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{{Infobox cryptographic hash function| name = MD5| image =| caption =| designers = Ronald Rivest, MD3, [MD4, MD5, MD5 (Message-Digest algorithm 5) is a widely used [cryptographic hash function with a 128-bit hash value. As an Internet standard (RFC 1321), MD5 has been employed in a wide variety of security applications, and is also commonly used to check the integrity of computer file. An MD5 hash is typically expressed as a 32-character hexadecimal number.

MD5 was designed by Ronald Rivest in 1991 to replace an earlier hash function, MD4. In 1996, a flaw was found with the design of MD5; while it was not a clearly fatal weakness, cryptographers began recommending the use of other algorithms, such as SHA hash functions (which has since been found vulnerable itself). In 2004, more serious flaws were discovered making further use of the algorithm for security purposes questionable.

History and cryptanalysis Message Digest is a series of message digest algorithms designed by Professor Ronald Rivest of Massachusetts Institute of Technology (Rivest, 1994). 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 "hash collision" of the MD5 One-way compression function; that is, two different initialization vectors which produce an identical digest.

In 1996, Dobbertin announced a Hash 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 WHIRLPOOL, SHA hash functions or RIPEMD-160.

The size of the hash—128 bits—is small enough to contemplate a birthday attack. MD5CRK was a distributed computing 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 hash collisions for the full MD5 were announced by Xiaoyun Wang, Dengguo Feng, Xuejia Lai, and Hongbo Yu.http://eprint.iacr.org/2004/199http://eprint.iacr.org/2004/264 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 demonstratedhttp://eprint.iacr.org/2005/067 construction of two X.509 certificates with different public keys and the same MD5 hash, a demonstrably practical collision. The construction included private keys for both public keys. A few days later, Vlastimil Klima describedhttp://eprint.iacr.org/2005/075 an improved algorithm, able to construct MD5 collisions in a few hours on a single notebook computer. On 18 March 2006, Klima published an algorithmhttp://eprint.iacr.org/2006/105 that can find a collision within one minute on a single notebook computer, using a method he calls tunneling.

Vulnerability Because MD5 makes only one pass over the data, if two prefixes with the same hash can be constructed, a common suffix can be added to both to make the collision more reasonable.

Because the current collision-finding techniques allow the preceding hash state to be specified arbitrarily, a collision can be found for any desired prefix; that is, for any given string of characters X, two colliding files can be determined which both begin with X.

All that is required 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.

Recently, a number of projects have created MD5 "rainbow tables" which are easily accessible online, and can be used to reverse many MD5 hashes into strings that collide with the original input, usually for the purposes of password cracking. However, if passwords are combined with a salt (cryptography) before the MD5 digest is generated, rainbow tables become much less useful.

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 checksum for the files, so that a user can compare the checksum of the downloaded file to it. Unix-based operating systems include MD5 sum utilities in their distribution packages, whereas Windows users use third-party applications.

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 is widely used to store Password#Form of stored passwords. To mitigate against the vulnerabilities mentioned above, one can add a Salt (cryptography) to the passwords before hashing them. Some implementations may apply the hashing function more than once—see key strengthening.

Algorithm s denotes a left bit rotation by s places; s varies for each operation. denotes addition modulo 232.

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 little endian integers); the message is padding (cryptography) 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 a 64-bit integer representing the length of the original message, in bits.

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 operates on each 512-bit message block in turn, each block modifying 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: F(X,Y,Z) = (X\wedge{Y}) \vee (\neg{X} \wedge{Z}) G(X,Y,Z) = (X\wedge{Z}) \vee (Y \wedge \neg{Z}) H(X,Y,Z) = X \oplus Y \oplus Z I(X,Y,Z) = Y \oplus (X \vee \neg{Z})

\oplus, \wedge, \vee, \neg denote the XOR, Logical conjunction, Logical disjunction and Negation operations respectively.

