# Post-Quantum Cryptography #4

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1 Post-Quantum Cryptography #4 Prof. Claude Crépeau McGill University 185

2 ( 186

3 Attack scenarios Ciphertext-only attack: This is the most basic type of attack and refers to the scenario where the adversary just observes a ciphertext (or multiple ciphertexts) and attempts to determine the underlying plaintext (or plaintexts). m? cwill you marry me? 187

4 cwill you marry me? Attack scenarios Known-plaintext attack: The adversary learns one or more pairs of plaintexts/ciphertexts encrypted under the same key. The aim is to determine the plaintext that was encrypted in some other ciphertext. m m? c Will you marry me? 188

5 Attack scenarios Chosen-plaintext attack: The adversary has the ability to obtain the encryption of plaintexts of its choice. It then attempts to determine the plaintext that was encrypted in some other ciphertext. m? m cwill you marry me? c Will you marry me? 189

6 Attack scenarios Chosen-ciphertext attack: The adversary is even given the capability to obtain the decryption of ciphertexts of its choice. The adversary s aim, once again, is to determine the plaintext that was encrypted in some other ciphertext. c cwill you marry me? m m? c Will you marry me? 190

7 What is secure encryption? Answer 1 an encryption scheme is secure if no adversary can find the secret key when given a ciphertext. 191

8 secure encryption. Answer 2 an encryption scheme is secure if no adversary can find the plaintext that corresponds to the ciphertext. 192

9 secure encryption. Answer 3 an encryption scheme is secure if no adversary can determine any character of the plaintext that corresponds to the ciphertext. 193

10 secure encryption. Answer 4 an encryption scheme is secure if no adversary can derive any meaningful information about the plaintext from the ciphertext. Definitions of security should suffice for all potential applications. 194

11 secure encryption. The Final Answer an encryption scheme is secure if no adversary can compute any function of the plaintext from the ciphertext. 195

12 Perfect Secrecy DEFINITION 2.1 An encryption scheme (Gen, Enc, Dec) over a message space M is perfectly secret if for every probability distribution over M, every message m M, and every ciphertext c C for which Pr[C = c] > 0 : Pr[M = m C = c] = Pr[M = m]. 196

13 An equivalent formulation LEMMA 2.2 An encryption scheme (Gen, Enc, Dec) over a message space M is perfectly secret if and only if for every probability distribution over M, every message m M, and every ciphertext c C : Pr[C = c M = m] = Pr[C = c]. 197

14 Perfect indistinguishability LEMMA 2.3 An encryption scheme (Gen, Enc, Dec) over a message space M is perfectly secret if and only if for every probability distribution over M, every m0, m1 M, and every c C : Pr[ C = c M = m0 ] = Pr[ C = c M = m1 ]. 198

16 Adversarial indistinguishability. This other definition is based on an experiment involving an adversary A, and formalizes A s inability to distinguish the encryption of one plaintext from the encryption of another; we thus call it adversarial indistinguishability. 199

17 Adversarial indistinguishability. This other definition is based on an experiment involving an adversary A, and formalizes A s inability to distinguish the encryption of one plaintext from the encryption of another; we thus call it adversarial indistinguishability. This definition will serve as our starting point when we introduce the notion of computational security in the next chapter. 199

19 Adversarial indistinguishability. The experiment is defined for any encryption scheme Π = (Gen, Enc, Dec) over message space M and for any adversary A. 200

20 Adversarial indistinguishability. The experiment is defined for any encryption scheme Π = (Gen, Enc, Dec) over message space M and for any adversary A. We let PrivK ea A, v denote an execution of the Π experiment for a given Π and A. The experiment is defined as follows: 200

