1 files changed, 81 insertions, 8 deletions

diff --git a/src/lib/libcrypto/ec/ec_lib.c b/src/lib/libcrypto/ec/ec_lib.c
index ed51582146..a8b74ce89d 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 1.109 2025/01/11 14:38:57 tb Exp $ */
+/* $OpenBSD: ec_lib.c,v 1.110 2025/01/11 15:02:42 tb Exp $ */
 /*
  * Originally written by Bodo Moeller for the OpenSSL project.
  */
@@ -1026,8 +1026,9 @@ LCRYPTO_ALIAS(EC_POINT_get_affine_coordinates_GFp);
 
 int
 EC_POINT_set_compressed_coordinates(const EC_GROUP *group, EC_POINT *point,
-    const BIGNUM *x, int y_bit, BN_CTX *ctx_in)
+    const BIGNUM *in_x, int y_bit, BN_CTX *ctx_in)
 {
+        BIGNUM *p, *a, *b, *w, *x, *y;
         BN_CTX *ctx;
         int ret = 0;
 
@@ -1036,18 +1037,90 @@ EC_POINT_set_compressed_coordinates(const EC_GROUP *group, EC_POINT *point,
         if (ctx == NULL)
                 goto err;
 
-        if (group->meth->point_set_compressed_coordinates == NULL) {
-                ECerror(ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
+        y_bit = (y_bit != 0);
+
+        BN_CTX_start(ctx);
+
+        if ((p = BN_CTX_get(ctx)) == NULL)
+                goto err;
+        if ((a = BN_CTX_get(ctx)) == NULL)
+                goto err;
+        if ((b = BN_CTX_get(ctx)) == NULL)
+                goto err;
+        if ((w = BN_CTX_get(ctx)) == NULL)
+                goto err;
+        if ((x = BN_CTX_get(ctx)) == NULL)
+                goto err;
+        if ((y = BN_CTX_get(ctx)) == NULL)
+                goto err;
+
+        /*
+         * Weierstrass equation: y^2 = x^3 + ax + b, so y is one of the
+         * square roots of x^3 + ax + b. The y-bit indicates which one.
+         */
+
+        if (!EC_GROUP_get_curve(group, p, a, b, ctx))
+                goto err;
+
+        /* XXX - should we not insist on 0 <= x < p instead? */
+        if (!BN_nnmod(x, in_x, p, ctx))
+                goto err;
+
+        /* y = x^3 */
+        if (!BN_mod_sqr(y, x, p, ctx))
+                goto err;
+        if (!BN_mod_mul(y, y, x, p, ctx))
+                goto err;
+
+        /* y += ax */
+        if (group->a_is_minus3) {
+                if (!BN_mod_lshift1_quick(w, x, p))
+                        goto err;
+                if (!BN_mod_add_quick(w, w, x, p))
+                        goto err;
+                if (!BN_mod_sub_quick(y, y, w, p))
+                        goto err;
+        } else {
+                if (!BN_mod_mul(w, a, x, p, ctx))
+                        goto err;
+                if (!BN_mod_add_quick(y, y, w, p))
+                        goto err;
+        }
+
+        /* y += b */
+        if (!BN_mod_add_quick(y, y, b, p))
+                goto err;
+
+        if (!BN_mod_sqrt(y, y, p, ctx)) {
+                ECerror(EC_R_INVALID_COMPRESSED_POINT);
                 goto err;
         }
-        if (group->meth != point->meth) {
-                ECerror(EC_R_INCOMPATIBLE_OBJECTS);
+
+        if (y_bit == BN_is_odd(y))
+                goto done;
+
+        if (BN_is_zero(y)) {
+                ECerror(EC_R_INVALID_COMPRESSION_BIT);
                 goto err;
         }
-        ret = group->meth->point_set_compressed_coordinates(group, point,
-            x, y_bit, ctx);
+        if (!BN_usub(y, p, y))
+                goto err;
+
+        if (y_bit != BN_is_odd(y)) {
+                /* Can only happen if p is even and should not be reachable. */
+                ECerror(ERR_R_INTERNAL_ERROR);
+                goto err;
+        }
+
+ done:
+        if (!EC_POINT_set_affine_coordinates(group, point, x, y, ctx))
+                goto err;
+
+        ret = 1;
 
  err:
+        BN_CTX_end(ctx);
+
         if (ctx != ctx_in)
                 BN_CTX_free(ctx);