Cryptographic hash function: Difference between revisions

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{{short description|Hash function that is suitable for use in cryptography}}

{{More citations needed|date=May 2016}}

0a1fc3e04ce6c6f0e365b45449cc5eb8b94e543af336fd243a0e59f9f9e3eefc59b4da753cabdd0568cdfb67d88a8aa7c410e58739d2398d6e3ab566def28451[[Image:Cryptographic Hash Function.svg|thumb|375px|right|A cryptographic hash function (specifically [[SHA-1]]) at work. A small change in the input (in the word "over") drastically changes the output (digest). This is called the [[avalanche effect]].]]

{{SHA-box}}

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In practice, collision resistance is insufficient for many practical uses. In addition to collision resistance, it should be impossible for an adversary to find two messages with substantially similar digests; or to infer any useful information about the data, given only its digest. In particular, a hash function should behave as much as possible like a [[random function]] (often called a [[random oracle]] in proofs of security) while still being deterministic and efficiently computable. This rules out functions like the [[SWIFFT]] function, which can be rigorously proven to be collision-resistant assuming that certain problems on ideal lattices are computationally difficult, but, as a linear function, does not satisfy these additional properties.{{sfn|Lyubashevsky|Micciancio|Peikert|Rosen|2008| pp=54–72}}

Checksum algorithms, such as [[CRC32CRC-32]] and other [[cyclic redundancy check]]s, are designed to meet much weaker requirements and are generally unsuitable as cryptographic hash functions. For example, a CRC was used for message integrity in the [[Wired Equivalent Privacy|WEP]] encryption standard, but an attack was readily discovered, which exploited the linearity of the checksum.

=== Degree of difficulty ===