MD5

MD5 is one in a series of message digest algorithms designed by Professor Ronald Rivest of MIT (Rivest, 1992). When analytic work indicated that MD5's predecessor MD4 was likely to be insecure, Rivest designed MD5 in 1991 as a secure replacement. (Hans Dobbertin did indeed later find weaknesses in MD4.)

In 1993, Den Boer and Bosselaers gave an early, although limited, result of finding a "pseudo-collision" of the MD5 compression function; that is, two different initialization vectors that produce an identical digest.

In 1996, Dobbertin announced a collision of the compression function of MD5 (Dobbertin, 1996). While this was not an attack on the full MD5 hash function, it was close enough for cryptographers to recommend switching to a replacement, such as SHA-1 (also compromised since) or RIPEMD-160.

The size of the hash value (128 bits) is small enough to contemplate a birthday attack. MD5CRK was a distributed project started in March 2004 to demonstrate that MD5 is practically insecure by finding a collision using a birthday attack.

MD5CRK ended shortly after 17 August 2004, when collisions for the full MD5 were announced by Xiaoyun Wang, Dengguo Feng, Xuejia Lai, and Hongbo Yu.{{cite journal |last1=Hawkes |first1=Philip |last2=Paddon |first2=Michael |last3=Rose |first3=Gregory G. |author-link3=Gregory G. Rose |title=Musings on the Wang et al. MD5 Collision |journal=Cryptology ePrint Archive |date=13 Oct 2004 |url=https://eprint.iacr.org/2004/264 |access-date=10 October 2018 |archive-url=https://web.archive.org/web/20181105220829/https://eprint.iacr.org/2004/264 |archive-date=5 November 2018 }} Their analytical attack was reported to take only one hour on an IBM p690 cluster.{{cite web|url=http://www.bishopfox.com/resources/tools/other-free-tools/md4md5-collision-code/|title=Fast MD5 and MD4 Collision Generators |website=BishopFox |date=26 September 2013|author=Bishop Fox|access-date=10 February 2014|archive-date=26 April 2017|archive-url=https://web.archive.org/web/20170426171733/http://www.bishopfox.com/resources/tools/other-free-tools/md4md5-collision-code/}}

On 1 March 2005, Arjen Lenstra, Xiaoyun Wang, and Benne de Weger demonstrated construction of two X.509 certificates with different public keys and the same MD5 hash value, a demonstrably practical collision.{{cite journal |last1=Lenstra |first1=Arjen |last2=Wang |first2=Xiaoyun |last3=Weger |first3=Benne de |author-link1=Arjen Lenstra |author-link2=Xiaoyun Wang |title=Colliding X.509 Certificates |journal=Cryptology ePrint Archive |date=1 Mar 2005 |url=http://eprint.iacr.org/2005/067 |access-date=10 October 2018 |archive-date=23 May 2017 |archive-url=https://web.archive.org/web/20170523000238/http://eprint.iacr.org/2005/067 |url-status=live }} The construction included private keys for both public keys. A few days later, Vlastimil Klima described an improved algorithm, able to construct MD5 collisions in a few hours on a single notebook computer.{{cite journal |last1=Klíma |first1=Vlastimil |author-link1=Vlastimil Klíma |title=Finding MD5 Collisions – a Toy For a Notebook |journal=Cryptology ePrint Archive |date=5 Mar 2005 |url=http://eprint.iacr.org/2005/075 |access-date=10 October 2018 |archive-date=17 May 2017 |archive-url=https://web.archive.org/web/20170517175404/http://eprint.iacr.org/2005/075 |url-status=live }} On 18 March 2006, Klima published an algorithm that could find a collision within one minute on a single notebook computer, using a method he calls tunneling.Vlastimil Klima: [http://eprint.iacr.org/2006/105 Tunnels in Hash Functions: MD5 Collisions Within a Minute] {{Webarchive|url=https://web.archive.org/web/20110806052832/http://eprint.iacr.org/2006/105 |date=6 August 2011 }}, Cryptology ePrint Archive Report 2006/105, 18 March 2006, revised 17 April 2006. Retrieved 27 July 2008.

Various MD5-related RFC errata have been published.

In 2009, the United States Cyber Command used an MD5 hash value of their mission statement as a part of their official emblem.{{cite magazine

|url= https://www.wired.com/dangerroom/2010/07/code-cracked-cyber-command-logos-mystery-solved/

|title= Code Cracked! Cyber Command Logo Mystery Solved

|magazine= USCYBERCOM

|publisher= Wired News

|date= 8 July 2010

|access-date= 29 July 2011

|archive-date= 17 February 2014

|archive-url= https://web.archive.org/web/20140217212938/http://www.wired.com/dangerroom/2010/07/code-cracked-cyber-command-logos-mystery-solved/

|url-status= live

}}

On 24 December 2010, Tao Xie and Dengguo Feng announced the first published single-block (512-bit) MD5 collision.{{cite web

|url=http://eprint.iacr.org/2010/643

|title=Construct MD5 Collisions Using Just A Single Block Of Message

|year=2010

|format=PDF

|author1=Tao Xie

|author2=Dengguo Feng

|access-date=28 July 2011

|archive-date=14 May 2017

|archive-url=https://web.archive.org/web/20170514185806/http://eprint.iacr.org/2010/643

|url-status=live

}} (Previous collision discoveries had relied on multi-block attacks.) For "security reasons", Xie and Feng did not disclose the new attack method. They issued a challenge to the cryptographic community, offering a US$10,000 reward to the first finder of a different 64-byte collision before 1 January 2013. Marc Stevens responded to the challenge and published colliding single-block messages as well as the construction algorithm and sources.{{cite web |url= http://marc-stevens.nl/research/md5-1block-collision/ |title= Marc Stevens – Research – Single-block collision attack on MD5 |publisher= Marc-stevens.nl |date= 2012 |access-date= 10 April 2014 |archive-date= 15 May 2017 |archive-url= https://web.archive.org/web/20170515070731/http://marc-stevens.nl/research/md5-1block-collision/ |url-status= live }}

