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Full Indifferentiable Security of the Xor of Two or More Random Permutations Using the 2 Method Srimanta Bhattacharya and Mridul Nandi Indian Statistical Institute, Kolkata. Eurocrypt 2018 Tel Aviv, Israel 30th April, 2018 Outline 1


  1. Introduction XORP and XORP [ k ] PRF-Security: Indistinguishability PRF-Security: Indistinguishability $ XORP A XORP ( A ) ∶ = ∣ Pr [A XORP → 1 ] − Pr [A $ → 1 ]∣ Adv prf Focus on information theoretic security of XORP . A comutationally unbounded. A deterministic. Restrict A to q queries. 1 = ( X 1 , 1 ,...,X 1 ,q ) , XORP and $ returns X q 2 = ( X 2 , 1 ,...,X 2 ,q ) ∈ Ω q X q Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  2. Introduction XORP and XORP [ k ] PRF-Security: Indistinguishability PRF-Security: Indistinguishability $ XORP A XORP ( A ) ∶ = ∣ Pr [A XORP → 1 ] − Pr [A $ → 1 ]∣ Adv prf Focus on information theoretic security of XORP . A comutationally unbounded. A deterministic. Restrict A to q queries. 1 = ( X 1 , 1 ,...,X 1 ,q ) , XORP and $ returns X q 2 = ( X 2 , 1 ,...,X 2 ,q ) ∈ Ω q X q Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  3. Introduction XORP and XORP [ k ] PRF-Security: Indistinguishability PRF-Security: Indistinguishability $ XORP A XORP ( A ) ∶ = ∣ Pr [A XORP → 1 ] − Pr [A $ → 1 ]∣ Adv prf Focus on information theoretic security of XORP . A comutationally unbounded. A deterministic. Restrict A to q queries. 1 = ( X 1 , 1 ,...,X 1 ,q ) , XORP and $ returns X q 2 = ( X 2 , 1 ,...,X 2 ,q ) ∈ Ω q X q XORP [ k ] (A) ≤ max E⊆ Ω q ∑ x q ∈E ( Pr [ X q 1 = x q ] − Pr [ X q 2 = x q ]) . Adv prf Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  4. Introduction XORP and XORP [ k ] PRF-Security Results Upper Bounds on Adv prf XORP (A) and Adv prf XORP [ k ] (A) 3 Bellare and Impagliazzo, 1999: O ( nq 2 ) for XORP 2 3 N Lucks, 2000: O ( q k + 1 N k ) for XORP [ k ] ,k ≥ 2 . Patarin, 2008, Patarin, 2013: O ( q N ) Cogliati et al., 2014: O ( q k + 2 N k + 1 ) , O (( kq 2 k + 2 N 2 k + 1 ) 3 ) for XORP [ k ] Dai et al., 2017: O ( q N ) for XORP . XORP [ k ] ( A ) = Adv prf XORP ( A ) Mennink and Preneel, 2015: Adv prf Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  5. Introduction Indifferentiability Moving from Secret to Public Permutation Moving from Secret to Public Permutation In PRF-security (indistinguishability) setting permuatations remain secret. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  6. Introduction Indifferentiability Moving from Secret to Public Permutation Moving from Secret to Public Permutation In PRF-security (indistinguishability) setting permuatations remain secret. Motivation behind making the permutations public Sometimes block ciphers are instantiated with fixed keys. Many unkeyed permutations are designed as an underlying primitive of encryption Bertoni et al., 2011a, MAC Bertoni et al., 2011b, hash functions Bertoni et al., 2013, Rivest et al., 2008, Wu, 2011, Gauravaram et al., 2009 CAESAR candidates have been analyzed in the public permutation model. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  7. Introduction Indifferentiability Indifferentiable-Security Notion Indifferentiable-Security Notion Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  8. Introduction Indifferentiability Indifferentiable-Security Notion Indifferentiable-Security Notion Real World Ideal World F T S G A Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  9. Introduction Indifferentiability Indifferentiable-Security Notion Indifferentiable-Security Notion Real World Ideal World F T S G A T F , G S ( A ) = ∣ Pr [A T , F → 1 ] − Pr [A G , S → 1 ]∣ . Adv diff Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  10. Introduction Indifferentiability Indifferentiable-Security Notion Indifferentiable-Security Notion Real World Ideal World F T S G A T F , G S ( A ) = ∣ Pr [A T , F → 1 ] − Pr [A G , S → 1 ]∣ . Adv diff Maurer et al., 2004 ∃ S s.t. Adv diff T F , G S (A) ⇒ T is indifferentiable is negligible ∀ adversary A . from G . Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  11. Introduction Indifferentiability Indifferentiability of XORP Indifferentiability of XORP Π = ( Π 0 , Π 1 , Π − 1 0 , Π − 1 1 ) $ XORP S A Purpose of S is to simulate Π such that ( XORP , Π ) is indistinguishable from ( $ , S ) . S has oracle access to $. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  12. Introduction Indifferentiability Indifferentiability of XORP Real World and Ideal World Real World: Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  13. Introduction Indifferentiability Indifferentiability of XORP Real World and Ideal World Real World: Construction Query: A queries with x . XORP returns Π 0 ( x ) ⊕ Π 1 ( x ) to A . Primitive Query: Forward Query: A queries Π 0 or Π 1 with x and gets Π 0 ( x ) or Π 1 ( x ) . Backward Query: A queries Π 0 or Π 1 with y 0 ( y ) or and obtains Π − 1 1 ( y ) . Π − 1 Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  14. Introduction Indifferentiability Indifferentiability of XORP Real World and Ideal World Real World: Ideal World: Construction Query: A queries with x . XORP returns Π 0 ( x ) ⊕ Π 1 ( x ) to A . Primitive Query: Forward Query: A queries Π 0 or Π 1 with x and gets Π 0 ( x ) or Π 1 ( x ) . Backward Query: A queries Π 0 or Π 1 with y 0 ( y ) or and obtains Π − 1 1 ( y ) . Π − 1 Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  15. Introduction Indifferentiability Indifferentiability of XORP Real World and Ideal World Real World: Ideal World: Random Function Query: $ returns $ ( x ) . Construction Query: A queries with x . XORP Simulator Query: returns Π 0 ( x ) ⊕ Π 1 ( x ) to Forward Query: A A . queries S with ( x,b ) . S returns V b ∈ { 0 , 1 } n . Primitive Query: Backward Query: A Forward Query: A queries S with ( y,b ) . S queries Π 0 or Π 1 with x and gets Π 0 ( x ) or Π 1 ( x ) . returns V b ∈ { 0 , 1 } n ∪ {�} . Backward Query: A � indicates that S queries Π 0 or Π 1 with y aborted after certain 0 ( y ) or and obtains Π − 1 number of iterations. 1 ( y ) . Π − 1 Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  16. Introduction Indifferentiability Indifferentiability of XORP Goal Purpose of S is to simulate Π such that ( XORP , Π ) is indistinguishable from ( $ , S ) . V b should be close to Π b (or Π − 1 b in case of backward query). Construct S such that XORP , $ ( A ) = ∣ Pr [ A XORP , Π → 1 ] − Pr [ A $ , S → 1 ]∣ Adv diff should be negligible. Restrict A to q queries and obtain a concrete upper bound on XORP , $ ( A ) (in terms of parameters q and n ) Adv diff Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  17. Introduction Indifferentiability Results Results Construction Best known bound Our bound √ q 3 / 2 2 n Mennink and Preneel, 2015 q / 2 n XORP √ XORP [ k ] 2 nk ( k ≥ 4 even) Lee, 2017 q / 2 n q k + 1 Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  18. Introduction Techniques Mirror Theory Mirror Theory and It’s Limitations Introduced in Patarin, 2010; motivated from the PRF-security of XORP [ k ] type constructions. Lower bound on the number of solutions satisfying a system of linear equations involving exactly two variables. ✓ Together with the × Complex: some stpes are not H-coefficient technique clear. × Limitation in indifferentiability provides a bound on the setting: PRF-security of XORP . × No equation in single variable ✓ Powerful: Optimal security × Adversary can make public of EDM, EWCDM, permutation calls. Need to etc. Mennink and Neves, 2017a, consider single variable Mennink and Neves, 2017b equations. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  19. χ 2 Method Introduction Techniques χ 2 Method X q ∶= ( X 1 ,...,X q ) and Z q ∶= ( Z 1 ,...,Z q ) distributed over Ω q = Ω × ⋯ × Ω according to P 0 and P 1 respectively. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  20. χ 2 Method Introduction Techniques χ 2 Method X q ∶= ( X 1 ,...,X q ) and Z q ∶= ( Z 1 ,...,Z q ) distributed over Ω q = Ω × ⋯ × Ω according to P 0 and P 1 respectively. P 0 ∣ x i − 1 ( x i ) = Pr [ X i = x i ∣ X 1 = x 1 ,...,X i − 1 = x i − 1 ] , P 1 ∣ x i − 1 ( x i ) = Pr [ Z i = x i ∣ Z 1 = x 1 ,...,Z i − 1 = x i − 1 ] . Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  21. χ 2 Method Introduction Techniques χ 2 Method X q ∶= ( X 1 ,...,X q ) and Z q ∶= ( Z 1 ,...,Z q ) distributed over Ω q = Ω × ⋯ × Ω according to P 0 and P 1 respectively. P 0 ∣ x i − 1 ( x i ) = Pr [ X i = x i ∣ X 1 = x 1 ,...,X i − 1 = x i − 1 ] , P 1 ∣ x i − 1 ( x i ) = Pr [ Z i = x i ∣ Z 1 = x 1 ,...,Z i − 1 = x i − 1 ] . Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  22. χ 2 Method Introduction Techniques χ 2 Method X q ∶= ( X 1 ,...,X q ) and Z q ∶= ( Z 1 ,...,Z q ) distributed over Ω q = Ω × ⋯ × Ω according to P 0 and P 1 respectively. P 0 ∣ x i − 1 ( x i ) = Pr [ X i = x i ∣ X 1 = x 1 ,...,X i − 1 = x i − 1 ] , P 1 ∣ x i − 1 ( x i ) = Pr [ Z i = x i ∣ Z 1 = x 1 ,...,Z i − 1 = x i − 1 ] . Definition Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  23. χ 2 Method Introduction Techniques χ 2 Method X q ∶= ( X 1 ,...,X q ) and Z q ∶= ( Z 1 ,...,Z q ) distributed over Ω q = Ω × ⋯ × Ω according to P 0 and P 1 respectively. P 0 ∣ x i − 1 ( x i ) = Pr [ X i = x i ∣ X 1 = x 1 ,...,X i − 1 = x i − 1 ] , P 1 ∣ x i − 1 ( x i ) = Pr [ Z i = x i ∣ Z 1 = x 1 ,...,Z i − 1 = x i − 1 ] . Definition ∥ P 0 − P 1 ∥ ∶= 1 2 ∑ x q ∈ Ω q ∣ P 0 ( x q ) − P 1 ( x q )∣ . Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  24. χ 2 Method Introduction Techniques χ 2 Method X q ∶= ( X 1 ,...,X q ) and Z q ∶= ( Z 1 ,...,Z q ) distributed over Ω q = Ω × ⋯ × Ω according to P 0 and P 1 respectively. P 0 ∣ x i − 1 ( x i ) = Pr [ X i = x i ∣ X 1 = x 1 ,...,X i − 1 = x i − 1 ] , P 1 ∣ x i − 1 ( x i ) = Pr [ Z i = x i ∣ Z 1 = x 1 ,...,Z i − 1 = x i − 1 ] . Definition ∥ P 0 − P 1 ∥ ∶= 1 2 ∑ x q ∈ Ω q ∣ P 0 ( x q ) − P 1 ( x q )∣ . ( P 0 ∣ xi − 1 ( x i )− P 1 ∣ xi − 1 ( x i )) 2 χ 2 ( x i − 1 ) = χ 2 ( P 0 ∣ x i − 1 , P 1 ∣ x i − 1 ) ∶= ∑ x i ∈ Ω . P 1 ∣ xi − 1 ( x i ) Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  25. χ 2 Method Introduction Techniques χ 2 Method X q ∶= ( X 1 ,...,X q ) and Z q ∶= ( Z 1 ,...,Z q ) distributed over Ω q = Ω × ⋯ × Ω according to P 0 and P 1 respectively. P 0 ∣ x i − 1 ( x i ) = Pr [ X i = x i ∣ X 1 = x 1 ,...,X i − 1 = x i − 1 ] , P 1 ∣ x i − 1 ( x i ) = Pr [ Z i = x i ∣ Z 1 = x 1 ,...,Z i − 1 = x i − 1 ] . Definition ∥ P 0 − P 1 ∥ ∶= 1 2 ∑ x q ∈ Ω q ∣ P 0 ( x q ) − P 1 ( x q )∣ . ( P 0 ∣ xi − 1 ( x i )− P 1 ∣ xi − 1 ( x i )) 2 χ 2 ( x i − 1 ) = χ 2 ( P 0 ∣ x i − 1 , P 1 ∣ x i − 1 ) ∶= ∑ x i ∈ Ω . P 1 ∣ xi − 1 ( x i ) Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  26. χ 2 Method Introduction Techniques χ 2 Method X q ∶= ( X 1 ,...,X q ) and Z q ∶= ( Z 1 ,...,Z q ) distributed over Ω q = Ω × ⋯ × Ω according to P 0 and P 1 respectively. P 0 ∣ x i − 1 ( x i ) = Pr [ X i = x i ∣ X 1 = x 1 ,...,X i − 1 = x i − 1 ] , P 1 ∣ x i − 1 ( x i ) = Pr [ Z i = x i ∣ Z 1 = x 1 ,...,Z i − 1 = x i − 1 ] . Definition ∥ P 0 − P 1 ∥ ∶= 1 2 ∑ x q ∈ Ω q ∣ P 0 ( x q ) − P 1 ( x q )∣ . ( P 0 ∣ xi − 1 ( x i )− P 1 ∣ xi − 1 ( x i )) 2 χ 2 ( x i − 1 ) = χ 2 ( P 0 ∣ x i − 1 , P 1 ∣ x i − 1 ) ∶= ∑ x i ∈ Ω . P 1 ∣ xi − 1 ( x i ) Theorem (Dai et al., 2017) ∥ P 0 − P 1 ∥ ≤ ( 1 i = 1 Ex [ χ 2 ( X i − 1 )]) 1 2 ∑ q 2 Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  27. χ 2 Method Introduction Techniques χ 2 Method(contd..) Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  28. χ 2 Method Introduction Techniques χ 2 Method(contd..) Ingredients Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  29. χ 2 Method Introduction Techniques χ 2 Method(contd..) Ingredients 1 Pinsker’s inequality, 2 chain rule of Kullback-Leibler divergence (KL divergence), and 3 Jensen’s inequality. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  30. χ 2 Method Introduction Techniques χ 2 Method(contd..) Ingredients 1 Pinsker’s inequality, 2 chain rule of Kullback-Leibler divergence (KL divergence), and 3 Jensen’s inequality. Applications Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  31. χ 2 Method Introduction Techniques χ 2 Method(contd..) Ingredients 1 Pinsker’s inequality, 2 chain rule of Kullback-Leibler divergence (KL divergence), and 3 Jensen’s inequality. Applications 1 PRF-security of the truncated random permutation in Stam, 1978. 2 Full PRF-security of XORP and improved PRF-security of EDM in Dai et al., 2017. 3 Full PRF-security of the variable output length XOR pseudorandom functions in Bhattacharya and Nandi, 2018. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  32. Simulator and Transcript Simulator for XORP Simulator Overview SIM FWD and SIM BCK S consists of a pair of stateful randomized algorithms Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  33. Simulator and Transcript Simulator for XORP Simulator Overview SIM FWD and SIM BCK S consists of a pair of stateful randomized algorithms SIM FWD - algorithm for forward queries Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  34. Simulator and Transcript Simulator for XORP Simulator Overview SIM FWD and SIM BCK S consists of a pair of stateful randomized algorithms SIM FWD - algorithm for forward queries SIM BCK - algorithm for backward queries Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  35. Simulator and Transcript Simulator for XORP Simulator Overview SIM FWD and SIM BCK S consists of a pair of stateful randomized algorithms SIM FWD - algorithm for forward queries SIM BCK - algorithm for backward queries S tries to be consistent with the XORP by ‘consulting’ with $. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  36. Simulator and Transcript Simulator for XORP Simulator Overview SIM FWD and SIM BCK S consists of a pair of stateful randomized algorithms SIM FWD - algorithm for forward queries SIM BCK - algorithm for backward queries S tries to be consistent with the XORP by ‘consulting’ with $. Tries to maintain $ ( x ) = SIM FWD ( x, 0 ) ⊕ SIM FWD ( x, 1 ) for x ∈ { 0 , 1 } n . Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  37. Simulator and Transcript Simulator for XORP Simulator Overview SIM FWD and SIM BCK S consists of a pair of stateful randomized algorithms SIM FWD - algorithm for forward queries SIM BCK - algorithm for backward queries S tries to be consistent with the XORP by ‘consulting’ with $. Tries to maintain $ ( x ) = SIM FWD ( x, 0 ) ⊕ SIM FWD ( x, 1 ) for x ∈ { 0 , 1 } n . If it fails (during backward queries only) after n attempts SIM BCK returns � . Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  38. Simulator and Transcript Simulator for XORP Simulator Overview Internal State Sets D , R 0 , and R 1 simulate the domain of Π 0 and Π 1 and their ranges respectively. Lists (indexed by elements of D ) L 0 , L 1 - simulate the input-output mappings of Π 0 and Π 1 respectively. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  39. Simulator and Transcript Simulator for XORP Simulator Overview Internal State Sets D , R 0 , and R 1 simulate the domain of Π 0 and Π 1 and their ranges respectively. Lists (indexed by elements of D ) L 0 , L 1 - simulate the input-output mappings of Π 0 and Π 1 respectively. For b ∈ { 0 , 1 } ,x ∈ D ,y ∈ R b , L b ( x ) = y implies Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  40. Simulator and Transcript Simulator for XORP Simulator Overview Internal State Sets D , R 0 , and R 1 simulate the domain of Π 0 and Π 1 and their ranges respectively. Lists (indexed by elements of D ) L 0 , L 1 - simulate the input-output mappings of Π 0 and Π 1 respectively. For b ∈ { 0 , 1 } ,x ∈ D ,y ∈ R b , L b ( x ) = y implies V b = y was output on a forward query ( x,b ) , or Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  41. Simulator and Transcript Simulator for XORP Simulator Overview Internal State Sets D , R 0 , and R 1 simulate the domain of Π 0 and Π 1 and their ranges respectively. Lists (indexed by elements of D ) L 0 , L 1 - simulate the input-output mappings of Π 0 and Π 1 respectively. For b ∈ { 0 , 1 } ,x ∈ D ,y ∈ R b , L b ( x ) = y implies V b = y was output on a forward query ( x,b ) , or V b = x was output on a backward query ( y,b ) Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  42. Simulator and Transcript Simulator for XORP Simulator Overview Internal State Sets D , R 0 , and R 1 simulate the domain of Π 0 and Π 1 and their ranges respectively. Lists (indexed by elements of D ) L 0 , L 1 - simulate the input-output mappings of Π 0 and Π 1 respectively. For b ∈ { 0 , 1 } ,x ∈ D ,y ∈ R b , L b ( x ) = y implies V b = y was output on a forward query ( x,b ) , or V b = x was output on a backward query ( y,b ) For all x ∈ D , the relationship L 0 ( x ) ⊕ L 1 ( x ) = $ ( x ) is always satisfied. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  43. Simulator and Transcript Simulator for XORP Simulator Detail SIM FWD Data : x ∈ { 0 , 1 } n ,b ∈ { 0 , 1 } Result : V b ∈ { 0 , 1 } n if x ∈ D then return L b ( x ) end Z ← $ ( x ) V b ← $ { 0 , 1 } n ∖ {R b ∪ { Z ⊕ R 1 − b }} R b ← R b ∪ { V b } , R 1 − b ← R 1 − b ∪ { Z ⊕ V b } D ← D ∪ { x } L b ( x ) ← V b , L 1 − b ( x ) ← Z ⊕ V b return V b Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  44. Simulator and Transcript Simulator for XORP Simulator Detail SIM FWD Data : x ∈ { 0 , 1 } n ,b ∈ { 0 , 1 } Result : V b ∈ { 0 , 1 } n if x ∈ D then return L b ( x ) end Z ← $ ( x ) V b ← $ { 0 , 1 } n ∖ {R b ∪ { Z ⊕ R 1 − b }} R b ← R b ∪ { V b } , R 1 − b ← R 1 − b ∪ { Z ⊕ V b } D ← D ∪ { x } L b ( x ) ← V b , L 1 − b ( x ) ← Z ⊕ V b return V b Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  45. Simulator and Transcript Simulator for XORP Simulator Detail SIM FWD Data : x ∈ { 0 , 1 } n ,b ∈ { 0 , 1 } Result : V b ∈ { 0 , 1 } n if x ∈ D then return L b ( x ) end Z ← $ ( x ) V b ← $ { 0 , 1 } n ∖ {R b ∪ { Z ⊕ R 1 − b }} R b ← R b ∪ { V b } , R 1 − b ← R 1 − b ∪ { Z ⊕ V b } D ← D ∪ { x } L b ( x ) ← V b , L 1 − b ( x ) ← Z ⊕ V b return V b Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  46. Simulator and Transcript Simulator for XORP Simulator Detail SIM FWD Data : x ∈ { 0 , 1 } n ,b ∈ { 0 , 1 } Result : V b ∈ { 0 , 1 } n if x ∈ D