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| CATEGORIES ABOUT CRYPTOGRAPHIC HASH FUNCTION | |
| cryptography | |
| cryptographic primitives | |
| cryptographic hash functions | |
| hashing | |
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In various standards and applications, the two most-commonly used hash functions are MD5 and SHA-1 . In 2005, security flaws were identified in both algorithms. OVERVIEW Broadly speaking, a cryptographic hash function should behave as much as possible like a Random Function while still being Deterministic and efficiently computable. A cryptographic hash function is considered insecure if either of the following is computationally feasible:
An attacker who can do either of these things might, for example, use them to substitute an unauthorized message for an authorized one. Ideally, it should not even be feasible to find two messages whose digests are substantially similar; nor would one want an Attacker to be able to learn anything useful about a message given only its digest. Of course the attacker learns at least one piece of information, the digest itself, which for instance gives the attacker the ability to recognise the same message should it occur again. RELATED ALGORITHMS Checksum s and Cyclic Redundancy Check s (CRCs) are quite distinct from cryptographic hash functions, and are used for different applications. If used for security, they are vulnerable to attack; for example, a CRC was used for message integrity in the WEP encryption standard, but an attack was readily discovered which exploited the linearity of the checksum specified. A Message Authentication Code or MAC takes a message and a secret key and generates a "MAC tag", such that it is difficult for an attacker to generate a valid pair (message, tag) that doesn't match one they've already seen; they are used to prevent attackers forging messages, among other uses. Though it is sometimes referred to as a "keyed hash function", a MAC serves a very different purpose and has very different security properties than a cryptographic hash function; for example, it is not considered a flaw if it is easy for someone who knows the MAC key to generate two messages that have the same MAC. Hash functions can be used to create MAC functions; see for example HMAC . CRYPTOGRAPHIC PROPERTIES There is no formal definition which captures all of the properties considered desirable for a cryptographic hash function. These properties below are generally considered prerequisites:
: This property is implied by collision-resistance.
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| It Is | "http://developersslashdotorg/commentsplsid=120193&cid=10130642" class="copylinks" target="_blank">often suggested that concatening multiple hash functions could produce a new hash function that is more secure than its component parts For example, one might concatenate the output of SHA-1 and RIPEMD-160 to produce a new function H(x) = SHA-1(x) RIPEMD-160(x) |
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