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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.
SHA-2 basically consists of two hash algorithms: SHA-256 and SHA-512. SHA-224 is a variant of SHA-256 with different starting values and truncated output. SHA-384 and the lesser-known SHA-512/224 and SHA-512/256 are all variants of SHA-512. SHA-512 is more secure than SHA-256 and is commonly faster than SHA-256 on 64-bit machines such as AMD64.
Merkle tree NLFSR (it is also a keyed hash function) RadioGatún: arbitrary ideal mangling function RIPEMD: 128 bits hash RIPEMD-128: 128 bits hash RIPEMD-160: 160 bits hash RIPEMD-256: 256 bits hash RIPEMD-320: 320 bits hash SHA-1: 160 bits Merkle–Damgård construction: SHA-224: 224 bits Merkle–Damgård construction: SHA-256: 256 bits ...
As of October 2012, CNSSP-15 [4] stated that the 256-bit elliptic curve (specified in FIPS 186-2), SHA-256, and AES with 128-bit keys are sufficient for protecting classified information up to the Secret level, while the 384-bit elliptic curve (specified in FIPS 186-2), SHA-384, and AES with 256-bit keys are necessary for the protection of Top ...
SHA-3 instances are drop-in replacements for SHA-2, intended to have identical security properties. SHAKE will generate as many bits from its sponge as requested, thus being extendable-output functions (XOFs). For example, SHAKE128(M, 256) can be used as a hash function with a 256 character bitstream with 128-bit security strength.
Not all cryptographic hash functions have this property. SHA-256 is a cryptographic hash function that has this property. Informally, a hash function is puzzle friendly if no solution exists, which is better than just making random guesses and the only way to find a solution is the brute force method.
Therefore, in a final step, a second compression function compresses the last internal hash value (2n bits) to the final hash value (n bits). This can be done as simply as discarding half of the last 2n-bit output. SHA-512/224 and SHA-512/256 take this form since they are derived from a variant of SHA-512.