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Computer Security

by: Tamia Bernhard

Computer Security 22C 169

Tamia Bernhard
GPA 3.87


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Class Notes
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This 17 page Class Notes was uploaded by Tamia Bernhard on Friday October 23, 2015. The Class Notes belongs to 22C 169 at University of Iowa taught by Staff in Fall. Since its upload, it has received 13 views. For similar materials see /class/228054/22c-169-university-of-iowa in ComputerScienence at University of Iowa.

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Date Created: 10/23/15
THE UvaEmnv OF IOWA Jan 31 2005 Lecture 6 22C2169 Computer Security Douglas W Jones Deparlmenl 1 Computer Science B cck Cyphere An Idea for Symmetric Key Cyphers Block of plaintext P1 P2 P3 P4 V Key Encypher V Cyphertext block C1 02 C3 C4 V Key Decypher V Block of plaintext P1 P2 P3 P4 Characteristics of Block Cyphers For each key encypher and decypher are One to One functions There are 2 one to one mapping on n bits Ideally key simply selects the mapping How do you select a mapping Hero work Block Cypher Issues Block size Same plaintext likely twice in message Too Small Much larger than key size Limits universe of mappings Typically Similar to key size DES First widely used block cypher 1974 adopted as FIPS 46 1977 Developed by IBM with NSA quothelpquot Block size 64 bits Key size 48 bits why so short Idea Multiround permutation and XOR EFF built a DES cracking engine 1998 cost under 250 000 speed 3 days to crack DES the idea Generate the key schedule 16 keys 48 bits each Each key is function of original key Apply keys in succession 16 rounds of encryption Each round looks relatively weak Design emphasis Easy hardware implementation Top level View of DES 2round version mwnu wvur Klanmlwum my kim m F I Pnzouwur km39ns um gm mm mm 7 mum Function blocks in each stage of DES W 4 5mm 7 mm 1 L j J mummyTum my Jimmy UJJU LLUH TU H UllUJ 7 17m 3 Key Schedule Generation m x n imuvw mm L o no 7 I 1 am in gum My 7 a 1 m T Mu m c a zn quot m m swns sums v v 7 o c an I Eiimuvrn y cumul Cracking DES RSA DES Challenge First public crack 1997 39 da 5 using over 10000 computers Team lead by Flocke Verser ol Loveland Colorado Second public crack 1998 3 clays using array of DeepCrack chips af r a 39 What To Do Triple DES DESk1 DESk2 DESkst Warning What if DESk1 DESk2 t DESfk1 k2 t Proofs are difficult AES Rijndael Joan Daemen and Vincent Rijmen Selected as AES in 2000 in open competition run by NIST As of 2003 Certi ed for classi ed information As of 2004 No recognized successful attacks Characteristics Block size 128 bits Key size 128 192 or 256 bits Multiround with key schedule One AES Round Substitute Bytes Uses a table lookup to do onetoone Shift Rows Shift each 4byte row Mix Columns Linear transformation of 4byte column Add Round Key Key for this round combined with bytes AES Substitute Bytes Step b Sa where Sis a 265 entry table AES Shift Rows Stage This step is as trivial as it looks The AES Mix Columns Step Fixed linear transform of 32bit column The AES Add Round Key Step


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