Dan Boneh Block ciphers The AES block cipher Online Cryptography Course Dan Boneh.

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Presentation transcript:

Dan Boneh Block ciphers The AES block cipher Online Cryptography Course Dan Boneh

Dan Boneh The AES process 1997: NIST publishes request for proposal 1998: 15 submissions. Five claimed attacks. 1999: NIST chooses 5 finalists 2000: NIST chooses Rijndael as AES (designed in Belgium) Key sizes: 128, 192, 256 bits. Block size: 128 bits

Dan Boneh AES is a Subs-Perm network (not Feistel) input S1S1 S1S1 S2S2 S2S2 S3S3 S3S3 S8S8 S8S8 ⋯ output subs. layer perm. layer inversion k1k1 S1S1 S1S1 S2S2 S2S2 S3S3 S3S3 S8S8 S8S8 ⋯ k2k2 S1S1 S1S1 S2S2 S2S2 S3S3 S3S3 S8S8 S8S8 ⋯ ⋯ knkn

Dan Boneh AES-128 schematic input rounds (1)ByteSub (2)ShiftRow (3)MixColumn (1)ByteSub (2)ShiftRow (3)MixColumn k2k2 k2k2 ⋯ k9k9 k9k9 (1)ByteSub (2)ShiftRow (3)MixColumn (1)ByteSub (2)ShiftRow (3)MixColumn k1k1 k1k1 k0k0 k0k0 (1)ByteSub (2)ShiftRow (1)ByteSub (2)ShiftRow output 4 4 k 10 key 16 bytes key expansion: invertible 16 bytes 176 bytes

Dan Boneh The round function ByteSub: a 1 byte S-box. 256 byte table (easily computable) ShiftRows: MixColumns:

Dan Boneh Code size/performance tradeoff Code sizePerformance Pre-compute round functions (24KB or 4KB) largest fastest: table lookups and xors Pre-compute S-box only (256 bytes) smallerslower No pre-computationsmallestslowest

Dan Boneh Example: Javascript AES AES library (6.4KB) no pre-computed tables AES in the browser: Prior to encryption: pre-compute tables Then encrypt using tables

Dan Boneh AES in hardware AES instructions in Intel Westmere: aesenc, aesenclast: do one round of AES 128-bit registers: xmm1=state, xmm2=round key aesenc xmm1, xmm2 ; puts result in xmm1 aeskeygenassist: performs AES key expansion Claim 14 x speed-up over OpenSSL on same hardware Similar instructions on AMD Bulldozer

Dan Boneh Attacks Best key recovery attack: four times better than ex. search [BKR’11] Related key attack on AES-256: [BK’09] Given 2 99 inp/out pairs from four related keys in AES-256 can recover keys in time ≈2 99

Dan Boneh End of Segment