Address a timing side channel whereby it is possible to determine some

[openssl.git] / crypto / dsa / dsa_ossl.c

diff --git a/crypto/dsa/dsa_ossl.c b/crypto/dsa/dsa_ossl.c
index 9c97ef949c0c5f02060b03bcd5d72b5313219cf6..d78c5f00cbabf325a3428823a1ae52bb774862c6 100644 (file)
--- a/crypto/dsa/dsa_ossl.c
+++ b/crypto/dsa/dsa_ossl.c
@@ -7,14 +7,11 @@
  * https://www.openssl.org/source/license.html
  */
 
-/* Original version from Steven Schoch <schoch@sheba.arc.nasa.gov> */
-
 #include <stdio.h>
 #include "internal/cryptlib.h"
 #include <openssl/bn.h>
 #include <openssl/sha.h>
 #include "dsa_locl.h"
-#include <openssl/rand.h>
 #include <openssl/asn1.h>
 
 static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa);
@@ -42,6 +39,18 @@ static DSA_METHOD openssl_dsa_meth = {
     NULL
 };
 
+static const DSA_METHOD *default_DSA_method = &openssl_dsa_meth;
+
+void DSA_set_default_method(const DSA_METHOD *meth)
+{
+    default_DSA_method = meth;
+}
+
+const DSA_METHOD *DSA_get_default_method(void)
+{
+    return default_DSA_method;
+}
+
 const DSA_METHOD *DSA_OpenSSL(void)
 {
     return &openssl_dsa_meth;
@@ -52,7 +61,6 @@ static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)
     BIGNUM *kinv = NULL;
     BIGNUM *m;
     BIGNUM *xr;
-    BIGNUM *r, *s;
     BN_CTX *ctx = NULL;
     int reason = ERR_R_BN_LIB;
     DSA_SIG *ret = NULL;
@@ -71,14 +79,16 @@ static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)
     ret = DSA_SIG_new();
     if (ret == NULL)
         goto err;
-
-    DSA_SIG_get0(&r, &s, ret);
+    ret->r = BN_new();
+    ret->s = BN_new();
+    if (ret->r == NULL || ret->s == NULL)
+        goto err;
 
     ctx = BN_CTX_new();
     if (ctx == NULL)
         goto err;
  redo:
-    if (!dsa_sign_setup(dsa, ctx, &kinv, &r, dgst, dlen))
+    if (!dsa_sign_setup(dsa, ctx, &kinv, &ret->r, dgst, dlen))
         goto err;
 
     if (dlen > BN_num_bytes(dsa->q))
@@ -92,21 +102,21 @@ static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)
         goto err;
 
     /* Compute  s = inv(k) (m + xr) mod q */
-    if (!BN_mod_mul(xr, dsa->priv_key, r, dsa->q, ctx))
+    if (!BN_mod_mul(xr, dsa->priv_key, ret->r, dsa->q, ctx))
         goto err;               /* s = xr */
-    if (!BN_add(s, xr, m))
+    if (!BN_add(ret->s, xr, m))
         goto err;               /* s = m + xr */
-    if (BN_cmp(s, dsa->q) > 0)
-        if (!BN_sub(s, s, dsa->q))
+    if (BN_cmp(ret->s, dsa->q) > 0)
+        if (!BN_sub(ret->s, ret->s, dsa->q))
             goto err;
-    if (!BN_mod_mul(s, s, kinv, dsa->q, ctx))
+    if (!BN_mod_mul(ret->s, ret->s, kinv, dsa->q, ctx))
         goto err;
 
     /*
      * Redo if r or s is zero as required by FIPS 186-3: this is very
      * unlikely.
      */
-    if (BN_is_zero(r) || BN_is_zero(s))
+    if (BN_is_zero(ret->r) || BN_is_zero(ret->s))
         goto redo;
 
     rv = 1;
@@ -136,7 +146,9 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
 {
     BN_CTX *ctx = NULL;
     BIGNUM *k, *kinv = NULL, *r = *rp;
+    BIGNUM *l, *m;
     int ret = 0;
+    int q_bits;
 
     if (!dsa->p || !dsa->q || !dsa->g) {
         DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PARAMETERS);
@@ -144,7 +156,9 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
     }
 
     k = BN_new();
-    if (k == NULL)
+    l = BN_new();
+    m = BN_new();
+    if (k == NULL || l == NULL || m == NULL)
         goto err;
 
     if (ctx_in == NULL) {
@@ -153,6 +167,13 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
     } else
         ctx = ctx_in;
 
