CoAP authentication using DTLS
It is the encryption using the key $Y_i$ of the constructed message.

The message consists of key $K_i$ XOR'ed (bit for bit) with the previous key $Y_i$, followed by ${nonce}_1$.


  where $K_i$ is the
     potential session key if the handshake is successful, '${nonce}_1$' is
     pseudo randomly generated nonce specific to one authentication
     session. Both $K_i$ and ${nonce}_1$ is 128 bit each. Total message size is
     256 bit.