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Validate DH peer public key subgroup before key agreement #460

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Validate DH peer public key subgroup before key agreement #460

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1d87f46
F-5914 - Validate DH peer public key subgroup before key agreement
aidangarske Aug 6, 2026
e22b86c
Harden DH subgroup regression coverage
aidangarske Aug 6, 2026
e7dcc23
Tighten DH subgroup rejection tests
aidangarske Aug 6, 2026
9f773cf
Preserve DH subgroup checks on older wolfSSL
aidangarske Aug 7, 2026
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66 changes: 65 additions & 1 deletion src/wp_dh_exch.c
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Original file line number Diff line number Diff line change
Expand Up @@ -254,6 +254,65 @@ static int wp_dh_kdf_derive(wp_DhCtx* ctx, unsigned char* key,
return ok;
}

#if (LIBWOLFSSL_VERSION_HEX < 0x05009002) || defined(WOLFSSL_KCAPI_DH)
/**
* Validate the peer's public key against our domain parameters.
*
* @param [in] key DH key object holding the domain parameters.
* @param [in] pub Peer's public key data.
* @param [in] pubSz Length of peer's public key data in bytes.
* @return 1 on success.
* @return 0 on failure.
*/
static int wp_dh_check_peer_pub(DhKey* key, const unsigned char* pub,
word32 pubSz)
{
int ok = 1;
int rc;
unsigned char* q = NULL;
word32 qSz;

WOLFPROV_ENTER(WP_LOG_COMP_DH, "wp_dh_check_peer_pub");

if (mp_iszero(&key->q)) {
rc = wc_DhCheckPubKey(key, pub, pubSz);
if (rc != 0) {
WOLFPROV_MSG_DEBUG_RETCODE(WP_LOG_LEVEL_DEBUG,
"wc_DhCheckPubKey", rc);
ok = 0;
}
}
else {
/* q must be passed explicitly for the y^q test to be performed. */
qSz = mp_unsigned_bin_size(&key->q);
q = OPENSSL_malloc(qSz);
if (q == NULL) {
ok = 0;
}
if (ok) {
rc = mp_to_unsigned_bin(&key->q, q);
if (rc != 0) {
WOLFPROV_MSG_DEBUG_RETCODE(WP_LOG_LEVEL_DEBUG,
"mp_to_unsigned_bin", rc);
ok = 0;
}
}
if (ok) {
rc = wc_DhCheckPubKey_ex(key, pub, pubSz, q, qSz);
if (rc != 0) {
WOLFPROV_MSG_DEBUG_RETCODE(WP_LOG_LEVEL_DEBUG,
"wc_DhCheckPubKey_ex", rc);
ok = 0;
}
}
OPENSSL_free(q);
}

WOLFPROV_LEAVE(WP_LOG_COMP_DH, __FILE__ ":" WOLFPROV_STRINGIZE(__LINE__), ok);
return ok;
}
#endif

/**
* Derive secret from DH keys.
*
Expand Down Expand Up @@ -288,6 +347,12 @@ static int wp_dh_derive_secret(wp_DhCtx* ctx, unsigned char* secret,
if (ok && (!wp_dh_get_pub(ctx->peer, &pub, &pubSz))) {
ok = 0;
}
#if (LIBWOLFSSL_VERSION_HEX < 0x05009002) || defined(WOLFSSL_KCAPI_DH)
/* Older wolfSSL and KCAPI paths do not validate the subgroup in agree. */
if (ok && (!wp_dh_check_peer_pub(wp_dh_get_key(ctx->key), pub, pubSz))) {
ok = 0;
}
#endif
if (ok) {
int rc;

Expand Down Expand Up @@ -708,4 +773,3 @@ const OSSL_DISPATCH wp_dh_keyexch_functions[] = {
};

#endif /* WP_HAVE_DH */

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