信息安全指南课件：Chapter 12 Hash and MAC Algorithms

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1、Computer Science&Technology School of Shandong UniversityInstructor:Hou Mengbo Email:houmb AT Office:Information Security Research GroupGuide to Information SecurityChapter 12 Hash and MAC AlgorithmsHash and MAC Algorithms Hash Functions condense arbitrary size message to fixed size by processing me

2、ssage in blocks through some compression function either custom or block cipher based Message Authentication Code(MAC)fixed sized authenticator for some message to provide authentication for message by using block cipher mode or hash functionHash Algorithms see similarities in the evolution of hash

3、functions&block ciphers increasing power of brute-force attacks leading to evolution in algorithms from DES to AES in block ciphers from MD4&MD5 to SHA-1&RIPEMD-160 in hash algorithms likewise tend to use common iterative structure as do block ciphersMD5*designed by Ronald Rivest(the R in RSA)latest

4、 in a series of MD2,MD4 produces a 128-bit hash value until recently was the most widely used hash algorithm in recent times have both brute-force&cryptanalytic concerns specified as Internet standard RFC1321MD5 Overview1.pad message so its length is 448 mod 512 2.append a 64-bit length value to mes

5、sage 3.initialise 4-word(128-bit)MD buffer(A,B,C,D)4.process message in 16-word(512-bit)blocks:using 4 rounds of 16 bit operations on message block&buffer add output to buffer input to form new buffer value 5.output hash value is the final buffer value MD5 OverviewMD5 Compression Function each round

6、 has 16 steps of the form:a=b+(a+g(b,c,d)+Xk+Ti)s)(a,b,c,d)cycled right shift one word a,b,c,d refer to the 4 words of the buffer,but used in varying permutations note this updates 1 word only of the buffer after 16 steps each word is updated 4 times where g(b,c,d)is a different nonlinear function i

7、n each round(F,G,H,I)Ti is a constant value derived from sinSingle Step of MD5 Compression FunctionMD4 precursor to MD5 also produces a 128-bit hash of message has 3 rounds of 16 steps vs 4 in MD5 design goals:collision resistant(hard to find collisions)direct security(no dependence on hard problems

8、)fast,simple,compact favours little-endian systems(eg PCs)Strength of MD5 MD5 hash is dependent on all message bits Rivest claims security is good as can be known attacks are:Berson 92 attacked any 1 round using differential cryptanalysis(but cant extend)Boer&Bosselaers 93 found a pseudo collision(a

9、gain unable to extend)Dobbertin 96 created collisions on MD compression function(but initial constants prevent exploit)conclusion is that MD5 looks vulnerable soonSecure Hash Algorithm(SHA-1)SHA was designed by NIST&NSA in 1993,revised 1995 as SHA-1 US standard for use with DSA signature scheme stan

10、dard is FIPS 180-1 1995,also Internet RFC3174 nb.the algorithm is SHA,the standard is SHS produces 160-bit hash values now the generally preferred hash algorithm based on design of MD4 with key differences SHA Overview1.pad message so its length is 448 mod 512 2.append a 64-bit length value to messa

11、ge3.initialise 5-word(160-bit)buffer(A,B,C,D,E)to(67452301,efcdab89,98badcfe,10325476,c3d2e1f0)4.process message in 16-word(512-bit)chunks:expand 16 words into 80 words by mixing&shifting use 4 rounds of 20 bit operations on message block&buffer add output to input to form new buffer value 5.output

12、hash value is the final buffer value SHA-1 Compression Function each round has 20 steps which replaces the 5 buffer words thus:(A,B,C,D,E)-(E+f(t,B,C,D)+(A5)+Wt+Kt),A,(B30),C,D)a,b,c,d refer to the 4 words of the buffer t is the step number f(t,B,C,D)is nonlinear function for round Wt is derived fro

13、m the message block Kt is a constant value derived from sinSingle Step of SHA-1 Compression FunctionSHA-1 verses MD5 brute force attack is harder(160 vs 128 bits for MD5)not vulnerable to any known attacks(compared to MD4/5)a little slower than MD5(80 vs 64 steps)both designed as simple and compact