21 PrivK e A a, v Π A 201

22 PrivK e A a, v Π m0, m1 M A 201

23 PrivK e A a, v Π k Gen m0, m1 M A 201

24 PrivK e A a, v Π k Gen b { 0, 1 } m0, m1 M A 201

25 PrivK e A a, v Π k Gen b { 0, 1 } c Enck(mb) m0, m1 M A 201

26 PrivK e A a, v Π k Gen b { 0, 1 } c Enck(mb) m0, m1 M c A 201

27 PrivK e A a, v Π k Gen b { 0, 1 } c Enck(mb) m0, m1 M c A b 201

28 PrivK e A a, v Π k Gen b { 0, 1 } c Enck(mb) m0, m1 M c A b b 201

29 PrivK e A a, v Π k Gen b { 0, 1 } c Enck(mb) m0, m1 M c A b b b = b? 201

31 Adversarial indistinguishability. PrivK e A a, v Π : 202

32 Adversarial indistinguishability. PrivK e A a, v Π : 1. Adversary A outputs a pair of messages m0, m1 M. 202

33 Adversarial indistinguishability. PrivK ea A, v : Π 1. Adversary A outputs a pair of messages m0, m1 M. 2. A random key k is generated by running Gen, and a random bit b { 0, 1 } is chosen (by some imaginary entity that is running the experiment with A.) A ciphertext c Enck(mb) is computed and given to A. 202

34 Adversarial indistinguishability. PrivK ea A, v : Π 1. Adversary A outputs a pair of messages m0, m1 M. 2. A random key k is generated by running Gen, and a random bit b { 0, 1 } is chosen (by some imaginary entity that is running the experiment with A.) A ciphertext c Enck(mb) is computed and given to A. 3. A outputs a bit b. 202

35 Adversarial indistinguishability. PrivK ea A, v : Π 1. Adversary A outputs a pair of messages m0, m1 M. 2. A random key k is generated by running Gen, and a random bit b { 0, 1 } is chosen (by some imaginary entity that is running the experiment with A.) A ciphertext c Enck(mb) is computed and given to A. 3. A outputs a bit b. 4. The output of the experiment is defined to be 1 if b = b, and 0 otherwise. 202

37 Adversarial indistinguishability. We write PrivK e A a, v Π = 1 if the output is 1 and in this case we say that A succeeded. 203

38 Adversarial indistinguishability. We write PrivK ea A, v = 1 if the output is 1 and in Π this case we say that A succeeded. One should think of A as trying to guess the value of b that is chosen in the experiment, and A succeeds when its guess b is correct. 203

39 Adversarial indistinguishability. We write PrivK ea A, v = 1 if the output is 1 and in Π this case we say that A succeeded. One should think of A as trying to guess the value of b that is chosen in the experiment, and A succeeds when its guess b is correct. The alternate definition we now give states that an encryption scheme is perfectly secret if no adversary A can succeed with probability any better than 1 /2. 203

40 PrivK e A a, v Π A 204

41 PrivK e A a, v Π k Gen b { 0, 1 } c Enck(mb) m0, m1 M c A 204

42 PrivK e A a, v Π k Gen b { 0, 1 } c Enck(mb) m0, m1 M c A b 204

43 PrivK e A a, v Π k Gen b { 0, 1 } c Enck(mb) m0, m1 M c A b b 204

44 PrivK e A a, v Π k Gen b { 0, 1 } c Enck(mb) m0, m1 M c A b b Pr[ b = b ] = 1 /2 204

45 PrivK e A a, v Π k Gen b { 0, 1 } c Enck(mb) m0, m1 M c A perfectly secret b b Pr[ b = b ] = 1 /2 204

46 Adversarial indistinguishability. DEFINITION 2.4 An encryption scheme Π = (Gen, Enc, Dec) over a message space M is perfectly secret if for every adversary A it holds that Pr[ PrivK ea A, v = 1 ] = 1 Π /2. 205

47 Adversarial indistinguishability. PROPOSITION 2.5 Let (Gen, Enc, Dec) be an encryption scheme over a message space M. Then (Gen, Enc, Dec) is perfectly secret with respect to Definition 2.1 if and only if it is perfectly secret with respect to Definition

