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SECURITY AND VERIFICATION Lecture 1: Why to prove cryptography? The origins of provable cryptography Tamara Rezk INDES TEAM, INRIA January 3 rd, 2012

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RSA INVENTORS GOT BORED AND DECIDED TO PLAY POKER Some history … Mental Poker Adi Shamir, Ronald Rivest, Leonard Adleman, ’81

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HOW TO PLAY MENTAL POKER?

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MENTAL POKER PROTOCOL Some history … Mental Poker Shamir, Rivest, Adleman, ’81 how to write a protocol for mental poker without using a third trusted party? in theory impossible

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MENTAL POKER PROTOCOL Some history … in theory impossible: no such protocol exists Information Theory: the ciphertext provides no information about the plaintext. Shannon’s entropy is a measure of this information. Mental Poker Shamir, Rivest, Adleman, ’81 how to write a protocol for mental poker without using a third trusted party?

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MENTAL POKER PROTOCOL Some history … in theory impossible Mental Poker Shamir, Rivest, Adleman, ’81 how to write a protocol for mental poker without using a third trusted party?

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MENTAL POKER PROTOCOL Some history … in theory impossible solution based on SRA Mental Poker Shamir, Rivest, Adleman, ’81 how to write a protocol for mental poker without using a third trusted party?

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MENTAL POKER PROTOCOL Some history … SRA Protocol relies on commutative encryption E ( E (x, a), b) = E ( E (x, b), a) in theory impossible solution based on SRA Mental Poker Shamir, Rivest, Adleman, ’81 how to write a protocol for mental poker without using a third trusted party?

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MENTAL POKER PROTOCOL Some history … Mental Poker Shamir, Rivest, Adleman, ’81 Encryption function E for SRA * q is a large prime number * (q) = q-1 * plaintext, ciphertext, key spaces all in Z q * * key a s.t. gcd(a, (q))= 1 E (x, a) = x a mod q D (c, a) = c -a mod q

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MENTAL POKER PROTOCOL Some history … Mental Poker Shamir, Rivest, Adleman, ’81 Cast : Alice and Bob

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MENTAL POKER PROTOCOL How SRA works

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MENTAL POKER PROTOCOL How SRA works

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MENTAL POKER PROTOCOL How SRA works

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MENTAL POKER PROTOCOL How SRA works E (, b ) E (, b ) E (, b )

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MENTAL POKER PROTOCOL How SRA works E (, b ) E (, b ) E (, b )

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MENTAL POKER PROTOCOL How SRA works E (, b ) E (, b) E (, b )

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MENTAL POKER PROTOCOL How SRA works E (, b ) E (, b) E (, b )

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MENTAL POKER PROTOCOL How SRA works E (, b ) E (, b ) E (, b )

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MENTAL POKER PROTOCOL How SRA works E (, b ) E (, b) E (, b) BobAlice E (, b ) E (, b ) E (, b )

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MENTAL POKER PROTOCOL How SRA works E (, b ) E (, b ) E (, b ) BobAlice E ( E (, b ),a )

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MENTAL POKER PROTOCOL How SRA works E (, b ) E (, b ) E (, b ) BobAlice E (, b ) E ( E (, b ),a )

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MENTAL POKER PROTOCOL How SRA works E (, b ) E (, b ) E (, b ) BobAlice E (, b ) E ( E (, b ),a ) D ( E ( E (, b ),a ), b)

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A simple programming language var::= x | y | z … op :: = + | - | * | < | = … expr :: = const | var | expr op expr c::= var := expr | skip | if ( expr ) then {c} else {c} | while ( expr ) do {c} | c; c

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Semantics of expressions [ e ] To define semantics of expressions, we need to define states . A state is a function that maps each variable into its value. We need to provide an interpretation for each operation op var::= x | y | z … op :: = + | - | * | < | = … expr :: = const| var | expr op expr

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Semantics of expressions [ e ] Example: If (x) = 3 and (y) = 0 then [ x+y ] = [ x ] + [ y ] = = 3 We say that the semantics of [ x/y ] is not defined. var::= x | y | z … op :: = + | - | * | < | = … expr :: = const| var | expr op expr

