Move block-alignment check before HMAC computation

Address review feedback:
- Move the alignment check before the HMAC-SHA256 computation to skip
  expensive crypto on obviously malformed packets
- Update MACThenDecrypt header comment to document block-alignment as a
  reason for returning zero

src/Utils.cpp

@@ -74,16 +74,17 @@ int Utils::encryptThenMAC(const uint8_t* shared_secret, uint8_t* dest, const uin
 int Utils::MACThenDecrypt(const uint8_t* shared_secret, uint8_t* dest, const uint8_t* src, int src_len) {
   if (src_len <= CIPHER_MAC_SIZE) return 0;  // invalid src bytes
 
+  int enc_len = src_len - CIPHER_MAC_SIZE;
+  if (enc_len % CIPHER_BLOCK_SIZE != 0) return 0;  // reject non-block-aligned ciphertext
+
   uint8_t hmac[CIPHER_MAC_SIZE];
   {
     SHA256 sha;
     sha.resetHMAC(shared_secret, PUB_KEY_SIZE);
-    sha.update(src + CIPHER_MAC_SIZE, src_len - CIPHER_MAC_SIZE);
+    sha.update(src + CIPHER_MAC_SIZE, enc_len);
     sha.finalizeHMAC(shared_secret, PUB_KEY_SIZE, hmac, CIPHER_MAC_SIZE);
   }
   if (memcmp(hmac, src, CIPHER_MAC_SIZE) == 0) {
-    int enc_len = src_len - CIPHER_MAC_SIZE;
-    if (enc_len % CIPHER_BLOCK_SIZE != 0) return 0;  // reject non-block-aligned ciphertext
     return decrypt(shared_secret, dest, src + CIPHER_MAC_SIZE, enc_len);
   }
   return 0; // invalid HMAC

src/Utils.h

@@ -50,7 +50,7 @@ public:
 
   /**
    * \brief  checks the MAC (in leading bytes of 'src'), then if valid, decrypts remaining bytes in src.
-   * \returns  zero if MAC is invalid, otherwise the length of decrypted bytes in 'dest'
+   * \returns  zero if MAC is invalid or ciphertext is not block-aligned, otherwise the length of decrypted bytes in 'dest'
   */
   static int MACThenDecrypt(const uint8_t* shared_secret, uint8_t* dest, const uint8_t* src, int src_len);