Fix AEAD-4 payload size check and nonce wrap-around

Author: weebl2000

src/Mesh.cpp

@@ -512,8 +512,9 @@ Packet* Mesh::createPathReturn(const uint8_t* dest_hash, const uint8_t* secret,
 
 Packet* Mesh::createDatagram(uint8_t type, const Identity& dest, const uint8_t* secret, const uint8_t* data, size_t data_len, uint16_t aead_nonce) {
   if (type == PAYLOAD_TYPE_TXT_MSG || type == PAYLOAD_TYPE_REQ || type == PAYLOAD_TYPE_RESPONSE) {
+    size_t hash_prefix = PATH_HASH_SIZE * 2;  // dest_hash + src_hash
     size_t max_overhead = aead_nonce ? (AEAD_NONCE_SIZE + AEAD_TAG_SIZE) : (CIPHER_MAC_SIZE + CIPHER_BLOCK_SIZE-1);
-    if (data_len + max_overhead > MAX_PACKET_PAYLOAD) return NULL;
+    if (data_len + hash_prefix + max_overhead > MAX_PACKET_PAYLOAD) return NULL;
   } else {
     return NULL;  // invalid type
   }

src/helpers/ContactInfo.h

@@ -20,7 +20,10 @@ struct ContactInfo {
   // Returns next AEAD nonce (post-increment) if peer supports AEAD, 0 otherwise.
   // When 0, callers use ECB encryption.
   uint16_t nextAeadNonce() const {
-    if (flags & CONTACT_FLAG_AEAD) return ++aead_nonce;
+    if (flags & CONTACT_FLAG_AEAD) {
+      if (++aead_nonce == 0) ++aead_nonce;  // skip 0 (sentinel for ECB)
+      return aead_nonce;
+    }
     return 0;
   }