In 2011 an informational RFC 6151{{cite journal|url=https://tools.ietf.org/html/rfc6151|title=RFC 6151 – Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms|website=Internet Engineering Task Force|date=March 2011|access-date=11 November 2013|last1=Turner|first1=Sean|doi=10.17487/RFC6151|archive-date=15 June 2017|archive-url=https://web.archive.org/web/20170615213134/https://tools.ietf.org/html///rfc6151|url-status=live}} was approved to update the security considerations in MD5{{cite journal |url=https://tools.ietf.org/html/rfc1321 |title=RFC 1321 – The MD5 Message-Digest Algorithm |website=Internet Engineering Task Force |date=April 1992 |access-date=5 October 2013 |last1=Rivest |first1=Ronald L. |doi=10.17487/RFC1321 |doi-access=free |archive-date=9 April 2021 |archive-url=https://web.archive.org/web/20210409200613/https://tools.ietf.org/html/rfc1321 |url-status=live |hdl=1721.1/149165 |hdl-access=free }} and HMAC-MD5.{{cite journal |url= https://tools.ietf.org/html/rfc2104 |title= RFC 2104 – HMAC: Keyed-Hashing for Message Authentication |website= Internet Engineering Task Force |date= February 1997 |access-date= 5 October 2013 |last1= Krawczyk |first1= Hugo |last2= Bellare |first2= Mihir |last3= Canetti |first3= Ran |doi= 10.17487/RFC2104 |archive-date= 15 April 2021 |archive-url= https://web.archive.org/web/20210415003434/https://tools.ietf.org/html/rfc2104 |url-status= live }}

One basic requirement of any cryptographic hash function is that it should be computationally infeasible to find two distinct messages that hash to the same value. MD5 fails this requirement catastrophically. On 31 December 2008, the CMU Software Engineering Institute concluded that MD5 was essentially "cryptographically broken and unsuitable for further use".{{cite web|first1=Chad R.|last1=Dougherty|title=Vulnerability Note VU#836068 MD5 vulnerable to collision attacks|url=https://www.kb.cert.org/vuls/id/836068|website=Vulnerability notes database|publisher=CERT Carnegie Mellon University Software Engineering Institute|access-date=3 February 2017|date=31 Dec 2008|archive-date=26 July 2011|archive-url=https://web.archive.org/web/20110726144513/http://www.kb.cert.org/vuls/id/836068|url-status=live}} The weaknesses of MD5 have been exploited in the field, most infamously by the Flame malware in 2012. {{As of|2019}}, MD5 continues to be widely used, despite its well-documented weaknesses and deprecation by security experts.

A collision attack exists that can find collisions within seconds on a computer with a 2.6 GHz Pentium 4 processor (complexity of 2^24.1).{{Cite thesis |degree=Master's |author=M.M.J. Stevens |date=June 2007 |title=On Collisions for MD5 |url=http://www.win.tue.nl/hashclash/On%20Collisions%20for%20MD5%20-%20M.M.J.%20Stevens.pdf |access-date=31 March 2010 |archive-date=17 May 2017 |archive-url=https://web.archive.org/web/20170517115509/http://www.win.tue.nl/hashclash/On%20Collisions%20for%20MD5%20-%20M.M.J.%20Stevens.pdf |url-status=live }} Further, there is also a chosen-prefix collision attack that can produce a collision for two inputs with specified prefixes within seconds, using off-the-shelf computing hardware (complexity 2³⁹).{{Cite web |author1=Marc Stevens |author2=Arjen Lenstra |author3=Benne de Weger |date=16 June 2009 |title=Chosen-prefix Collisions for MD5 and Applications |website=École Polytechnique Fédérale de Lausanne |url=https://documents.epfl.ch/users/l/le/lenstra/public/papers/lat.pdf |access-date=31 March 2010 |archive-url=https://web.archive.org/web/20111109092157/https://documents.epfl.ch/users/l/le/lenstra/public/papers/lat.pdf |archive-date=9 November 2011 }}

The ability to find collisions has been greatly aided by the use of off-the-shelf GPUs. On an NVIDIA GeForce 8400GS graphics processor, 16–18 million hashes per second can be computed. An NVIDIA GeForce 8800 Ultra can calculate more than 200 million hashes per second.{{cite web

| url = http://bvernoux.free.fr/md5/index.php

| title = New GPU MD5 cracker cracks more than 200 million hashes per second.

