check for zero K value from curve25519

common-kex.c

@@ -760,6 +760,7 @@ void kexcurve25519_comb_key(struct kex_curve25519_param *param, buffer *buf_pub_
 	unsigned char out[CURVE25519_LEN];
 	const unsigned char* Q_C = NULL;
 	const unsigned char* Q_S = NULL;
+	char zeroes[CURVE25519_LEN] = {0};
 
 	if (buf_pub_them->len != CURVE25519_LEN)
 	{
@@ -767,6 +768,11 @@ void kexcurve25519_comb_key(struct kex_curve25519_param *param, buffer *buf_pub_
 	}
 
 	curve25519_donna(out, param->priv, buf_pub_them->data);
+
+	if (constant_time_memcmp(zeroes, out, CURVE25519_LEN) == 0) {
+		dropbear_exit("Bad curve25519");
+	}
+
 	m_mp_alloc_init_multi(&ses.dh_K, NULL);
 	bytes_to_mp(ses.dh_K, out, CURVE25519_LEN);
 	m_burn(out, sizeof(out));