small performance patch for header encryption (#500)

src/header_encryption.c

@@ -35,12 +35,6 @@ uint32_t packet_header_decrypt (uint8_t packet[], uint16_t packet_len,
   // to full 128 bit IV
   memcpy (iv, packet, 12);
 
-  // extract time stamp (first 64 bit) and checksum (last 16 bit) blended in IV
-  speck_he_iv_decrypt (iv, (speck_context_t*)ctx_iv);
-  *checksum = be16toh (((uint16_t*)iv)[5]);
-  *stamp    = be64toh (((uint64_t*)iv)[0]);
-
-  memcpy (iv, packet, 12);
 
   // try community name as possible key and check for magic bytes
   uint32_t magic = 0x6E326E00; // ="n2n_"
@@ -54,12 +48,20 @@ uint32_t packet_header_decrypt (uint8_t packet[], uint16_t packet_len,
        ) {
     // decrypt the complete header
     speck_he (&packet[12], &packet[12], (uint8_t)(test_magic) - 12, iv, (speck_context_t*)ctx);
+
+    // extract time stamp (first 64 bit) and checksum (last 16 bit) blended in IV
+    speck_he_iv_decrypt (iv, (speck_context_t*)ctx_iv);
+    *checksum = be16toh (((uint16_t*)iv)[5]);
+    *stamp    = be64toh (((uint64_t*)iv)[0]);
+
     // restore original packet order
     memcpy (&packet[0], &packet[16], 4);
     memcpy (&packet[4], community_name, N2N_COMMUNITY_SIZE);
+
     return (1); // successful
-  } else
+  } else {
     return (0); // unsuccessful
+  }
 }
 
 /* ********************************************************************** */