| access-date = 25 March 2011

| archive-date = 11 May 2011

| archive-url = https://web.archive.org/web/20110511121239/http://bvernoux.free.fr/md5/index.php

| url-status = live

}}

These hash and collision attacks have been demonstrated in the public in various situations, including colliding document files{{cite web |author=Magnus Daum, Stefan Lucks |title=Hash Collisions (The Poisoned Message Attack) |work=Eurocrypt 2005 rump session |url=http://th.informatik.uni-mannheim.de/People/lucks/HashCollisions/ |archive-url=https://web.archive.org/web/20100327141611/http://th.informatik.uni-mannheim.de/people/lucks/HashCollisions/ |archive-date=27 March 2010 |df=dmy-all }}{{Cite web |author1=Max Gebhardt |author2=Georg Illies |author3=Werner Schindler |title=A Note on the Practical Value of Single Hash Collisions for Special File Formats |website=National Institute of Standards and Technology |url=http://csrc.nist.gov/groups/ST/hash/documents/Illies_NIST_05.pdf |archive-url=https://web.archive.org/web/20080917182949/http://csrc.nist.gov/groups/ST/hash/documents/Illies_NIST_05.pdf |archive-date=2008-09-17 |date=31 October 2005}} and digital certificates. As of 2015, MD5 was demonstrated to be still quite widely used, most notably by security research and antivirus companies.{{Cite web|title = Poisonous MD5 – Wolves Among the Sheep {{!}} Silent Signal Techblog|date = 10 June 2015|url = http://blog.silentsignal.eu/2015/06/10/poisonous-md5-wolves-among-the-sheep/|access-date = 2015-06-10|archive-date = 10 June 2015|archive-url = https://web.archive.org/web/20150610115424/http://blog.silentsignal.eu/2015/06/10/poisonous-md5-wolves-among-the-sheep/|url-status = live}}

As of 2019, one quarter of widely used content management systems were reported to still use MD5 for password hashing.{{Cite web|url=https://www.zdnet.com/article/a-quarter-of-major-cmss-use-outdated-md5-as-the-default-password-hashing-scheme/|title=A quarter of major CMSs use outdated MD5 as the default password hashing scheme|last=Cimpanu|first=Catalin|website=ZDNet|language=en|access-date=2019-06-17|archive-date=24 January 2021|archive-url=https://web.archive.org/web/20210124103125/https://www.zdnet.com/article/a-quarter-of-major-cmss-use-outdated-md5-as-the-default-password-hashing-scheme/|url-status=live}}

In 1996, a flaw was found in the design of MD5. While it was not deemed a fatal weakness at the time, cryptographers began recommending the use of other algorithms, such as SHA-1, which has since been found to be vulnerable as well.{{cite web|url=http://ftp.arnes.si/packages/crypto-tools/rsa.com/cryptobytes/crypto2n2.pdf.gz|title=The Status of MD5 After a Recent Attack|author=Hans Dobbertin|work=CryptoBytes|volume=2|issue=2|date=Summer 1996|access-date=22 October 2013}}

In 2004 it was shown that MD5 is not collision-resistant.{{cite web|url=http://merlot.usc.edu/csac-f06/papers/Wang05a.pdf|title=How to Break MD5 and Other Hash Functions|author1=Xiaoyun Wang|author2=Hongbo Yu |work=Advances in Cryptology – Lecture Notes in Computer Science|volume=3494|pages=19–35|year=2005|access-date=21 December 2009|archive-url=https://web.archive.org/web/20090521024709/http://merlot.usc.edu/csac-f06/papers/Wang05a.pdf|archive-date=21 May 2009}} As such, MD5 is not suitable for applications like SSL certificates or digital signatures that rely on this property for digital security. Researchers additionally discovered more serious flaws in MD5, and described a feasible collision attack—a method to create a pair of inputs for which MD5 produces identical checksums.J. Black, M. Cochran, T. Highland: [http://www.cs.colorado.edu/~jrblack/papers/md5e-full.pdf A Study of the MD5 Attacks: Insights and Improvements] {{Webarchive|url=https://web.archive.org/web/20150101093005/http://www.cs.colorado.edu/%7Ejrblack/papers/md5e-full.pdf |date=1 January 2015 }}, 3 March 2006. Retrieved 27 July 2008.Xiaoyun Wang, Dengguo ,k.,m.,m, HAVAL-128 and RIPEMD, Cryptology ePrint Archive Report 2004/199, 16 August 2004, revised 17 August 2004. Retrieved 27 July 2008. Further advances were made in breaking MD5 in 2005, 2006, and 2007.Marc Stevens, Arjen Lenstra, Benne de Weger: [http://www.win.tue.nl/hashclash/SoftIntCodeSign/ Vulnerability of software integrity and code signing applications to chosen-prefix collisions for MD5] {{Webarchive|url=https://web.archive.org/web/20071213023720/http://www.win.tue.nl/hashclash/SoftIntCodeSign/ |date=13 December 2007 }}, 30 November 2007. Retrieved 27 July 2008. In December 2008, a group of researchers used this technique to fake SSL certificate validity.{{cite web |url=http://www.win.tue.nl/hashclash/rogue-ca/ |title=MD5 considered harmful today |last=Sotirov |first=Alexander |author2=Marc Stevens |author3=Jacob Appelbaum |author4=Arjen Lenstra |author5=David Molnar |author6=Dag Arne Osvik |author7=Benne de Weger |date=30 December 2008 |access-date=30 December 2008 |archive-date=25 March 2017 |archive-url=https://web.archive.org/web/20170325033522/http://www.win.tue.nl/hashclash/rogue-ca/ |url-status=live }} [https://events.ccc.de/congress/2008/Fahrplan/events/3023.en.html Announced] {{Webarchive|url=https://web.archive.org/web/20181116081156/https://events.ccc.de/congress/2008/Fahrplan/events/3023.en.html |date=16 November 2018 }} at the 25th Chaos Communication Congress.{{cite web