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Operational semantics Semantics precisely defines the meaning of programs: We will define a “small-step operational semantics” Basic idea: execution of a program can be formalize as a sequence of configurations: c0 c1 c2 …. A configuration is a pair command and a state Example of configuration:

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The operational semantics is defined by a transition system (Configurations, ). Configurations = {,,,, } The relation can be represented by a picture but it should be formally defined by a set of rules. Operational semantics

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In this example: Configurations = {, }

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Operational semantics The operational semantics is defined by a transition system (Configurations, ). The relation is defined by a set of semantic rules of the form: [ e ] =v _________________________

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Operational semantics We need to define relation for each command in the programming language: c::= var := expr | skip | if ( expr ) then {c} else {c} | while ( expr ) do {c} | c; c

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Operational semantics [ e ] =v _________________________

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Operational semantics _________________________

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Operational semantics [ e ] =0 _________________________

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Operational semantics [ e ] =0 _________________________ [ e ] 0 _________________________

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Operational semantics [ e ] =0 _________________________ [ e ] 0 _________________________

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Operational semantics c’ _________________________ _________________________

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Operational semantics ________________

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Probabilistic programming language var::= x | y | z … op :: = + | - | * | < | = … expr :: = var | expr op expr probFun::= f | g | E |G | D | … c::= var := expr | skip | var:= probFun ( var, …,var) | if ( expr ) then {c} else {c} | while ( expr ) do {c} | c ; c

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Markov Chain

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Markov Chain The sum is equal to 1 This forms a distribution for configurations reachable from

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Markov Chain Distribution d induced by d ( ) = 0.4 d ( ) = 0.1 d ( ) = 0.2 …

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Markov Chain What is the probability of reaching from Pr[ ] ? What is the probability of reaching from Pr[ ] ?

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Markov Chain What is the probability of reaching from Pr[ ] ? What is the probability of reaching from Pr[ ] ?

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Markov Chain T he probability of reaching from Pr[ ] = 0.2 * 0.7 = 0.14 T he probability of reaching from Pr[ ] = 0.2 * 0.7 = 0.14

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Markov Chain T he probability of reaching from Pr[ ] = 0.2 * 0.7 = 0.14 T he probability of reaching from Pr[ ] = 0.2 * 0.7 = 0.14

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Probabilistic semantics Given by a sequence of probability distributions

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Probabilistic Transition System And more formally, we need to provide a set of rules to define the probabilistic transition system Now relation is probabilistic, annotated with a probability p p

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Operational semantics [ e ] =v _________________________ 1 _________________________ 1 [ e ] =0 _________________________ 1 [ e ] 0 _________________________ 1 [ e ] =0 _________________________ 1 [ e ] 0 _________________________ 1 p c’ _________________________ p p _________________________ p ________________ 1

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Mental Poker in While p shuffle ( ) = c := {0,1,2,3,4,5} ; if c=0 then b 0,b 1,b 2 := ; else ….

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MP = cards:= shuffle( ); for c := 1 to 3 do ce[c]:= E (cards[c],b); lce:= ce ecards:= shuffle( E (, b ) E (, b ) E (, b ) ); aliceCard := randomPick(lce, nil); bobCard := randomPick(lce,aliceCard ); aliceCard:= E (aliceCard,a); laliceCard:= aliceCard; lbobCard:= bobCard laliceCard:= D (laliceCard,b -1 ); bobCard:= D (lbobCard,b -1 ); MENTAL POKER

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MP = cards:= shuffle( ); for c := 1 to 3 do ce[c]:= E (cards[c],b); lce:= ce laliceCard:= D (laliceCard,b -1 ); bobCard:= D (lbobCard,b -1 ); HOW TO CHEAT IN MENTAL POKER ?

