Block Cyphers in Computer Security: Understanding the Idea, Characteristics, and Issues, Study notes of Computer Science

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Jan 31, 2005 -- Lecture 6

22C:

Computer Security

Douglas W. Jones Department of Computer Science

Block Cyphers

An Idea for Symmetric Key Cyphers Block of plaintext: P 1 P 2 P 3 P 4 | | V Key - Encypher | | V Cyphertext block: C 1 C 2 C 3 C 4 | | V Key - Decypher | | V Block of plaintext: P 1 P 2 P 3 P 4

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 "help" 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, speed: 3 days to crack

Top level view of DES (2-round version)

Function blocks in each stage of DES

Cracking DES (RSA DES Challenge) First public crack, 1997 39 days using over 10,000 computers Team lead by Rocke Verser of Loveland Colorado Second public crack, 1998 3 days using array of Deep-Crack chips

What To Do? Triple DES: DES(k 1 , DES(k 2 , DES(k 3 ,t))) Warning : What if DES(k 1 , DES(k 2 , t)) = DES(f(k 1 , k 2 ), t) Proofs are difficult!

One AES Round Substitute Bytes Uses a table lookup to do one-to-one Shift Rows Shift each 4-byte row Mix Columns Linear transformation of 4-byte column Add Round Key Key for this round combined with bytes

AES Substitute Bytes Step b = S[a], where S is a 265 entry table

The AES Mix Columns Step Fixed linear transform of 32-bit column

The AES Add Round Key Step XOR