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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-256 hash function. Smart contracts use 256- or 257-bit integers; 256-bit words for the Ethereum Virtual Machine. "We realize that a 257 bits byte is quite unusual, but for smart contracts it is ok to have at least 256 bits numbers. The leading VM for smart contracts, Ethereum VM, introduced this practice and other blockchain VMs followed." [8]
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.
SHA-256 SHA-384 SHA-512: 2002 SHA-224: 2004 SHA-3 (Keccak) 2008 Guido Bertoni Joan Daemen Michaël Peeters Gilles Van Assche: RadioGatún: Website Specification: Streebog: 2012 FSB, InfoTeCS JSC RFC 6986: Tiger: 1995 Ross Anderson Eli Biham: Website Specification: Whirlpool: 2004 Vincent Rijmen Paulo Barreto: Website
Function Compress Input: h Persistent state vector chunk 128-byte (16 double word) chunk of message to compress t: Number, 0..2 128 Count of bytes that have been fed into the Compression IsLastBlock: Boolean Indicates if this is the final round of compression Output: h Updated persistent state vector Setup local work vector V V 0..7 ← h 0..7 ...
This should be acceptable for up to 2 64 × 16 B = 256 exabytes of data, and would suffice for many years after introduction. The winner of the AES contest, Rijndael , supports block and key sizes of 128, 192, and 256 bits, but in AES the block size is always 128 bits.
The second property requires dividing the disk into several sectors, usually 512 bytes (4096 bits) long, which are encrypted and decrypted independently of each other. In turn, if the data is to stay confidential, the encryption method must be tweakable ; no two sectors should be processed in exactly the same way.