Lecture - Data Encryption Standard 4

Transcription

1 The Data Encrypton Standard For an encrypton algorthm we requre: secrecy of the key and not of the algorthm tself s the only thng that s needed to ensure the prvacy of the data the best cryptographc algorthms can prevent the attacker from deducng the key even when the ndvdual knows both the plantext and cphertext The followng sldes gve an overvew for DES For CNPA A famlarse yourself wth the general dagram on followng page and use these sldes to gan an overvew of the algorthm Block Cpher Desgn Claude Shannon (Paper 1945/49) Confuson and Dffuson consdered cornerstone of modern Block Cpher Desgn Confuson a process employed to obscure the relatonshp between plantext and cphertext Obscures the relatonshp between the statstcs of the cphertext and the encrypton key any observer knowng some nformaton regardng round n a Festel Cpher System (eg DES) but not the round key K should know less and less about subsequent rounds Block Cpher Desgn Dffuson a process that dsspates the statstcal structure of the plantext nto long range statstcs of the cphertext such that the frequences are more unform n the cphertext than was the case wth the plantext that process by whch each bt n a block has the chance to affect other bts n the block The smplest form of: confuson s acheved through substtuton dffuson s acheved through transposton Data Encrypton Standard May 15th 1973 US Federal Department of Commerce call for cryptographc standard for storng, processng and dstrbutng nformaton Specfcatons hgh level of securty comprehensve and transparent specfcaton securty may not rely upon the secrecy of the algorthm avalable and accessble to all users sutable for a varety of applcatons low cost mplementaton able to be exported accessble for valdaton Data Encrypton Standard No Proposals resulted! A new s call made August 6th 1974 IBM respond wth varaton on the Lucfer Cpher System encphers blocks of 128 bts wth a 128 bt key after analyss Natonal Bureau of Standards wth the Natonal Securty Agency make numerous modfcatons July 15th Algorthm publshed under the name of DES DES adopted by ANSI - Amercan Natonal Standards Agency: ANSI X392 - sets standard for bankng The DES Algorthm - Overvew Input Permutaton IP L 0 R 0 Round1 L 1 R 1 Round 2 L 2 R 2 Fnal Permutaton IP -1 Output 4

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3 DES Algorthm Data enters n 64 bt blocks 1 Intal Permutaton IP s appled to the data: IP: Here: the 58th bt s placed n poston 1 the 50th bt s placed n poston 2 DES Algorthm 2 The 64 bts are splt nto 32 left hand bts L 0, 32 rght hand bts R 0 and the 16 DES ROUNDS are appled 3 The Inverse Permutaton IP -1 s appled to the result: IP -1 : Here the 40th bt s placed n poston 1 DES - Intal Permutaton DES - Fnal Permutaton IP: IP -1 : DES The Intermedate Steps DES Round/Cpher Functon F(R -1, K ) The Rounds: L= R 1 R = L F(R, K ) 1 1 K = G( K, ) where G(, ) denotes that the subkey K s a functon of the orgnal (64/56 bt) key K and the processng step or round 32 bt 48 bt R -1 E E(R -1 ) 48 bt 32 bt Note: the Round / Cpher Functon F(R-1, K) s also referred to as the Combner Functon S - Boxes P Output K 48 bt 5

4 Round/Cpher Functon F(R -1, K ) R - The nput ( the rght half of the 64 bt block) -1 E - The Expanson Operator K - The subkey generated at the 'th round - The exclusve - OR (XOR) operator P - The Permutaton Operator Output - The output at the end of the round functon Round/Cpher Functon F(R -1, K ) 1 The cpher functon F(R -1, K ) commences wth R -1 The Expanson operator E s appled to the 32 bt R -1 whch t expands to 48 bts (Shannon dffuson) E: Round/Cpher Functon F(R -1, K ) 2 Followng the expanson operator the resultng 48 bt block s XOR d wth the subkey K 3 The result s then broken up nto 8 blocks of length 6 bts each Each of these are fed nto the S Boxes The frst block of sx are fed nto the frst S - Box S 1 The second block of sx are fed nto the second S - Box S 2, and so forth (See S-Box Hand out) Out of each S - Box result 4 bts gvng 8 x 4 = 32 bts Round/Cpher Functon F(R -1, K ) 4 The resultng 32 bts are then subject to the permutaton P: 5 The resultng output XOR d wth L -1 becomes R Manpulaton at each round of DES L - 1 R -1 L R F F(R -1, K ) K See Fgure 85 p585 S-Box Method The S-Boxes (Substtuton Boxes) Heart of the DES Algorthm Transforms 6 nput bts to 4 output bts Method Let(x 1, x 2, x 3, x 4, x 5, x 6 ) denote 6 nput bts to n S-Box Use x 1 and x 6 to determne whch row of the S-Box to use Use x 2, x 3, x 4 and x 5 to determne whch column of the S-Box to use Output = ntersecton of the column and row 6

5 S-Box Example for S - Box 4 For (111011) Row = 11 = 3 Column = 1101 = 13 Intersecton of row 3 and column 13 s 7 = 0111 Hence nput s and output s S-Box Generatng the Sub-Keys K Sub-key algorthm DES - Expanson Operator DES - Permutaton after S - Boxes E: DES - Sub Key Permutatons DES - Sub Key Permutatons PC-1: PC-2: