Symmetric Key Block Ciphers - Northern Kentucky University
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Jeff Ehme Spelman College jehme@spelman.edu Chris Christensen Northern Kentucky University christensen@nku.edu Encipher(plaintext, key) plaintext CIPHERTEXT Decipher(CIPHERTEXT, key) function patterned "random" inverse Skytale Caesar cipher Playfair Cipher 19th century Vigenere Cipher 16th century Transposition Cipher KL47 KW-7 ATM IFF Data Encryption Standard 1977 “New Directions in Cryptography” 1976 Symmetric key block ciphers Public key ciphers Stream ciphers Post-quantum ciphers Substitution Transposition DROSX MDCYP ONSXD SXDRO WKDRO XEXNO BONRY OIMSZ QBKZR BOOAE DSXQ DOBOC ZELVS OBOCD COMSZ WKDSM BQBKN GOFOB ROBCD IWYNO KVVIW DSXQW FUOIM LIWKD ROBCK CDRKD EKDOM KBODR ROGYB BXZBS KDROW KDROW SZROB ROWKD CKZZV MKXLO YEBCO OWYNO URYBC FKDOS KDSMK KDSMK CRKFO SMCPK SMKDS ZBOCO CYPDO BXZBS OCYPM OIMSZ VVISX VKCZO CZKBU MEVDI YXCYP XDONS XSQXY FKDOU BIZDY ROBCK DOBOC Lester Hill (1891 – 1961) Symmetric Key vs Asymmetric key Symmetric Key Asymmetric Key If the solution to a problem can be quickly verified by a computer, can the computer also solve that problem quickly? Horst Feistel (1915 – 1990) Simplified DES: Wade Trapp and Lawrence Washington DES Simplified DES 64-bit block 56-bit key (8 parity bits) 8 S-boxes 6 x 4 16 rounds Feistel cipher 12-bit block 9-bit key 2 S-boxes 4 x 3 4 rounds Feistel cipher 9 bits of key 111 010 110 Use the 8 bits on the left for k1 1110 1011 Rotate to the left one bit 110 101 101 Use the 8 bits on the left for k12 1101 0110 Rotate to the left 1 bit 101 011 011 Use the 8 bits on the left for k3 1010 1101 Rotate to the left 1 bit 010 110 111 Use the 8 bits on the left for k4 0101 1011 S1 0 1 0 1 2 3 4 5 6 7 101 010 001 110 011 100 111 000 001 100 110 010 000 111 101 011 S2 0 1 0 1 2 3 4 5 6 7 100 000 110 101 111 001 011 010 101 011 000 111 110 010 001 100 Li Li1 Ri Ri Ri1 Li f Ri, Ki Encryption Li Ri Li1 Ri Ri1 Li f Ri , Ki Li1 Ri Ri1 Li f Ri , Ki Decryption Li Ri1 f Ri , Ki Ri Li1 Encryption Decryption Backdoor? EK 3 DK 2 EK1 plaintext Perfect Security Random key, as long as the message, and used only once. Computational Security Symmetric Key RSA and DH 80 1024 112 2048 128 3072 192 7680 256 15360 Today 112 ECC 160 224 256 384 521 64-bit block 80-bit key 32 rounds Unbalanced Feistel cipher Skipjack declassified 1998 Clipper Chip 1993 -1996 Claude Shannon (1916 – 2001) “A Mathematical Theory of Communication” 1948 “The Theory of Secrecy Systems” 1949 Diffusion: “… the statistical structure of [the message] which leads to its redundancy is dissipated into long range statistics.” Wire crossing, S-box Confusion: “… make the relation between the simple statistics of [the ciphertext] and the simple description of [the key] a very complex and involved one.” Permutation, P-box Many 64-bit block 80- or 128-bit key 1 S-box 4 x 4 31 Rounds Substitution-Permutation Network How do they know it’s secure? Joan Daemen and Vincent Rijmen Simplified AES: Mohammad Musa, Edward Schaefer, and Stephen Wedig AES Simplified AES 128-bit block 128-bit, 192-bit, or 256-bit key 10, 12, or 14 rounds SPN 16-bit block 16-bit key 2 rounds SPN y 1x xy 1 k0k1k2k3 k4k5k6k7 k8k9k10k11 k12k13k14k15 W[0] W[1] pt A NS SR MC A NS SR A CT K K K 0 1 2 Serpent Twofish RC6 MARS Simplified IDEA Hoffman, N. 2007. “A Simplified IDEA Algorithm,” Cryptologia 31(2), 143 – 151. TEA Holden, J. 2013. “Demitasse: A ‘Small’ Version of the Tiny Encryption Algorithm and Its Use in a classroom Setting.” Cryptologia 37(1), 74 – 83. Stream ciphers Simplified DES Trappe, W. and Washington, L. Introduction to Cryptography with Coding Theory Simplified IDEA Hoffman, N. 2007. “A Simplified IDEA Algorithm,” Cryptologia 31(2), 143 – 151. PRESENT Boganov, A. and others. 2007. “PRESENT: An Ultra-Lightweight Block Cipher.” (Search the net.) Simplified AES Musa, A., Schaefer, E., and Wedig, S. 2010. “A Simplified AES Algorithm and Its Linear and Differential Cryptanalysis,” Cryptologia 27(12), 148 – 177. TEA Holden, J. 2013. “Demitasse: A ‘Small’ Version of the Tiny Encryption Algorithm and Its Use in a classroom Setting.” Cryptologia 37(1), 74 – 83. Cryptology course notes and student-produced software http://www.nku.edu/~christensen/1301cscmat483.htm
