Check discriminant directly in EC_GROUP_set_discriminant()

After possibly decoding a and b in EC_GROUP_get_curve(), this is a pure
calculation in GFp and as such doesn't make use of any method-specifics.
Let's perform this calculation directly in the public API implementation
rather than redirecting through the methods and remove yet another method
handler.

ok jsing

1 files changed, 45 insertions, 5 deletions

diff --git a/src/lib/libcrypto/ec/ec_lib.c b/src/lib/libcrypto/ec/ec_lib.c
index 7a82eb23f8..a50b1e5633 100644
--- a/src/lib/libcrypto/ec/ec_lib.c
+++ b/src/lib/libcrypto/ec/ec_lib.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ec_lib.c,v 1.102 2025/01/06 19:23:25 tb Exp $ */
+/* $OpenBSD: ec_lib.c,v 1.103 2025/01/07 08:30:52 tb Exp $ */
 /*
  * Originally written by Bodo Moeller for the OpenSSL project.
  */
@@ -600,6 +600,7 @@ int
 EC_GROUP_check_discriminant(const EC_GROUP *group, BN_CTX *ctx_in)
 {
         BN_CTX *ctx;
+        BIGNUM *p, *a, *b, *discriminant;
         int ret = 0;
 
         if ((ctx = ctx_in) == NULL)
@@ -607,11 +608,50 @@ EC_GROUP_check_discriminant(const EC_GROUP *group, BN_CTX *ctx_in)
         if (ctx == NULL)
                 goto err;
 
-        if (group->meth->group_check_discriminant == NULL) {
-                ECerror(ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
+        BN_CTX_start(ctx);
+
+        if ((p = BN_CTX_get(ctx)) == NULL)
                 goto err;
-        }
-        ret = group->meth->group_check_discriminant(group, ctx);
+        if ((a = BN_CTX_get(ctx)) == NULL)
+                goto err;
+        if ((b = BN_CTX_get(ctx)) == NULL)
+                goto err;
+        if ((discriminant = BN_CTX_get(ctx)) == NULL)
+                goto err;
+
+        if (!EC_GROUP_get_curve(group, p, a, b, ctx))
+                goto err;
+
+        /*
+         * Check that the discriminant 4a^3 + 27b^2 is non-zero modulo p.
+         */
+
+        if (BN_is_zero(a) && BN_is_zero(b))
+                goto err;
+        if (BN_is_zero(a) || BN_is_zero(b))
+                goto done;
+
+        /* Compute the discriminant: first 4a^3, then 27b^2, then their sum. */
+        if (!BN_mod_sqr(discriminant, a, p, ctx))
+                goto err;
+        if (!BN_mod_mul(discriminant, discriminant, a, p, ctx))
+                goto err;
+        if (!BN_lshift(discriminant, discriminant, 2))
+                goto err;
+
+        if (!BN_mod_sqr(b, b, p, ctx))
+                goto err;
+        if (!BN_mul_word(b, 27))
+                goto err;
+
+        if (!BN_mod_add(discriminant, discriminant, b, p, ctx))
+                goto err;
+
+        if (BN_is_zero(discriminant))
+                goto err;
+
+ done:
+        ret = 1;
 
  err:
         if (ctx != ctx_in)