Part 14
Table 98 – Layout of the counter block for UADP message security
BlockCounter
The counter for each encrypted block of the NetworkMessage.
The counter is a 32-bit big endian integer (the opposite of the normal encoding for Uint32 values in OPC UA. This convention comes from the AES-CTR RFC).
The counter starts with 0 at the first block. The counter is incremented by 1 for each block.

https://tools.ietf.org/html/rfc3686
Block Counter
The block counter field is the least significant 32 bits of the
counter block. The block counter begins with the value of one,
and it is incremented to generate subsequent portions of the key
stream. The block counter is a 32-bit big-endian integer value.

The starting value for the BlockCounter is different in Part 14 (0) and RFC (1). This is a problem if crypto libraries are used that have a comlete implementation for AES-CRT.

Part 14 should be in sync with the RFC.