From 8bbda20016e5c5fe4b795ed53292cc98a0c9232f Mon Sep 17 00:00:00 2001
From: tb <>
Date: Sat, 11 Jan 2025 15:02:42 +0000
Subject: Move compressed coordinate setting into public API

Now that it is method-agnostic, we can remove the method and move the
implementation to the body of the public API function. And another
method goes away. We're soon down to the ones we really need.

discussed with jsing
---
 src/lib/libcrypto/ec/ec_lib.c | 89 +++++++++++++++++++++++++++++++++++++++----
 1 file changed, 81 insertions(+), 8 deletions(-)

(limited to 'src/lib/libcrypto/ec/ec_lib.c')

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);
 
-- 
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