14、optimised for big endian CPUs(vs MD5 which is optimised for little endian CPUs)Revised Secure Hash Standard NIST have issued a revision FIPS 180-2 adds 3 additional hash algorithms SHA-256,SHA-384,SHA-512 designed for compatibility with increased security provided by the AES cipher structure&detail

15、is similar to SHA-1 hence analysis should be similar but security levels are rather higherSHA-512 OverviewSHA-512 Compression Function heart of the algorithm processing message in 1024-bit blocks consists of 80 rounds updating a 512-bit buffer using a 64-bit value Wt derived from the current message

16、 block and a round constant based on cube root of first 80 prime numbersSHA-512 Round FunctionSHA-512 Round FunctionWhirlpool now examine the Whirlpool hash function endorsed by European NESSIE project uses modified AES internals as compression function addressing concerns on use of block ciphers se

17、en previously with performance comparable to dedicated algorithms like SHAWhirlpool OverviewWhirlpool Block Cipher W designed specifically for hash function use with security and efficiency of AES but with 512-bit block size and hence hash similar structure&functions as AES but input is mapped row w

18、ise has 10 rounds a different primitive polynomial for GF(28)uses different S-box design&valuesWhirlpool Block Cipher WWhirlpool Performance&Security Whirlpool is a very new proposal hence little experience with use but many AES findings should apply does seem to need more h/w than SHA,but with bett

19、er resulting performanceRIPEMD-160*RIPEMD-160 was developed in Europe as part of RIPE project in 96 by researchers involved in attacks on MD4/5 initial proposal strengthen following analysis to become RIPEMD-160 somewhat similar to MD5/SHA uses 2 parallel lines of 5 rounds of 16 steps creates a 160-

20、bit hash value slower,but probably more secure,than SHARIPEMD-160 Overview1.pad message so its length is 448 mod 512 2.append a 64-bit length value to message3.initialise 5-word(160-bit)buffer(A,B,C,D,E)to(67452301,efcdab89,98badcfe,10325476,c3d2e1f0)4.process message in 16-word(512-bit)chunks:use 1

21、0 rounds of 16 bit operations on message block&buffer in 2 parallel lines of 5add output to input to form new buffer value 5.output hash value is the final buffer valueRIPEMD-160 RoundSingle step of RIPEMD-160 Compression FunctionRIPEMD-160 Design Criteria use 2 parallel lines of 5 rounds for increa

22、sed complexity for simplicity the 2 lines are very similar step operation very close to MD5 permutation varies parts of message used circular shifts designed for best resultsRIPEMD-160 verses MD5&SHA-1 brute force attack harder(160 like SHA-1 vs 128 bits for MD5)not vulnerable to known attacks,like

23、SHA-1 though stronger(compared to MD4/5)slower than MD5(more steps)all designed as simple and compact SHA-1 optimised for big endian CPUs vs RIPEMD-160&MD5 optimised for little endian CPUsKeyed Hash Functions as MACs have desire to create a MAC using a hash function rather than a block cipher becaus

24、e hash functions are generally faster Can use existing Hash library.not limited by export controls unlike block ciphers hash includes a key along with the message original proposal:KeyedHash=Hash(Key|Message)some weaknesses were found with this eventually led to development of HMAC HMAC specified as

25、 Internet standard RFC2104 uses hash function on the message:HMACK=Hash(K+XOR opad)|Hash(K+XOR ipad)|M)where K+is the key padded out to size and opad,ipad are specified padding constants overhead is just 3 more hash calculations than the message needs alone any of MD5,SHA-1,RIPEMD-160,Whirlpool can

26、be used HMAC OverviewHMAC Security know that the security of HMAC relates to that of the underlying hash algorithm attacking HMAC requires either:brute force attack on key used birthday attack(but since keyed would need to observe a very large number of messages)choose hash function used based on sp

27、eed verses security constraintsCMAC previously saw the DAA(CBC-MAC)widely used in govt&industry but has message size limitation can overcome using 2 keys&padding thus forming the Cipher-based Message Authentication Code(CMAC)adopted by NIST SP800-38BCMAC OverviewSummary have considered:some current hash algorithms SHA-512&Whirlpool HMAC authentication using hash function CMAC authentication using a block cipher