48 4 Equivalent Formulations DEFINITION 2.1 An encryption scheme (Gen, Enc, Dec) over a message space M is perfectly secret if for every probability distribution over M, every message m M, and every ciphertext c C for which Pr[C = c] > 0 : Pr[M = m C = c] = Pr[M = m]. LEMMA 2.3 An encryption scheme (Gen, Enc, Dec) over a message space M is perfectly secret if and only if for every probability distribution over M, every m0, m1 M, and every c C : Pr[ C = c M = m0 ] = Pr[ C = c M = m1 ]. LEMMA 2.2 An encryption scheme (Gen, Enc, Dec) over a message space M is perfectly secret if and only if for every probability distribution over M, every message m M, and every ciphertext c C : Pr[C = c M = m] = Pr[C = c]. DEFINITION 2.4 An encryption scheme Π = (Gen, Enc, Dec) over a message space M is perfectly secret if for every adversary A it holds that Pr[ PrivK e a v A, Π = 1 ] = 1 /2. 207

49 3.2 Defining Computationally- Secure Encryption DEFINITION 3.7 A private-key encryption scheme is a tuple of probabilistic polynomial-time algorithms (Gen, Enc, Dec) such that: 1/3. The key-generation algorithm Gen takes as input the security parameter 1 n and outputs a key k; we write this as k Gen(1 n ) (thus emphasizing the fact that Gen is a randomized algorithm). We will assume without loss of generality that any key k Gen(1 n ) satisfies k n. 208

50 Defining Computationally- Secure Encryption DEFINITION 3.7 A private-key encryption scheme is a tuple of probabilistic polynomial-time algorithms (Gen, Enc, Dec) such that: 2/3. The encryption algorithm Enc takes as input a key k and a plaintext message m {0,1}, and outputs a ciphertext c. Since Enc may be randomized, we write c Enck(m). 209

51 Defining Computationally- Secure Encryption DEFINITION 3.7 A private-key encryption scheme is a tuple of probabilistic polynomial-time algorithms (Gen, Enc, Dec) such that: 3/3. The decryption algorithm Dec takes as input a key k and a ciphertext c, and outputs a message m. We assume that Dec is deterministic, and so write this as m Deck(c). 210

52 Defining Computationally- Secure Encryption It is required that for every n, every key k output by Gen(1 n ), and every m {0,1}, it holds that Deck(Enck(m)) = m. If (Gen, Enc, Dec) is such that for k output by Gen(1 n ), algorithm Enck is only defined for m {0,1} (n), then we say that (Gen, Enc, Dec) is a fixed-length private-key encryption scheme for messages of length (n). 211

53 Indistinguishability in the presence of an eavesdropper An experiment is defined for any private-key encryption scheme Π = (Gen, Enc, Dec), any PPT adversary A and any value n for the security parameter. The eavesdropping indistinguishability experiment PrivK e A a, v Π(n) : 212

54 PrivK e A a, v Π 1 n A 213

55 PrivK e A a, v Π 1 n m0, m1 M A 213

56 PrivK e A a, v Π 1 n k Gen(1 n ) m0, m1 M A 213

57 PrivK e A a, v Π 1 n k Gen(1 n ) b { 0, 1 } m0, m1 M A 213

58 PrivK e A a, v Π 1 n k Gen(1 n ) b { 0, 1 } c Enck(mb) m0, m1 M A 213

59 PrivK e A a, v Π 1 n k Gen(1 n ) b { 0, 1 } c Enck(mb) m0, m1 M c A 213

60 PrivK e A a, v Π 1 n k Gen(1 n ) b { 0, 1 } c Enck(mb) m0, m1 M c A b 213

61 PrivK e A a, v Π 1 n k Gen(1 n ) b { 0, 1 } c Enck(mb) m0, m1 M c A b b 213

62 PrivK e A a, v Π 1 n k Gen(1 n ) b { 0, 1 } c Enck(mb) m0, m1 M c A b b Pr[ b = b ] ½ + negl(n) 213

63 PrivK e A a, v Π 1 n k Gen(1 n ) b { 0, 1 } c Enck(mb) m0, m1 M c A computationally secret b b Pr[ b = b ] ½ + negl(n) 213

