 0)) {
+ /*-
+ * In this case we want to compute scalar * GeneratorPoint: this
+ * codepath is reached most prominently by (ephemeral) key
generation
+ * of EC cryptosystems (i.e. ECDSA keygen and sign setup, ECDH
+ * keygen/first half), where the scalar is always secret. This is
why
+ * we ignore if BN_FLG_CONSTTIME is actually set and we always
call the
+ * constant time version.
+ */
+ return ec_mul_consttime(group, r, scalar, NULL, ctx);
+ }
+ if ((scalar == NULL) && (num == 1)) {
+ /*-
+ * In this case we want to compute scalar * GenericPoint: this
codepath
+ * is reached most prominently by the second half of ECDH, where
the
+ * secret scalar is multiplied by the peer's public point. To
protect
+ * the secret scalar, we ignore if BN_FLG_CONSTTIME is actually
set and
+ * we always call the constant time version.
+ */
+ return ec_mul_consttime(group, r, scalars[0], points[0], ctx);
+ }
+ }
+
for (i = 0; i < num; i++) {
if (group->meth != points[i]->meth) {
ECerr(EC_F_EC_WNAF_MUL, EC_R_INCOMPATIBLE_OBJECTS);
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