View source for SHA-1

{{short description|Cryptographic hash function}}
{{SHA-box}}
{{Infobox encryption method
| name           = SHA-1
| image          = 
| caption        = 
<!-- General --> 
| designers      = [[National Security Agency]]
| publish date   = 1993 (SHA-0),<br />1995 (SHA-1)
| series         = ([[SHA-0]]), SHA-1, [[SHA-2]], [[SHA-3]]
| derived from   = 
| derived to     = 
| related to     = 
| certification  = [[Federal Information Processing Standard|FIPS]] PUB 180-4, [[CRYPTREC]] (Monitored)
<!-- Detail -->
| digest size    = 160 bits
| block size     = 512 bits
| structure      = [[Merkle–Damgård construction]]
| rounds         = 80
| cryptanalysis  = A 2011 attack by Marc Stevens can produce hash collisions with a complexity between 2<sup>60.3</sup> and 2<sup>65.3</sup> operations.<ref name="stevens-attacks">{{Cite thesis|last1=Stevens|first1=Marc|author-link1=Marc Stevens (cryptology)|title=Attacks on Hash Functions and Applications|url=https://marc-stevens.nl/research/papers/PhD%20Thesis%20Marc%20Stevens%20-%20Attacks%20on%20Hash%20Functions%20and%20Applications.pdf|date=2012-06-19|degree=PhD |institution=[[Leiden University]]|isbn=9789461913173|oclc=795702954|hdl=1887/19093|df=mdy-all}}</ref> The first public collision was published on 23 February 2017.<ref name="sha1-shattered">{{Cite conference|date=2017|last1=Stevens|first1=Marc|author-link1=Marc Stevens (cryptology)|last2=Bursztein|first2=Elie|author-link2=Elie Bursztein|last3=Karpman|first3=Pierre|last4=Albertini|first4=Ange|last5=Markov|first5=Yarik|editor-last=Katz|editor-first=Jonathan|editor-link1=Jonathan Katz (computer_scientist)|editor2-last=Shacham|editor2-first=Hovav|title=The First Collision for Full SHA-1|url=https://shattered.io/static/shattered.pdf|conference=Advances in Cryptology – [[International Cryptology Conference|CRYPTO]] 2017|series= [[Lecture Notes in Computer Science]]|publisher=[[Springer Publishing|Springer]]|volume=10401|pages=570–596|doi=10.1007/978-3-319-63688-7_19|isbn=9783319636870|df=mdy-all |access-date=2017-02-23|archive-url=https://web.archive.org/web/20180515222208/http://shattered.io/static/shattered.pdf|archive-date=2018-05-15|url-status=dead}}
*{{cite web |author=Marc Stevens |author2=Elie Bursztein |author3=Pierre Karpman |author4=Ange Albertini |author5=Yarik Markov |author6=Alex Petit Bianco |author7=Clement Baisse |date=February 23, 2017 |title=Announcing the first SHA1 collision |website=Google Security Blog |url=https://security.googleblog.com/2017/02/announcing-first-sha1-collision.html}}</ref> SHA-1 is prone to [[length extension attack]]s.
}}
{{Wikifunctions|Z10148|SHA-1}}
In [[cryptography]], '''SHA-1''' ('''Secure Hash Algorithm 1''') is a [[hash function]] which takes an input and produces a 160-[[bit]] (20-[[byte]]) hash value known as a [[message digest]] – typically rendered as 40 [[hexadecimal]] digits. It was designed by the United States [[National Security Agency]], and is a U.S. [[Federal Information Processing Standard]].<ref name=":0">{{Cite web |url=https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.180-4.pdf |title=Secure Hash Standard (SHS) | id= Federal Information Processing Standards Publication 180-4 | doi= 10.6028/NIST.FIPS.180-4 | date= 2015|access-date=2019-09-23 |archive-date=2020-01-07 |archive-url=https://web.archive.org/web/20200107195003/https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.180-4.pdf |url-status=dead | publisher= National Institute of Standards and Technology }}</ref> The algorithm has been cryptographically broken<ref name=":1" /><ref name="autogenerated1" /><ref name=":2" /><ref name=":3" /><ref name="leurent-peyrin-sha1-shambles" /><ref name=":4" /><ref name="shappening" /> but is still widely used.

Since 2005, SHA-1 has not been considered secure against well-funded opponents;<ref>{{cite web|url=https://www.schneier.com/blog/archives/2005/02/cryptanalysis_o.html |title=Schneier on Security: Cryptanalysis of SHA-1|date=February 18, 2005|last=Schneier|first=Bruce }}</ref> as of 2010 many organizations have recommended its replacement.<ref>{{cite web|url=http://csrc.nist.gov/groups/ST/toolkit/secure_hashing.html|title=NIST.gov – Computer Security Division – Computer Security Resource Center|access-date=2019-01-05|archive-url=https://web.archive.org/web/20110625054822/http://csrc.nist.gov/groups/ST/toolkit/secure_hashing.html|archive-date=2011-06-25|url-status=dead}}</ref><ref name="shappening" /><ref>{{cite web|url=https://www.schneier.com/blog/archives/2015/10/sha-1_freestart.html |first=Bruce |last=Schneier |date=8 October 2015 |title=SHA-1 Freestart Collision |work=Schneier on Security}}</ref> [[NIST]] formally deprecated use of SHA-1 in 2011 and disallowed its use for digital signatures in 2013, and declared that it should be phased out by 2030.<ref>{{Cite press release |date=2022-12-15 |title=NIST Retires SHA-1 Cryptographic Algorithm |url=https://www.nist.gov/news-events/news/2022/12/nist-retires-sha-1-cryptographic-algorithm |publisher=NIST |language=en}}</ref> {{As of|2020}}, [[chosen-prefix attack]]s against SHA-1 are practical.<ref name=":2">{{Cite web |title=Critical flaw demonstrated in common digital security algorithm |url=https://www.ntu.edu.sg/news/detail/critical-flaw-demonstrated-in-common-digital-security-algorithm |website=Nanyang Technological University, Singapore |date=24 January 2020 |language=en-US}}</ref><ref name="leurent-peyrin-sha1-shambles">{{cite web|last1=Leurent |first1=Gaëtan |last2=Peyrin |first2=Thomas |date=2020-01-05 |title=SHA-1 is a Shambles First Chosen-Prefix Collision on SHA-1 and Application to the PGP Web of Trust |url=https://eprint.iacr.org/2020/014.pdf |work=Cryptology ePrint Archive, Report 2020/014}}</ref> As such, it is recommended to remove SHA-1 from products as soon as possible and instead use [[SHA-2]] or [[SHA-3]]. Replacing SHA-1 is urgent where it is used for [[digital signatures]].

