LectureNote12_DES

LectureNote12_DES - S ECRET K EY C RYPTOSYSTEMS C...

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Unformatted text preview: S ECRET K EY C RYPTOSYSTEMS C RYPTOGRAPHY S ECRET K EY C RYPTOSYSTEMS K EY E SCROW CS 556 - Computer Security - c circlecopyrt 2009 Colorado State University – 31 / 94 Secret Key Cryptosystem C RYPTOGRAPHY S ECRET K EY C RYPTOSYSTEMS K EY E SCROW CS 556 - Computer Security - c circlecopyrt 2009 Colorado State University – 32 / 94 Cryptanalyst Encrypt M with Source Message Message Destination Key Source Key Source Generates Random Key Decrypt C with Provides Or Produces Key Secure Key Channel M M C Insecure Communications Channel C key K 1 key K 2 C = E[M, K 1 ] M = D[C, K 2 ] K 1 K 1 K 2 Basic Secret-Key Techniques C RYPTOGRAPHY S ECRET K EY C RYPTOSYSTEMS K EY E SCROW CS 556 - Computer Security - c circlecopyrt 2009 Colorado State University – 33 / 94 ● Substitution ● Permutation or transposition ✦ Reverse Cipher ✦ Column Transposition ✦ Rail Fence ✦ Scytale Cipher ✦ Nihilist Cipher ● Combination and iterations of these - Product ciphers Simple Alphabetic Substitution C RYPTOGRAPHY S ECRET K EY C RYPTOSYSTEMS K EY E SCROW CS 556 - Computer Security - c circlecopyrt 2009 Colorado State University – 34 / 94 Plaintext ABCDEFGHIJKLMNOPQRSTUVWXYZ Ciphertext PZQSGIMBWXDFKJVCHAOLUTERYN ● Also called Caesar cipher ● Huge key space: 26! greatermuch 10 26 ● Trivially broken for known plaintext attacks ● Easily broken for ciphertext only attacks (for natural language plaintext) ● Multiple encipherment does not help (no point in doing two substitutions in sequence) Simple Permutation C RYPTOGRAPHY S ECRET K EY C RYPTOSYSTEMS K EY E SCROW CS 556 - Computer Security - c circlecopyrt 2009 Colorado State University – 35 / 94 Plaintext 1 2 3 4 5 Ciphertext 1 2 3 5 4 ● Key space N! for block size N ● Trivially broken for known plaintext attack ● Easily broken for ciphertext only attack (for natural language plaintext) ● Multiple encipherment does not help Reverse Cipher C RYPTOGRAPHY S ECRET K EY C RYPTOSYSTEMS K EY E SCROW CS 556 - Computer Security - c circlecopyrt 2009 Colorado State University – 36 / 94 ● Reverse the order of the letters in a message ✦ Plaintext – ICAMEISAWICONQUERED ✦ Ciphertext – DERDUQNOCIWASIEMACI Column Transposition C RYPTOGRAPHY S ECRET K EY C RYPTOSYSTEMS K EY E SCROW CS 556 - Computer Security - c circlecopyrt 2009 Colorado State University – 37 / 94 1 2 3 4 5 6 7 l a s e f b e a m s c a n b e m o d u l a r r y m o r e i n t e l l i g e n c e t h a n r a d i o Key; 4523617 Ciphertext: merne nasso dytnr vbnlc rltiq laetr igawe baaei hox ecdtm ecaef auool edsam w a v e s q x t e d t o c a Rail Fance Cipher C RYPTOGRAPHY S ECRET K EY C RYPTOSYSTEMS K EY E SCROW CS 556 - Computer Security - c circlecopyrt 2009 Colorado State University – 38 / 94 ● Write the message alternating letters in two rows ● Write the ciphertext from the rows N T M F R G N E D O L A O I E I O W S T H E L O M Plaintext Ciphertext NWSHTMFRLGOMNOITEIEOALODE Scytale Cipher C RYPTOGRAPHY S ECRET K EY C RYPTOSYSTEMS...
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LectureNote12_DES - S ECRET K EY C RYPTOSYSTEMS C...

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