Introduce ec_decode_scalar()

This is a helper that decodes a scalar from field-internal representation
to a representation as a BIGNUM in the interval [0, p). This simplifies
EC_GROUP_get_curve() and EC_POINT_get_Jprojective_coordinates() to a few
obvious lines and prepares cleanup in EC_POINT_get_affine_coordinates().

ok jsing

1 files changed, 39 insertions, 81 deletions

diff --git a/src/lib/libcrypto/ec/ecp_smpl.c b/src/lib/libcrypto/ec/ecp_smpl.c
index d270d495b8..21d0d32ef9 100644
--- a/src/lib/libcrypto/ec/ecp_smpl.c
+++ b/src/lib/libcrypto/ec/ecp_smpl.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ecp_smpl.c,v 1.49 2023/07/26 11:58:34 tb Exp $ */
+/* $OpenBSD: ecp_smpl.c,v 1.50 2023/07/26 12:12:13 tb Exp $ */
 /* Includes code written by Lenka Fibikova <fibikova@exp-math.uni-essen.de>
  * for the OpenSSL project.
  * Includes code written by Bodo Moeller for the OpenSSL project.
@@ -114,6 +114,18 @@ ec_GFp_simple_group_copy(EC_GROUP *dest, const EC_GROUP *src)
         return 1;
 }
 
+static int
+ec_decode_scalar(const EC_GROUP *group, BIGNUM *bn, const BIGNUM *x, BN_CTX *ctx)
+{
+        if (bn == NULL)
+                return 1;
+
+        if (group->meth->field_decode != NULL)
+                return group->meth->field_decode(group, bn, x, ctx);
+
+        return bn_copy(bn, x);
+}
+
 int
 ec_GFp_simple_group_set_curve(EC_GROUP *group,
     const BIGNUM *p, const BIGNUM *a, const BIGNUM *b, BN_CTX *ctx)
@@ -167,31 +179,17 @@ ec_GFp_simple_group_set_curve(EC_GROUP *group,
 }
 
 int
-ec_GFp_simple_group_get_curve(const EC_GROUP *group, BIGNUM *p, BIGNUM *a, BIGNUM *b, BN_CTX *ctx)
+ec_GFp_simple_group_get_curve(const EC_GROUP *group, BIGNUM *p, BIGNUM *a,
+    BIGNUM *b, BN_CTX *ctx)
 {
         if (p != NULL) {
                 if (!bn_copy(p, &group->field))
                         return 0;
         }
-        if (group->meth->field_decode != NULL) {
-                if (a != NULL) {
-                        if (!group->meth->field_decode(group, a, &group->a, ctx))
-                                return 0;
-                }
-                if (b != NULL) {
-                        if (!group->meth->field_decode(group, b, &group->b, ctx))
-                                return 0;
-                }
-        } else {
-                if (a != NULL) {
-                        if (!bn_copy(a, &group->a))
-                                return 0;
-                }
-                if (b != NULL) {
-                        if (!bn_copy(b, &group->b))
-                                return 0;
-                }
-        }
+        if (!ec_decode_scalar(group, a, &group->a, ctx))
+                return 0;
+        if (!ec_decode_scalar(group, b, &group->b, ctx))
+                return 0;
 
         return 1;
 }
@@ -363,33 +361,12 @@ ec_GFp_simple_get_Jprojective_coordinates(const EC_GROUP *group,
 {
         int ret = 0;
 