All major [[web browser]] vendors ceased acceptance of SHA-1 [[SSL certificate]]s in 2017.<ref>{{Cite web|url=https://arstechnica.com/information-technology/2016/05/microsoft-to-retire-support-for-sha1-certificates-in-the-next-4-months/|title=Microsoft to retire support for SHA1 certificates in the next 4 months|last=Goodin|first=Dan|date=2016-05-04|website=Ars Technica|language=en-us|access-date=2019-05-29}}</ref><ref name=":4">{{Cite web|url=https://venturebeat.com/2015/12/18/google-will-drop-sha-1-encryption-from-chrome-by-january-1-2017/|title=Google will drop SHA-1 encryption from Chrome by January 1, 2017|date=2015-12-18|website=VentureBeat|language=en-US|access-date=2019-05-29}}</ref><ref name=":1">{{Cite web|url=https://blog.mozilla.org/security/2017/02/23/the-end-of-sha-1-on-the-public-web/|title=The end of SHA-1 on the Public Web|website=Mozilla Security Blog|date=23 February 2017 |language=en-US|access-date=2019-05-29}}</ref> In February 2017, [[CWI Amsterdam]] and [[Google]] announced they had performed a [[collision attack]] against SHA-1, publishing two dissimilar PDF files which produced the same SHA-1 hash.<ref>{{Cite news|url=https://phys.org/news/2017-02-cwi-google-collision-industry-standard.html|title=CWI, Google announce first collision for Industry Security Standard SHA-1|access-date=2017-02-23}}</ref><ref name="sha1-shattered" /> However, SHA-1 is still secure for [[HMAC]].<ref>{{Citation| mode= cs1|title=Recommendation for Key Management: Part 1 – General, Table 3.|doi=10.6028/NIST.SP.800-57pt1r5|last1=Barker|first1=Elaine|publisher=NIST|type= Technical Report|date=May 2020|pages=56|doi-access=free}}</ref>

[[Microsoft]] has discontinued SHA-1 code signing support for [[Windows Update]] on August 3, 2020,<ref>{{cite web|url=https://techcommunity.microsoft.com/t5/windows-it-pro-blog/sha-1-windows-content-to-be-retired-august-3-2020/ba-p/1544373|title=SHA-1 Windows content to be retired August 3, 2020|website=techcommunity.microsoft.com|access-date=2024-02-28}}</ref> which also effectively ended the update servers for versions of [[Microsoft Windows|Windows]] that have not been updated to SHA-2, such as [[Windows 2000]] up to [[Windows Vista|Vista]], as well as [[Windows Server]] versions from [[Windows 2000#Editions|Windows 2000 Server]] to [[Windows Server 2003|Server 2003]].

==Development==
[[File:SHA-1.svg|thumbnail|right|300px|One iteration within the SHA-1 compression function:{{ubli
 | A, B, C, D and E are 32-bit [[Word (data type)|words]] of the state;
 | ''F'' is a nonlinear function that varies;
 | {{tmath|\lll_n}} denotes a left bit rotation by ''n'' places;
 | ''n'' varies for each operation;
 | W<sub>''t''</sub> is the expanded message word of round ''t'';
 | K<sub>''t''</sub> is the round constant of round ''t'';
 | [[Image:Boxplus.png|alt=⊞|Addition]] denotes addition modulo 2<sup>32</sup>.
}}]]

SHA-1 produces a [[message digest]] based on principles similar to those used by [[Ron Rivest|Ronald L. Rivest]] of [[Massachusetts Institute of Technology|MIT]] in the design of the [[MD2 (hash function)|MD2]], [[MD4]] and [[MD5]] message digest algorithms, but generates a larger hash value (160 bits vs. 128 bits).

SHA-1 was developed as part of the U.S. Government's [[Capstone (cryptography)|Capstone project]].<ref>{{cite web| url = http://x5.net/faqs/crypto/q150.html| title = RSA FAQ on Capstone}}</ref> The original specification of the algorithm was published in 1993 under the title ''Secure Hash Standard'', [[Federal Information Processing Standard|FIPS]] PUB 180, by U.S. government standards agency [[NIST]] (National Institute of Standards and Technology).<ref>{{cite book|last1=Selvarani|first1=R.|last2=Aswatha|first2=Kumar|last3=T V Suresh|first3=Kumar|title=Proceedings of International Conference on Advances in Computing|page=551|url=https://books.google.com/books?id=L2OFg7OiV9YC&pg=PA551|date=2012|publisher=Springer Science & Business Media|isbn=978-81-322-0740-5}}</ref><ref>{{citation|title=Secure Hash Standard, Federal Information Processing Standards Publication FIPS PUB 180|institution=National Institute of Standards and Technology|date=11 May 1993}}</ref> This version is now often named ''SHA-0''. It was withdrawn by the [[NSA]] shortly after publication and was superseded by the revised version, published in 1995 in FIPS PUB 180-1 and commonly designated ''SHA-1''. SHA-1 differs from SHA-0 only by a single bitwise rotation in the message schedule of its [[One-way compression function|compression function]]. According to the NSA, this was done to correct a flaw in the original algorithm which reduced its cryptographic security, but they did not provide any further explanation.<ref>{{cite web |last1=Kramer |first1=Samuel |title=Proposed Revision of Federal Information Processing Standard (FIPS) 180, Secure Hash Standard |url=https://www.federalregister.gov/documents/1994/07/11/94-16666/proposed-revision-of-federal-information-processing-standard-fips-180-secure-hash-standard |website=Federal Register |date=11 July 1994}}</ref><ref>{{cite web |last1=fgrieu |title=Where can I find a description of the SHA-0 hash algorithm? |url=https://crypto.stackexchange.com/a/62071 |website=Cryptography Stack Exchange}}</ref> Publicly available techniques did indeed demonstrate a compromise of SHA-0, in 2004, before SHA-1 in 2017 (''see [[#Attacks|§Attacks]]'').

==Applications==

===Cryptography===
{{Further|Cryptographic hash function#Applications}}
SHA-1 forms part of several widely used security applications and protocols, including [[Transport Layer Security|TLS]] and [[Secure Sockets Layer|SSL]], [[Pretty Good Privacy|PGP]], [[Secure Shell|SSH]], [[S/MIME]], and [[IPsec]]. Those applications can also use [[MD5]]; both MD5 and SHA-1 are descended from [[MD4]].

