Ensure SRP BN_mod_exp follows the constant time path

SRP_Calc_client_key calls BN_mod_exp with private data. However it was
not setting BN_FLG_CONSTTIME and therefore not using the constant time
implementation. This could be exploited in a side channel attack to
recover the password.

Since the attack is local host only this is outside of the current OpenSSL
threat model and therefore no CVE is assigned.

Thanks to Mohammed Sabt and Daniel De Almeida Braga for reporting this
issue.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13888)

diff --git a/CHANGES.md b/CHANGES.md
index 8ae1c7470a304202a078f9b277b23b427b726d0a..a298a0590cbf36374ab5a8adea92a497d5b1b701 100644 (file)
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1395,7 +1395,20 @@ OpenSSL 3.0
 OpenSSL 1.1.1
 -------------
 
-### Changes between 1.1.1h and 1.1.1i [xx XXX xxxx]
+### Changes between 1.1.1i and 1.1.1j [xx XXX xxxx]
+
+ * Fixed SRP_Calc_client_key so that it uses constant time. The previous
+   implementation called BN_mod_exp without setting BN_FLG_CONSTTIME. This
+   could be exploited in a side channel attack to recover the password. Since
+   the attack is local host only this is outside of the current OpenSSL
+   threat model and therefore no CVE is assigned.
+
+   Thanks to Mohammed Sabt and Daniel De Almeida Braga for reporting this
+   issue.
+
+   *Matt Caswell*
+
+### Changes between 1.1.1h and 1.1.1i [8 Dec 2020]
 
  * Fixed NULL pointer deref in the GENERAL_NAME_cmp function
    This function could crash if both GENERAL_NAMEs contain an EDIPARTYNAME.

diff --git a/crypto/srp/srp_lib.c b/crypto/srp/srp_lib.c
index 092cc159aabcda1f20d5a69ba0abbe593a8b061e..39113d53ec6e7eb672193126bd03cef68e66abbe 100644 (file)
--- a/crypto/srp/srp_lib.c
+++ b/crypto/srp/srp_lib.c
@@ -211,6 +211,7 @@ BIGNUM *SRP_Calc_client_key_ex(const BIGNUM *N, const BIGNUM *B, const BIGNUM *g
                             OSSL_LIB_CTX *libctx, const char *propq)
 {
     BIGNUM *tmp = NULL, *tmp2 = NULL, *tmp3 = NULL, *k = NULL, *K = NULL;
+    BIGNUM *xtmp = NULL;
     BN_CTX *bn_ctx;
 
     if (u == NULL || B == NULL || N == NULL || g == NULL || x == NULL
@@ -219,10 +220,13 @@ BIGNUM *SRP_Calc_client_key_ex(const BIGNUM *N, const BIGNUM *B, const BIGNUM *g
 
     if ((tmp = BN_new()) == NULL ||
         (tmp2 = BN_new()) == NULL ||
-        (tmp3 = BN_new()) == NULL)
+        (tmp3 = BN_new()) == NULL ||
+        (xtmp = BN_new()) == NULL)
         goto err;
 
-    if (!BN_mod_exp(tmp, g, x, N, bn_ctx))
+    BN_with_flags(xtmp, x, BN_FLG_CONSTTIME);
+    BN_set_flags(tmp, BN_FLG_CONSTTIME);
+    if (!BN_mod_exp(tmp, g, xtmp, N, bn_ctx))
         goto err;
     if ((k = srp_Calc_k(N, g, libctx, propq)) == NULL)
         goto err;
@@ -230,7 +234,7 @@ BIGNUM *SRP_Calc_client_key_ex(const BIGNUM *N, const BIGNUM *B, const BIGNUM *g
         goto err;
     if (!BN_mod_sub(tmp, B, tmp2, N, bn_ctx))
         goto err;
-    if (!BN_mul(tmp3, u, x, bn_ctx))
+    if (!BN_mul(tmp3, u, xtmp, bn_ctx))
         goto err;
     if (!BN_add(tmp2, a, tmp3))
         goto err;
@@ -242,6 +246,7 @@ BIGNUM *SRP_Calc_client_key_ex(const BIGNUM *N, const BIGNUM *B, const BIGNUM *g
 
  err:
     BN_CTX_free(bn_ctx);
+    BN_free(xtmp);
     BN_clear_free(tmp);
     BN_clear_free(tmp2);
     BN_clear_free(tmp3);