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MP = cards:= shuffle( ); for c := 1 to 3 do ce[c]:= E (cards[c],b); lce:= ce laliceCard:= D (laliceCard,b -1 ); bobCard:= D (lbobCard,b -1 ); HOW TO CHEAT IN MENTAL POKER A

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How SRA works Some facts to break the protocol:

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HOW TO CHEAT IN MENTAL POKER How SRA works Some facts to break the protocol: If “Alice” can decrypt without the key

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HOW TO CHEAT IN MENTAL POKER How SRA works Some facts to break the protocol: If “Alice” can decrypt without the key An adversary that breaks the protocol by using brute force : A = for k = 1 to 2 ɳ do y:= D (lce[c],k); if y = then aliceCard := lce[c]

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HOW TO CHEAT IN MENTAL POKER How SRA works Some facts to break the protocol: If “Alice” can decrypt without the key A is polynomial, factorization is a hard problem

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HOW TO CHEAT IN MENTAL POKER How SRA works Some facts to break the protocol: If “Alice” can decrypt without the key A is polynomial, factorization is a hard problem

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HOW TO CHEAT IN MENTAL POKER How SRA works Some facts to break the protocol: If “Alice” can decrypt without the key If Bob plays twice with the same key

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HOW TO CHEAT IN MENTAL POKER How SRA works Some facts to break the protocol: If “Alice” can decrypt without the key If Bob plays twice with the same key An adversary that breaks the protocol if Bob plays twice with the same key

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HOW TO CHEAT IN MENTAL POKER How SRA works Some facts to break the protocol: If “Alice” can decrypt without the key If Bob plays twice with the same key A = if oldEncACE = lce[c] then aliceCard := lce[c];

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HOW TO CHEAT IN MENTAL POKER How SRA works Some facts to break the protocol: If “Alice” can decrypt without the key If Bob plays twice with the same key Observing quadratic residues!! (R.J. Lipton) x Q q b x 2 b (mod q) x Q q x k (mod q) Q q

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HOW TO CHEAT IN MENTAL POKER How SRA works Observing quadratic residues!! (R.J. Lipton) x Q q b x 2 b (mod q) x Q q x k (mod q) Q q How to cheat in Mental Poker Lipton’81

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PROVABLE CRYPTOGRAPHY how to prove security of encryption algorithms? PROVABLE CRYPTOGRAPHY

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how to prove security of encryption algorithms? Probabilistic Encryption and How to Play Mental Poker Keeping Secret All Partial Information Goldwasser and Micali ’82 PROVABLE CRYPTOGRAPHY

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Probabilistic Encryption and How to Play Mental Poker … Goldwasser and Micali ’82 The fact that f is a trapdoor function does not rule out: 1.the possibility of computing x from f(x) when x is of a special form. 2.the possibility of computing some partial information about x (even every other bit of x) from f(x). TRAPDOOR FUNCTION x f(x) easy hard

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PROVABLE CRYPTOGRAPHY how to prove security of encryption algorithms? Probabilistic Encryption and How to Play Mental Poker … Goldwasser and Micali ’82 - probabilistic setting - notion of semantic security PROVABLE CRYPTOGRAPHY

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Semantic security or Chosen Plaintext Attack (a.k.a. CPA) E ( message1, b) message1, message 2 E ( message2, b) “I guess that the coin was tail” | Pr[CPA; g = c] - ½ | is negligible for |b| (|b| is called security parameter)

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NEGLIGIBLE FUNCTION A function f (x) is negligible for x when for all c>0, there is a constant n c such that n c ≤ x implies f(x) ≤ 1/x c

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NEGLIGIBLE FUNCTION A function f (x) is negligible for x when for all c>0, there is a constant n c such that n c ≤ x implies f(x) ≤ 1/x c Are the following functions negligible? f(x) = x 2 f(x) =1/x f(x) =1/x 2 f(x)= 1/3 x

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encryption scheme Definition of encryption scheme An encryption scheme is a triple ( G, E, D ) of probabilistic polynomial-time algorithms such that: - On input ɳ, algorithm G outputs a pair e, d of bitstrings - D ( E (x,e),d) = x

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PROVABLE CRYPTOGRAPHY Chosen-plaintext attack (CPA) E (x 0, x 1 ) = if (c = 0) then {m := E (x 0, k e )} else {m := E (x 1,k e )}; CPA = c := {0,1}; k e, k d := G e (); A [ E ] | Pr[CPA; g =c] - ½ | is negligible for ɳ ( ɳ is called security parameter)

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READING Slides, Notes, Bibliography Slides and lecture notes: www-sop.inria.fr/members/Tamara.Rezk/teaching Mental Poker – Shamir, Rivest, Adleman Probabilistic Encryption & How to Play Mental Poker Keeping Secret all Partial Information – Goldwasser, Micali

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