The MD5 message-digest technique, which produces a 128-bit hash value, is cryptographically broken yet nevertheless extensively used. Although MD5 was created with the intention of being used as a cryptographic hash function, it has been discovered to have numerous flaws.
It can still be used to validate data integrity as a checksum, but only for unintended corruption. It is still acceptable for non-cryptographic tasks, such as detecting the partition for a specific key in a partitioned database, and may be favored over more contemporary Secure Hash Techniques algorithms due to lower computing requirements.
Ronald Rivest created MD5 in 1991 to replace an earlier hash function, MD4,, and RFC 1321 was published in 1992.
One of the most basic requirements of any cryptographic hash function is that finding two separate messages that hash to the same value should be computationally impossible. MD5 fails miserably on this criterion; such collisions can be discovered in seconds on a standard home computer.
The CMU Software Engineering Institute ruled on December 31, 2008 that MD5 was “cryptographically flawed and unfit for further use.”
 MD5’s flaws have been exploited in the field before, most notably by the Flame virus in 2012. Despite its well-documented flaws and deprecation by security experts, MD5 is still frequently used in 2019.
Professor Ronald Rivest of MIT created a series of message digest algorithms, including MD5 (Rivest, 1992). Rivest devised MD5 in 1991 as a secure substitute after analytic studies revealed that MD4’s predecessor was likely insecure. (MD4 was later found to have flaws by Hans Dobbertin.)
Den Boer and Bosselaers published an early, albeit limited, discovery in 1993: they discovered a “pseudo-collision” of the MD5 compression function, that is, two alternative initialization vectors that generate the same digest.
In 1996, Dobbertin stated that the MD5 compression function had collided (Dobbertin, 1996). While this was not a full-fledged attack on MD5, it was close enough for cryptographers to propose upgrading to an other hash function, such as SHA-1 (which was also compromised) or RIPEMD-160.
The hash value’s size (128 bits) is short enough for a birthday attack to be considered. MD5CRK was a distributed initiative that began in March 2004 with the goal of demonstrating that MD5 is practically vulnerable by employing a birthday attack to locate a collision.
MD5CRK came to an end on August 17, 2004, when Xiaoyun Wang, Dengguo Feng, Xuejia Lai, and Hongbo Yu published full MD5 collisions. On an IBM p690 cluster, their analytical attack was said to take only one hour.
Arjen Lenstra, Xiaoyun Wang, and Benne de Weger demonstrated the creation of two X.509 certificates with separate public keys but the same MD5 hash value on March 1, 2005, demonstrating a demonstrably viable collision. Private keys for both public keys were included in the construction. Vlastimil Klima described an improved approach a few days later, capable of constructing MD5 collisions in a matter of hours on a single notebook computer.  Klima proposed an algorithm on March 18, 2006, that, using a process he calls tunneling, could locate a collision in under a minute on a single notebook computer.
Several RFC errata pertaining to MD5 have been published. The United States Cyber Command’s official symbol in 2009 included an MD5 hash value of their mission statement.
Tao Xie and Dengguo Feng revealed the first single-block (512-bit) MD5 collision on December 24, 2010. (Multi-block attacks were used in previous collision discoveries.)
Xie and Feng did not reveal the new assault method for “security considerations.” They issued a challenge to the cryptographic community, giving a reward of $10,000 to the first person to discover a distinct 64-byte collision by January 1, 2013.
In response to the challenge, Marc Stevens shared colliding single-block messages, as well as the building algorithm and sources.
In 2011, RFC 6151 was approved as an informational RFC to update the security issues in MD5 and HMAC-MD5.
The MD5 hash function’s security has been significantly harmed. On a computer with a 2.6 GHz Pentium 4 processor, a collision attack can identify collisions in seconds (complexity of 224.1).
Furthermore, utilizing off-the-shelf computational power, a chosen-prefix collision attack can establish a collision for two inputs with specified prefixes in seconds (complexity 239).
The introduction of off-the-shelf GPUs has substantially increased the capacity to locate collisions. 16–18 million hashes per second can be computed using an NVIDIA GeForce 8400GS graphics processor. A GeForce 8800 Ultra from NVIDIA can calculate over 200 million hashes per second.
These hash and collision attacks have been publicly demonstrated in a variety of scenarios, including colliding document files and digital certificates.
MD5 was still frequently used in 2015, most notably by security research and antivirus businesses.
As of 2019, MD5 was still being utilized for password hashing by one-quarter of commonly used content management systems.
