Brute force attack

A brute force or exhaustive search attack is an attempt to break a cipher by trying all possible keys. This is always possible in theory (except against a one-time pad), but it becomes practical only if the key size is inadequate.

Symmetric ciphers

For a symmetric cipher longer keys protect against brute force attacks. Each extra bit in the key doubles the number of possible keys and therefore doubles the work a brute force attack must do. In general, with an n-bit key a brute force attack requires 2^n-1 encryptions on average and 2ⁿ in the worst case. A large enough key defeats any brute force attack.

For example, the EFF's DES Cracker searched a 56-bit key space in an average of a few days. Assume an attacker that can find a 64-bit key (256 times harder) by brute force search in a second (a few hundred thousand times faster). For a 96-bit key, that attacker needs 2³² seconds, about 135 years. Against a 128-bit key, he needs 2³² times that, over 500,000,000,000 years. Your data is then obviously secure against brute force attacks. Even if our estimate of the attacker's speed is off by a factor of a million, it still takes him over 500,000 years to crack a message.

This is why single DES is now considered dangerously insecure, all of the current generation of block ciphers use a 128-bit or longer key, and AES ciphers support key sizes 128, 192 and 256 bits.

The question of how large a key is "large enough" has been extensively studied. A summary recommended a minimum of 90 bits for any new ciphers deployed as of 1996. Computers improve roughly in accord with Moore's Law, twice as fast every 18 months, so ciphers need about one extra bit of key every 18 months to keep up.

Public-key Systems

For public key systems the relation between key size and security is more complex. Here an attacker has the public key and that is mathematically related to the private key. He need not try all possible keys, only solve a math problem. For example, to break a 256-bit RSA key, he has to factor a 256-bit number.

The question then is not how big the key needs to be to defeat brute force, but how big it needs to be to make the math problem hard enough for the security requirement. In general, the difficulty of such math problems does not increase exponentially — doubling for each extra key bit — as for symmetric ciphers, but more slowly. Assymetric keys therefore need to be larger than symmetric keys for the same security levels.

Cautions

Inadequate keylength always indicates a weak cipher but it is important to note that adequate keylength does not necessarily indicate a strong cipher. There are many attacks other than brute force, and adequate keylength only guarantees resistance to brute force. Any cipher, whatever its key size, will be weak if design or implementation flaws allow other attacks.

Also, once you have adequate keylength, adding more key bits make no practical difference , even against brute force. Consider our 128-bit example above that takes 500,000,000,000 years to break by brute force. We really don't care how many zeroes there are on the end of that, as long as the number remains ridiculously large. That is, we don't care exactly how large the key is as long as it is large enough.

There may be reasons of convenience in the design of the cipher to support larger keys. For example Blowfish allows up to 448 bits and RC4 up to 2048, but beyond 100-odd bits it makes no difference to practical security.