|url=http://news.cnet.com/8301-1009_3-10129693-83.html

|title=Web browser flaw could put e-commerce security at risk

|last=Stray

|first=Jonathan

|date=30 December 2008

|access-date=24 February 2009

|publisher=CNET.com

|archive-date=28 August 2013

|archive-url=https://web.archive.org/web/20130828142658/http://news.cnet.com/8301-1009_3-10129693-83.html

}}

As of 2010, the CMU Software Engineering Institute considers MD5 "cryptographically broken and unsuitable for further use", and most U.S. government applications now require the SHA-2 family of hash functions.{{cite web |url=http://csrc.nist.gov/groups/ST/hash/policy.html |title=NIST.gov — Computer Security Division — Computer Security Resource Center |publisher=Csrc.nist.gov |access-date=9 August 2010 |archive-url=https://web.archive.org/web/20110609064344/http://csrc.nist.gov/groups/ST/hash/policy.html |archive-date=9 June 2011 }} In 2012, the Flame malware exploited the weaknesses in MD5 to fake a Microsoft digital signature.{{cite web|url=http://blogs.technet.com/b/srd/archive/2012/06/06/more-information-about-the-digital-certificates-used-to-sign-the-flame-malware.aspx|title=Flame malware collision attack explained|access-date=7 June 2012|archive-url=https://web.archive.org/web/20120608225029/http://blogs.technet.com/b/srd/archive/2012/06/06/more-information-about-the-digital-certificates-used-to-sign-the-flame-malware.aspx|archive-date=8 June 2012}}

{{Further|Collision attack}}

In 1996, collisions were found in the compression function of MD5, and Hans Dobbertin wrote in the RSA Laboratories technical newsletter, "The presented attack does not yet threaten practical applications of MD5, but it comes rather close ... in the future MD5 should no longer be implemented ... where a collision-resistant hash function is required."{{Cite FTP |url=ftp://ftp.rsasecurity.com/pub/cryptobytes/crypto2n2.pdf

|date=Summer 1996

|volume=2

|issue=2

|page=1

|title=The Status of MD5 After a Recent Attack

|last=Dobbertin

|first=Hans

|access-date=10 August 2010

|server=RSA Laboratories CryptoBytes

|url-status=dead

|quote=The presented attack does not yet threaten practical applications of MD5, but it comes rather close. ....{{sic}} in the future MD5 should no longer be implemented...{{sic}} where a collision-resistant hash function is required.

}}

In 2005, researchers were able to create pairs of PostScript documents{{cite web |url=http://www.schneier.com/blog/archives/2005/06/more_md5_collis.html |title=Schneier on Security: More MD5 Collisions |publisher=Schneier.com |access-date=9 August 2010 |archive-date=11 April 2021 |archive-url=https://web.archive.org/web/20210411035935/https://www.schneier.com/blog/archives/2005/06/more_md5_collis.html |url-status=live }} and X.509 certificates{{cite web |url=http://www.win.tue.nl/~bdeweger/CollidingCertificates/ |title=Colliding X.509 Certificates |publisher=Win.tue.nl |access-date=9 August 2010 |archive-date=15 May 2017 |archive-url=https://web.archive.org/web/20170515022608/http://www.win.tue.nl/~bdeweger/CollidingCertificates/ |url-status=live }} with the same hash. Later that year, MD5's designer Ron Rivest wrote that "md5 and sha1 are both clearly broken (in terms of collision-resistance)".{{cite web |url=http://mail.python.org/pipermail/python-dev/2005-December/058850.html |title=[Python-Dev] hashlib — faster md5/sha, adds sha256/512 support |date=16 December 2005 |publisher=Mail.python.org |access-date=9 August 2010 |archive-date=6 May 2021 |archive-url=https://web.archive.org/web/20210506232819/https://mail.python.org/pipermail/python-dev/2005-December/058850.html |url-status=live }}

