**Definition 6**

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

A chosen plaintext pair is a pair whose plaintext can be chosen by a certain principal. Chosen-plaintext pairs can lead to vulnerabilities that enable the adversary to forge messages using the key, read messages, or learn the key. Messages can be forged by exercising the system to create new messages. The key can be learned by precomputing a dictionary of a chosen plaintext under the key space. Note, however, it must be possible for the adversary to obtain the corresponding ciphertext.

**Definition 7**

A pair,
, is a
chosen ciphertext pair
with respect to a principal, *I* , and key, *k* , if and only if
.

A chosen ciphertext pair is a pair whose ciphertext can be chosen by a certain principal.