64 PrivK e A a, v Π(n) 1. The adversary A is given input 1 n, and outputs a pair of messages m0, m1 of the same length. 2. A key k is generated by running Gen(1 n ), and a random bit b {0,1} is chosen. A (challenge) ciphertext c Enck(mb) is computed and given to A. 3. A outputs a bit b. 4. The output of the experiment is defined to be 1 if b = b, and 0 otherwise. (If PrivK e A a, v Π(n) = 1, we say that A succeeded.) 214

65 PrivK e A a, v Π(n) If Π is a fixed-length scheme for messages of length (n), the previous experiment is modified by requiring m0, m1 {0,1} (n). 215

66 Defining Computationally- Secure Encryption DEFINITION 3.8 A private-key encryption scheme Π = (Gen, Enc, Dec) has indistinguishable encryptions in the presence of an eavesdropper if for all PPT adversaries A there exists a negligible function negl such that Pr[ PrivK e A a, v Π(n) = 1 ] ½ + negl(n), where the probability is taken over the random coins used by A, as well as the random coins used in the experiment (for choosing the key, the random bit b, and any random coins used in the encryption process). 216

67 3.2.2* Properties of the Definition DEFINITION 3.12 A private-key encryption scheme (Gen, Enc, Dec) is semantically secure in the presence of an eavesdropper if for every PPT algorithm A there exists a PPT algorithm A such that for all efficiently-sampleable distributions X = (X1,...) and all polynomial-time computable functions f and h, there exists a negligible function negl s.t. Pr[ A(1 n, Enck(m), h(m)) = f(m) ] Pr[ A (1 n, h(m)) = f(m) ] negl(n), where m is chosen according to distribution Xn, and the probabilities are taken over the choice of m and the key k, and any random coins used by A, A, and the encryption process. 217

68 A 218

69 1 n A 218

70 k Gen(1 n ) 1 n A 218

71 k Gen(1 n ) 1 n c Enck(m) A 218

72 k Gen(1 n ) 1 n h(m) c Enck(m) A 218

73 k Gen(1 n ) 1 n c Enck(m) h(m) c A 218

74 k Gen(1 n ) 1 n c Enck(m) h(m) c A z 218

75 k Gen(1 n ) 1 n c Enck(m) h(m) c A z 218

76 k Gen(1 n ) 1 n c Enck(m) h(m) c A z A 218

77 k Gen(1 n ) 1 n c Enck(m) h(m) c A z 1 n A 218

78 k Gen(1 n ) 1 n c Enck(m) h(m) c A z 1 n h(m) A 218

79 k Gen(1 n ) 1 n c Enck(m) h(m) c A z 1 n h(m) z A 218

80 k Gen(1 n ) 1 n c Enck(m) h(m) c A Pr[z = f(m)] Pr[z = f(m)] negl(n), z 1 n h(m) z A 218

81 Semantic Security THEOREM 3.13 A private-key encryption scheme has indistinguishable encryptions in the presence of an eavesdropper if and only if it is semantically secure in the presence of an eavesdropper. Shafi Goldwasser Silvio Micali 219

82 ) 220

83 Post-Quantum Cryptography Finite Fields based cryptography Codes Multi-variate Polynomials Integers based cryptography Approximate Integer GCD Lattices 221

84 Lattice based cryptography x 3b1+2b2 b2 0 b1 222

85 Lattices Given n-linearly independent vectors b 1,...,b n R n, the lattice they generate is the set of vectors L(b 1,...,b n ) = i n =1 x i b i :x i Z. The vectors b 1,...,b n are known as a basis of the lattice. 223

86 Lattices x 3b1+2b2 b2 0 b1 224

87 Integer Lattices Given n-linearly independent vectors b 1,...,b n Z n, the lattice they generate is the set of vectors L(b 1,...,b n ) = i n =1 x i b i :x i Z. The vectors b 1,...,b n are known as a basis of the lattice. 225

