An attacker is trying to discover the contents of encrypted messages that he can easily intercept. The attacker attempts to break messages by intercepting and substituting his own messages in a...

An attacker is trying to discover the contents of encrypted messages that he can easily intercept. The attacker attempts to break messages by intercepting and substituting his own messages in a communications stream between two parties. This type of attack is known as:

a. Unknown plaintext

b. Known cipher text

c. Man in the middle

d. Replay