Attacking a cipher or a cryptographic system may lead to breaking it fully or only partially. After compromising the security, the attacker may obtain various amounts and kinds of information.
Lars Knudsen, a Danish researcher, proposed the following division for determining the scale of attacker's success:
- Total break: deducing and obtaining a secret key.
- Global deduction: discovering an algorithm, which allows to decrypt many messages, without knowing the actual secret key.
- Local deduction: discovering an original plaintext of the specific given ciphertext.
- Information deduction: obtaining some information about the secret key or original message (for example, a few bits of the key or information about a plaintext format).
The best ciphers should protect against all the cipher's failures levels mentioned above. No attack should be able to reveal any information related to the secret key and plaintext messages.