On 30 December 2008, a group of researchers announced at the 25th Chaos Communication Congress how they had used MD5 collisions to create an intermediate certificate authority certificate that appeared to be legitimate when checked by its MD5 hash. The researchers used a PS3 cluster at the EPFL in Lausanne, Switzerland{{cite magazine|url=http://blog.wired.com/27bstroke6/2008/12/berlin.html|title=Researchers Use PlayStation Cluster to Forge a Web Skeleton Key|date=31 December 2008|magazine=Wired|access-date=31 December 2008|archive-date=21 April 2009|archive-url=https://web.archive.org/web/20090421023048/http://blog.wired.com/27bstroke6/2008/12/berlin.html|url-status=live}} to change a normal SSL certificate issued by RapidSSL into a working CA certificate for that issuer, which could then be used to create other certificates that would appear to be legitimate and issued by RapidSSL. Verisign, the issuers of RapidSSL certificates, said they stopped issuing new certificates using MD5 as their checksum algorithm for RapidSSL once the vulnerability was announced.{{cite web|url=https://blogs.verisign.com/ssl-blog/2008/12/on_md5_vulnerabilities_and_mit.php|title=This morning's MD5 attack — resolved|last=Callan|first=Tim|date=31 December 2008|publisher=Verisign|access-date=31 December 2008|archive-url=https://web.archive.org/web/20090116180944/http://blogs.verisign.com/ssl-blog/2008/12/on_md5_vulnerabilities_and_mit.php|archive-date=16 January 2009}} Although Verisign declined to revoke existing certificates signed using MD5, their response was considered adequate by the authors of the exploit (Alexander Sotirov, Marc Stevens, Jacob Appelbaum, Arjen Lenstra, David Molnar, Dag Arne Osvik, and Benne de Weger). Bruce Schneier wrote of the attack that "we already knew that MD5 is a broken hash function" and that "no one should be using MD5 anymore".{{cite web |author=Bruce Schneier |url=http://www.schneier.com/blog/archives/2008/12/forging_ssl_cer.html |title=Forging SSL Certificates |publisher=Schneier on Security |date=31 December 2008 |access-date=10 April 2014 |archive-date=9 November 2020 |archive-url=https://web.archive.org/web/20201109014745/https://www.schneier.com/blog/archives/2008/12/forging_ssl_cer.html |url-status=live }} The SSL researchers wrote, "Our desired impact is that Certification Authorities will stop using MD5 in issuing new certificates. We also hope that use of MD5 in other applications will be reconsidered as well."

In 2012, according to Microsoft, the authors of the Flame malware used an MD5 collision to forge a Windows code-signing certificate.

MD5 uses the Merkle–Damgård construction, so if two prefixes with the same hash can be constructed, a common suffix can be added to both to make the collision more likely to be accepted as valid data by the application using it. Furthermore, current collision-finding techniques allow specifying an arbitrary prefix: an attacker can create two colliding files that both begin with the same content. All the attacker needs to generate two colliding files is a template file with a 128-byte block of data, aligned on a 64-byte boundary, that can be changed freely by the collision-finding algorithm. An example MD5 collision, with the two messages differing in 6 bytes, is:

d131dd02c5e6eec4 693d9a0698aff95c 2fcab5{{Background color|#87CEEB|8}}712467eab 4004583eb8fb7f89

55ad340609f4b302 83e4888325{{Background color|#87CEEB|7}}1415a 085125e8f7cdc99f d91dbd{{Background color|#87CEEB|f}}280373c5b

d8823e3156348f5b ae6dacd436c919c6 dd53e2{{Background color|#87CEEB|b}}487da03fd 02396306d248cda0

e99f33420f577ee8 ce54b67080{{Background color|#87CEEB|a}}80d1e c69821bcb6a88393 96f965{{Background color|#87CEEB|2}}b6ff72a70

d131dd02c5e6eec4 693d9a0698aff95c 2fcab5{{Background color|#87CEEB|0}}712467eab 4004583eb8fb7f89

55ad340609f4b302 83e4888325{{Background color|#87CEEB|f}}1415a 085125e8f7cdc99f d91dbd{{Background color|#87CEEB|7}}280373c5b

d8823e3156348f5b ae6dacd436c919c6 dd53e2{{Background color|#87CEEB|3}}487da03fd 02396306d248cda0

e99f33420f577ee8 ce54b67080{{Background color|#87CEEB|2}}80d1e c69821bcb6a88393 96f965{{Background color|#87CEEB|a}}b6ff72a70

Both produce the MD5 hash 79054025255fb1a26e4bc422aef54eb4.{{cite web

|url=http://www.rtfm.com/movabletype/archives/2004_08.html#001055

|title=A real MD5 collision

|author=Eric Rescorla

|date=2004-08-17

|archive-url=https://web.archive.org/web/20140815234704/http://www.rtfm.com/movabletype/archives/2004_08.html#001055

|archive-date=2014-08-15

|work=Educated Guesswork (blog)

|access-date=2015-04-13}}

The difference between the two samples is that the leading bit in each nibble has been flipped. For example, the 20th byte (offset 0x13) in the top sample, 0x87, is 10000111 in binary. The leading bit in the byte (also the leading bit in the first nibble) is flipped to make 00000111, which is 0x07, as shown in the lower sample.

Later it was also found to be possible to construct collisions between two files with separately chosen prefixes. This technique was used in the creation of the rogue CA certificate in 2008. A new variant of parallelized collision searching using MPI was proposed by Anton Kuznetsov in 2014, which allowed finding a collision in 11 hours on a computing cluster.{{cite web | url=http://eprint.iacr.org/2014/871.pdf | title=An algorithm for MD5 single-block collision attack using high performance computing cluster | publisher=IACR | access-date=2014-11-03 | author=Anton A. Kuznetsov | archive-date=4 June 2016 | archive-url=https://web.archive.org/web/20160604093753/https://eprint.iacr.org/2014/871.pdf | url-status=live }}

