Sentence:  The protocol opens with "A" communicating in clear to "AS" his own claimed identity and the identity of the desired correspondent, "B", together with A's <WSD>nonce</WSD> identifier for this transaction, "I<sub>A1</sub>". ("<WSD>Nonce</WSD>" means "used only once.") 
Meaning of the word marked with <WSD> tags:  A value constructed so as to be unique to a particular message in a stream, in order to prevent replay attacks.