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SHA-2 (Secure Hash Algorithm 2) is a set of cryptographic hash functions designed by the United States National Security Agency (NSA) and first published in 2001. [3] [4] They are built using the Merkle–Damgård construction, from a one-way compression function itself built using the Davies–Meyer structure from a specialized block cipher.
SHA-2: A family of two similar hash functions, with different block sizes, known as SHA-256 and SHA-512. They differ in the word size; SHA-256 uses 32-bit words where SHA-512 uses 64-bit words. There are also truncated versions of each standard, known as SHA-224, SHA-384, SHA-512/224 and SHA-512/256. These were also designed by the NSA.
In hash tables, since hash collisions are inevitable, hash tables have mechanisms of dealing with them, known as collision resolutions. Two of the most common strategies are open addressing and separate chaining. The cache-conscious collision resolution is another strategy that has been discussed in the past for string hash tables.
Collisions originally reported in 2004, [13] followed up by cryptanalysis paper in 2005. [19] RadioGatún: Up to 2 608 [20] 2 704: 2008-12-04 For a word size w between 1-64 bits, the hash provides a security claim of 2 9.5w. The attack can find a collision in 2 11w time. [21] RIPEMD-160 2 80: 48 of 80 rounds (2 51 time) 2006 Paper. [22] SHA-0: ...
The MD construction is inherently sequential. There is a parallel algorithm [13] which constructs a collision-resistant hash function from a collision-resistant compression function. The hash function PARSHA-256 [14] was designed using the parallel algorithm and the compression function of SHA-256.
This property is sometimes referred to as weak collision resistance. Functions that lack this property are vulnerable to second pre-image attacks. Collision resistance: it should be hard to find two different messages m 1 and m 2 such that hash(m 1) = hash(m 2). Such a pair is called a (cryptographic) hash collision.
[5] [6] Cryptographic hash functions with output size of n bits usually have a collision resistance security level n/2 and a preimage resistance level n. This is because the general birthday attack can always find collisions in 2 n/2 steps. [7] For example, SHA-256 offers 128-bit collision resistance and 256-bit preimage resistance.
In words, when given an x, it is not possible to find another x' such that the hashing function would create a collision. A hash function has strong collision resistance when, given a hashing function H, no arbitrary x and x' can be found where H(x)=H(x'). In words, no two x's can be found where the hashing function would create a collision.