What is a Nonce?
A nonce is a value, N, that is used only once: Nonce. Nonces or nonce values are encountered in cryptography.
The initialization vector used for AES in CBC mode are typically nonces:
C1 = CIPHK(P1 ⊕ Nonce)
Cj = CIPHK(Pj ⊕ Cj-1) for 2 <= j <= n
Here, cipher block 1 is the result of the cipher block function keyed on K applied to the XOR value of plaintext block 1 and a nonce. The following cipher blocks are the result of the cipher block function keyed on K applied to the XOR value of the corresponding plaintext blocks and the previous cipher block.