88 Lattices x b1+b2 b2 0 b1 226

89 Closest Vector Problem Given a basis b 1,...,b n R n, and a vector t R n find the closest vector in the lattice L(b 1,...,b n ) (x 1,...,x n ) Z n : d(t, i n =1 x i b i ) is minimal. d(u,v) is Euclidean distance i n =1 (u i -v i ) 2 227

90 CVP t b2 0 b1 Analoguous to correcting errors in codes 228

91 CVP t b2 0 b1 Analoguous to correcting errors in codes 229

92 Shortest Vector Problem Given a basis b 1,...,b n R n find the shortest vector in the lattice L(b 1,...,b n ) (x 1,...,x n ) Z n \0 : d(0, i n =1 x i b i ) is minimal. d(u,v) is Euclidean distance i n =1 (u i -v i ) 2 230

93 SVP shortest b2 b1 0 shortest Analoguous to finding min distance in code 231

94 GGH 232

95 GGH The GGH cryptosystem, proposed by Goldreich, Goldwasser, and Halevi is essentially a lattice analogue of the McEliece/Niederreiter cryptosystem 232

96 GGH The GGH cryptosystem, proposed by Goldreich, Goldwasser, and Halevi is essentially a lattice analogue of the McEliece/Niederreiter cryptosystem The private key is a good lattice basis B. 232

97 GGH The GGH cryptosystem, proposed by Goldreich, Goldwasser, and Halevi is essentially a lattice analogue of the McEliece/Niederreiter cryptosystem The private key is a good lattice basis B. Typically, a good basis consists of short, almost orthogonal vectors. 232

98 GGH The GGH cryptosystem, proposed by Goldreich, Goldwasser, and Halevi is essentially a lattice analogue of the McEliece/Niederreiter cryptosystem The private key is a good lattice basis B. Typically, a good basis consists of short, almost orthogonal vectors. Algorithmically, good bases allow to efficiently solve certain instances of the closest vector problem in L(B), e.g., instances where the target is very close to the lattice. 232

99 GGH/HNF 233

100 GGH/HNF The public key H is a bad basis for the same lattice L(H) = L(B). 233

101 GGH/HNF The public key H is a bad basis for the same lattice L(H) = L(B). Micciancio proposed to use the Hermite Normal Form (HNF) of B. This normal form gives a lower triangular basis for L(B). 233

102 GGH/HNF The public key H is a bad basis for the same lattice L(H) = L(B). Micciancio proposed to use the Hermite Normal Form (HNF) of B. This normal form gives a lower triangular basis for L(B). Notice that any attack on the HNF public key can be easily adapted to work with any other basis B of L(B) by first computing H from B. 233

103 GGH/HNF 234

104 GGH/HNF The encryption process consists of adding a short noise vector r (somehow encoding the message to be encrypted) to a properly chosen lattice point v. 234

105 GGH/HNF The encryption process consists of adding a short noise vector r (somehow encoding the message to be encrypted) to a properly chosen lattice point v. It was proposed to select the vector v such that all the coordinates of (r + v) are reduced modulo the corresponding element along the diagonal of the HNF public basis H. 234

106 GGH/HNF The encryption process consists of adding a short noise vector r (somehow encoding the message to be encrypted) to a properly chosen lattice point v. It was proposed to select the vector v such that all the coordinates of (r + v) are reduced modulo the corresponding element along the diagonal of the HNF public basis H. The resulting vector is denoted r mod H, and it provably makes cryptanalysis hardest because r mod H can be efficiently computed from any vector of the form (r + v) with v L(B). 234

107 GGH/HNF 235

108 GGH/HNF The decryption problem corresponds to finding the lattice point v closest to the target ciphertext c = (r mod H) = v+r, and the error vector r = c v. 235