In April 2009, an attack against MD5 was published that breaks MD5's preimage resistance. This attack is only theoretical, with a computational complexity of 2^123.4 for full preimage.{{Cite book|author1=Yu Sasaki |title=Advances in Cryptology - EUROCRYPT 2009 |volume=5479 |pages=134–152 |author2=Kazumaro Aoki |date=16 April 2009 |chapter=Finding Preimages in Full MD5 Faster Than Exhaustive Search |publisher=Springer Berlin Heidelberg |doi=10.1007/978-3-642-01001-9_8 |series=Lecture Notes in Computer Science |isbn=978-3-642-01000-2 }}{{cite book

|chapter=Construction of the Initial Structure for Preimage Attack of MD5

|publisher=IEEE Computer Society

|year=2009|volume=1|pages=442–445|author=Ming Mao and Shaohui Chen and Jin Xu

|title=2009 International Conference on Computational Intelligence and Security

|doi=10.1109/CIS.2009.214|isbn=978-0-7695-3931-7

|s2cid=16512325

}}

MD5 digests have been widely used in the software world to provide some assurance that a transferred file has arrived intact. For example, file servers often provide a pre-computed MD5 (known as md5sum) checksum for the files, so that a user can compare the checksum of the downloaded file to it. Most unix-based operating systems include MD5 sum utilities in their distribution packages; Windows users may use the included PowerShell function "Get-FileHash", the included command line function "certutil -hashfile md5",{{cite web |title=Finding Checksum Values in Windows 10 |url=https://answers.microsoft.com/en-us/windows/forum/all/finding-checksum-values-in-windows-10/dbc3c569-4b5a-4967-8810-c25255cdc1fd |publisher=Microsoft Community |access-date=23 November 2023 |archive-date=11 January 2024 |archive-url=https://web.archive.org/web/20240111185213/https://answers.microsoft.com/en-us/windows/forum/all/finding-checksum-values-in-windows-10/dbc3c569-4b5a-4967-8810-c25255cdc1fd |url-status=live }}{{cite web |title=certutil |url=https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/certutil |website=certutil |publisher=Microsoft Learn |access-date=23 November 2023 |archive-date=23 November 2023 |archive-url=https://web.archive.org/web/20231123190431/https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/certutil |url-status=live }} install a Microsoft utility,{{cite web |url=https://support.microsoft.com/kb/841290/en-us |title=Availability and description of the File Checksum Integrity Verifier utility |publisher=Microsoft Support |date=17 June 2013 |access-date=10 April 2014 |archive-date=15 February 2015 |archive-url=https://web.archive.org/web/20150215153720/http://support.microsoft.com/kb/841290/en-us |url-status=live }}{{cite web |url=https://support.microsoft.com/kb/889768/en-us |title=How to compute the MD5 or SHA-1 cryptographic hash values for a file |publisher=Microsoft Support |date=23 January 2007 |access-date=10 April 2014 |archive-date=9 March 2015 |archive-url=https://web.archive.org/web/20150309230754/http://support.microsoft.com/kb/889768/en-us |url-status=live }} or use third-party applications. Android ROMs also use this type of checksum.

File:CPT-Hashing-File-Transmission.svg

As it is easy to generate MD5 collisions, it is possible for the person who created the file to create a second file with the same checksum, so this technique cannot protect against some forms of malicious tampering. In some cases, the checksum cannot be trusted (for example, if it was obtained over the same channel as the downloaded file), in which case MD5 can only provide error-checking functionality: it will recognize a corrupt or incomplete download, which becomes more likely when downloading larger files.

Historically, MD5 has been used to store a one-way hash of a password, often with key stretching.{{Cite web|url = https://www.freebsd.org/cgi/man.cgi?crypt(3)|title = FreeBSD Handbook, Security – DES, Blowfish, MD5, and Crypt|access-date = 2014-10-19|archive-date = 18 February 2017|archive-url = https://web.archive.org/web/20170218081024/https://www.freebsd.org/cgi/man.cgi?crypt(3)|url-status = live}}{{cite web |url=http://docs.oracle.com/cd/E26505_01/html/816-5174/policy.conf-4.html |title=Synopsis – man pages section 4: File Formats |publisher=Docs.oracle.com |date=1 January 2013 |access-date=10 April 2014 |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304204917/http://docs.oracle.com/cd/E26505_01/html/816-5174/policy.conf-4.html |url-status=live }} NIST does not include MD5 in their list of recommended hashes for password storage.[http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-132.pdf NIST SP 800-132] {{Webarchive|url=https://web.archive.org/web/20161201044854/http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-132.pdf |date=1 December 2016 }} Section 5.1

MD5 is also used in the field of electronic discovery, to provide a unique identifier for each document that is exchanged during the legal discovery process. This method can be used to replace the Bates stamp numbering system that has been used for decades during the exchange of paper documents. As above, this usage should be discouraged due to the ease of collision attacks.

Image:MD5 algorithm.svg

MD5 processes a variable-length message into a fixed-length output of 128 bits. The input message is broken up into chunks of 512-bit blocks (sixteen 32-bit words); the message is padded so that its length is divisible by 512. The padding works as follows: first, a single bit, 1, is appended to the end of the message. This is followed by as many zeros as are required to bring the length of the message up to 64 bits fewer than a multiple of 512. The remaining bits are filled up with 64 bits representing the length of the original message, modulo 2⁶⁴.

