md5算法.docx

1、md5算法Network Working Group R. RivestRequest for Comments: 1321 MIT Laboratory for Computer Science and RSA Data Security, Inc. April 1992 The MD5 Message-Digest AlgorithmStatus of this Memo This memo provides information for the Internet community. It does not specify an Internet standard. Distribut

2、ion of this memo is unlimited.Acknowlegements We would like to thank Don Coppersmith, Burt Kaliski, Ralph Merkle, David Chaum, and Noam Nisan for numerous helpful comments and suggestions.Table of Contents 1. Executive Summary 1 2. Terminology and Notation 2 3. MD5 Algorithm Description 3 4. Summary

3、 6 5. Differences Between MD4 and MD5 6 References 7 APPENDIX A - Reference Implementation 7 Security Considerations 21 Authors Address 211. Executive Summary This document describes the MD5 message-digest algorithm. The algorithm takes as input a message of arbitrary length and produces as output a

4、 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 applic

5、ations, 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.Rivest Page 1RFC 1321 MD5 Message-Digest Algorithm April 1992 The MD5 algorithm is designed to be quite fast on 32-bit machines. In addition

6、, the MD5 algorithm does not require any large substitution tables; the algorithm can be coded quite compactly. The MD5 algorithm is an extension of the MD4 message-digest algorithm 1,2. MD5 is slightly slower than MD4, but is more conservative in design. MD5 was designed because it was felt that MD

7、4 was perhaps being adopted for use more quickly than justified by the existing critical review; because MD4 was designed to be exceptionally fast, it is at the edge in terms of risking successful cryptanalytic attack. MD5 backs off a bit, giving up a little in speed for a much greater likelihood of

8、 ultimate security. It incorporates some suggestions made by various reviewers, and contains additional optimizations. The MD5 algorithm is being placed in the public domain for review and possible adoption as a standard. For OSI-based applications, MD5s object identifier is md5 OBJECT IDENTIFIER :=

9、 iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5 In the X.509 type AlgorithmIdentifier 3, the parameters for MD5 should have type NULL.2. Terminology and Notation In this document a word is a 32-bit quantity and a byte is an eight-bit quantity. A sequence of bits can be interpreted

10、 in a natural manner as a sequence of bytes, where each consecutive group of eight bits is interpreted as a byte with the high-order (most significant) bit of each byte listed first. Similarly, a sequence of bytes can be interpreted as a sequence of 32-bit words, where each consecutive group of four

11、 bytes is interpreted as a word with the low-order (least significant) byte given first. Let x_i denote x sub i. If the subscript is an expression, we surround it in braces, as in x_i+1. Similarly, we use for superscripts (exponentiation), so that xi denotes x to the i-th power. Let the symbol + den

12、ote addition of words (i.e., modulo-232 addition). Let X s denote the 32-bit value obtained by circularly shifting (rotating) X left by s bit positions. Let not(X) denote the bit-wise complement of X, and let X v Y denote the bit-wise OR of X and Y. Let X xor Y denote the bit-wise XOR of X and Y, an

13、d let XY denote the bit-wise AND of X and Y.Rivest Page 2RFC 1321 MD5 Message-Digest Algorithm April 19923. MD5 Algorithm Description We begin by supposing that we have a b-bit message as input, and that we wish to find its message digest. Here b is an arbitrary nonnegative integer; b may be zero, i

14、t need not be a multiple of eight, and it may be arbitrarily large. We imagine the bits of the message written down as follows: m_0 m_1 . m_b-1 The following five steps are performed to compute the message digest of the message.3.1 Step 1. Append Padding Bits The message is padded (extended) so that

15、 its length (in bits) is congruent to 448, modulo 512. That is, the message is extended so that it is just 64 bits shy of being a multiple of 512 bits long. Padding is always performed, even if the length of the message is already congruent to 448, modulo 512. Padding is performed as follows: a sing

16、le 1 bit is appended to the message, and then 0 bits are appended so that the length in bits of the padded message becomes congruent to 448, modulo 512. In all, at least one bit and at most 512 bits are appended.3.2 Step 2. Append Length A 64-bit representation of b (the length of the message before

17、 the padding bits were added) is appended to the result of the previous step. In the unlikely event that b is greater than 264, then only the low-order 64 bits of b are used. (These bits are appended as two 32-bit words and appended low-order word first in accordance with the previous conventions.)

