Informational Constraints (idea) by ink-

Informational constraints in cryptography will tell us the path we will take when we are attacking a certain cipher or ciphertext.

Ciphertext Only: We have only ciphertext to work with. Sometimes the statistics of the ciphertext provide insight and can lead to a break. Other times, perhaps due to a good avalanche effect of the cipher, we will gain more from looking at /dev/random.
Known Plaintext: We have some, or even an extremely large amount, of plaintext and the associated ciphertext. <\li>
Defined Plaintext: We can submit arbitrary messages to be ciphered and capture the resulting ciphertext. <\li>
Defined Ciphertext: We can submit arbitrary messages to be deciphered and see the resulting plaintext. <\li>
Chosen Key: We can specify a change in any particular key bit, or some other relationship between keys. <\li>
Timing: We can measure the duration of ciphering operations and use that to reveal the key or plaintext. <\li>
Differential Fault Analysis: We can induce random faults into the ciphering machinery, and use those to expose the key. <\li>
Man in the Middle: We can subvert the routing capabilities of a computer network, and pose as the other side to each of the communicators. (Usually a key authentication attack on public key systems.)<\li>