Only support >= 256-bit elliptic curves with ecdh_auto (server) or by default (client).

Also reorder preferences to prefer prime curves to binary curves, and P-256 to everything else.

The result:

$ openssl s_server -named_curves "auto"

This command will negotiate an ECDHE ciphersuite with P-256:

$ openssl s_client

This command will negotiate P-384:

$ openssl s_client -curves "P-384"

This command will not negotiate ECDHE because P-224 is disabled with "auto":

$ openssl s_client -curves "P-224"
Reviewed-by: NKurt Roeckx <kurt@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>

CHANGES

@@ -396,7 +396,14 @@
      whose return value is often ignored. 
      [Steve Henson]
 
- Changes between 1.0.2 and 1.0.2a [xx XXX xxxx]
+ Changes between 1.0.2a and 1.0.2b [xx XXX xxxx]
+
+  *) Only support 256-bit or stronger elliptic curves with the
+     'ecdh_auto' setting (server) or by default (client). Of supported
+     curves, prefer P-256 (both).
+     [Emilia Kasper]
+
+ Changes between 1.0.2 and 1.0.2a [19 Mar 2015]
 
   *) ClientHello sigalgs DoS fix
 

ssl/t1_lib.c

@@ -268,37 +268,65 @@ static const unsigned char ecformats_default[] = {
     TLSEXT_ECPOINTFORMAT_ansiX962_compressed_char2
 };
 
-static const unsigned char eccurves_default[] = {
-    0, 14,                      /* sect571r1 (14) */
-    0, 13,                      /* sect571k1 (13) */
+/* The client's default curves / the server's 'auto' curves. */
+static const unsigned char eccurves_auto[] = {
+    /* Prefer P-256 which has the fastest and most secure implementations. */
+    0, 23,                      /* secp256r1 (23) */
+    /* Other >= 256-bit prime curves. */
     0, 25,                      /* secp521r1 (25) */
     0, 28,                      /* brainpool512r1 (28) */
-    0, 11,                      /* sect409k1 (11) */
-    0, 12,                      /* sect409r1 (12) */
     0, 27,                      /* brainpoolP384r1 (27) */
     0, 24,                      /* secp384r1 (24) */
+    0, 26,                      /* brainpoolP256r1 (26) */
+    0, 22,                      /* secp256k1 (22) */
+    /* >= 256-bit binary curves. */
+    0, 14,                      /* sect571r1 (14) */
+    0, 13,                      /* sect571k1 (13) */
+    0, 11,                      /* sect409k1 (11) */
+    0, 12,                      /* sect409r1 (12) */
     0, 9,                       /* sect283k1 (9) */
     0, 10,                      /* sect283r1 (10) */
+};
+
+static const unsigned char eccurves_all[] = {
+    /* Prefer P-256 which has the fastest and most secure implementations. */
+    0, 23,                      /* secp256r1 (23) */
+    /* Other >= 256-bit prime curves. */
+    0, 25,                      /* secp521r1 (25) */
+    0, 28,                      /* brainpool512r1 (28) */
+    0, 27,                      /* brainpoolP384r1 (27) */
+    0, 24,                      /* secp384r1 (24) */
     0, 26,                      /* brainpoolP256r1 (26) */
     0, 22,                      /* secp256k1 (22) */
-    0, 23,                      /* secp256r1 (23) */
+    /* >= 256-bit binary curves. */
+    0, 14,                      /* sect571r1 (14) */
+    0, 13,                      /* sect571k1 (13) */
+    0, 11,                      /* sect409k1 (11) */
+    0, 12,                      /* sect409r1 (12) */
+    0, 9,                       /* sect283k1 (9) */
+    0, 10,                      /* sect283r1 (10) */
+    /*
+     * Remaining curves disabled by default but still permitted if set
+     * via an explicit callback or parameters.
+     */
+    0, 20,                      /* secp224k1 (20) */
+    0, 21,                      /* secp224r1 (21) */
+    0, 18,                      /* secp192k1 (18) */
+    0, 19,                      /* secp192r1 (19) */
+    0, 15,                      /* secp160k1 (15) */
+    0, 16,                      /* secp160r1 (16) */
+    0, 17,                      /* secp160r2 (17) */
     0, 8,                       /* sect239k1 (8) */
     0, 6,                       /* sect233k1 (6) */
     0, 7,                       /* sect233r1 (7) */
-    0, 20,                      /* secp224k1 (20) */
-    0, 21,                      /* secp224r1 (21) */
     0, 4,                       /* sect193r1 (4) */
     0, 5,                       /* sect193r2 (5) */
-    0, 18,                      /* secp192k1 (18) */
-    0, 19,                      /* secp192r1 (19) */
     0, 1,                       /* sect163k1 (1) */
     0, 2,                       /* sect163r1 (2) */
     0, 3,                       /* sect163r2 (3) */
-    0, 15,                      /* secp160k1 (15) */
-    0, 16,                      /* secp160r1 (16) */
-    0, 17,                      /* secp160r2 (17) */
 };
 
+
 static const unsigned char suiteb_curves[] = {
     0, TLSEXT_curve_P_256,
     0, TLSEXT_curve_P_384
@@ -419,8 +447,13 @@ static int tls1_get_curvelist(SSL *s, int sess,
             pcurveslen = s->tlsext_ellipticcurvelist_length;
         }
         if (!*pcurves) {
-            *pcurves = eccurves_default;
-            pcurveslen = sizeof(eccurves_default);
+            if (!s->server || (s->cert && s->cert->ecdh_tmp_auto)) {
+                *pcurves = eccurves_auto;
+                pcurveslen = sizeof(eccurves_auto);
+            } else {
+                *pcurves = eccurves_all;
+                pcurveslen = sizeof(eccurves_all);
+            }
         }
     }
 

test/ssltest.c

@@ -1494,12 +1494,9 @@ int main(int argc, char *argv[])
                 BIO_printf(bio_err, "unknown curve name (%s)\n", named_curve);
                 goto end;
             }
-        } else
-# ifdef OPENSSL_NO_EC2M
+        } else {
             nid = NID_X9_62_prime256v1;
-# else
-            nid = NID_sect163r2;
-# endif
+        }
 
         ecdh = EC_KEY_new_by_curve_name(nid);
         if (ecdh == NULL) {