109 GGH/HNF The decryption problem corresponds to finding the lattice point v closest to the target ciphertext c = (r mod H) = v+r, and the error vector r = c v. The correctness of the GGH/HNF cryptosystem rests on the fact that the error vector r is short enough so that the lattice point v can be recovered from the ciphertext v+r using the private basis B, e.g., by using Babai s rounding procedure, which gives v = B[B 1 (v + r)] where [x] stands for the nearest integer to x 235

110 236

111 q-ary Lattices Given n-linearly independent vectors b 1,...,b n Z n, the q-ary lattice they generate is the set of vectors L(b 1,...,b n,q 1,...,q n ) = i n =1 x i b i mod q:x i Z where each vector q i is of the form (0,...,0,q,0,...,0) 237

112 q-ary Lattices mod q x 3b1+2b2 b2 0 b1 238

113 q-ary Lattices 239

114 q-ary Lattices Structure very similar to linear codes 239

115 q-ary Lattices Structure very similar to linear codes We define two types of q-ary lattices from a matrix A Z nxm q q (A)={y Z m q : y = A T s mod q, s Z qn } q(a)={y Z m q : Ay = 0 mod q} 239

116 Learning With Errors LWE uses a discrete normal distribution - - with mean 0 and standard deviation q / 2π defined as [ ] mod q 240

117 Learning With Errors LWE uses a discrete normal distribution - - with mean 0 and standard deviation q / 2π defined as [ ] mod q -q/2 +q/2 241

118 Learning With Errors A generalization of Learning Parity with Noise where q=2 and Bernouilli errors. 242

119 Learning With Errors A generalization of Learning Parity with Noise where q=2 and Bernouilli errors. LWE is parametrized by n and q=poly(n) 242

120 Learning With Errors A generalization of Learning Parity with Noise where q=2 and Bernouilli errors. LWE is parametrized by n and q=poly(n) A: Z q mxn, a uniform public matrix 242

121 Learning With Errors A generalization of Learning Parity with Noise where q=2 and Bernouilli errors. LWE is parametrized by n and q=poly(n) A: Z q mxn, a uniform public matrix S: Z qn, a uniform secret (trapdoor) vector 242

122 Learning With Errors A generalization of Learning Parity with Noise where q=2 and Bernouilli errors. LWE is parametrized by n and q=poly(n) A: Z q mxn, a uniform public matrix S: Z qn, a uniform secret (trapdoor) vector E: Z qm, a secret vector where each entry has distribution - - with s.t. q n (reductions & there is an exp(( q) 2 )-time attack) 242

123 Learning With Errors A generalization of Learning Parity with Noise where q=2 and Bernouilli errors. LWE is parametrized by n and q=poly(n) A: Z q mxn, a uniform public matrix S: Z qn, a uniform secret (trapdoor) vector E: Z qm, a secret vector where each entry has distribution - - with s.t. q n (reductions & there is an exp(( q) 2 )-time attack) (search-)lwe: Given A and P=AS+E find S. 242

124 Learning With Errors 243

125 Learning With Errors Decision-LWE is made of 243

126 Learning With Errors Decision-LWE is made of A: Z q mxn, a uniform public matrix 243

127 Learning With Errors Decision-LWE is made of A: Z q mxn, a uniform public matrix S: Z qn, a uniform secret (trapdoor) vector 243

128 Learning With Errors Decision-LWE is made of A: Z q mxn, a uniform public matrix S: Z qn, a uniform secret (trapdoor) vector E: Z qm, a secret vector where each entry has distribution

129 Learning With Errors Decision-LWE is made of A: Z q mxn, a uniform public matrix S: Z qn, a uniform secret (trapdoor) vector E: Z qm, a secret vector where each entry has distribution - -. Decision LWE : Given either A and P=AS+E or A,P for unfiorm P, identify which is the case. 243

130 Learning With Errors Decision-LWE is made of A: Z q mxn, a uniform public matrix S: Z qn, a uniform secret (trapdoor) vector E: Z qm, a secret vector where each entry has distribution - -. Decision LWE : Given either A and P=AS+E or A,P for unfiorm P, identify which is the case. Equivalent to the search problem. 243

