Indifferentiability of Confusion- Diffusion Networks Yevgeniy Dodis (NYU), Martijn Stam (Bristol), John Steinberger (Tsinghua), Tianren Liu (MIT) Wednesday, May 11, 16
Indifferentiability of Confusion- Diffusion Networks Yevgeniy Dodis (NYU), Martijn Stam (Bristol), John Steinberger (Tsinghua), Liu Tianren (MIT) Wednesday, May 11, 16
Substitution-Permutation Network (ex: AES): S S S S S S S S π π π π S S S S k 0 k 1 k 2 k 3 k 4 S S S S Wednesday, May 11, 16
Substitution-Permutation Network (ex: AES): S S S S S S S S π π π π S S S S k 0 k 1 k 2 k 3 k 4 S S S S S-boxes (small input size) (permutations!) Wednesday, May 11, 16
Substitution-Permutation Network (ex: AES): S S S S S S S S π π π π S S S S k 0 k 1 k 2 k 3 k 4 S S S S S-boxes “Diffusion” (small input size) (permutations!) Permutations (big input size, simple structure) Wednesday, May 11, 16
Substitution-Permutation Network (ex: AES): S S S S S S S S π π π π S S S S k 0 k 1 k 2 k 3 k 4 S S S S S-boxes Subkeys! “Diffusion” (small input size) (permutations!) Permutations (big input size, simple structure) Wednesday, May 11, 16
Confusion-Diffusion Network: S S S S S S S S S S π π π π S S S S S S S S S S Wednesday, May 11, 16
Confusion-Diffusion Network: S S S S S S S S S S π π π π S S S S S S S S S S ...same, but no keys! Wednesday, May 11, 16
Confusion-Diffusion Network: S S S S S S S S S S π π π π S S S S S S S S S S ...same, but no keys! • can be seen as a domain extension mechanism for permutations (from S-box to “full” permutation) Wednesday, May 11, 16
Confusion-Diffusion Network: S S S S S S S S S S π π π π S S S S S S S S S S ...same, but no keys! • can be seen as a domain extension mechanism for permutations (from S-box to “full” permutation) • terminology goes back to Shannon (1949), but the design paradigm seems to be Feistel’s (1970) Wednesday, May 11, 16
This work’s high-level goals • Investigate the theoretical soundness of CD (confusion-diffusion!) networks as a design paradigm for cryptographic permutations • Fundamental question: (efficient) domain extension of a public random permutation • Work in an ideal model (S-boxes are independent random permutations, D-boxes are fixed, explicit permutations) • Does the network “emulate” a random permutation? How many rounds are necessary, and what kinds of D-boxes do we need?? Wednesday, May 11, 16
This work’s high-level goals • Investigate the theoretical soundness of CD (confusion-diffusion!) networks as a design paradigm for cryptographic permutations • Fundamental question: (efficient) domain extension of a public random permutation • Work in an ideal model (S-boxes are independent random permutations, D-boxes are fixed, explicit permutations) • Does the network “emulate” a random permutation? How many rounds are necessary, and what kinds of D-boxes do we need?? Wednesday, May 11, 16
This work’s high-level goals • Investigate the theoretical soundness of CD (confusion-diffusion!) networks as a design paradigm for cryptographic permutations • Fundamental question: (efficient) domain extension of a public random permutation • Work in an ideal model (S-boxes are independent random permutations, D-boxes are fixed, explicit permutations) • Does the network “emulate” a random permutation? How many rounds are necessary, and what kinds of D-boxes do we need?? Wednesday, May 11, 16
This work’s high-level goals • Investigate the theoretical soundness of CD (confusion-diffusion!) networks as a design paradigm for cryptographic permutations • Fundamental question: (efficient) domain extension of a public random permutation • Work in an ideal model (S-boxes are independent random permutations, D-boxes are fixed, explicit permutations) • Does the network “emulate” a random permutation? How many rounds are necessary, and what kinds of D-boxes do we need?? Wednesday, May 11, 16
vaguely related work • Miles & Viola prove an indistinguishability result for SPN networks where the S-boxes are secret (part of the key) and one-way (so not really an SPN network after all) Wednesday, May 11, 16
vaguely related work CD indifferentiability • Miles & Viola prove an indistinguishability result for SPN networks where the S-boxes are secret (part of the key) and one-way (so not really an SPN network after all) public two-way Wednesday, May 11, 16
S 1 , 3 S 2 , 3 S 3 , 3 S 4 , 3 S 1 , 2 S 2 , 2 S 3 , 2 S 4 , 2 π 1 π 2 π 3 S 1 , 1 S 2 , 1 S 3 , 1 S 4 , 1 Wednesday, May 11, 16
