Avoid a few unnecessary contortions

Turns out that after ~40 years of practice I still can't do addition
with carry correctly :S

diff --git a/regress/lib/libcrypto/curve25519/ed25519test.c b/regress/lib/libcrypto/curve25519/ed25519test.c
index 4239997..da14070 100644 (file)
--- a/regress/lib/libcrypto/curve25519/ed25519test.c
+++ b/regress/lib/libcrypto/curve25519/ed25519test.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: ed25519test.c,v 1.5 2022/11/17 21:19:43 tb Exp $ */
+/*     $OpenBSD: ed25519test.c,v 1.6 2022/11/17 22:45:48 tb Exp $ */
 /*
  * Copyright (c) 2019, 2022 Theo Buehler <tb@openbsd.org>
  *
@@ -387,22 +387,6 @@ dump_info(const uint8_t *message, size_t message_len, const uint8_t *public_key,
        }
 }
 
-static void
-dump_once(const char *description, const uint8_t *message, size_t message_len,
-    const uint8_t *public_key, const uint8_t *private_key,
-    const uint8_t *signature)
-{
-       static int dumped = 0;
-
-       if (dumped)
-               return;
-
-       fprintf(stderr, "%s\n", description);
-       dump_info(message, message_len, public_key, private_key, signature);
-
-       dumped = 1;
-}
-
 /*
  * Little-endian representation of the order of edwards25519,
  * see https://www.rfc-editor.org/rfc/rfc7748#section-4.1
@@ -421,18 +405,22 @@ static const uint8_t order[] = {
 static void
 modify_signature(uint8_t *signature)
 {
+       uint16_t sum;
        uint8_t *upper_half = &signature[32];
+       uint16_t carry = 0;
        size_t i;
 
        for (i = 0; i < sizeof(order); i++) {
-               if (i < sizeof(order) - 1 && 0xff - order[i] < upper_half[i])
-                       upper_half[i + 1] += 1;
-               upper_half[i] += order[i];
+               sum = carry + order[i] + upper_half[i];
+               carry = (sum > 0xff);
+               upper_half[i] = sum & 0xff;
        }
+
+       /* carry == 0 since 0 <= upper_half < order and 2 * order < 2^256. */
 }
 
 static int
-test_signature_malleability(void)
+test_ED25519_signature_malleability(void)
 {
        uint8_t public_key[ED25519_PUBLIC_KEY_LENGTH];
        uint8_t private_key[ED25519_PRIVATE_KEY_LENGTH];
@@ -461,8 +449,9 @@ test_signature_malleability(void)
        modify_signature(signature);
 
        if (ED25519_verify(message, sizeof(message), signature, public_key)) {
-               dump_once("Verified with modified signature", message,
-                   sizeof(message), public_key, private_key, signature);
+               fprintf(stderr, "Verified with modified signature\n");
+               dump_info(message, sizeof(message), public_key, private_key,
+                   signature);
                goto err;
        }
 
@@ -472,18 +461,6 @@ test_signature_malleability(void)
        return failed;
 }
 
-static int
-test_ED25519_signature_malleability(void)
-{
-       int i;
-       int failed = 0;
-
-       for (i = 0; i < 128; i++)
-               failed |= test_signature_malleability();
-
-       return failed;
-}
-
 int
 main(int argc, char *argv[])
 {