SHA-1 and SHA-2 are the 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 from most government uses; the U.S. National Institute of Standards and Technology said, "Federal agencies should stop using SHA-1 for...applications that require collision resistance as soon as practical, and must use the [[SHA-2]] family of hash functions for these applications after 2010",<ref name="Computer Security Division">{{Cite web |last=Computer Security Division |first=Information Technology Laboratory |date=2017-01-04 |title=NIST Policy on Hash Functions – Hash Functions |url=https://csrc.nist.gov/Projects/Hash-Functions/NIST-Policy-on-Hash-Functions |access-date=2023-08-27 |website=CSRC, NIST |language=EN-US}}</ref> though that was later relaxed to allow SHA-1 to be used for verifying old digital signatures and time stamps.<ref name="Computer Security Division"/>

A prime motivation for the publication of the [[Secure Hash Algorithm]] was the [[Digital Signature Algorithm|Digital Signature Standard]], in which it is incorporated.

The SHA hash functions have been used for the basis of the [[SHACAL]] [[block cipher]]s.

===Data integrity<span class="anchor" id="Data Integrity"></span>===
[[Revision control]] systems such as [[Git (software)|Git]], [[Mercurial]], and [[Monotone (software)|Monotone]] use SHA-1, not for security, but to identify revisions and to ensure that the data has not changed due to accidental corruption. [[Linus Torvalds]] said about Git in 2007:
:If you have disk corruption, if you have DRAM corruption, if you have any kind of problems at all, Git will notice them. It's not a question of ''if'', it's a guarantee. You can have people who try to be malicious. They won't succeed. [...] Nobody has been able to break SHA-1, but the point is the SHA-1, as far as Git is concerned, isn't even a security feature. It's purely a consistency check. The security parts are elsewhere, so a lot of people assume that since Git uses SHA-1 and SHA-1 is used for cryptographically secure stuff, they think that, Okay, it's a huge security feature. It has nothing at all to do with security, it's just the best hash you can get. ...
:I guarantee you, if you put your data in Git, you can trust the fact that five years later, after it was converted from your hard disk to DVD to whatever new technology and you copied it along, five years later you can verify that the data you get back out is the exact same data you put in. [...]
:One of the reasons I care is for the kernel, we had a break in on one of the [[BitKeeper]] sites where people tried to corrupt the kernel source code repositories.<ref>{{cite web | url = https://www.youtube.com/watch?v=4XpnKHJAok8&t=56m20s | title = Tech Talk: Linus Torvalds on git | website = [[YouTube]] |access-date=November 13, 2013}}</ref> 
However Git does not require the [[second preimage resistance]] of SHA-1 as a security feature, since it will always prefer to keep the earliest version of an object in case of collision, preventing an attacker from surreptitiously overwriting files.<ref>{{cite web |last1=Torvalds |first1=Linus |title=Re: Starting to think about sha-256? |url=https://marc.info/?l=git&m=115678778717621&w=2 |website=marc.info |access-date=30 May 2016}}</ref> The known attacks (as of 2020) also do not break second preimage resistance.<ref>{{cite web |date=2020 |last1=Walfield |first1=Neal H. |title=openpgp: Pass the hash algo's security reqs to Policy::signature  |url=https://gitlab.com/sequoia-pgp/sequoia/-/commit/35119b755db270ab43a8e1ec13577bc0f9846546 |website=gitlab.com/sequoia-pgp |language=en}} &ndash; see section "Background" in the [https://docs.rs/sequoia-openpgp/1.18.0/sequoia_openpgp/policy/enum.HashAlgoSecurity.html rendered documentation]</ref>

==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 approximately 2<sup>''L''</sup> evaluations. This is called a [[preimage attack]] and may or may not be practical depending on ''L'' and the particular computing environment. However, a ''collision'', consisting of finding two different messages that produce the same message digest, requires on average only about {{nowrap|1.2 × 2<sup>''L''/2</sup>}} evaluations using a [[birthday attack]]. Thus the [[Security level|strength]] of a hash function is usually compared to a symmetric cipher of half the message digest length. SHA-1, which has a 160-bit message digest, was originally thought to have 80-bit strength.

Some of the applications that use cryptographic hashes, like 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 (e.g. to obtain a password to try against a user's account elsewhere) is not made possible by the attacks. However, even a secure password hash can't prevent brute-force attacks on [[password strength|weak passwords]]. ''See'' [[Password cracking]].

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 [[Transport Layer Security|SSL]] certificates using an [[MD5]] collision.<ref>{{cite web|first1=Alexander |last1=Sotirov|first2=Marc |last2=Stevens|first3=Jacob |last3=Appelbaum|first4=Arjen |last4=Lenstra|first5=David |last5=Molnar|first6=Dag Arne |last6=Osvik|first7=Benne |last7=de Weger|url=http://www.win.tue.nl/hashclash/rogue-ca/|title=MD5 considered harmful today: Creating a rogue CA certificate|access-date=March 29, 2009|date=December 30, 2008}}</ref>

Due to the block and iterative structure of the algorithms and the absence of additional final steps, all SHA functions (except SHA-3)<ref>{{Cite web|url = http://keccak.noekeon.org/|title = Strengths of Keccak – Design and security|access-date = 20 September 2015|website = The Keccak sponge function family|publisher = Keccak team|quote = Unlike SHA-1 and SHA-2, Keccak does not have the length-extension weakness, hence does not need the HMAC nested construction. Instead, MAC computation can be performed by simply prepending the message with the key.}}</ref> are vulnerable to [[Length extension attack|length-extension]] and partial-message collision attacks.<ref>{{Cite web |title=Schneier on Security: Cryptography Engineering |url=https://www.schneier.com/books/cryptography-engineering |access-date=2023-08-27 |website=www.schneier.com}}</ref> These attacks allow an attacker to forge a message signed only by a keyed hash –  {{nowrap|SHA(''key'' {{!!}} ''message'')}}, but not {{nowrap|SHA(''message'' {{!!}} ''key'')}} – by extending the message and recalculating the hash without knowing the key. A simple improvement to prevent these attacks is to hash twice: {{nowrap|1=SHA<sub>d</sub>(''message'') = SHA(SHA(0<sup>''b''</sup> {{!!}} ''message''))}} (the length of 0<sup>''b''</sup>, zero block, is equal to the block size of the hash function).

