Editing Cryptanalysis (section)

===Partial breaks===
The results of cryptanalysis can also vary in usefulness. Cryptographer [[Lars Knudsen]] (1998) classified various types of attack on [[block cipher]]s according to the amount and quality of secret information that was discovered:
* ''Total break'' – the attacker deduces the secret [[key (cryptography)|key]].
* ''Global deduction'' – the attacker discovers a functionally equivalent [[algorithm]] for encryption and decryption, but without learning the key.
* ''Instance (local) deduction'' – the attacker discovers additional plaintexts (or ciphertexts) not previously known.
* ''Information deduction'' – the attacker gains some [[Information entropy|Shannon information]] about plaintexts (or ciphertexts) not previously known.
* ''Distinguishing algorithm'' – the attacker can distinguish the cipher from a random [[permutation]].

Academic attacks are often against weakened versions of a cryptosystem, such as a block cipher or hash function with some rounds removed. Many, but not all, attacks become exponentially more difficult to execute as rounds are added to a cryptosystem,<ref>For an example of an attack that cannot be prevented by additional rounds, see [[slide attack]].</ref> so it's possible for the full cryptosystem to be strong even though reduced-round variants are weak. Nonetheless, partial breaks that come close to breaking the original cryptosystem may mean that a full break will follow; the successful attacks on [[Data Encryption Standard|DES]], [[MD5]], and [[SHA-1]] were all preceded by attacks on weakened versions.

In academic cryptography, a ''weakness'' or a ''break'' in a scheme is usually defined quite conservatively: it might require impractical amounts of time, memory, or known plaintexts. It also might require the attacker be able to do things many real-world attackers can't: for example, the attacker may need to choose particular plaintexts to be encrypted or even to ask for plaintexts to be encrypted using several keys related to the [[secret key]]. Furthermore, it might only reveal a small amount of information, enough to prove the cryptosystem imperfect but too little to be useful to real-world attackers. Finally, an attack might only apply to a weakened version of cryptographic tools, like a reduced-round block cipher, as a step towards breaking the full system.<ref name=schneier>{{Harvnb|Schneier|2000}}</ref>