分组密码和数据加密标准课件

,Click to edit Master title style,*,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,第,3,章 分组密码与数据加密标准,第3章 分组密码与数据加密标准,1,Modern Block Ciphers,modern block ciphers,one of the most widely used types of cryptographic algorithms,provide,secrecy/authentication,services,focus on,DES,(Data Encryption Standard)to illustrate block cipher design principles,Modern Block Ciphersmodern blo,2,Block vs Stream Ciphers,block ciphers,process messages,in blocks,each of which is then en/decrypted,like a substitution on very big characters,64-bits or more,stream ciphers,process messages,a bit or byte,at a time when en/decrypting,many current ciphers are block ciphers,broader range of applications,Block vs Stream Ciphersblock c,3,Block Cipher Principles,most symmetric block ciphers are based on a,Feistel Cipher Structure,needed since must be able to,decrypt,ciphertext to recover messages efficiently,block ciphers look like,an extremely large substitution,would need table of 2,64,entries for a 64-bit block,instead create from smaller building blocks using idea of a product cipher,Block Cipher Principlesmost sy,4,Ideal Block Cipher,Ideal Block Cipher,5,Claude Shannon and Substitution-Permutation Ciphers,Claude Shannon introduced,idea of substitution-permutation(S-P)networks,in 1949 paper,form basis of modern block ciphers,S-P nets are based on the,two primitive cryptographic operations,seen before:,substitution,(S-box),permutation,(P-box),provide,confusion,&,diffusion,of message&key,Claude Shannon and Substitutio,6,Confusion and Diffusion,cipher needs to,completely obscure statistical properties,of original message,a one-time pad does this,more practically Shannon suggested combining S&P elements to obtain:,diffusion,dissipates statistical structure of plaintext over bulk of ciphertext,confusion,makes relationship between ciphertext and key as complex as possible,Confusion and Diffusioncipher,7,Feistel Cipher Structure,Feistel devised,Feistel cipher,based on,concept of invertible product cipher,partitions input block into two halves,process through multiple rounds which,perform a substitution on left data half,based on,round function of right half&subkey,then have,permutation swapping halves,implements Shannons S-P net concept,Feistel Cipher StructureFeiste,8,Feistel Cipher Structure,Feistel Cipher Structure,9,Feistel Cipher Design Elements,block size,key size,number of rounds,subkey generation algorithm,round function,fast software en/decryption,ease of analysis,Feistel Cipher Design Elements,10,Feistel Cipher Decryption,Feistel Cipher Decryption,11,Data Encryption Standard(DES),most widely,used,block cipher in world,adopted in 1977 by NBS(now NIST),as FIPS PUB 46,encrypts,64-bit data using 56-bit key,has widespread use,has been considerable controversy over its security,Data Encryption Standard(DES),12,DES History,IBM developed,Lucifer cipher,by team led by Feistel in late 60s,used 64-bit data blocks with 128-bit key,then redeveloped as,a commercial cipher,with input from NSA and others,in 1973 NBS issued request for proposals for a national cipher standard,IBM submitted their,revised Lucifer,which was eventually accepted as the DES,DES HistoryIBM developed Lucif,13,DES Design Controversy,although DES standard is public,it was,considerable controversy over design,in choice of,56-bit key,(vs Lucifer 128-bit),because,design criteria were classified,subsequent events and public analysis show in fact,design was appropriate,use of DES has flourished,especially in,financial applications,still standardised for,legacy application use,DES Design Controversyalthough,14,DES Encryption Overview,DES Encryption Overview,15,Initial Permutation IP,first step of the data computation,IP,reorders,the input data bits,even bits to LH half,odd bits to RH half,quite regular in structure(easy in h/w),example:,IP(675a6967 5e5a6b5a)=(ffb2194d 004df6fb),Initial Permutation IPfirst st,16,DES Round Structure,uses,two 32-bit,L&R halves,as for any,Feistel cipher,can describe as:,L,i,=,R,i,1,R,i,=,L,i,1,F(,R,i,1,K,i,),F takes 32-bit R half and 48-bit subkey:,expands R to 48-bits using perm E,adds to subkey using XOR,passes through 8 S-boxes to get 32-bit result,finally permutes using 32-bit perm P,DES Round Structureuses two 32,17,DES Round Structure,DES Round Structure,18,Substitution Boxes S,have,eight S-boxes,which,map 6 to 4 bits,each S-box is actually 4 little 4 bit boxes,outer bits 1&6(,row,bits)select one row of 4,inner bits 2-5(,col,bits)are substituted,result is 8 lots of 4 bits,or 32 bits,row selection depends on both data&key,feature known as autoclaving(autokeying),example:,S(18 09 12 3d 11 17 38 39)=5fd25e03,Substitution Boxes Shave eight,19,DES Key Schedule,forms,subkeys,used in each round,initial permutation of the key(PC1)which selects 56-bits in two 28-bit halves,16 stages consisting of:,rotating,each half,separately either 1 or 2 places depending on the,key rotation schedule,K,selecting,24-bits,from each half