===SHA-0===

At [[CRYPTO (conference)|CRYPTO]] 98, two French researchers, [[Florent Chabaud]] and [[Antoine Joux]], presented an attack on SHA-0: [[hash collision|collisions]] can be found with complexity 2<sup>61</sup>, fewer than the 2<sup>80</sup> for an ideal hash function of the same size.<ref name="sha0-chabaud">{{Cite book|chapter-url=https://link.springer.com/chapter/10.1007/BFb0055720|chapter=Differential collisions in SHA-0|first1=Florent|last1=Chabaud|first2=Antoine|last2=Joux|title=Advances in Cryptology – CRYPTO '98 |series=Lecture Notes in Computer Science |editor-first=Hugo|editor-last=Krawczyk|date=October 3, 1998|volume=1462 |publisher=Springer|pages=56–71|via=Springer Link|doi=10.1007/BFb0055720|isbn=978-3-540-64892-5 }}</ref>

In 2004, [[Eli Biham|Biham]] and Chen found near-collisions for SHA-0 – two messages that hash to nearly the same value; in this case, 142 out of the 160 bits are equal. They also found full collisions of SHA-0 reduced to 62 out of its 80 rounds.<ref>{{Cite web|url=https://www.iacr.org/archive/crypto2004/31520290/biham-chen-sha0-proc-real-one.pdf|title=Near-Collisions of SHA-0|last1=Biham|first1=Eli|last2=Chen|first2=Rafi}}</ref>

Subsequently, on 12 August 2004, a collision for the full SHA-0 algorithm was announced by Joux, Carribault, Lemuet, and Jalby. This was done by using a generalization of the Chabaud and Joux attack. Finding the collision had complexity 2<sup>51</sup> and took about 80,000 processor-hours on a [[supercomputer]] with 256 [[Itanium 2]] processors (equivalent to 13 days of full-time use of the computer).

On 17 August 2004, at the Rump Session of CRYPTO 2004, preliminary results were announced by [[Xiaoyun Wang|Wang]], Feng, Lai, and Yu, about an attack on [[MD5]], SHA-0 and other hash functions. The complexity of their attack on SHA-0 is 2<sup>40</sup>, significantly better than the attack by Joux ''et al.''<ref>{{cite web|url=http://www.freedom-to-tinker.com/archives/000664.html|title=Report from Crypto 2004|url-status=dead|archive-url=https://web.archive.org/web/20040821031401/http://www.freedom-to-tinker.com/archives/000664.html|archive-date=2004-08-21|access-date=2004-08-23}}</ref><ref>{{Cite newsgroup|first=Francois |last=Grieu|newsgroup=sci.crypt|title=Re: Any advance news from the crypto rump session?|date=18 August 2004 |time=05:06:02 +0200|message-id=fgrieu-05A994.05060218082004@individual.net}}</ref>

In February 2005, an attack by [[Xiaoyun Wang]], [[Yiqun Lisa Yin]], and Hongbo Yu was announced which could find collisions in SHA-0 in 2<sup>39</sup> operations.<ref name="autogenerated1">{{Cite web |title=SHA-1 Broken – Schneier on Security |url=https://www.schneier.com/blog/archives/2005/02/sha1_broken.html |website=www.schneier.com}}</ref><ref>[http://www.infosec.sdu.edu.cn/paper/sha0-crypto-author-new.pdf Efficient Collision Search Attacks on SHA-0] {{webarchive|url=https://web.archive.org/web/20050910132832/http://www.infosec.sdu.edu.cn/paper/sha0-crypto-author-new.pdf |date=2005-09-10 }}, [[Shandong University]]</ref>

Another attack in 2008 applying the [[boomerang attack]] brought the complexity of finding collisions down to 2<sup>33.6</sup>, which was estimated to take 1 hour on an average PC from the year 2008.<ref>{{Cite conference|first1=Stéphane |volume=5086|pages=16–35|last1=Manuel|first2=Thomas |last2=Peyrin |date=2008-02-11 |url= https://link.springer.com/content/pdf/10.1007%2F978-3-540-71039-4_2.pdf |title=Collisions on SHA-0 in One Hour |conference=Fast Software Encryption 2008 |doi=10.1007/978-3-540-71039-4_2|series=Lecture Notes in Computer Science|isbn=978-3-540-71038-7|doi-access=free }}</ref>

In light of the results for SHA-0, some experts{{who|date=September 2015}} suggested that plans for the use of SHA-1 in new [[cryptosystem]]s should be reconsidered. After the CRYPTO 2004 results were published, NIST announced that they planned to phase out the use of SHA-1 by 2010 in favor of the SHA-2 variants.<ref>{{Cite web |url=https://csrc.nist.gov/News/2004/NIST-Brief-Comments-on-Recent-Cryptanalytic-Attack |title= NIST Brief Comments on Recent Cryptanalytic Attacks on Secure Hashing Functions and the Continued Security Provided by SHA-1 |date= 23 August 2017 |access-date=2022-03-16 }}</ref>

===Attacks===
In early 2005, [[Vincent Rijmen]] and [[Elisabeth Oswald]] published an attack on a reduced version of SHA-1 – 53 out of 80 rounds – which finds collisions with a computational effort of fewer than 2<sup>80</sup> operations.<ref>{{cite journal |url=http://eprint.iacr.org/2005/010|title=Update on SHA-1 |journal=Cryptology ePrint Archive |year=2005|last1=Rijmen|first1=Vincent|last2=Oswald|first2=Elisabeth}}</ref>

In February 2005, an attack by [[Xiaoyun Wang]], Yiqun Lisa Yin, and Hongbo Yu was announced.<ref name="autogenerated1" /> The attacks can find collisions in the full version of SHA-1, requiring fewer than 2<sup>69</sup> operations. (A [[brute-force search]] would require 2<sup>80</sup> operations.)

The authors write: "In particular, our analysis is built upon the original differential attack on SHA-0, the near collision attack on SHA-0, the multiblock collision techniques, as well as the message modification techniques used in the collision search attack on MD5. Breaking SHA-1 would not be possible without these powerful analytical techniques."<ref>[http://theory.csail.mit.edu/~yiqun/shanote.pdf Collision Search Attacks on SHA1] {{Webarchive|url=https://web.archive.org/web/20050219180957/http://theory.csail.mit.edu/~yiqun/shanote.pdf |date=2005-02-19 }}, [[Massachusetts Institute of Technology]]</ref> The authors have presented a collision for 58-round SHA-1, found with 2<sup>33</sup> hash operations. The paper with the full attack description was published in August 2005 at the CRYPTO conference.

In an interview, Yin states that, "Roughly, we exploit the following two weaknesses: One is that the file preprocessing step is not complicated enough; another is that certain math operations in the first 20 rounds have unexpected security problems."<ref>{{cite web|url=https://www.zdnet.com/article/fixing-a-hole-in-security/|title=Fixing a hole in security|first=Robert |last=Lemos|work=ZDNet}}</ref>

On 17 August 2005, an improvement on the SHA-1 attack was announced on behalf of [[Xiaoyun Wang]], [[Andrew Yao]] and [[Frances Yao]] at the CRYPTO 2005 Rump Session, lowering the complexity required for finding a collision in SHA-1 to 2<sup>63</sup>.<ref name=":3">{{Cite web |title=New Cryptanalytic Results Against SHA-1 – Schneier on Security |url=https://www.schneier.com/blog/archives/2005/08/new_cryptanalyt.html |website=www.schneier.com}}</ref> On 18 December 2007 the details of this result were explained and verified by Martin Cochran.<ref>{{cite journal| url = http://eprint.iacr.org/2007/474| title = Notes on the Wang et al. 2<sup>63</sup> SHA-1 Differential Path| year = 2007| last1 = Cochran| first1 = Martin| journal = Cryptology ePrint Archive}}</ref>

