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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.
G (key-generator) gives the key k on input 1 n, where n is the security parameter. S (signing) outputs a tag t on the key k and the input string x. V (verifying) outputs accepted or rejected on inputs: the key k, the string x and the tag t. S and V must satisfy the following: Pr [ k ← G(1 n), V( k, x, S(k, x) ) = accepted] = 1. [5]
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.
The global public key is the single node at the very top of the Merkle tree. Its value is an output of the selected hash function, so a typical public key size is 32 bytes. The validity of this global public key is related to the validity of a given one-time public key using a sequence of tree nodes. This sequence is called the authentication path.
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
The identity string and the KGC's public key are used to verify that the recipient's public key belongs to the recipient (was generated from the identity string and the KGC's public key). To decrypt, a user just needs to use their private key.
Each key pair consists of a public key and a corresponding private key. [1] [2] Key pairs are generated with cryptographic algorithms based on mathematical problems termed one-way functions. Security of public-key cryptography depends on keeping the private key secret; the public key can be openly distributed without compromising security. [3]