A summary of security concerns
MD5 was revealed to have a vulnerability in its design in 1996. While the flaw was not considered serious at the time, cryptographers began advising the adoption of alternate algorithms, such as SHA-1, which has now been discovered to be vulnerable as well. MD5 was discovered to be collision-prone in 2004.
As a result, MD5 is unsuitable for applications that rely on this attribute for digital security, such as SSL certificates or digital signatures.
More major weaknesses in MD5 were also uncovered, as well as a possible collision attack — a means of creating a pair of inputs for which MD5 outputs identical checksums.
In 2005, 2006, and 2007, more progress was made in breaking MD5. A group of researchers utilized this technique to falsify the validity of an SSL certificate in December 2008.
MD5 is now considered “cryptographically flawed and inappropriate for continuing use” by the CMU Software Engineering Institute, and most U.S. government applications now need the SHA-2 family of hash functions. Flame malware used MD5’s flaws to impersonate a Microsoft digital signature in 2012.
Vulnerabilities in collisions
Collision assault (detailed details)
“The disclosed attack does not yet threaten practical uses of MD5, but it comes quite near… in the future MD5 should no longer be employed… where a collision-resistant hash function is necessary,” Hans Dobbertin said in the RSA Laboratories technical newsletter in 1996.
Researchers were able to use the same hash to construct pairs of PostScript documents and X.509 certificates in 2005.
“md5 and sha1 are both manifestly defective (in terms of collision-resistance),” MD5 designer Ron Rivest wrote later that year.
At the 25th Chaos Communication Congress on December 30, 2008, a group of researchers announced that they had utilized MD5 collisions to construct an intermediate certificate authority certificate that seemed to be authentic when checked by its MD5 hash.
The researchers used a PS3 cluster at the EPFL in Lausanne, Switzerland, to convert a regular RapidSSL SSL certificate into a functioning CA certificate for that issuer, which could then be used to construct further certificates that looked authentic and were issued by RapidSSL.
VeriSign, the RapidSSL certificate issuer, stated that immediately the vulnerability was discovered, they stopped issuing new certificates using MD5 as the checksum algorithm.
Despite Verisign’s refusal to cancel existing MD5-signed certificates, the exploit’s creators deemed their reaction satisfactory (Alexander Sotirov, Marc Stevens, Jacob Appelbaum, Arjen Lenstra, David Molnar, Dag Arne Osvik, and Benne de Weger).
“We previously knew that MD5 is a flawed hash function,” Bruce Schneier noted of the assault, adding that “no one should be using MD5 anymore.”
According to the SSL researchers, “Our objective outcome is for Certification Authorities to stop issuing new certificates using MD5. We also hope that the use of MD5 will be revisited in future applications.”
The creators of the Flame malware used an MD5 collision to fake a Windows code-signing certificate in 2012, according to Microsoft.
If two prefixes with the same hash can be created, a common suffix can be added to each to make the collision more likely to be accepted as acceptable data by the application using it.
Furthermore, existing collision-finding approaches allow an attacker to specify an arbitrary prefix, allowing him to generate two colliding files with the same content at the start.
To create two colliding files, all the attacker needs is a template file with a 128-byte block of data aligned on a 64-byte border that the collision-finding algorithm can adjust at will. The following is an example of an MD5 collision, with the two messages differing by 6 bits:
The MD5 hash for both is 79054025255fb1a26e4bc422aef54eb4.
The leading bit in each nibble has been inverted, which is the difference between the two samples. For example, the top sample’s 20th byte (offset 0x13), 0x87, is 10000111 in binary. As demonstrated in the lower sample, the leading bit in the byte (also the leading bit in the first nibble) is flipped to form 00000111, which is 0x07.
Later, it was discovered that collisions between two files with different prefixes may be created. In 2008, this method was used to create the rogue CA certificate. Anton Kuznetsov introduced a new type of parallelized collision searching using MPI in 2014, which allowed a computing cluster to locate a collision in 11 hours.
Vulnerability before the image
MD5’s preimage resistance was broken in April 2009 when an assault against it was released. With a computational complexity of 2123.4 for full preimage, this assault is only theoretical.
MD5 digests are commonly used in the software industry to ensure that a transferred file arrives in good condition. Item servers, for example, frequently give a pre-computed MD5 (also known as md5sum) checksum for files, which a user can compare to the checksum of the downloaded file.
MD5 sum programs are available in the distribution packages of most unix-based operating systems; Windows users can use the inbuilt PowerShell function “Get-FileHash,” install a Microsoft utility, or utilize third-party apps. This type of checksum is also used in Android ROMs.
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