Christophe De Cannière and Christian Rechberger further improved the attack on SHA-1 in "Finding SHA-1 Characteristics: General Results and Applications,"<ref>{{Cite book |first1=Christophe |title=Advances in Cryptology – ASIACRYPT 2006 |volume=4284 |pages=1–20 |last1=De Cannière |first2=Christian |last2=Rechberger |date=2006-11-15 |chapter=Finding SHA-1 Characteristics: General Results and Applications |doi=10.1007/11935230_1 |series=Lecture Notes in Computer Science |isbn=978-3-540-49475-1 }}</ref> receiving the Best Paper Award at [[ASIACRYPT]] 2006. A two-block collision for 64-round SHA-1 was presented, found using unoptimized methods with 2<sup>35</sup> compression function evaluations. Since this attack requires the equivalent of about 2<sup>35</sup> evaluations, it is considered to be a significant theoretical break.<ref>{{cite web
 | url = http://www.iaik.tugraz.at/content/research/krypto/sha1/SHA1Collision_Description.php
 | title = IAIK Krypto Group&nbsp;— Description of SHA-1 Collision Search Project
 | access-date = 2009-06-30
 | archive-url = https://web.archive.org/web/20130115071715/http://www.iaik.tugraz.at/content/research/krypto/sha1/SHA1Collision_Description.php
 | archive-date = 2013-01-15
 | url-status = dead
 }}</ref> Their attack was extended further to 73 rounds (of 80) in 2010 by Grechnikov.<ref>{{cite web
 | url = http://eprint.iacr.org/2010/413
 | title = Collisions for 72-step and 73-step SHA-1: Improvements in the Method of Characteristics
 | access-date = 2010-07-24
}}</ref> In order to find an actual collision in the full 80 rounds of the hash function, however, tremendous amounts of computer time are required. To that end, a collision search for SHA-1 using the volunteer computing platform [[BOINC]] began August 8, 2007, organized by the [[Graz University of Technology]]. The effort was abandoned May 12, 2009 due to lack of progress.<ref>{{cite web
 |url         = http://boinc.iaik.tugraz.at/sha1_coll_search/
 |title       = SHA-1 Collision Search Graz
 |access-date  = 2009-06-30
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20090225115007/http://boinc.iaik.tugraz.at/sha1_coll_search/
 |archive-date = 2009-02-25
}}</ref>

At the Rump Session of CRYPTO 2006, Christian Rechberger and Christophe De Cannière claimed to have discovered a collision attack on SHA-1 that would allow an attacker to select at least parts of the message.<ref>{{cite web|url=http://www.heise-online.co.uk/security/SHA-1-hash-function-under-pressure--/news/77244|title=heise online – IT-News, Nachrichten und Hintergründe|work=heise online|date=27 August 2023 }}</ref><ref>{{Cite web|url=https://www.iacr.org/conferences/crypto2006/rumpsched.html|title=Crypto 2006 Rump Schedule|website=www.iacr.org}}</ref>

In 2008, an attack methodology by Stéphane Manuel reported hash collisions with an estimated theoretical complexity of 2<sup>51</sup> to 2<sup>57</sup> operations.<ref>{{Cite web | first=Stéphane |last=Manuel | title=Classification and Generation of Disturbance Vectors for Collision Attacks against SHA-1 |website=Cryptology ePrint Archive |url=http://eprint.iacr.org/2008/469.pdf | access-date = 2011-05-19 }}</ref> However he later retracted that claim after finding that local collision paths were not actually independent, and finally quoting for the most efficient a collision vector that was already known before this work.<ref>{{Cite journal | first=Stéphane | last=Manuel | title=Classification and Generation of Disturbance Vectors for Collision Attacks against SHA-1 | journal=Designs, Codes and Cryptography | volume=59 | issue=1–3 | pages=247–263 | doi=10.1007/s10623-010-9458-9 | year=2011 | s2cid=47179704 }} ''the most efficient disturbance vector is Codeword2 first reported by Jutla and Patthak''</ref>

Cameron McDonald, Philip Hawkes and Josef Pieprzyk presented a hash collision attack with claimed complexity 2<sup>52</sup> at the Rump Session of Eurocrypt 2009.<ref>{{cite web| url = http://eurocrypt2009rump.cr.yp.to/837a0a8086fa6ca714249409ddfae43d.pdf| title = SHA-1 collisions now 2^52 }}</ref> However, the accompanying paper, "Differential Path for SHA-1 with complexity [[Big O notation|''O'']](2<sup>52</sup>)" has been withdrawn due to the authors' discovery that their estimate was incorrect.<ref>{{cite journal |url=http://eprint.iacr.org/2009/259|title=Differential Path for SHA-1 with complexity O(<sup>252</sup>) |journal=Cryptology ePrint Archive |year=2009|last1=McDonald|first1=Cameron|last2=Hawkes|first2=Philip|last3=Pieprzyk|first3=Josef}} (withdrawn)</ref>

One attack against SHA-1 was Marc Stevens<ref name="Cryptanalysis of MD5 & SHA-1">{{cite web| url = http://2012.sharcs.org/slides/stevens.pdf| title = Cryptanalysis of MD5 & SHA-1}}</ref> with an estimated cost of $2.77M (2012) to break a single hash value by renting CPU power from cloud servers.<ref>{{Cite web |title=When Will We See Collisions for SHA-1? – Schneier on Security |url=https://www.schneier.com/blog/archives/2012/10/when_will_we_se.html |website=www.schneier.com}}</ref> Stevens developed this attack in a project called HashClash,<ref>{{Cite web |title=Google Code Archive – Long-term storage for Google Code Project Hosting. |url=https://code.google.com/archive/p/hashclash |website=code.google.com}}</ref> implementing a differential path attack. On 8 November 2010, he claimed he had a fully working near-collision attack against full SHA-1 working with an estimated complexity equivalent to 2<sup>57.5</sup> SHA-1 compressions. He estimated this attack could be extended to a full collision with a complexity around 2<sup>61</sup>.