+    /* Preallocate space */
+    q_bits = BN_num_bits(dsa->q);
+    if (!BN_set_bit(k, q_bits)
+        || !BN_set_bit(l, q_bits)
+        || !BN_set_bit(m, q_bits))
+        goto err;
+
     /* Get random k */
     do {
         if (dgst != NULL) {
@@ -163,7 +184,7 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
             if (!BN_generate_dsa_nonce(k, dsa->q, dsa->priv_key, dgst,
                                        dlen, ctx))
                 goto err;
-        } else if (!BN_rand_range(k, dsa->q))
+        } else if (!BN_priv_rand_range(k, dsa->q))
             goto err;
     } while (BN_is_zero(k));
 
@@ -179,17 +200,19 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
 
     /*
      * We do not want timing information to leak the length of k, so we
-     * compute g^k using an equivalent exponent of fixed length. (This
-     * is a kludge that we need because the BN_mod_exp_mont() does not
-     * let us specify the desired timing behaviour.)
+     * compute G^k using an equivalent scalar of fixed bit-length.
+     *
+     * We unconditionally perform both of these additions to prevent a
+     * small timing information leakage.  We then choose the sum that is
+     * one bit longer than the modulus.
+     *
+     * TODO: revisit the BN_copy aiming for a memory access agnostic
+     * conditional copy.
      */
-
-    if (!BN_add(k, k, dsa->q))
+    if (!BN_add(l, k, dsa->q)
+        || !BN_add(m, l, dsa->q)
+        || !BN_copy(k, BN_num_bits(l) > q_bits ? l : m))
         goto err;
-    if (BN_num_bits(k) <= BN_num_bits(dsa->q)) {
-        if (!BN_add(k, k, dsa->q))
-            goto err;
-    }
 
     if ((dsa)->meth->bn_mod_exp != NULL) {
             if (!dsa->meth->bn_mod_exp(dsa, r, dsa->g, k, dsa->p, ctx,
@@ -217,6 +240,8 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
     if (ctx != ctx_in)
         BN_CTX_free(ctx);
     BN_clear_free(k);
+    BN_clear_free(l);
+    BN_clear_free(m);
     return ret;
 }
 
@@ -226,7 +251,7 @@ static int dsa_do_verify(const unsigned char *dgst, int dgst_len,
     BN_CTX *ctx;
     BIGNUM *u1, *u2, *t1;
     BN_MONT_CTX *mont = NULL;
-    BIGNUM *r, *s;
+    const BIGNUM *r, *s;
     int ret = -1, i;
     if (!dsa->p || !dsa->q || !dsa->g) {
         DSAerr(DSA_F_DSA_DO_VERIFY, DSA_R_MISSING_PARAMETERS);
@@ -251,7 +276,7 @@ static int dsa_do_verify(const unsigned char *dgst, int dgst_len,
     if (u1 == NULL || u2 == NULL || t1 == NULL || ctx == NULL)
         goto err;
 
-    DSA_SIG_get0(&r, &s, sig);
+    DSA_SIG_get0(sig, &r, &s);
 
     if (BN_is_zero(r) || BN_is_negative(r) ||
         BN_ucmp(r, dsa->q) >= 0) {
@@ -322,17 +347,17 @@ static int dsa_do_verify(const unsigned char *dgst, int dgst_len,
     BN_free(u1);
     BN_free(u2);
     BN_free(t1);
-    return (ret);
+    return ret;
 }
 
 static int dsa_init(DSA *dsa)
 {
     dsa->flags |= DSA_FLAG_CACHE_MONT_P;
-    return (1);
+    return 1;
 }
 
 static int dsa_finish(DSA *dsa)
 {
     BN_MONT_CTX_free(dsa->method_mont_p);
-    return (1);
+    return 1;
 }