The OFB mode uses an initial chaining vector (ICV) in its processing. OFB mode requires that the ICV is a nonce (the ICV must be unique for each execution of the mode under the given key). Each encryption step takes an input block, enciphers it with the key provided to generate an output block, and then exclusive ORs the output block with the plaintext block. The first input block is the ICV and each subsequent input block is the previous output block. The input text can be of any length. The output text will have the same length as the input text.