131 LWE hardness GapSVP SIVP search-lwe decision-lwe crypto 244

132 LWE hardness Quantum!!! GapSVP SIVP search-lwe decision-lwe crypto 244

133 LWE based cryptography 245

134 LWE based cryptography Private key: S: Z qn, E: Z q m sampled using

135 LWE based cryptography Private key: S: Z qn, E: Z q m sampled using - - Public Key: A: Z q mxn, P=AS+E 245

136 LWE based cryptography Private key: S: Z qn, E: Z q m sampled using - - Public Key: A: Z q mxn, P=AS+E Input message: b: {0,1} 245

137 LWE based cryptography Private key: S: Z qn, E: Z q m sampled using - - Public Key: A: Z q mxn, P=AS+E Input message: b: {0,1} Enc AP (v) := (A T a,p T a+bq/2) where a: {0,1} m 245

138 LWE based cryptography Private key: S: Z qn, E: Z q m sampled using - - Public Key: A: Z q mxn, P=AS+E Input message: b: {0,1} Enc AP (v) := (A T a,p T a+bq/2) where a: {0,1} m Dec S (u,c) := 1 (0) iff c-s T u is closer to q/2 (0) c-s T u = P T a+bq/2-s T A T a = P T a+bq/2-p T a+ea = bq/2+ea 245

139 LWE based cryptography 246

140 LWE based cryptography In the first part, one shows that distinguishing between public keys (A,P) as generated by the cryptosystem and pairs chosen uniformly at random from Z q mxn Z q m implies a solution to the LWE problem with parameters n,m,q,

141 LWE based cryptography In the first part, one shows that distinguishing between public keys (A,P) as generated by the cryptosystem and pairs chosen uniformly at random from Z q mxn Z q m implies a solution to the LWE problem with parameters n,m,q, - -. The second part consists of showing that if one tries to encrypt with a public key (A,P) chosen at random, then with very high probability, the result carries essentially no statistical information about the encrypted message. (m > n log q) 246

142 LWE based cryptography In the first part, one shows that distinguishing between public keys (A,P) as generated by the cryptosystem and pairs chosen uniformly at random from Z q mxn Z q m implies a solution to the LWE problem with parameters n,m,q, - -. The second part consists of showing that if one tries to encrypt with a public key (A,P) chosen at random, then with very high probability, the result carries essentially no statistical information about the encrypted message. (m > n log q) Together, these two parts establish the security of the cryptosystem (under chosen plaintext attacks). 246

143 LWE-2 based cryptography 247

144 LWE-2 based cryptography Private key: S,E: Z q n both sampled using - -, 247

145 LWE-2 based cryptography Private key: S,E: Z q n both sampled using - -, Public Key: A: Z q nxn, P=AS+E 247

146 LWE-2 based cryptography Private key: S,E: Z q n both sampled using - -, Public Key: A: Z q nxn, P=AS+E Input message: b: {0,1} 247

147 LWE-2 based cryptography Private key: S,E: Z q n both sampled using - -, Public Key: A: Z q nxn, P=AS+E Input message: b: {0,1} Enc AP (v) := (A T a+x,p T a+bq/2+e ), a,x,e : Z q n using

148 LWE-2 based cryptography Private key: S,E: Z q n both sampled using - -, Public Key: A: Z q nxn, P=AS+E Input message: b: {0,1} Enc AP (v) := (A T a+x,p T a+bq/2+e ), a,x,e : Z q n using - - Dec S (u,c) := 1 (0) iff c-s T u is closer to q/2 (0) c-s T u = P T a+bq/2+e -S T A T a-s T x = P T a+bq/2+e -P T a+ea-s T x = bq/2+ea+e -S T x 247

149 LWE based cryptography 8 7 feb Peikert

150 Lattice based cryptography 249

151 Post-Quantum Cryptography Prof. Claude Crépeau McGill University 250

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