====The SHAppening====
On 8 October 2015, Marc Stevens, Pierre Karpman, and Thomas Peyrin published a freestart collision attack on SHA-1's compression function that requires only 2<sup>57</sup> SHA-1 evaluations. This does not directly translate into a collision on the full SHA-1 hash function (where an attacker is ''not'' able to freely choose the initial internal state), but undermines the security claims for SHA-1. In particular, it was the first time that an attack on full SHA-1 had been ''demonstrated''; all earlier attacks were too expensive for their authors to carry them out. The authors named this significant breakthrough in the [[cryptanalysis]] of SHA-1 ''The SHAppening''.<ref name="shappening" />

The method was based on their earlier work, as well as the auxiliary paths (or boomerangs) speed-up technique from Joux and Peyrin, and using high performance/cost efficient GPU cards from [[Nvidia]]. The collision was found on a 16-node cluster with a total of 64 graphics cards. The authors estimated that a similar collision could be found by buying US$2,000 of GPU time on [[Amazon Elastic Compute Cloud|EC2]].<ref name="shappening" />

The authors estimated that the cost of renting enough of EC2 CPU/GPU time to generate a full collision for SHA-1 at the time of publication was between US$75K and $120K, and noted that was well within the budget of criminal organizations, not to mention national [[intelligence agency|intelligence agencies]]. As such, the authors recommended that SHA-1 be deprecated as quickly as possible.<ref name="shappening">{{cite web |url=https://sites.google.com/site/itstheshappening/ |title=The SHAppening: freestart collisions for SHA-1 |access-date=2015-10-09 |first1=Marc |last1=Stevens |first2=Pierre |last2=Karpman |first3=Thomas |last3=Peyrin}}</ref>

==== SHAttered – first public collision ====
On 23 February 2017, the [[Centrum Wiskunde & Informatica|CWI (Centrum Wiskunde & Informatica)]] and Google announced the ''SHAttered'' attack, in which they generated two different PDF files with the same SHA-1 hash in roughly 2<sup>63.1</sup> SHA-1 evaluations. This attack is about 100,000 times faster than brute forcing a SHA-1 collision with a [[birthday attack]], which was estimated to take 2<sup>80</sup> SHA-1 evaluations. The attack required "the equivalent processing power of 6,500 years of single-CPU computations and 110 years of single-GPU computations".<ref name="sha1-shattered"/>

==== Birthday-Near-Collision Attack – first practical chosen-prefix attack ====
On 24 April 2019 a paper by Gaëtan Leurent and Thomas Peyrin presented at Eurocrypt 2019 described an enhancement to the previously best [[Collision attack#Chosen-prefix collision attack|chosen-prefix attack]] in [[Merkle–Damgård construction|Merkle–Damgård]]–like digest functions based on [[One-way compression function#Davies–Meyer|Davies–Meyer]] block ciphers. With these improvements, this method is capable of finding chosen-prefix collisions in approximately 2<sup>68</sup> SHA-1 evaluations. This is approximately 1 billion times faster (and now usable for many targeted attacks, thanks to the possibility of choosing a prefix, for example malicious code or faked identities in signed certificates) than the previous attack's 2<sup>77.1</sup> evaluations (but without chosen prefix, which was impractical for most targeted attacks because the found collisions were almost random)<ref name="stevens-attacks"/> and is fast enough to be practical for resourceful attackers, requiring approximately $100,000 of cloud processing. This method is also capable of finding chosen-prefix collisions in the [[MD5]] function, but at a complexity of 2<sup>46.3</sup> does not surpass the prior best available method at a theoretical level (2<sup>39</sup>), though potentially at a practical level (≤2<sup>49</sup>).<ref name="leurent-peyrin-sha1">{{cite conference |chapter-url=https://hal.inria.fr/hal-02424900/file/SHA1_EC19.pdf |last1=Leurent |first1=Gaëtan |last2=Peyrin |first2=Thomas |title=Advances in Cryptology – EUROCRYPT 2019 |chapter=From Collisions to Chosen-Prefix Collisions Application to Full SHA-1 | conference= 38th Annual International Conference on the Theory and Applications of Cryptographic Techniques, Darmstadt, Germany, May 19–23, 2019 |date=2019 |url=https://eprint.iacr.org/2019/459.pdf | publisher= Springer |series=Lecture Notes in Computer Science |volume=11478 |pages=527–555 |doi=10.1007/978-3-030-17659-4_18 |isbn=978-3-030-17658-7 |s2cid=153311244 | editor1= Yuval Ishai| editor2= Vincent Rijmen}}</ref> This attack has a memory requirement of 500+ GB.

On 5 January 2020 the authors published an improved attack called "shambles".<ref name="leurent-peyrin-sha1-shambles"/> In this paper they demonstrate a chosen-prefix collision attack with a complexity of 2<sup>63.4</sup>, that at the time of publication would cost US$45K per generated collision.

===Official validation===
{{Main|Cryptographic Module Validation Program}}
Implementations of all FIPS-approved security functions can be officially validated through the [[Cryptographic Module Validation Program|CMVP program]], jointly run by the [[National Institute of Standards and Technology]] (NIST) and the [[Communications Security Establishment]] (CSE). For informal verification, a package to generate a high number of test vectors is made available for download on the NIST site; the resulting verification, however, does not replace the formal CMVP validation, which is required by law for certain applications.

{{As of|2013|alt=As of December 2013}}, there are over 2000 validated implementations of SHA-1, with 14 of them capable of handling messages with a length in bits not a multiple of eight (see [http://csrc.nist.gov/groups/STM/cavp/documents/shs/shaval.htm SHS Validation List] {{Webarchive|url=https://web.archive.org/web/20110823092514/http://csrc.nist.gov/groups/STM/cavp/documents/shs/shaval.htm |date=2011-08-23 }}).

==Examples and pseudocode==

===Example hashes===
These are examples of SHA-1 [[message digest]]s in hexadecimal and in [[Base64]] binary to [[ASCII]] text encoding.

* <code>SHA1("The quick brown fox jumps over the lazy {{Background color|#87CEEB|d}}og")</code>
** Outputted hexadecimal: <code>2fd4e1c67a2d28fced849ee1bb76e7391b93eb12</code>
** Outputted [[Base64]] binary to [[ASCII]] text encoding: <code>L9ThxnotKPzthJ7hu3bnORuT6xI=</code>

Even a small change in the message will, with overwhelming probability, result in many bits changing due to the [[avalanche effect]]. For example, changing <code>dog</code> to <code>cog</code> produces a hash with different values for 81 of the 160 bits:

* <code>SHA1("The quick brown fox jumps over the lazy {{Background color|#87CEEB|c}}og")</code>
** Outputted hexadecimal: <code>de9f2c7fd25e1b3afad3e85a0bd17d9b100db4b3</code>
** Outputted [[Base64]] binary to [[ASCII]] text encoding: <code>3p8sf9JeGzr60+haC9F9mxANtLM=</code>

The hash of the zero-length string is:

* <code>SHA1("")</code>
** Outputted hexadecimal: <code>da39a3ee5e6b4b0d3255bfef95601890afd80709</code>
** Outputted [[Base64]] binary to [[ASCII]] text encoding: <code>2jmj7l5rSw0yVb/vlWAYkK/YBwk=</code>

===SHA-1 pseudocode===
[[Pseudocode]] for the SHA-1 algorithm follows:
 
 <span style="color: green;">''Note 1: All variables are unsigned 32-bit quantities and wrap modulo 2<sup>32</sup> when calculating, except for''</span>
         <span style="color: green;">''ml, the message length, which is a 64-bit quantity, and''</span>
         <span style="color: green;">''hh, the message digest, which is a 160-bit quantity.''</span>
 <span style="color: green;">''Note 2: All constants in this pseudo code are in [[endianness|big endian]].''</span>
         <span style="color: green;">''Within each word, the most significant byte is stored in the leftmost byte position''</span>
 
 <span style="color: green;">''Initialize variables:''</span>
 
 h0 = 0x67452301
 h1 = 0xEFCDAB89
 h2 = 0x98BADCFE
 h3 = 0x10325476
 h4 = 0xC3D2E1F0
 
 ml = message length in bits (always a multiple of the number of bits in a character).
 
