GH367: Fix dsa keygen for too-short seed

If the seed value for dsa key generation is too short (< qsize), return an error. Also update the documentation. Signed-off-by: Rich Salz <[email protected]> Reviewed-by: Emilia Käsper <[email protected]>
handcraftedcode1 · Aug 28, 2015 · f00a10b · f00a10b
1 parent 3c65047
commit f00a10b
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Showing 3 changed files with 21 additions and 23 deletions.
diff --git a/CHANGES b/CHANGES
@@ -4,6 +4,10 @@
 
  Changes between 1.0.2 and 1.1.0  [xx XXX xxxx]
 
+  *) In DSA_generate_parameters_ex, if the provided seed is too short,
+     return an error
+     [Rich Salz and Ismo Puustinen <[email protected]>]
+
   *) Rewrite PSK to support ECDHE_PSK, DHE_PSK and RSA_PSK. Add ciphersuites
      from RFC4279, RFC4785, RFC5487, RFC5489.
 

diff --git a/crypto/dsa/dsa_gen.c b/crypto/dsa/dsa_gen.c
@@ -132,18 +132,15 @@ int dsa_builtin_paramgen(DSA *ret, size_t bits, size_t qbits,
 
     bits = (bits + 63) / 64 * 64;
 
-    /*
-     * NB: seed_len == 0 is special case: copy generated seed to seed_in if
-     * it is not NULL.
-     */
-    if (seed_len && (seed_len < (size_t)qsize))
-        seed_in = NULL;         /* seed buffer too small -- ignore */
-    if (seed_len > (size_t)qsize)
-        seed_len = qsize;       /* App. 2.2 of FIPS PUB 186 allows larger
-                                 * SEED, but our internal buffers are
-                                 * restricted to 160 bits */
-    if (seed_in != NULL)
+    if (seed_in != NULL) {
+        if (seed_len < (size_t)qsize)
+            return 0;
+        if (seed_len > (size_t)qsize) {
+            /* Don't overflow seed local variable. */
+            seed_len = qsize;
+        }
         memcpy(seed, seed_in, seed_len);
+    }
 
     if ((ctx = BN_CTX_new()) == NULL)
         goto err;
@@ -166,20 +163,18 @@ int dsa_builtin_paramgen(DSA *ret, size_t bits, size_t qbits,
 
     for (;;) {
         for (;;) {              /* find q */
-            int seed_is_random;
+            int seed_is_random = seed_in == NULL;
 
             /* step 1 */
             if (!BN_GENCB_call(cb, 0, m++))
                 goto err;
 
-            if (!seed_len) {
+            if (seed_is_random) {
                 if (RAND_bytes(seed, qsize) <= 0)
                     goto err;
-                seed_is_random = 1;
             } else {
-                seed_is_random = 0;
-                seed_len = 0;   /* use random seed if 'seed_in' turns out to
-                                 * be bad */
+                /* If we come back through, use random seed next time. */
+                seed_in = NULL;
             }
             memcpy(buf, seed, qsize);
             memcpy(buf2, seed, qsize);

diff --git a/doc/crypto/DSA_generate_parameters.pod b/doc/crypto/DSA_generate_parameters.pod
@@ -23,13 +23,12 @@ Deprecated:
 DSA_generate_parameters_ex() generates primes p and q and a generator g
 for use in the DSA and stores the result in B<dsa>.
 
-B<bits> is the length of the prime to be generated; the DSS allows a
-maximum of 1024 bits.
+B<bits> is the length of the prime p to be generated.
+For lengths under 2048 bits, the length of q is 160 bits; for lengths
+at least 2048, it is set to 256 bits.
 
-If B<seed> is B<NULL> or B<seed_len> E<lt> 20, the primes will be
-generated at random. Otherwise, the seed is used to generate
-them. If the given seed does not yield a prime q, a new random
-seed is chosen and placed at B<seed>.
+If B<seed> is NULL, the primes will be generated at random.
+If B<seed_len> is less than the length of q, an error is returned.
 
 DSA_generate_parameters_ex() places the iteration count in
 *B<counter_ret> and a counter used for finding a generator in