CoAP authentication using DTLS what is the meaning of this notation ?? 


  AES{Yi, (Yi XOR Ki | nonce1)}


https://tools.ietf.org/html/draft-bhattacharyya-core-coap-lite-auth-00
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.