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Home > MD5

 Contents
1 History
2 MD5 hashes
3 Algorithm
4 A description of MD5
5 Security
6 Integrity checking
7 See also
8 References
9 External links
In cryptography, MD5 (Message-Digest algorithm 5) is a widely-used cryptographic hash function with a 128-bit hash value. MD5 was designed by Ronald Rivest in 1991. Significant security flaws in MD5 were reported in 2004, and many systems that use it will probably switch to more secure alternatives.

1 History

MD5 is one of a series of message digest algorithms designed by Professor Ronald Rivest of MIT (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 subsequently found in MD4 by Hans Dobbertin .

In 1996, Dobbertin announced a collision of the compression function of MD5 (Dobbertin, 1996). This was not quite an attack on the full MD5 hash function, but it was close enough for cryptographers to recommend switching to a replacement, such as SHA-1 or RIPEMD-160. In August 2004, Chinese researchers found collisions for MD5. It is still unknown how this discovery will affect the widespread use of MD5.

2 MD5 hashes

The 128-bit MD5 hashes (also termed message digests) are typically represented as 32-digit hexadecimal numbers. 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, e.g. changing d to c:

MD5("The quick brown fox jumps over the lazy cog") = 1055d3e698d289f2af8663725127bd4b

The hash of the zero-length string is:

MD5("") = d41d8cd98f00b204e9800998ecf8427e

3 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; 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 a 64-bit integer representing the length of the original message.

The main MD5 algorithm operates on a 128-bit state, divided into four 32-bit words, denoted A, B, C and D. These are initialised 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:

denote the XOR, AND, OR and NOT operations respectively.

4 A description of MD5

Pseudocode for the MD5 algorithm follows: Note: All variables are unsigned 32 bits and wrap modulo 2^32 when calculating Define r(i) as the following function: r( 0..15) := {7, 12, 17, 22, 7, 12, 17, 22, 7, 12, 17, 22, 7, 12, 17, 22} r(16..31) := {5, 9, 14, 20, 5, 9, 14, 20, 5, 9, 14, 20, 5, 9, 14, 20} r(32..47) := {4, 11, 16, 23, 4, 11, 16, 23, 4, 11, 16, 23, 4, 11, 16, 23} r(48..63) := {6, 10, 15, 21, 6, 10, 15, 21, 6, 10, 15, 21, 6, 10, 15, 21} Use binary fractional part of the sines of integers as constants: for i from 0 to 63 k(i) := floor( abs(sin(i + 1)) * 2^32 ) Initialize variables: h0 := 0x67452301 h1 := 0xEFCDAB89 h2 := 0x98BADCFE h3 := 0x10325476 Pre-processing: append "1" bit to message append "0" bits until message length ≡ 448 (mod 512) append length of message as 64-bit little-endian integer to message Process the message in successive 512-bit chunks: break message into 512-bit chunks for each chunk break chunk into sixteen 32-bit little-endian words w(i), 0 ≤ i ≤ 15 Initialize hash value for this chunk: a := h0 b := h1 c := h2 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 := ((a + f + k(i) + w(g)) leftrotate r(i)) + b d := c c := b b := a a := temp Add this chunk's hash to result so far: h0 := h0 + a h1 := h1 + b h2 := h2 + c h3 := h3 + d digest = hash = h0 append h1 append h2 append h3 (expressed as little-endian)

Note: Instead of the formulation from the original RFC 1321 shown, the following may be used for improved efficiency:

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

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Topics: MD5, History, MD5 Hashes, Algorithm, A Description Of MD5...

MD5In cryptography, MD5 Message-Digest algorithm 5 is a widely-used cryptographic hash function with a 128-bit hash value. MD5 was designed by Ronald Rivest in 1991. Significant security flaws in MD5 were reported in 2004, and many systems that use it will pMD5CRKIn cryptography, MD5CRK is a distributed effort (similar to distributed. net) launched by Jean-Luc Cooke and his company, CertainKey Cryptosystems, to demonstrate that the MD5 message digest algorithm is insecure by finding a collision — two messages that 
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