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Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,Cryptography and Network Security(Various Hash Algorithms),Fourth Edition,by William Stallings,Lecture slides by Lawrie Brown,(Changed by Somesh Jha),1,Cryptography and Network Secur,Birthday Attacks,might think a 64-bit hash is secure,but by,Birthday Paradox,is not,birthday attack,works thus:,opponent generates 2,m,/,2,variations of a valid message all with essentially the same meaning,opponent also generates 2,m,/,2,variations of a desired fraudulent message,two sets of messages are compared to find pair with same hash(probability 0.5 by birthday paradox),have user sign the valid message,then substitute the forgery which will have a valid signature,conclusion is that need to use larger MACs,2,Birthday Attacksmight think a,Hash Function Properties,a Hash Function produces a fingerprint of some file/message/data,h=H(M),condenses a variable-length message M,to a fixed-sized fingerprint,assumed to be public,3,Hash Function Propertiesa Hash,Requirements for Hash Functions,can be applied to any sized message,M,produces fixed-length output,h,is easy to compute,h=H(M),for any message,M,given,h,is infeasible to find,x,s.t.,H(x)=h,one-way property,given,x,is infeasible to find,y,s.t,.H(y)=H(x),weak collision resistance,is infeasible to find any,x,y,s.t,.H(y)=H(x),strong collision resistance,4,Requirements for Hash Function,Block Ciphers as Hash Functions,can use block ciphers as hash functions,using H,0,=0 and zero-pad of final block,compute:H,i,=E,M,i,H,i-1,and use final block as the hash value,similar to CBC but without a key,resulting hash is too small(64-bit),both due to direct birthday attack,and to“meet-in-the-middle”attack,other variants also susceptible to attack,5,Block Ciphers as Hash Function,Hash Algorithms,similarities in the evolution of hash 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 ciphers,6,Hash Algorithmssimilarities in,MD5,designed by,Ronald Rivest,(the“,R,”in RSA),latest 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 RFC1321,7,MD5designed by Ronald Rivest(,MD5 Overview,pad message so its length is 448 mod 512,append a 64-bit length value to message,initialise 4-word(128-bit)MD buffer(A,B,C,D),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,output hash value is the final buffer value,8,MD5 Overviewpad message so its,MD5 Overview,9,MD5 Overview9,MD5 Compression Function,each round has 16 steps of the form:,a=b+(a+g(b,c,d)+Xk+Ti)s),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 in each round(F,G,H,I),Ti is a constant value derived from sin,10,MD5 Compression Functioneach r,MD5 Compression Function,11,MD5 Compression Function11,MD4,precursor to MD5,also produces a 128-bit hash of message,has 3 rounds of 16 steps versus 4 in MD5,design goals:,collision resistant(hard to find collisions),direct security(no dependence on hard problems),fast,simple,compact,favors little-endian systems(eg PCs),12,MD4precursor to MD512,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(again unable to extend),Dobbertin 96 created collisions on MD compression function(but initial constants prevent exploit),conclusion is that MD5 looks vulnerable soon,13,Strength of MD5MD5 hash is dep,Secure HashAlgorithm(SHA-1),SHA was designed by NIST&NSA in 1993,revised 1995 as SHA-1,US standard for use with DSA signature scheme,standard is FIPS 180-1 1995,also Internet RFC3174,note:,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,14,Secure HashAlgorithm(SHA-1)S,SHA Overview,pad message so its length is 448 mod 512,append a 64-bit length value to message,initialise 5-word(160-bit)buffer(A,B,C,D,E)to,(67452301,efcdab89,98badcfe,10325476,c3d2e1f0),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,output hash value is the final buffer value,15,SHA Overviewpad message so its,SHA-1 Compression Function,each round has 20 steps which replaces the 5 buffer words thus:,(A,B,C,D,
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