Brute Force Attack Strategies

Question

A Brute Force Attack obviously involves attempting to decrypt ciphertext (with the associated plaintext being known) using all possible encryption keys.

Aside from attempting all possible encryption keys in sequence, have there been, or is there any alternative methods of performing a brute-force attack?

I'm aware of the Distributed.NET attack on DES (which leveraged parallel/distributed computing) whereby the entire keyspace of DES was subdivided and allocated to a number of different computers, with each computer decrypting a piece of ciphertext using the section of the keyspace it was allocated.

Answer 1

No, you can divide and distribute, but if you do anything else (such as filtering for candidates) then it would not officially be a brute force attack anymore. You will have to iterate over the full key space one way or another.

With DES I guess it is acceptable to forget about parity while iterating over the key space, only effective key bits have to be traversed.

Finally, there may be specific methods required to filter out possible candidates if it isn't a known plaintext attack (which is however assumed in the question).

Answer 2

Often a dictionary attack is also classified as brute force, e.g. according to RFC 4949 (the Internet Security Glossary). That doesn't apply to encryption with random keys, but does apply where the key is derived from a password, or when the cipher is used for password hashing, like DES-based crypt.