 <span style="color: green;">''Pre-processing:''</span>
 append the bit '1' to the message e.g. by adding 0x80 if message length is a multiple of 8 bits.
 append 0 ≤ k &lt; 512 bits '0', such that the resulting message length in ''bits''
    is [[modular arithmetic|congruent]] to −64 ≡ 448 (mod 512)
 append ml, the original message length in bits, as a 64-bit [[Endianness|big-endian]] integer. 
    Thus, the total length is a multiple of 512 bits.
 
 <span style="color: green;">''Process the message in successive 512-bit chunks:''</span>
 break message into 512-bit chunks
 '''for''' each chunk
     break chunk into sixteen 32-bit big-endian words w[i], 0 ≤ i ≤ 15
 
     <span style="color: green;">''Message schedule: extend the sixteen 32-bit words into eighty 32-bit words:''</span>
     '''for''' i '''from''' 16 to 79
         <span style="color: green;">''Note 3: SHA-0 differs by not having this leftrotate.''</span>
         w[i] = (w[i-3] '''xor''' w[i-8] '''xor''' w[i-14] '''xor''' w[i-16]) '''[[Circular shift|leftrotate]]''' 1
 
     <span style="color: green;">''Initialize hash value for this chunk:''</span>
     a = h0
     b = h1
     c = h2
     d = h3
     e = h4
 
     <span style="color: green;">''Main loop:''</span><ref name=":0" /><ref>{{Cite web|url=http://www.faqs.org/rfcs/rfc3174.html|title=RFC 3174 - US Secure Hash Algorithm 1 (SHA1) (RFC3174)|website=www.faqs.org}}</ref>
     '''for''' i '''from''' 0 '''to''' 79
         '''if''' 0 ≤ i ≤ 19 '''then'''
             f = (b '''and''' c) '''or''' (('''not''' b) '''and''' d)
             k = 0x5A827999
         '''else if''' 20 ≤ i ≤ 39
             f = b '''xor''' c '''xor''' d
             k = 0x6ED9EBA1
         '''else if''' 40 ≤ i ≤ 59
             f = (b '''and''' c) '''or''' (b '''and''' d) '''or''' (c '''and''' d) 
             k = 0x8F1BBCDC
         '''else if''' 60 ≤ i ≤ 79
             f = b '''xor''' c '''xor''' d
             k = 0xCA62C1D6
 
         temp = (a '''leftrotate''' 5) + f + e + k + w[i]
         e = d
         d = c
         c = b '''leftrotate''' 30
         b = a
         a = temp
 
     <span style="color: green;">''Add this chunk's hash to result so far:''</span>
     h0 = h0 + a
     h1 = h1 + b 
     h2 = h2 + c
     h3 = h3 + d
     h4 = h4 + e
 
 <span style="color:green;">''Produce the final hash value (big-endian) as a 160-bit number:''</span>
 hh = (h0 '''leftshift''' 128) '''or''' (h1 '''leftshift''' 96) '''or''' (h2 '''leftshift''' 64) '''or''' (h3 '''leftshift''' 32) '''or''' h4

The number <code>hh</code> is the message digest, which can be written in hexadecimal (base 16).
 
The chosen constant values used in the algorithm were assumed to be [[nothing up my sleeve number]]s:
* The four round constants <code>k</code> are 2<sup>30</sup> times the square roots of 2, 3, 5 and 10. However they were incorrectly rounded to the nearest integer instead of being rounded to the nearest odd integer, with equilibrated proportions of zero and one bits. As well, choosing the square root of 10 (which is not a prime) made it a common factor for the two other chosen square roots of primes 2 and 5, with possibly usable arithmetic properties across successive rounds, reducing the strength of the algorithm against finding collisions on some bits.
* The first four starting values for <code>h0</code> through <code>h3</code> are the same with the MD5 algorithm, and the fifth (for <code>h4</code>) is similar. However they were not properly verified for being resistant against inversion of the few first rounds to infer possible collisions on some bits, usable by multiblock differential attacks.

Instead of the formulation from the original FIPS PUB 180-1 shown, the following equivalent expressions may be used to compute <code>f</code> in the main loop above:

 <span style="color: green;">''Bitwise choice between ''c'' and ''d'', controlled by ''b''.''</span>
 (0  ≤ i ≤ 19): f = d '''xor''' (b '''and''' (c '''xor''' d))                <span style="color: green;">''(alternative 1)''</span>
 (0  ≤ i ≤ 19): f = (b '''and''' c) '''or''' (('''not''' b) '''and''' d)           <span style="color: green;">''(alternative 2)''</span>
 (0  ≤ i ≤ 19): f = (b '''and''' c) '''xor''' (('''not''' b) '''and''' d)          <span style="color: green;">''(alternative 3)''</span>
 (0  ≤ i ≤ 19): f = vec_sel(d, c, b)                       <span style="color: green;">''(alternative 4)''</span>
 &nbsp;[premo08]
 <span style="color: green;">''Bitwise majority function.''</span>
 (40 ≤ i ≤ 59): f = (b '''and''' c) '''or''' (d '''and''' (b '''or''' c))          <span style="color: green;">''(alternative 1)''</span>
 (40 ≤ i ≤ 59): f = (b '''and''' c) '''or''' (d '''and''' (b '''xor''' c))         <span style="color: green;">''(alternative 2)''</span>
 (40 ≤ i ≤ 59): f = (b '''and''' c) '''xor''' (d '''and''' (b '''xor''' c))        <span style="color: green;">''(alternative 3)''</span>
 (40 ≤ i ≤ 59): f = (b '''and''' c) '''xor''' (b '''and''' d) '''xor''' (c '''and''' d)  <span style="color: green;">''(alternative 4)''</span>
 (40 ≤ i ≤ 59): f = vec_sel(c, b, c '''xor''' d)                 <span style="color: green;">''(alternative 5)''</span>