n S 1 , 3 S 2 , 3 S 3 , 3 S 4 , 3 n S 1 , 2 S 2 , 2 S 3 , 2 S 4 , 2 π 1 π 2 π 3 n S 1 , 1 S 2 , 1 S 3 , 1 S 4 , 1 n = wire length Wednesday, May 11, 16
n S 1 , 3 S 2 , 3 S 3 , 3 S 4 , 3 n w = 3 S 1 , 2 S 2 , 2 S 3 , 2 S 4 , 2 π 1 π 2 π 3 n S 1 , 1 S 2 , 1 S 3 , 1 S 4 , 1 n = wire length w = “width” (no. S-boxes per round) Wednesday, May 11, 16
r = 4 n S 1 , 3 S 2 , 3 S 3 , 3 S 4 , 3 n w = 3 S 1 , 2 S 2 , 2 S 3 , 2 S 4 , 2 π 1 π 2 π 3 n S 1 , 1 S 2 , 1 S 3 , 1 S 4 , 1 n = wire length w = “width” (no. S-boxes per round) r = number of rounds Wednesday, May 11, 16
{ 0 , 1 } wn = domain of CD network r = 4 n S 1 , 3 S 2 , 3 S 3 , 3 S 4 , 3 n w = 3 S 1 , 2 S 2 , 2 S 3 , 2 S 4 , 2 π 1 π 2 π 3 n S 1 , 1 S 2 , 1 S 3 , 1 S 4 , 1 n = wire length w = “width” (no. S-boxes per round) r = number of rounds Wednesday, May 11, 16
Security Model (indifferentiability) ? D ? ? Wednesday, May 11, 16
Security Model (indifferentiability) REAL WORLD ? D ? IDEAL WORLD ? Wednesday, May 11, 16
Security Model (indifferentiability) REAL WORLD S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 2 S 2 , 2 S 3 , 2 π 1 π 2 ? S 1 , 1 S 2 , 1 S 3 , 1 D ? IDEAL WORLD ? Wednesday, May 11, 16
Security Model (indifferentiability) REAL WORLD S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 2 S 2 , 2 S 3 , 2 S 1 , 2 S 2 , 2 S 3 , 2 π 1 π 2 ? S 1 , 1 S 2 , 1 S 3 , 1 S 1 , 1 S 2 , 1 S 3 , 1 D ? IDEAL WORLD ? Wednesday, May 11, 16
Security Model (indifferentiability) REAL WORLD S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 2 S 2 , 2 S 3 , 2 S 1 , 2 S 2 , 2 S 3 , 2 π 1 π 2 ? S 1 , 1 S 2 , 1 S 3 , 1 S 1 , 1 S 2 , 1 S 3 , 1 D ? IDEAL WORLD ? Q Wednesday, May 11, 16
Security Model (indifferentiability) REAL WORLD S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 2 S 2 , 2 S 3 , 2 S 1 , 2 S 2 , 2 S 3 , 2 π 1 π 2 ? S 1 , 1 S 2 , 1 S 3 , 1 S 1 , 1 S 2 , 1 S 3 , 1 D ? Insert Simulator IDEAL WORLD ? Here Q Wednesday, May 11, 16
Security Model (indifferentiability) REAL WORLD S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 2 S 2 , 2 S 3 , 2 S 1 , 2 S 2 , 2 S 3 , 2 π 1 π 2 ? S 1 , 1 S 2 , 1 S 3 , 1 S 1 , 1 S 2 , 1 S 3 , 1 D ? Insert Simulator IDEAL WORLD ? S S S Here Q S S S S S S Wednesday, May 11, 16
Security Model (indifferentiability) REAL WORLD S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 2 S 2 , 2 S 3 , 2 S 1 , 2 S 2 , 2 S 3 , 2 π 1 π 2 ? S 1 , 1 S 2 , 1 S 3 , 1 S 1 , 1 S 2 , 1 S 3 , 1 D ? Insert Simulator IDEAL WORLD ? S S S Here Q S S S S S S S Q Goal: By using oracle access to the simulator has to make up answers that look “consistent” with Q Wednesday, May 11, 16
Security Model (indifferentiability) REAL WORLD S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 2 S 2 , 2 S 3 , 2 S 1 , 2 S 2 , 2 S 3 , 2 π 1 π 2 “For so-and-so many rounds, for such-and-such ? S 1 , 1 S 2 , 1 S 3 , 1 S 1 , 1 S 2 , 1 S 3 , 1 diffusion permutations, and with such-and-such D a simulator, the distinguisher cannot distinguish ? Insert Simulator using so-and-so-many queries.” IDEAL WORLD ? S S S Here Q S S S S S S ⊆ { Q Goal: By using oracle access to the simulator has to make up answers that look “consistent” with Q Wednesday, May 11, 16
Security Model (indifferentiability) REAL WORLD S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 3 S 2 , 3 S 3 , 3 S 1 , 2 S 2 , 2 S 3 , 2 S 1 , 2 S 2 , 2 S 3 , 2 π 1 π 2 “For so-and-so many rounds, for such-and-such ? S 1 , 1 S 2 , 1 S 3 , 1 S 1 , 1 S 2 , 1 S 3 , 1 diffusion permutations, and with such-and-such D a simulator, the distinguisher cannot distinguish ? Insert Simulator using so-and-so-many queries.” IDEAL WORLD ? S S S Here Q S S S S S S ⊆ { Q Goal: By using oracle access to the simulator has to make up answers that look “consistent” with Q Wednesday, May 11, 16
Combinatorial Properties of the Diffusion Permutations, by name: 1. Entry-Wise Randomized Preimage Resistance (RPR) 2. Entry-Wise Randomized Collision Resistance (RCR) 3. Conductance (& “all-but-one Conductance”) Wednesday, May 11, 16
RPR $ → x 1 ? = y ∗ x 2 y 2 2 π x 3 x 4 Wednesday, May 11, 16
RPR $ → x 1 ? = y ∗ x 2 y 2 2 π x 3 x 4 For any fixed values of , and x 2 x 3 , and for any , there is low y ∗ x 4 2 probability that over the y 2 = y ∗ 2 randomness in x 1 Wednesday, May 11, 16
RCR $ → x 1 ? = y ′ x 2 , x ′ y 2 2 2 π x 3 , x ′ 3 x 4 , x ′ 4 Wednesday, May 11, 16
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