CS470, A.SelcukLucifer & DES1 Block Ciphers Lucifer & DES CS 470 Introduction to Applied Cryptography Instructor: Ali Aydin Selcuk.

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CS470, A.SelcukLucifer & DES1 Block Ciphers Lucifer & DES CS 470 Introduction to Applied Cryptography Instructor: Ali Aydin Selcuk

CS470, A.SelcukLucifer & DES2 Block Ciphers & S-P Networks Block Ciphers: Substitution ciphers with large block size (≥ 64 bits) How to define a good substitution for such large blocks? “SP Networks” (Shannon, 1949) –small, carefully designed substitution boxes (“confusion”) –their output mixed by a permutation box (“diffusion”) –iterated a certain number of times

CS470, A.SelcukLucifer & DES3 Lucifer Early 1970s: First serious needs for civilian encryption (in electronic banking) IBM’s response: Lucifer, an iterated SP cipher Lucifer (v0): –Two fixed, 4x4 s-boxes, S 0 & S 1 –A fixed permutation P –Key bits determine which s-box is to be used at each position –8 x 64/4 = 128 key bits (for 64-bit block, 8 rounds)..... P S0S0 S1S1 S0S0 S1S1 S0S0 S1S1 S0S0 S1S1 S0S0 S1S1 S0S0 S1S1 P S0S0 S1S1 S0S0 S1S1 S0S0 S1S1 x E K (x)

CS470, A.SelcukLucifer & DES4 Feistel Ciphers A straightforward SP cipher needs twice the hardware: one for encryption (S, P), one for decryption (S -1, P -1 ). Feistel’s solution: Lucifer v1: Feistel SP cipher; 64-bit block, 128-bit key, 16 rounds. x E K (x) LR f  ...   f f f where the f function is SP: x SSS.. P f(x, k i ) kiki (xhy?)

CS470, A.SelcukLucifer & DES5 Data Encryption Standard (DES) Need for a standardized cipher to protect computer and communications data NBS’ request for proposals (1973) IBM’s submission Lucifer is adopted after a revision by NSA.

CS470, A.SelcukLucifer & DES6 From Lucifer to DES 8 fixed, 6x4 s-boxes (non-invertible) expansion E (simple duplication of 16 bits) round keys are used only for xor with the input 56-bit key size 16 x 48 round key bits are selected from the 56-bit master key by the “key schedule”. x S1S1 S2S2 S8S8.. P f(x, k i ) kiki 32  48 E 32 bits

CS470, A.SelcukLucifer & DES7 The DES Contraversy Design process not made public. Any hidden trapdoors in the s-boxes? 56-bit key length is too short. Is it so that NSA can break it?

CS470, A.SelcukLucifer & DES8 Strengthening DES Multiple DES encryption 3DES: E K3 (D K2 (E K1 (x))) –Why not 2DES? (112-bit key not long enough?) –Why “D”? –Two-key 3DES: K3 = K1 DES-X (Rivest, 1995) E K (x  K1)  K2 –overhead cost minimal –construction is provably secure (Rogaway & Killian) –Why not E K (x)  K2 or E K (x  K1) ?