Pseudocode Pseudocode for the MD5 algorithm follows.

//''Note: All variables are unsigned 32 bits and wrap modulo 2^32 when calculating'' '''var''' ''int'' r, k

//''r specifies the per-round shift amounts'' r 0..15 := {7, 12, 17, 22, 7, 12, 17, 22, 7, 12, 17, 22, 7, 12, 17, 22} r16..31 := {5, 9, 14, 20, 5, 9, 14, 20, 5, 9, 14, 20, 5, 9, 14, 20} r32..47 := {4, 11, 16, 23, 4, 11, 16, 23, 4, 11, 16, 23, 4, 11, 16, 23} r48..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 as constants:'' '''for''' i '''from''' 0 '''to''' 63 ki := floor(abs(sin(i + 1)) × (2 '''pow''' 32))

//''Initialize variables:'' '''var''' ''int'' h0 := 0x67452301 '''var''' ''int'' h1 := 0xEFCDAB89 '''var''' ''int'' h2 := 0x98BADCFE '''var''' ''int'' h3 := 0x10325476

//''Pre-processing:'' '''append''' "1" bit '''to''' message '''append''' "0" bits '''until''' message length in bits ≡ 448 (mod 512) '''append''' bit (bit, not byte) length of unpadded message '''as''' ''64-bit little-endian integer'' '''to''' message

//''Process the message in successive 512-bit chunks:'' '''for each''' ''512-bit'' chunk '''of''' message break chunk into sixteen 32-bit little-endian words wi, 0 ≤ i ≤ 15

//''Initialize hash value for this chunk:'' '''var''' ''int'' a := h0 '''var''' ''int'' b := h1 '''var''' ''int'' c := h2 '''var''' ''int'' d := h3

//''Main loop:'' '''for''' i '''from''' 0 '''to''' 63 '''if''' 0 ≤ i ≤ 15 '''then''' f := (b '''and''' c) '''or''' (('''not''' b) '''and''' d) g := i '''else if''' 16 ≤ i ≤ 31 f := (d '''and''' b) '''or''' (('''not''' d) '''and''' c) g := (5×i + 1) '''mod''' 16 '''else if''' 32 ≤ i ≤ 47 f := b '''xor''' c '''xor''' d g := (3×i + 5) '''mod''' 16 '''else if''' 48 ≤ i ≤ 63 f := c '''xor''' (b '''or''' ('''not''' d)) g := (7×i) '''mod''' 16

temp := d d := c c := b b := b + '''leftrotate'''((a + f + ki + wg) , ri) a := temp

//''Add this chunk's hash to result so far:'' h0 := h0 + a h1 := h1 + b h2 := h2 + c h3 := h3 + d

'''var''' ''int'' digest := h0 '''append''' h1 '''append''' h2 '''append''' h3 //''(expressed as little-endian)''

//''leftrotate function definition'' '''leftrotate''' (x, c) return (x > (32-c));

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):(0 ≤ i ≤ 15): f := d '''xor''' (b '''and''' (c '''xor''' d)) (16 ≤ i ≤ 31): f := c '''xor''' (d '''and''' (b '''xor''' c))

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:

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 completely different hash, due to the avalanche effect. For example, changing d to e:

MD5("The quick brown fox jumps over the lazy '''e'''og") = ffd93f16876049265fbaef4da268dd0e

The hash of the zero-length string is:

MD5("") = d41d8cd98f00b204e9800998ecf8427e

References | first = Thomas A. | last = Berson | title = Differential Cryptanalysis Mod 232 with Applications to MD5 | booktitle = EUROCRYPT | year = 1992 | pages = 71–80 | id = ISBN 3-540-56413-6 --> | author = Bert den Boer; Antoon Bosselaers | title = Collisions for the Compression Function of MD5 | booktitle = EUROCRYPT | year = 1993 | pages = 293–304 | id = ISBN 3-540-57600-2 -->
 

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