**Definition 5**

A pair,
, is a
known plaintext-ciphertext pair
with respect to a principal, *I* , and key, *k* if and only if

We will use the term *known pair* to refer to a known plaintext-ciphertext pair.
A principal may know that two pairs are encrypted under the same key even though the key is not known to
the principal. For a particular block encryption function, it may be the case that for any
*p* and *c* . However, in our analysis, it is of interest to know this fact in the context of receiving
particular *c* and associating it with sets of under the same *k* . Knowledge of a pair does not imply
knowledge of the corresponding key. That is knowing or does not imply knowing
*k* .