18、At this point the resulting message (after padding with bits and with b) has a length that is an exact multiple of 512 bits. Equivalently, this message has a length that is an exact multiple of 16 (32-bit) words. Let M0 . N-1 denote the words of the resulting message, where N is a multiple of 16.3.3

19、 Step 3. Initialize MD Buffer A four-word buffer (A,B,C,D) is used to compute the message digest. Here each of A, B, C, D is a 32-bit register. These registers are initialized to the following values in hexadecimal, low-order bytes first):Rivest Page 3RFC 1321 MD5 Message-Digest Algorithm April 1992

20、 word A: 01 23 45 67 word B: 89 ab cd ef word C: fe dc ba 98 word D: 76 54 32 103.4 Step 4. Process Message in 16-Word Blocks We first define four auxiliary functions that each take as input three 32-bit words and produce as output one 32-bit word. F(X,Y,Z) = XY v not(X) Z G(X,Y,Z) = XZ v Y not(Z) H

21、(X,Y,Z) = X xor Y xor Z I(X,Y,Z) = Y xor (X v not(Z) In each bit position F acts as a conditional: if X then Y else Z. The function F could have been defined using + instead of v since XY and not(X)Z will never have 1s in the same bit position.) It is interesting to note that if the bits of X, Y, an

22、d Z are independent and unbiased, the each bit of F(X,Y,Z) will be independent and unbiased. The functions G, H, and I are similar to the function F, in that they act in bitwise parallel to produce their output from the bits of X, Y, and Z, in such a manner that if the corresponding bits of X, Y, an

23、d Z are independent and unbiased, then each bit of G(X,Y,Z), H(X,Y,Z), and I(X,Y,Z) will be independent and unbiased. Note that the function H is the bit-wise xor or parity function of its inputs. This step uses a 64-element table T1 . 64 constructed from the sine function. Let Ti denote the i-th el

24、ement of the table, which is equal to the integer part of 4294967296 times abs(sin(i), where i is in radians. The elements of the table are given in the appendix. Do the following: /* Process each 16-word block. */ For i = 0 to N/16-1 do /* Copy block i into X. */ For j = 0 to 15 do Set Xj to Mi*16+

25、j. end /* of loop on j */ /* Save A as AA, B as BB, C as CC, and D as DD. */ AA = A BB = BRivest Page 4RFC 1321 MD5 Message-Digest Algorithm April 1992 CC = C DD = D /* Round 1. */ /* Let abcd k s i denote the operation a = b + (a + F(b,c,d) + Xk + Ti) s). */ /* Do the following 16 operations. */ AB

26、CD 0 7 1 DABC 1 12 2 CDAB 2 17 3 BCDA 3 22 4 ABCD 4 7 5 DABC 5 12 6 CDAB 6 17 7 BCDA 7 22 8 ABCD 8 7 9 DABC 9 12 10 CDAB 10 17 11 BCDA 11 22 12 ABCD 12 7 13 DABC 13 12 14 CDAB 14 17 15 BCDA 15 22 16 /* Round 2. */ /* Let abcd k s i denote the operation a = b + (a + G(b,c,d) + Xk + Ti) s). */ /* Do t

27、he following 16 operations. */ ABCD 1 5 17 DABC 6 9 18 CDAB 11 14 19 BCDA 0 20 20 ABCD 5 5 21 DABC 10 9 22 CDAB 15 14 23 BCDA 4 20 24 ABCD 9 5 25 DABC 14 9 26 CDAB 3 14 27 BCDA 8 20 28 ABCD 13 5 29 DABC 2 9 30 CDAB 7 14 31 BCDA 12 20 32 /* Round 3. */ /* Let abcd k s t denote the operation a = b + (

28、a + H(b,c,d) + Xk + Ti) s). */ /* Do the following 16 operations. */ ABCD 5 4 33 DABC 8 11 34 CDAB 11 16 35 BCDA 14 23 36 ABCD 1 4 37 DABC 4 11 38 CDAB 7 16 39 BCDA 10 23 40 ABCD 13 4 41 DABC 0 11 42 CDAB 3 16 43 BCDA 6 23 44 ABCD 9 4 45 DABC 12 11 46 CDAB 15 16 47 BCDA 2 23 48 /* Round 4. */ /* Let

29、 abcd k s t denote the operation a = b + (a + I(b,c,d) + Xk + Ti) s). */ /* Do the following 16 operations. */ ABCD 0 6 49 DABC 7 10 50 CDAB 14 15 51 BCDA 5 21 52 ABCD 12 6 53 DABC 3 10 54 CDAB 10 15 55 BCDA 1 21 56 ABCD 8 6 57 DABC 15 10 58 CDAB 6 15 59 BCDA 13 21 60 ABCD 4 6 61 DABC 11 10 62 CDAB

30、2 15 63 BCDA 9 21 64 /* Then perform the following additions. (That is increment each of the four registers by the value it had before this block was started.) */ A = A + AA B = B + BB C = C + CC D = D + DD end /* of loop on i */Rivest Page 5RFC 1321 MD5 Message-Digest Algorithm April 19923.5 Step 5

31、. Output The message digest produced as output is A, B, C, D. That is, we begin with the low-order byte of A, and end with the high-order byte of D. This completes the description of MD5. A reference implementation in C is given in the appendix.4. Summary The MD5 message-digest algorithm is simple to implement, and provides a fingerprint or message digest of a message of arbitrary length. It is conjectured that the difficulty of coming up with two messages having the same message digest i