then return L b ( x ) end Z ← $ ( x ) V b ← $ { 0 , 1 } n ∖ {R b ∪ { Z ⊕ R 1 − b }} R b ← R b ∪ { V b } , R 1 − b ← R 1 − b ∪ { Z ⊕ V b } D ← D ∪ { x } L b ( x ) ← V b , L 1 − b ( x ) ← Z ⊕ V b return V b Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  47. Simulator and Transcript Simulator for XORP Simulator Detail SIM BCK Data : y ∈ { 0 , 1 } n ,b ∈ { 0 , 1 } Result : V b ∈ { 0 , 1 } n ∪ {⊥} if y = L b ( x ) for x ∈ D then return x D ′ ← D repeat V b ← $ { 0 , 1 } n ∖ D ′ , Z ← $ ( V b ) if Z ⊕ y ∉ R 1 − b then D ← D ∪ { V b } , R b ← R b ∪ { y } , L b ( V b ) ← y, R 1 − b ← R 1 − b ∪ { Z ⊕ y } , L 1 − b ( V b ) ← Z ⊕ y return V b end D ′ ← D ′ ∪ { V b } until n times ; return � Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  48. Simulator and Transcript Simulator for XORP Simulator Detail SIM BCK Data : y ∈ { 0 , 1 } n ,b ∈ { 0 , 1 } Result : V b ∈ { 0 , 1 } n ∪ {⊥} if y = L b ( x ) for x ∈ D then return x D ′ ← D repeat V b ← $ { 0 , 1 } n ∖ D ′ , Z ← $ ( V b ) if Z ⊕ y ∉ R 1 − b then D ← D ∪ { V b } , R b ← R b ∪ { y } , L b ( V b ) ← y, R 1 − b ← R 1 − b ∪ { Z ⊕ y } , L 1 − b ( V b ) ← Z ⊕ y return V b end D ′ ← D ′ ∪ { V b } until n times ; return � Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  49. Simulator and Transcript Simulator for XORP Simulator Detail SIM BCK Data : y ∈ { 0 , 1 } n ,b ∈ { 0 , 1 } Result : V b ∈ { 0 , 1 } n ∪ {⊥} if y = L b ( x ) for x ∈ D then return x D ′ ← D repeat V b ← $ { 0 , 1 } n ∖ D ′ , Z ← $ ( V b ) if Z ⊕ y ∉ R 1 − b then D ← D ∪ { V b } , R b ← R b ∪ { y } , L b ( V b ) ← y, R 1 − b ← R 1 − b ∪ { Z ⊕ y } , L 1 − b ( V b ) ← Z ⊕ y return V b end D ′ ← D ′ ∪ { V b } until n times ; return � Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  50. Simulator and Transcript Simulator for XORP Simulator Detail SIM BCK Data : y ∈ { 0 , 1 } n ,b ∈ { 0 , 1 } Result : V b ∈ { 0 , 1 } n ∪ {⊥} if y = L b ( x ) for x ∈ D then return x D ′ ← D repeat V b ← $ { 0 , 1 } n ∖ D ′ , Z ← $ ( V b ) if Z ⊕ y ∉ R 1 − b then D ← D ∪ { V b } , R b ← R b ∪ { y } , L b ( V b ) ← y, R 1 − b ← R 1 − b ∪ { Z ⊕ y } , L 1 − b ( V b ) ← Z ⊕ y return V b end D ′ ← D ′ ∪ { V b } until n times ; return � Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  51. Simulator and Transcript Simulator for XORP Simulator Detail SIM BCK Data : y ∈ { 0 , 1 } n ,b ∈ { 0 , 1 } Result : V b ∈ { 0 , 1 } n ∪ {⊥} if y = L b ( x ) for x ∈ D then return x D ′ ← D repeat V b ← $ { 0 , 1 } n ∖ D ′ , Z ← $ ( V b ) if Z ⊕ y ∉ R 1 − b then D ← D ∪ { V b } , R b ← R b ∪ { y } , L b ( V b ) ← y, R 1 − b ← R 1 − b ∪ { Z ⊕ y } , L 1 − b ( V b ) ← Z ⊕ y return V b end D ′ ← D ′ ∪ { V b } until n times ; return � Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  52. Simulator and Transcript Simulator for XORP Simulator Detail SIM BCK Data : y ∈ { 0 , 1 } n ,b ∈ { 0 , 1 } Result : V b ∈ { 0 , 1 } n ∪ {⊥} if y = L b ( x ) for x ∈ D then return x D ′ ← D repeat V b ← $ { 0 , 1 } n ∖ D ′ , Z ← $ ( V b ) if Z ⊕ y ∉ R 1 − b then D ← D ∪ { V b } , R b ← R b ∪ { y } , L b ( V b ) ← y, R 1 − b ← R 1 − b ∪ { Z ⊕ y } , L 1 − b ( V b ) ← Z ⊕ y return V b end D ′ ← D ′ ∪ { V b } until n times ; return � Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  53. Simulator and Transcript Simulator for XORP Simulator Detail SIM BCK Data : y ∈ { 0 , 1 } n ,b ∈ { 0 , 1 } Result : V b ∈ { 0 , 1 } n ∪ {⊥} if y = L b ( x ) for x ∈ D then return x D ′ ← D repeat V b ← $ { 0 , 1 } n ∖ D ′ , Z ← $ ( V b ) if Z ⊕ y ∉ R 1 − b then D ← D ∪ { V b } , R b ← R b ∪ { y } , L b ( V b ) ← y, R 1 − b ← R 1 − b ∪ { Z ⊕ y } , L 1 − b ( V b ) ← Z ⊕ y return V b end D ′ ← D ′ ∪ { V b } until n times ; return � Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  54. Simulator and Transcript Transcript to the Adversary Additional Information Additional Information Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  55. Simulator and Transcript Transcript to the Adversary Additional Information Additional Information After the interation with real/ideal world is over Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  56. Simulator and Transcript Transcript to the Adversary Additional Information Additional Information After the interation with real/ideal world is over A is given additional information. Real World Query: A knows the tuple ( x i , Π 0 ( x i ) , Π 1 ( x i )) = S i . Distributions: p fwd and p bck for forward and backward queries. 0 0 Ideal World Query: A knows the tuple ( x i ,V 0 ,i ,V 1 ,i ) (In case of ‘abort’ ( x i ,V 0 ,i ,V 1 ,i ) = � ). Distributions: p fwd and p bck for forward 1 1 and backward queries. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  57. Main Result: Indifferentiability of XORP Result and Outline Outline Indifferentiability of XORP : Outline Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  58. Main Result: Indifferentiability of XORP Result and Outline Outline Indifferentiability of XORP : Outline √ Theorem XORP , $ ( q ) ≤ 1 . 25 q Let N ≥ 16 and q < N 2 . Then Adv diff N . Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  59. Main Result: Indifferentiability of XORP Result and Outline Outline Indifferentiability of XORP : Outline √ Theorem XORP , $ ( q ) ≤ 1 . 25 q Let N ≥ 16 and q < N 2 . Then Adv diff N . Goal is to calculate Ex [ χ 2 ( S i − 1 )] over the real world distributions ( p fwd and p bck ). 0 0 Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  60. Main Result: Indifferentiability of XORP Result and Outline Outline Indifferentiability of XORP : Outline √ Theorem XORP , $ ( q ) ≤ 1 . 25 q Let N ≥ 16 and q < N 2 . Then Adv diff N . Goal is to calculate Ex [ χ 2 ( S i − 1 )] over the real world distributions ( p fwd and p bck ). 0 0 Need to consider two cases. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  61. Main Result: Indifferentiability of XORP Result and Outline Outline Indifferentiability of XORP : Outline √ Theorem XORP , $ ( q ) ≤ 1 . 25 q Let N ≥ 16 and q < N 2 . Then Adv diff N . Goal is to calculate Ex [ χ 2 ( S i − 1 )] over the real world distributions ( p fwd and p bck ). 0 0 Need to consider two cases. s i is a forward query Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  62. Main Result: Indifferentiability of XORP Result and Outline Outline Indifferentiability of XORP : Outline √ Theorem XORP , $ ( q ) ≤ 1 . 25 q Let N ≥ 16 and q < N 2 . Then Adv diff N . Goal is to calculate Ex [ χ 2 ( S i − 1 )] over the real world distributions ( p fwd and p bck ). 0 0 Need to consider two cases. s i is a forward query s i is a backward query Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  63. Main Result: Indifferentiability of XORP Result and Outline Forward Query Forward Query Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  64. Main Result: Indifferentiability of XORP Result and Outline Forward Query Forward Query ( p fwd ( s i ∣ s i − 1 )− p fwd ( s i ∣ s i − 1 )) 2 χ 2 ( s i − 1 ) = ∑ s i . 0 1 ( s i ∣ s i − 1 ) p fwd 1 Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  65. Main Result: Indifferentiability of XORP Result and Outline Forward Query Forward Query ( p fwd ( s i ∣ s i − 1 )− p fwd ( s i ∣ s i − 1 )) 2 χ 2 ( s i − 1 ) = ∑ s i . 0 1 ( s i ∣ s i − 1 ) p fwd 1 To consider χ 2 ( S i − 1 ) for real world distribution S i − 1 . Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  66. Main Result: Indifferentiability of XORP Result and Outline Forward Query Forward Query ( p fwd ( s i ∣ s i − 1 )− p fwd ( s i ∣ s i − 1 )) 2 χ 2 ( s i − 1 ) = ∑ s i . 0 1 ( s i ∣ s i − 1 ) p fwd 1 To consider χ 2 ( S i − 1 ) for real world distribution S i − 1 . Each S j ∈ { S i − 1 } may correspond to a forward or a backward query. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  67. Main Result: Indifferentiability of XORP Result and Outline Forward Query Forward Query ( p fwd ( s i ∣ s i − 1 )− p fwd ( s i ∣ s i − 1 )) 2 χ 2 ( s i − 1 ) = ∑ s i . 0 1 ( s i ∣ s i − 1 ) p fwd 1 To consider χ 2 ( S i − 1 ) for real world distribution S i − 1 . Each S j ∈ { S i − 1 } may correspond to a forward or a backward query. The distributions p fwd and p bck are identical; the distribution of S i − 1 0 0 does not depend on the query type of each individual S j . Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  68. Main Result: Indifferentiability of XORP Result and Outline Forward Query Forward Query ( p fwd ( s i ∣ s i − 1 )− p fwd ( s i ∣ s i − 1 )) 2 χ 2 ( s i − 1 ) = ∑ s i . 0 1 ( s i ∣ s i − 1 ) p fwd 1 To consider χ 2 ( S i − 1 ) for real world distribution S i − 1 . Each S j ∈ { S i − 1 } may correspond to a forward or a backward query. The distributions p fwd and p bck are identical; the distribution of S i − 1 0 0 does not depend on the query type of each individual S j . Allows to treat χ 2 ( S i − 1 ) as a random variable and take its expectation under the distribution of S i − 1 . Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  69. Main Result: Indifferentiability of XORP Result and Outline Forward Query Forward Query ( p fwd ( s i ∣ s i − 1 )− p fwd ( s i ∣ s i − 1 )) 2 χ 2 ( s i − 1 ) = ∑ s i . 0 1 ( s i ∣ s i − 1 ) p fwd 1 To consider χ 2 ( S i − 1 ) for real world distribution S i − 1 . Each S j ∈ { S i − 1 } may correspond to a forward or a backward query. The distributions p fwd and p bck are identical; the distribution of S i − 1 0 0 does not depend on the query type of each individual S j . Allows to treat χ 2 ( S i − 1 ) as a random variable and take its expectation under the distribution of S i − 1 . Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  70. Main Result: Indifferentiability of XORP Result and Outline Forward Query Forward Query ( p fwd ( s i ∣ s i − 1 )− p fwd ( s i ∣ s i − 1 )) 2 χ 2 ( s i − 1 ) = ∑ s i . 0 1 ( s i ∣ s i − 1 ) p fwd 1 To consider χ 2 ( S i − 1 ) for real world distribution S i − 1 . Each S j ∈ { S i − 1 } may correspond to a forward or a backward query. The distributions p fwd and p bck are identical; the distribution of S i − 1 0 0 does not depend on the query type of each individual S j . Allows to treat χ 2 ( S i − 1 ) as a random variable and take its expectation under the distribution of S i − 1 . Forward Query Bound i = 1 Ex [ χ 2 ( S i − 1 )] ≤ 8 q 3 ∑ q N 3 Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  71. Main Result: Indifferentiability of XORP Result and Outline Backward Query Backward Query Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  72. Main Result: Indifferentiability of XORP Result and Outline Backward Query Backward Query Steps are similar to the backward query case. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

  73. Main Result: Indifferentiability of XORP Result and Outline Backward Query Backward Query Steps are similar to the backward query case. s i ≠ ⊥ and s i = ⊥ are treated separately. Srimanta Bhattacharya and Mridul Nandi Full Indifferentiable Security of the Xor of Random Permutations

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