SHA-2

SHA-2 is a set of cryptographic hash functions designed by the United States National Security Agency and first published in 2001. 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 includes significant changes from its predecessor, SHA-1. The SHA-2 family consists of six hash functions with digests that are 224, 256, 384 or 512 bits: SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, SHA-512/256. SHA-256 and SHA-512 are hash functions whose digests are eight 32-bit and 64-bit words, respectively. They use different shift amounts and additive constants, but their structures are otherwise virtually identical, differing only in the number of rounds. SHA-224 and SHA-384 are truncated versions of SHA-256 and SHA-512 respectively, computed with different initial values. SHA-512/224 and SHA-512/256 are also truncated versions of SHA-512, but the initial values are generated using the method described in Federal Information Processing Standards PUB 180-4.
SHA-2 was first published by the National Institute of Standards and Technology as a U.S. federal standard. The SHA-2 family of algorithms are patented in the U.S. The United States has released the patent under a royalty-free license.
As of 2011 the best public attacks break preimage resistance for 52 out of 64 rounds of SHA-256 or 57 out of 80 rounds of SHA-512, and collision resistance for 46 out of 64 rounds of SHA-256.

Hash standard

With the publication of FIPS PUB 180-2, NIST added three additional hash functions in the SHA family. The algorithms are collectively known as SHA-2, named after their digest lengths : SHA-256, SHA-384, and SHA-512.
The algorithms were first published in 2001 in the draft FIPS PUB 180-2, at which time public review and comments were accepted. In August 2002, FIPS PUB 180-2 became the new Secure Hash Standard, replacing FIPS PUB 180-1, which was released in April 1995. The updated standard included the original SHA-1 algorithm, with updated technical notation consistent with that describing the inner workings of the SHA-2 family.
In February 2004, a change notice was published for FIPS PUB 180-2, specifying an additional variant, SHA-224, defined to match the key length of two-key Triple DES. In October 2008, the standard was updated in FIPS PUB 180-3, including SHA-224 from the change notice, but otherwise making no fundamental changes to the standard. The primary motivation for updating the standard was relocating security information about the hash algorithms and recommendations for their use to Special Publications 800-107 and 800-57. Detailed test data and example message digests were also removed from the standard, and provided as separate documents.
In January 2011, NIST published SP800-131A, which specified a move from the then-current minimum of 80-bit security allowable for federal government use until the end of 2013, to 112-bit security being both the minimum requirement and the recommended security level.
In March 2012, the standard was updated in FIPS PUB 180-4, adding the hash functions SHA-512/224 and SHA-512/256, and describing a method for generating initial values for truncated versions of SHA-512. Additionally, a restriction on padding the input data prior to hash calculation was removed, allowing hash data to be calculated simultaneously with content generation, such as a real-time video or audio feed. Padding the final data block must still occur prior to hash output.
In July 2012, NIST revised SP800-57, which provides guidance for cryptographic key management. The publication disallowed creation of digital signatures with a hash security lower than 112 bits after 2013. The previous revision from 2007 specified the cutoff to be the end of 2010. In August 2012, NIST revised SP800-107 in the same manner.
In March 2023, NIST announced its decision to revise FIPS 180-4. FIPS 180-5 will remove the SHA-1 specification, add guidance from SP 800-107, and include editorial updates.
The NIST hash function competition selected a new hash function, SHA-3, in 2012. The SHA-3 algorithm is not derived from SHA-2.

Applications

The SHA-2 hash function is implemented in some widely used security applications and protocols, including TLS and SSL, PGP, SSH, S/MIME, and IPsec. The inherent computational demand of SHA-2 algorithms has driven the proposal of more efficient solutions, such as those based on application-specific integrated circuits hardware accelerators.
SHA-256 is used for authenticating Debian software packages and in the DKIM message signing standard; SHA-512 is part of a system to authenticate archival video from the International Criminal Tribunal of the Rwandan genocide. SHA-256 and SHA-512 are used in DNSSEC. Linux distributions usually use 512-bit SHA-2 for secure password hashing.
Several cryptocurrencies, including Bitcoin, use SHA-256 for verifying transactions and calculating proof of work or proof of stake. The rise of ASIC SHA-2 accelerator chips has led to the use of scrypt-based proof-of-work schemes.
In both 4G and 5G mobile networks, HMAC-SHA-256 is utilized as a key derivation function to generate cryptographic keys essential for securing communications. This process is defined in the 3rd Generation Partnership Project Technical Specifications TS 33.401 and TS 33.501, which outline the security architecture and procedures for these networks.
SHA-1, SHA-2, and SHA-3 are the Secure Hash Algorithms required by law for use in certain U.S. Government applications, including use within other cryptographic algorithms and protocols, for the protection of sensitive unclassified information. FIPS PUB 180-1 also encouraged adoption and use of SHA-1 by private and commercial organizations. SHA-1 is being retired for most government uses; the U.S. National Institute of Standards and Technology says, "NIST recommends that federal agencies transition away from SHA-1 for all applications as soon as possible. Federal agencies should use SHA-2 or SHA-3 as an alternative to SHA-1.". NIST's directive that U.S. government agencies ought to, but not explicitly must, stop uses of SHA-1 after 2010 was hoped to accelerate migration away from SHA-1.
The SHA-2 functions were not quickly adopted initially, despite better security than SHA-1. Reasons might include lack of support for SHA-2 on systems running Windows XP SP2 or older and a lack of perceived urgency since SHA-1 collisions had not yet been found. The Google Chrome team announced a plan to make their web browser gradually stop honoring SHA-1-dependent TLS certificates over a period from late 2014 and early 2015. Similarly, Microsoft announced that Internet Explorer and Microsoft Edge Legacy|Edge would stop honoring public SHA-1-signed TLS certificates from February 2017. Mozilla disabled SHA-1 in Firefox during early January 2016, but had to re-enable it temporarily via an update, after problems with web-based user interfaces of some router models and security appliances.