The main MD5 algorithm operates on a 128-bit state, divided into four 32-bit words, denoted {{mvar|A}}, {{mvar|B}}, {{mvar|C}}, and {{mvar|D}}. These are initialized to certain fixed constants. The main algorithm then uses each 512-bit message block in turn to modify the state. The processing of a message block consists of four similar stages, termed rounds; each round is composed of 16 similar operations based on a non-linear function {{mvar|F}}, modular addition, and left rotation. Figure 1 illustrates one operation within a round. There are four possible functions; a different one is used in each round:

:$\backslash begin\{align\}$

F(B,C,D) &= (B\wedge{C}) \vee (\neg{B} \wedge{D}) \\

G(B,C,D) &= (B\wedge{D}) \vee (C \wedge \neg{D}) \\

H(B,C,D) &= B \oplus C \oplus D \\

I(B,C,D) &= C \oplus (B \vee \neg{D})

\end{align}

$\backslash oplus,\; \backslash wedge,\; \backslash vee,\; \backslash neg$ denote the XOR, AND, OR and NOT operations respectively.

The MD5 hash is calculated according to this algorithm.{{Cite web|url=https://referencesource.microsoft.com/#System.Workflow.Runtime/MD5HashHelper.cs,5a97802b6014fccc,references|title=Reference Source|access-date=23 December 2020|archive-date=21 June 2021|archive-url=https://web.archive.org/web/20210621192842/https://referencesource.microsoft.com/#System.Workflow.Runtime/MD5HashHelper.cs,5a97802b6014fccc,references|url-status=live}} All values are in little-endian.

// : All variables are unsigned 32 bit and wrap modulo 2^32 when calculating

var int s[64], K[64]

var int i

// s specifies the per-round shift amounts

s[ 0..15] := { 7, 12, 17, 22, 7, 12, 17, 22, 7, 12, 17, 22, 7, 12, 17, 22 }

s[16..31] := { 5, 9, 14, 20, 5, 9, 14, 20, 5, 9, 14, 20, 5, 9, 14, 20 }

s[32..47] := { 4, 11, 16, 23, 4, 11, 16, 23, 4, 11, 16, 23, 4, 11, 16, 23 }

s[48..63] := { 6, 10, 15, 21, 6, 10, 15, 21, 6, 10, 15, 21, 6, 10, 15, 21 }

// Use binary integer part of the sines of integers (Radians) as constants:

for i from 0 to 63 do

K[i] := floor(2³² × abs(sin(i + 1)))

end for

// (Or just use the following precomputed table):

K[ 0.. 3] := { 0xd76aa478, 0xe8c7b756, 0x242070db, 0xc1bdceee }

K[ 4.. 7] := { 0xf57c0faf, 0x4787c62a, 0xa8304613, 0xfd469501 }

K[ 8..11] := { 0x698098d8, 0x8b44f7af, 0xffff5bb1, 0x895cd7be }

K[12..15] := { 0x6b901122, 0xfd987193, 0xa679438e, 0x49b40821 }

K[16..19] := { 0xf61e2562, 0xc040b340, 0x265e5a51, 0xe9b6c7aa }

K[20..23] := { 0xd62f105d, 0x02441453, 0xd8a1e681, 0xe7d3fbc8 }

K[24..27] := { 0x21e1cde6, 0xc33707d6, 0xf4d50d87, 0x455a14ed }

K[28..31] := { 0xa9e3e905, 0xfcefa3f8, 0x676f02d9, 0x8d2a4c8a }

K[32..35] := { 0xfffa3942, 0x8771f681, 0x6d9d6122, 0xfde5380c }

K[36..39] := { 0xa4beea44, 0x4bdecfa9, 0xf6bb4b60, 0xbebfbc70 }

K[40..43] := { 0x289b7ec6, 0xeaa127fa, 0xd4ef3085, 0x04881d05 }

K[44..47] := { 0xd9d4d039, 0xe6db99e5, 0x1fa27cf8, 0xc4ac5665 }

K[48..51] := { 0xf4292244, 0x432aff97, 0xab9423a7, 0xfc93a039 }

K[52..55] := { 0x655b59c3, 0x8f0ccc92, 0xffeff47d, 0x85845dd1 }

K[56..59] := { 0x6fa87e4f, 0xfe2ce6e0, 0xa3014314, 0x4e0811a1 }

K[60..63] := { 0xf7537e82, 0xbd3af235, 0x2ad7d2bb, 0xeb86d391 }

// Initialize variables:

var int a0 := 0x67452301 // A

var int b0 := 0xefcdab89 // B

var int c0 := 0x98badcfe // C

var int d0 := 0x10325476 // D

// Pre-processing: adding a single 1 bit

append "1" bit to message<

// Notice: the input bytes are considered as bit strings,

// where the first bit is the most significant bit of the byte.RFC 1321, section 2, "Terminology and Notation", Page 2.

