Use BN_bn2binpad() instead of handrolling it

As ugly as the BN_bn2binpad() internals are, what it does is quite handy
with all sorts of EC stuff. So use it here too and eliminate some ugly
manual pointer zeroing and offsets. Also switch len and buflen from size_t
to int to remove an iffy cast: both are set by functions that return a
non-negative int.

ok jsing

diff --git a/lib/libcrypto/ecdh/ech_key.c b/lib/libcrypto/ecdh/ech_key.c
index 1dfb3c0..b364b31 100644 (file)
--- a/lib/libcrypto/ecdh/ech_key.c
+++ b/lib/libcrypto/ecdh/ech_key.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ech_key.c,v 1.20 2023/07/01 14:39:34 tb Exp $ */
+/* $OpenBSD: ech_key.c,v 1.21 2023/07/01 14:48:01 tb Exp $ */
 /* ====================================================================
  * Copyright 2002 Sun Microsystems, Inc. ALL RIGHTS RESERVED.
  *
@@ -98,8 +98,8 @@ ossl_ecdh_compute_key(void *out, size_t outlen, const EC_POINT *pub_key,
        const BIGNUM *priv_key;
        const EC_GROUP* group;
        int ret = -1;
-       size_t buflen, len;
        unsigned char *buf = NULL;
+       int buflen, len;
 
        if (outlen > INT_MAX) {
                /* Sort of, anyway. */
@@ -156,9 +156,7 @@ ossl_ecdh_compute_key(void *out, size_t outlen, const EC_POINT *pub_key,
                ECDHerror(ERR_R_MALLOC_FAILURE);
                goto err;
        }
-
-       memset(buf, 0, buflen - len);
-       if (len != (size_t)BN_bn2bin(x, buf + buflen - len)) {
+       if (BN_bn2binpad(x, buf, buflen) != buflen) {
                ECDHerror(ERR_R_BN_LIB);
                goto err;
        }