The MD5 (Message-Digest algorithm) hashing algorithm is a one-way cryptographic function that accepts a message of any length as input and returns as output a fixed-length digest value to be used for authenticating the original message.
The MD5 hash function was originally designed for use as a secure cryptographic hash algorithm for authenticating digital signatures. MD5 is a cryptographically broken, it has been found to suffer from extensive vulnerabilities. Because MD5 hashing is no longer considered reliable for use as a cryptographic checksum because security experts have demonstrated techniques capable of easily producing MD5 collisions on commercial off-the-shelf computers. An encryption collision means two files have the same hash. Hash functions are used for message security, password security, computer forensics and cryptocurrency. MD5 still remains suitable for other non-cryptographic purposes, for example for determining the partition for a particular key in a partitioned database, and may be preferred due to lower computational requirements than more recent Secure Hash Algorithms.
MD5 was designed by the foundern, Ronald Rivest of RSA Data Security LLC and professor at Massachusetts Institute of Technology in 1991 to replace an earlier hash function MD4, and was specified in 1992 as RFC 1321.
Describing it in Internet Engineering Task Force (IETF) Request for Comments (RFC) 1321, “The MD5 Message-Digest Algorithm,” Rivest wrote:
“The algorithm takes as input a message of arbitrary length and produces as output a 128-bit ‘fingerprint’ or ‘message digest’ of the input. It is conjectured that it is computationally infeasible to produce two messages having the same message digest, or to produce any message having a given prespecified target message digest. The MD5 algorithm is intended for digital signature applications, where a large file must be ‘compressed’ in a secure manner before being encrypted with a private (secret) key under a public-key cryptosystem such as RSA.”
IETF suggests MD5 hashing can still be used for integrity protection, noting: “Where the MD5 checksum is used inline with the protocol solely to protect against errors, an MD5 checksum is still an acceptable use.” However, it added that “any application and protocol that employs MD5 for any purpose needs to clearly state the expected security services from their use of MD5.”
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; such collisions can be found in seconds on an ordinary home computer.
On 31 December 2008, the CMU Software Engineering Institute concluded that MD5 was essentially “cryptographically broken and unsuitable for further use”. 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.
History and cryptanalysis
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) 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. Their analytical attack was reported to take only one hour on an IBM p690 cluster.
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. 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. 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.
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.
On 24 December 2010, Tao Xie and Dengguo Feng announced the first published single-block (512-bit) MD5 collision. (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.
In 2011 an informational RFC 6151 was approved to update the security considerations in MD5 and HMAC-MD5.
The security of the MD5 hash function is severely compromised. A collision attack exists that can find collisions within seconds on a computer with a 2.6 GHz Pentium 4 processor (complexity of 224.1). 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). 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.
These hash and collision attacks have been demonstrated in the public in various situations, including colliding document files and digital certificates. As of 2015, MD5 was demonstrated to be still quite widely used, most notably by security research and antivirus companies.
As of 2019, one quarter of widely used content management systems were reported to still use MD5 for password hashing.
Overview of security issues
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. In 2004 it was shown that MD5 is not collision-resistant. 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. Further advances were made in breaking MD5 in 2005, 2006, and 2007. In December 2008, a group of researchers used this technique to fake SSL certificate validity.
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. In 2012, the Flame malware exploited the weaknesses in MD5 to fake a Microsoft digital signature.
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.”
In 2005, researchers were able to create pairs of PostScript documents and X.509 certificates 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)”.
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 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. 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“. 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 to specify 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.