// Pre-processing: padding with zeros

append "0" bit until message length in bits ≡ 448 (mod 512)

// Notice: the two padding steps above are implemented in a simpler way

// in implementations that only work with complete bytes: append 0x80

// and pad with 0x00 bytes so that the message length in bytes ≡ 56 (mod 64).

append original length in bits mod 2⁶⁴ to message

// Process the message in successive 512-bit chunks:

for each 512-bit chunk of padded message do

break chunk into sixteen 32-bit words M[j], 0 ≤ j ≤ 15

// Initialize hash value for this chunk:

var int A := a0

var int B := b0

var int C := c0

var int D := d0

// Main loop:

for i from 0 to 63 do

var int F, g

if 0 ≤ i ≤ 15 then

F := (B and C) or ((not B) and D)

g := i

else if 16 ≤ i ≤ 31 then

F := (D and B) or ((not D) and C)

g := (5×i + 1) mod 16

else if 32 ≤ i ≤ 47 then

F := B xor C xor D

g := (3×i + 5) mod 16

else if 48 ≤ i ≤ 63 then

F := C xor (B or (not D))

g := (7×i) mod 16

// Be wary of the below definitions of a,b,c,d

F := F + A + K[i] + M[g] // M[g] must be a 32-bit block

A := D

D := C

C := B

B := B + leftrotate(F, s[i])

end for

// Add this chunk's hash to result so far:

a0 := a0 + A

b0 := b0 + B

c0 := c0 + C

d0 := d0 + D

end for

var char digest[16] := a0 append b0 append c0 append d0 // (Output is in little-endian)

Instead of the formulation from the original RFC 1321 shown, the following may be used for improved efficiency (useful if assembly language is being used – otherwise, the compiler will generally optimize the above code. Since each computation is dependent on another in these formulations, this is often slower than the above method where the nand/and can be parallelised):

( 0 ≤ i ≤ 15): F := D xor (B and (C xor D))

(16 ≤ i ≤ 31): F := C xor (D and (B xor C))

The 128-bit (16-byte) MD5 hashes (also termed message digests) are typically represented as a sequence of 32 hexadecimal digits. The following demonstrates a 43-byte ASCII input and the corresponding MD5 hash:

MD5("The quick brown fox jumps over the lazy dog") =

9e107d9d372bb6826bd81d3542a419d6

Even a small change in the message will (with overwhelming probability) result in a mostly different hash, due to the avalanche effect. For example, adding a period to the end of the sentence:

MD5("The quick brown fox jumps over the lazy dog{{Background color|#87CEEB|.}}") =

e4d909c290d0fb1ca068ffaddf22cbd0

The hash of the zero-length string is:

MD5("") =

d41d8cd98f00b204e9800998ecf8427e

The MD5 algorithm is specified for messages consisting of any number of bits; it is not limited to multiples of eight bits (octets, bytes). Some MD5 implementations such as md5sum might be limited to octets, or they might not support streaming for messages of an initially undetermined length.

Below is a list of cryptography libraries that support MD5:

• Botan
• Bouncy Castle
• cryptlib
• Crypto++
• Libgcrypt
• Nettle
• OpenSSL
• wolfSSL

• Comparison of cryptographic hash functions
• Hash function security summary
• HashClash
• MD5Crypt
• md5deep
• md5sum
• MD6
• SHA-1
• SHA-2

{{Reflist|30em}}

• {{Cite conference

| first = Thomas A.

| last = Berson

| title = Differential Cryptanalysis Mod 2³² with Applications to MD5

| book-title = EUROCRYPT

| year = 1992

| pages = 71–80

| isbn = 3-540-56413-6

}}

• {{Cite conference

|author1=Bert den Boer |author2=Antoon Bosselaers | title = Collisions for the Compression Function of MD5

| conference = EUROCRYPT

| book-title = Advances in Cryptology – EUROCRYPT '93

| year = 1993

| pages = 293–304

| isbn = 978-3-540-57600-6

| publisher = Springer

| location = Berlin; London

}}

• Hans Dobbertin, Cryptanalysis of MD5 compress. Announcement on Internet, May 1996. {{cite web |url=http://citeseer.ist.psu.edu/dobbertin96cryptanalysis.html |title=CiteSeerX |publisher=Citeseer.ist.psu.edu |access-date=9 August 2010 |archive-date=24 June 2008 |archive-url=https://web.archive.org/web/20080624043846/http://citeseer.ist.psu.edu/dobbertin96cryptanalysis.html |url-status=live }}
• {{Cite journal

| first = Hans

| last = Dobbertin

| title = The Status of MD5 After a Recent Attack

| journal = CryptoBytes

| volume = 2

| issue = 2

| year = 1996

| url = http://ftp.arnes.si/packages/crypto-tools/rsa.com/cryptobytes/crypto2n2.pdf.gz

}}

• {{Cite conference

| author1 = Xiaoyun Wang

| author2 = Hongbo Yu

| title = How to Break MD5 and Other Hash Functions

| book-title = EUROCRYPT

| year = 2005

| url = http://www.infosec.sdu.edu.cn/uploadfile/papers/How%20to%20Break%20MD5%20and%20Other%20Hash%20Functions.pdf

| isbn = 3-540-25910-4

| access-date = 6 March 2008

| archive-url = https://web.archive.org/web/20090521001714/http://www.infosec.sdu.edu.cn/uploadfile/papers/How%20to%20Break%20MD5%20and%20Other%20Hash%20Functions.pdf

| archive-date = 21 May 2009

}}

• [http://www.w3.org/TR/1998/REC-DSig-label/MD5-1_0 W3C recommendation on MD5] {{Webarchive|url=https://web.archive.org/web/20141228122551/http://www.w3.org/TR/1998/REC-DSig-label/MD5-1_0 |date=28 December 2014 }}
• [https://fe-tool.com/en-us/hash/md5 MD5 Calculator] {{Webarchive|url=https://web.archive.org/web/20221116025344/https://fe-tool.com/en-us/hash/md5 |date=16 November 2022 }}

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