It was also shown<ref>{{Citation |first1=Max |last1=Locktyukhin |url=https://www.intel.com/content/www/us/en/developer/articles/technical/improving-the-performance-of-the-secure-hash-algorithm-1.html |title=Improving the Performance of the Secure Hash Algorithm (SHA-1) |journal=Intel Software Knowledge Base |date=2010-03-31 |access-date=2010-04-02}}</ref> that for the rounds 32–79 the computation of:

 w[i] = (w[i-3] '''xor''' w[i-8] '''xor''' w[i-14] '''xor''' w[i-16]) '''[[Circular shift|leftrotate]]''' 1

can be replaced with:

 w[i] = (w[i-6] '''xor''' w[i-16] '''xor''' w[i-28] '''xor''' w[i-32]) '''[[Circular shift|leftrotate]]''' 2

This transformation keeps all operands 64-bit aligned and, by removing the dependency of <code>w[i]</code> on <code>w[i-3]</code>, allows efficient SIMD implementation with a vector length of 4 like [[x86]] [[Streaming SIMD Extensions|SSE]] instructions.

==Comparison of SHA functions==
In the table below, ''internal state'' means the "internal hash sum" after each compression of a data block.
{{Further|Merkle–Damgård construction}}

{{Comparison of SHA functions}}

== Implementations ==
Below is a list of cryptography libraries that support SHA-1:

* [[Botan (programming library)|Botan]]
* [[Bouncy Castle (cryptography)|Bouncy Castle]]
* [[cryptlib]]
* [[Crypto++]]
* [[Libgcrypt]]
* [[Mbed TLS]]
* [[Nettle (cryptographic library)|Nettle]]
* [[LibreSSL]]
* [[OpenSSL]]
* [[GnuTLS]]

Hardware acceleration is provided by the following processor extensions:

* [[Intel SHA extensions]]: Available on some Intel and AMD x86 processors.
* [[VIA PadLock]]
* IBM [[z/Architecture]]:  Available since 2003 as part of the Message-Security-Assist Extension<ref>IBM z/Architecture Principles of Operation, publication number SA22-7832.  See KIMD and KLMD instructions in Chapter 7.</ref>

=== Collision countermeasure ===
In the wake of SHAttered, Marc Stevens and Dan Shumow published "sha1collisiondetection" (SHA-1CD), a variant of SHA-1 that detects collision attacks and changes the hash output when one is detected. The false positive rate is 2<sup>−90</sup>.<ref>{{cite web |last1=Stevens |first1=Marc |title=cr-marcstevens/sha1collisiondetection:  Library and command line tool to detect SHA-1 collision in a file  |url=https://github.com/cr-marcstevens/sha1collisiondetection |date=2017}}</ref> SHA-1CD is used by [[GitHub]] since March 2017 and [[git]] since version 2.13.0 of May 2017.<ref>{{cite web |last1=King |first1=Jeff |title=Git 2.13 has been released |url=https://github.blog/2017-05-10-git-2-13-has-been-released/ |website=The GitHub Blog |date=10 May 2017}}</ref>

== See also ==
* [[Comparison of cryptographic hash functions]]
* [[Hash function security summary]]
* [[International Association for Cryptologic Research]]
* [[Secure Hash Algorithms]]

== Notes ==
{{Reflist|30em}}

== References ==
{{Refbegin}}
* [[Eli Biham]], Rafi Chen, Near-Collisions of SHA-0, Cryptology ePrint Archive, Report 2004/146, 2004 (appeared on CRYPTO 2004), [http://eprint.iacr.org/2004/146/ IACR.org]
* [[Xiaoyun Wang]], Hongbo Yu and Yiqun Lisa Yin, [https://web.archive.org/web/20050910132832/http://www.infosec.sdu.edu.cn/paper/sha0-crypto-author-new.pdf Efficient Collision Search Attacks on SHA-0], Crypto 2005
* [[Xiaoyun Wang]], Yiqun Lisa Yin and Hongbo Yu, [https://www.iacr.org/archive/crypto2005/36210017/36210017.pdf Finding Collisions in the Full SHA-1], Crypto 2005 
* [[Henri Gilbert]], [[Helena Handschuh]]: [https://link.springer.com/content/pdf/10.1007/978-3-540-24654-1_13.pdf Security Analysis of SHA-256 and Sisters]. [[Selected Areas in Cryptography]] 2003: pp175–193
* [http://www.unixwiz.net/techtips/iguide-crypto-hashes.html An Illustrated Guide to Cryptographic Hashes]
* {{cite journal|title=Proposed Revision of Federal Information Processing Standard (FIPS) 180, Secure Hash Standard|journal=Federal Register|date=1994-07-11|volume=59|issue=131|pages=35317–35318|url=http://frwebgate1.access.gpo.gov/cgi-bin/waisgate.cgi?WAISdocID=5963452267+0+0+0&WAISaction=retrieve|access-date=2007-04-26}}{{dead link|date=April 2018 |bot=InternetArchiveBot |fix-attempted=yes }}
* A. Cilardo, L. Esposito, A. Veniero, A. Mazzeo, V. Beltran, E. Ayugadé, [https://web.archive.org/web/20170830045350/http://people.ac.upc.edu/vbeltran/papers/HPCC2010.pdf A CellBE-based HPC application for the analysis of vulnerabilities in cryptographic hash functions], High Performance Computing and Communication international conference, August 2010
* SECURE HASH STANDARD. (1995). https://nvlpubs.nist.gov/nistpubs/Legacy/FIPS/fipspub180-1.pdf
{{Refend}}

==External links==
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* [https://web.archive.org/web/20110625054822/http://csrc.nist.gov/groups/ST/toolkit/secure_hashing.html CSRC Cryptographic Toolkit] – Official [[National Institute of Standards and Technology|NIST]] site for the Secure Hash Standard
* [https://web.archive.org/web/20161126003357/http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.180-4.pdf FIPS 180-4: Secure Hash Standard (SHS)]
* {{IETF RFC|3174|link=no}} (with sample C implementation)
* [https://www.zdnet.com/article/fixing-a-hole-in-security/ Interview with Yiqun Lisa Yin concerning the attack on SHA-1]
* [http://www.heise-online.co.uk/security/Hash-cracked--/features/75686 Explanation of the successful attacks on SHA-1] (3 pages, 2006)
* [http://www.cryptography.com/cnews/hash.html Cryptography Research – Hash Collision Q&amp;A]
* {{YouTube|5q8q4PhN0cw|Lecture on SHA-1 (1h 18m)}} by [http://www.emsec.rub.de/chair/_staff/christof-paar/ Christof Paar] {{Webarchive|url=https://web.archive.org/web/20170424134012/http://www.emsec.rub.de/chair/_staff/christof-paar/ |date=2017-04-24 }}

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