1. Adversary Model

Five adversary models from week to powerful are:

• Ciphertext only

• Known plaintext

• Chosen plaintext

• Chosen ciphertext

• Chosen text

In a network, Alice would like to send some encrypted messages to Bob.

• If an attacker only has access to the encrypted messages, the ciphertexts, it is doing Ciphertext only attack.

• If an attacker can also see what message Alice sent, say by seeing Alice’s screen, it is doing Known plaintext attack.

• If an attacker uses a backdoor program and can use Alice’s machine to send arbitrary plaintexts, it can self-chose the plaintext and know the corresponding ciphertext, it is doing Chosen plaintext attack.

• If an attacker also hacks Bob’s machine and knows which ciphtertext, even some self-generated ciphertext, can be decrypted to which plaintext, it is doing Chosen ciphertext attack.

• If an attacker can create arbitrary plaintexts and know their ciphertexts and also create arbitrary ciphtertexts and decrypt them, it is doing Chosen text attack.