Brute force attack: Difference between revisions
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==Related attacks== | ==Related attacks== | ||
Sometimes brute force is used as the final stage of another attack. For example, in the original paper <ref>{{cite paper| | Sometimes brute force is used as the final stage of another attack. For example, in the original paper <ref>{{cite paper|author=Eli Biham and Adi Shamir|title=Differential cryptanalysis of DES-like cryptosystems|publisher=Journal of Cryptology|date=1991}}</ref> on [[differential cryptanalysis]], the differential attack gives 48 bits of the [[DES]] key and ther 8 are found by brute force. | ||
Some ways of combining of ciphers are vulnerable to a [[meet-in-the-middle attack]. Against double DES with two independent 56-bit keys, for example, the attacker need not search among the 2<sup>122</sup> possible key combinations; there is a meet-in-the middle attack with cost only 2<sup>57</sup> if you have enough memory, and not too much more if memory is constrained. | Some ways of combining of ciphers are vulnerable to a [[meet-in-the-middle attack]. Against double DES with two independent 56-bit keys, for example, the attacker need not search among the 2<sup>122</sup> possible key combinations; there is a meet-in-the middle attack with cost only 2<sup>57</sup> if you have enough memory, and not too much more if memory is constrained. | ||
Revision as of 12:33, 8 August 2008
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. With an n-bit key, there are 2n possible keys. On average, a brute force attack must test half of them, performing 2n-1 encryptions, to find the key. A large enough key makes any brute force attack wildly impractical.
For example, the EFF's DES Cracker [1] 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 with its 56-bit key 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. An analysis by a group of well-known people [2] 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 symmetric 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. This not easy, but it is far better for the attacker than a brute force search.
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. Asymmetric keys therefore need to be larger than symmetric keys for the same security levels. For example, RSA keys of 1024 bits or more are commonly used.
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, and even a strong cipher will not provide security unless it is used correctly.
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.
Related attacks
Sometimes brute force is used as the final stage of another attack. For example, in the original paper [3] on differential cryptanalysis, the differential attack gives 48 bits of the DES key and ther 8 are found by brute force.
Some ways of combining of ciphers are vulnerable to a [[meet-in-the-middle attack]. Against double DES with two independent 56-bit keys, for example, the attacker need not search among the 2122 possible key combinations; there is a meet-in-the middle attack with cost only 257 if you have enough memory, and not too much more if memory is constrained.
References
- ↑ Electronic Frontier Foundation (1998). Cracking DES - Secrets of Encryption Research, Wiretap Politics & Chip Design. Oreilly & Associates Inc. ISBN 1-56592-520-3.
- ↑ Blaze, Diffie, Rivest, Schneier, Shimomura, Thompson & Wiener (1996). Minimal Key Lengths for Symmetric Ciphers to Provide Adequate Commercial Security.
- ↑ Eli Biham and Adi Shamir (1991). "Differential cryptanalysis of DES-like cryptosystems". Journal of Cryptology.