Cryptanalysis and validation

For a hash function for which L is the number of bits in the message digest, finding a message that corresponds to a given message digest can always be done using a brute force search in 2^L evaluations. This is called a preimage attack and may or may not be practical depending on L and the particular computing environment. The second criterion, finding two different messages that produce the same message digest, known as a collision, requires on average only 2^L/2 evaluations using a birthday attack.
Some of the applications that use cryptographic hashes, such as password storage, are only minimally affected by a collision attack. Constructing a password that works for a given account requires a preimage attack, as well as access to the hash of the original password which may or may not be trivial. Reversing password encryption is not made possible by the attacks.
In the case of document signing, an attacker could not simply fake a signature from an existing document—the attacker would have to produce a pair of documents, one innocuous and one damaging, and get the private key holder to sign the innocuous document. There are practical circumstances in which this is possible; until the end of 2008, it was possible to create forged SSL certificates using an MD5 collision which would be accepted by widely used web browsers.
Increased interest in cryptographic hash analysis during the SHA-3 competition produced several new attacks on the SHA-2 family, the best of which are given in the table below. Only the collision attacks are of practical complexity; none of the attacks extend to the full round hash function.
At FSE 2012, researchers at Sony gave a presentation suggesting pseudo-collision attacks could be extended to 52 rounds on SHA-256 and 57 rounds on SHA-512 by building upon the biclique pseudo-preimage attack.

Published in	Year	Attack method	Attack	Variant	Rounds	Complexity
New Collision Attacks Against Up To 24-step SHA-2	2008	Differential	Collision	SHA-256	24/64	2^15.5
New Collision Attacks Against Up To 24-step SHA-2	2008	Differential	Collision	SHA-512	24/80	2^22.5
Preimages for step-reduced SHA-2	2009	Meet-in-the-middle	Preimage	SHA-256	42/64	2^251.7
Preimages for step-reduced SHA-2	2009	Meet-in-the-middle	Preimage	SHA-256	43/64	2^254.9
Preimages for step-reduced SHA-2	2009	Meet-in-the-middle	Preimage	SHA-512	42/80	2^502.3
Preimages for step-reduced SHA-2	2009	Meet-in-the-middle	Preimage	SHA-512	46/80	2^511.5
Advanced meet-in-the-middle preimage attacks	2010	Meet-in-the-middle	Preimage	SHA-256	42/64	2^248.4
Advanced meet-in-the-middle preimage attacks	2010	Meet-in-the-middle	Preimage	SHA-512	42/80	2^494.6
Higher-Order Differential Attack on Reduced SHA-256	2011	Differential	Pseudo-collision	SHA-256	46/64	2¹⁷⁸
Higher-Order Differential Attack on Reduced SHA-256	2011	Differential	Pseudo-collision	SHA-256	33/64	2⁴⁶
Bicliques for Preimages: Attacks on Skein-512 and the SHA-2 family	2011	Biclique	Preimage	SHA-256	45/64	2^255.5
Bicliques for Preimages: Attacks on Skein-512 and the SHA-2 family	2011	Biclique	Preimage	SHA-512	50/80	2^511.5
Bicliques for Preimages: Attacks on Skein-512 and the SHA-2 family	2011	Biclique	Pseudo-preimage	SHA-256	52/64	2²⁵⁵
Bicliques for Preimages: Attacks on Skein-512 and the SHA-2 family	2011	Biclique	Pseudo-preimage	SHA-512	57/80	2⁵¹¹
Improving Local Collisions: New Attacks on Reduced SHA-256	2013	Differential	Collision	SHA-256	31/64	2^65.5
Improving Local Collisions: New Attacks on Reduced SHA-256	2013	Differential	Pseudo-collision	SHA-256	38/64	2³⁷
Branching Heuristics in Differential Collision Search with Applications to SHA-512	2014	Heuristic differential	Pseudo-collision	SHA-512	38/80	2^40.5
Analysis of SHA-512/224 and SHA-512/256	2016	Differential	Collision	SHA-256	28/64	practical
Analysis of SHA-512/224 and SHA-512/256	2016	Differential	Collision	SHA-512	27/80	practical
Analysis of SHA-512/224 and SHA-512/256	2016	Differential	Pseudo-collision	SHA-512	39/80	practical
New Records in Collision Attacks on SHA-2	2024	Differential	Collision	SHA-256	31/64	2^49.8
New Records in Collision Attacks on SHA-2	2024	Differential	Collision	SHA-512	31/80	2^115.6
New Records in Collision Attacks on SHA-2	2024	Differential	Pseudo-collision	SHA-256	39/64	practical