-        if (group->meth->field_decode != NULL) {
-                if (x != NULL) {
-                        if (!group->meth->field_decode(group, x, &point->X, ctx))
-                                goto err;
-                }
-                if (y != NULL) {
-                        if (!group->meth->field_decode(group, y, &point->Y, ctx))
-                                goto err;
-                }
-                if (z != NULL) {
-                        if (!group->meth->field_decode(group, z, &point->Z, ctx))
-                                goto err;
-                }
-        } else {
-                if (x != NULL) {
-                        if (!bn_copy(x, &point->X))
-                                goto err;
-                }
-                if (y != NULL) {
-                        if (!bn_copy(y, &point->Y))
-                                goto err;
-                }
-                if (z != NULL) {
-                        if (!bn_copy(z, &point->Z))
-                                goto err;
-                }
-        }
+        if (!ec_decode_scalar(group, x, &point->X, ctx))
+                goto err;
+        if (!ec_decode_scalar(group, y, &point->Y, ctx))
+                goto err;
+        if (!ec_decode_scalar(group, z, &point->Z, ctx))
+                goto err;
 
         ret = 1;
 
@@ -411,11 +388,10 @@ ec_GFp_simple_point_set_affine_coordinates(const EC_GROUP *group, EC_POINT *poin
 }
 
 int
-ec_GFp_simple_point_get_affine_coordinates(const EC_GROUP *group, const EC_POINT *point,
-    BIGNUM *x, BIGNUM *y, BN_CTX *ctx)
+ec_GFp_simple_point_get_affine_coordinates(const EC_GROUP *group,
+    const EC_POINT *point, BIGNUM *x, BIGNUM *y, BN_CTX *ctx)
 {
-        BIGNUM *Z, *Z_1, *Z_2, *Z_3;
-        const BIGNUM *Z_;
+        BIGNUM *z, *Z, *Z_1, *Z_2, *Z_3;
         int ret = 0;
 
         if (EC_POINT_is_at_infinity(group, point) > 0) {
@@ -425,6 +401,8 @@ ec_GFp_simple_point_get_affine_coordinates(const EC_GROUP *group, const EC_POINT
 
         BN_CTX_start(ctx);
 
+        if ((z = BN_CTX_get(ctx)) == NULL)
+                goto err;
         if ((Z = BN_CTX_get(ctx)) == NULL)
                 goto err;
         if ((Z_1 = BN_CTX_get(ctx)) == NULL)
@@ -434,38 +412,18 @@ ec_GFp_simple_point_get_affine_coordinates(const EC_GROUP *group, const EC_POINT
         if ((Z_3 = BN_CTX_get(ctx)) == NULL)
                 goto err;
 
-        /* transform  (X, Y, Z)  into  (x, y) := (X/Z^2, Y/Z^3) */
+        /* Convert from projective coordinates (X, Y, Z) into (X/Z^2, Y/Z^3). */
 
-        if (group->meth->field_decode) {
-                if (!group->meth->field_decode(group, Z, &point->Z, ctx))
-                        goto err;
-                Z_ = Z;
-        } else {
-                Z_ = &point->Z;
-        }
+        if (!ec_decode_scalar(group, z, &point->Z, ctx))
+                goto err;
 
-        if (BN_is_one(Z_)) {
-                if (group->meth->field_decode) {
-                        if (x != NULL) {
-                                if (!group->meth->field_decode(group, x, &point->X, ctx))
-                                        goto err;
-                        }
-                        if (y != NULL) {
-                                if (!group->meth->field_decode(group, y, &point->Y, ctx))
-                                        goto err;
-                        }
-                } else {
-                        if (x != NULL) {
-                                if (!bn_copy(x, &point->X))
-                                        goto err;
-                        }
-                        if (y != NULL) {
-                                if (!bn_copy(y, &point->Y))
-                                        goto err;
-                        }
-                }
+        if (BN_is_one(z)) {
+                if (!ec_decode_scalar(group, x, &point->X, ctx))
+                        goto err;
+                if (!ec_decode_scalar(group, y, &point->Y, ctx))
+                        goto err;
         } else {
-                if (BN_mod_inverse_ct(Z_1, Z_, &group->field, ctx) == NULL) {
+                if (BN_mod_inverse_ct(Z_1, z, &group->field, ctx) == NULL) {
                         ECerror(ERR_R_BN_LIB);
                         goto err;
                 }