Block Ciphers: DES and AES

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Presentation transcript:

Block Ciphers: DES and AES CSCI284 Spring 2004 GWU

One round of DES: Feistel Cipher Equal length Li-1 Ri-1 f Ki Li Ri 2/25/2019 CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES

CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES Diagram cut from FIPS standard f in DES Expansion Input 6 bits permutation Output 4 bits 2/25/2019 CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES

CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES Diagram cut from FIPS standard 2/25/2019 CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES

CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES Modes Electronic Codebook (ECB) Mode Regular, each 64-bit plaintext encrypted with the same key Cipher Block Chaining (CBC) Mode 64-bit ciphertext XORed with next plaintext, then encrypted yi = eK(yi-1 xi) Stream Cipher Modes: yi = xi  zi Output Feedback (OFB) Mode: zi = eK(zi-1) Cipher Feedback (CFB) Mode: zi = eK(yi-1) 2/25/2019 CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES

CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES Chosen for security, efficiency, implementation Key lengths: 128 bits (10 rounds) 192 bits (12 rounds) 256 bits (14 rounds) Consists of: XOR with key, S-box substitution, permutation, mixcolumns 2/25/2019 CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES

High-level AES (all byte operations, 1 round shown) XOR with key S-box defined as an algebraic operation S-box Shift Rows Mix Columns 2/25/2019 CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES

CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES Shift Rows x0 x4 x8 x12 x1 x5 x9 x13 x2 x6 x10 x14 x3 x7 x11 x15 x0 x4 x8 x12 x5 x9 x13 x1 x10 x14 x2 x6 x15 x3 x7 x11 Arrow wrong direction in text 2/25/2019 CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES

CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES Mix Columns a b c d x0 x4 x8 x12 x5 x9 x13 x1 x10 x14 x2 x6 x15 x3 x7 x11 (Aa)0 (Ab)0 (Ac)0 (Ad)0 (Aa)1 (Ab)1 (Ac)1 (Ad)1 (Aa)2 (Ab)2 (Ac)2 (Ad)2 (Aa)3 (Ab)3 (Ac)3 (Ad)3 Multiplication by A is a multiplication in a finite field, not a regular multiplication 2/25/2019 CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES

CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES Key Schedule A key is 4 words; each word is 4 bytes The key has to generate 10 other keys to get a total of 11 for a 10-round AES The 11 keys are represented by 44 words: w[0, ..43] 2/25/2019 CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES

CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES Algorithm 3.6 in the book First 4 words = given key; i.e. first round key = given key for i=0 to 3 w[i] = (key[4i], key[4i+1], key[4i + 2], key[4i +3]) Thereafter, if word is not first word in key, i.e. i  0 mod4 word = corresponding word in previous key  previous word w[i] = w[i-4]w[i-1] 2/25/2019 CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES

When word is first word of key word = first word of previous key  stuff w[i] = w[i-4]SUBWORD(ROTWORD(w[i-1])Rcon[i/4] SUBWORD: AES S-box to each byte ROTWORD: rotate word to left Rcon: constant array of 64-bit values 2/25/2019 CS284/Spring04/GWU/Vora/Block Ciphers: DES and AES