Increasing the time needed to test a cryptographic key to protect against brute-force attack
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In cryptography, key stretching techniques are used to make a possibly weak key, typically a password or passphrase, more secure against a brute-force attack by increasing the resources (time and possibly space) it takes to test each possible key. Passwords or passphrases created by humans are often short or predictable enough to allow password cracking, and key stretching is intended to make such attacks more difficult by complicating a basic step of trying a single password candidate. Key stretching also improves security in some real-world applications where the key length has been constrained, by mimicking a longer key length from the perspective of a brute-force attacker.[1]
There are several ways to perform key stretching. One way is to apply a cryptographic hash function or a block cipher repeatedly in a loop. For example, in applications where the key is used for a cipher, the key schedule in the cipher may be modified so that it takes a specific length of time to perform. Another way is to use cryptographic hash functions that have large memory requirements – these can be effective in frustrating attacks by memory-bound adversaries.
^Kelsey, John; Schneier, Bruce; Hall, Chris; Wagner, David A. (1997). "Secure Applications of Low-Entropy Keys". In Okamoto, Eiji; Davida, George I.; Mambo, Masahiro (eds.). Information Security, First International Workshop, ISW '97, Tatsunokuchi, Japan, September 17-19, 1997, Proceedings. Lecture Notes in Computer Science. Vol. 1396. Springer. pp. 121–134. doi:10.1007/BFb0030415. ISBN 978-3-540-64382-1.
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