MD5 The MD5 message-digest algorithm is a widely
used cryptographic hash function producing a 128-bit hash value, typically expressed
in text format as a 32 digit hexadecimal number. MD5 has been utilized in a wide variety of
cryptographic applications, and is also commonly used to verify data integrity.
MD5 was designed by Ron Rivest in 1991 to replace an earlier hash function, MD4. The
source code in RFC 1321 contains a "by attribution" RSA license.
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. Also in 2004 more serious
flaws were discovered in MD5, making further use of the algorithm for security purposes
questionable; specifically, a group of researchers described how to create a pair of files that
share the same MD5 checksum. 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, and CMU Software Engineering Institute now says that MD5 "should be considered 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. 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, MD5 was designed in 1991 to be a secure replacement. (Weaknesses were indeed
later found in MD4 by Hans Dobbertin.) 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 which 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 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 with the aim of demonstrating 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 can 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 relied on multi-block attacks. For "security reasons", Xie and Feng did not
disclose the new attack method. They have 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. Security
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 hours, 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. Collision vulnerabilities
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, "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 which appeared to be legitimate
when checked via its MD5 hash. The researchers used a cluster of Sony PlayStation 3 units
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 "e 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. An example MD5 collision, with
the two messages differing in 6 bits, is: Both produce the MD5 hash 79054025255fb1a26e4bc422aef54eb4.
The difference between the two samples is 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. Preimage vulnerability
In April 2009, a preimage attack against MD5 was published that breaks MD5's preimage resistance.
This attack is only theoretical, with a computational complexity of 2123.4 for full preimage.
Other vulnerabilities A number of projects have published MD5 rainbow
tables online, which can be used to reverse many MD5 hashes into strings that collide
with the original input, usually for the purposes of password cracking.
The use of MD5 in some websites' URLs means that search engines such as Google can also
sometimes function as a limited tool for reverse lookup of MD5 hashes.
Both these techniques are rendered ineffective by the use of a sufficiently long salt.
Applications 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 install a Microsoft
utility, or use third-party applications. Android ROMs also utilize this type of checksum.
However, now that 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. Also, 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.
MD5 can be used to store a one-way hash of a password, often with key stretching. Along
with other hash functions, it is also used in the field of electronic discovery, in order
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.
Algorithm 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
A, B, C and 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 F, modular addition, and left rotation. Figure 1 illustrates
one operation within a round. There are four possible functions F; a different one is used
in each round: denote the XOR, AND, OR and NOT operations
respectively. Pseudocode
The MD5 hash is calculated according to this algorithm. All values are in little-endian.
Note: 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): MD5 hashes
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: 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:
The hash of the zero-length string is: The MD5 algorithm is specified for messages
consisting of any number of bits; it is not limited to multiples of eight bit (octets,
bytes) as shown in the examples above. Some MD5 implementations such as md5sum might be
limited to octets, or they might not support streaming for messages of an initially undetermined
length Simple implementation
A simple MD5 implementation in C++, that follows the pseudocode:
The message to hash is passed via the first argument of the command line.