introduction to practical cryptography forward key security zero knowledge oblivious transfer...

Introduction to Practical Cryptography

Forward Key SecurityZero Knowledge

Oblivious TransferMulti-Party Computation

2

Overview

• Intended as overview of specific areas

• Solutions/instances of some require background not covered in this class

3

Agenda

• Forward Key Security

• Zero Knowledge

• Oblivious Transfer

• Multi-party Computation

4

Forward Secure Encryption Schemes

• Attacker learns key at time t• Should not be able to decrypt anything

from prior to time t• Encryption algorithm – add an input, time

– Esk(m) becomes E’sk(m,t)

• Current key is a function of prior key – kt = F(sk,t) = G(kt-1,t)– Given sk, can compute entire sequence of kt’s

• c = Ekt(m)

5

Digital Signatures

• Alice has a secret key• Everyone else has the corresponding public key• Alice can sign message with her secret key:

• Given a signature and a message, everyone can verify correctness using Alice’s public key:

• Desirable property: non-repudiation. If Alice signed a contract, she can’t deny it later.

6

Digital Signature Schemes• Three algorithms Key-Gen, Sign, Verify:

Key-Gen inputs: security parameter (“key size”) k output: keys (PK, SK)

Sign inputs: message M, secret key SK output: signature

Verify inputs: M, signature , public key PK output: valid/invalid

• Strong security notion:even given signatures on messages of its choice,adversary cannot forge signatures on new messages

7

Problem with Signatures

• Can’t tell when a signature was really generated– Not even if you include current time into the document – doesn’t say when it

was signed

• Therefore,

signing key disclosed all signatures worthless

(even if produced before disclosure)

• Inconvenience: your notarized document is no longer valid

• Repudiation: Alice gets out of past contracts by anonymously leaking her SK

• Key revocation doesn’t help: it merely informs of the leak

8

Fixing the Problem

• Attempt 1: re-sign all the past messages with a new key– Expensive

– What if the signer doesn’t cooperate?

• Attempt 2: change keys frequently, erasing past SK– Disclosure of current SK doesn’t affect past SK

– However, changing PK is expensive, requires certification

• Attempt 3: Employ time stamping authority (third party)

9

Forward Security• Idea: change SK but not PK [And97]

• Divide the lifetime of PK into T time periods

• Each SKj is for a particular time period, erased thereafter

• If current SKj is compromised, signatures with previous SKj-t

remain secure

• Note: can’t be done without assuming secure erasure

10

Definitions: Key-Evolving Scheme [BM99]

The usual three algorithms Key-Gen, Sign, Verify:Key-Gen inputs: security parameter (“key size”) k

total number T of time periodsoutput: (PK, SK1)

Sign inputs: message M, current secret key SKj output: signature S (time period j included)

Verify inputs: M, time period j, signature S, public key PK output: valid/invalid

11

Definitions: Key-Evolving Scheme [BM99]

• The usual three algorithms Key-Gen, Sign, Verify:Key-Gen inputs: security parameter (“key size”) k

total number T of time periodsoutput: (PK, SK1)

Sign inputs: message M, current secret key SKj output: signature S (time period j included)

Verify inputs: M, time period j, signature S, public key PK output: valid/invalid

• A new algorithm Update:Update input: current secret key SKj

output: new secret key SKj+1

12

Definitions: Forward-Security [BM99]

Given:

• Signatures on messages and time periods of its

choice

• The ability to “break-in” in any time period b and get

SKb

adversary can’t forge a new signature for a time period

j<b

13

Simple Schemes: Efficiency?

• Long Public and Long Private Keys– T pairs (p1, s1), (p2,s2),……. (pt, st)– PK= (p1,p2,….,pt) – SK= (s1,s2,…..,st) – Update= erase si for period i– Drawback: public and private key linear in t =

number of periods

14

Anderson: Long Secret Key Only

• T pairs as before and an additional pair (p,s)• Sig(j)=SIG( j || pj) with key s, j =1,…,t [“certificate”]• Public key now = p (only)• Secret key = (sj, Sig(j)) j=1,..,t [Still linear]• The public key p is like a CA key and a signature will

include the period, the certificate, the message, signature on the message with period’s secret key

15

Long Signatures Only

• Have (p,s)• In period j get (pj,sj) and • Let Cert(j)= sig_s[j-1] (j || pj)• A signature is the entire certificate chain +

signature, it is of the form: (j, sig_sj(m), p1, Cert(1),…pj, Cert(j))

• Think about it as a tree of degree one and height t for t periods

• (signer memory still linear in t)

16

Binary Certification tree [BM]

• Now (s0,p0)• Each element certifies two children left and right• We have a tree of height log t (for t periods)• At each point only a log branch of certificate is used in

the signature • Only leaves are used to sign• Keys whose children are not going to be used in future

periods are erased.• We get O(log t) key sizes, signature size

17

Concrete Scheme

• Use a scheme based on Fiat-Shamir variant• Have the public key be a point x raised to 2t

• Have the initial private key at period zero be x• Sign based on FS paradigm• Update by squaring the current key• The verifier is aware of the period so it knows that

currently the private key is raised to the power 2t-1 (and the identification proofs are adjusted accordingly).

18

Replace keys Pseudorandomness

• Have future keys derived from a forward secure pseudorandom generator

• Pseudorandom generators which are forward secure are easy: replace the seed with an iteration of the function.

• Possibilities in practice:– Block cipher – use previous output as next input– Stream cipher – previous output as next state

19

Agenda


• Zero Knowledge



20

Zero Knowledge

XProve it

I know X

Some exchange, but which does not provide X

• Interactive method for Alice to prove to Bob that she has/knows x without revealing x to Bob.

• Motivation: authentication

• Alice wants to prove her identity to Bob via some secret but doesn't want Bob to learn anything about this secret– Login methods where password is not stored on server (maybe hash of password is

stored)– Alice (user/client) proves to Bob (server) that she knows the password without giving Bob

the password

21

Zero Knowledge

• Interactive method for Alice to prove to Bob that she has/knows x without revealing x to Bob.

• Zero-knowledge proof must satisfy three properties:• Statement is “alice knows x”

– Completeness: if the statement is true, the honest verifier (that is, one following the protocol properly) will be convinced of this fact by an honest prover.

– Soundness: if the statement is false, no cheating prover can convince the honest verifier that it is true, except with some small probability.

– Zero-knowledge: if the statement is true, no cheating verifier learns anything other than this fact.

• Completeness and soundness needed for any interactive proof

22

Zero Knowledge

• Example use: enforce honest behavior while maintaining privacy

• Force a user to prove that its behavior is correct according to the protocol.

• Used in secure multiparty computation

23

Zero Knowledge• Jean-Jacques Quisquater, et. al "How to Explain Zero-Knowledge

Protocols to Your Children“• Peggy (prover) has uncovered the secret password to open a magic

door in a cave. The cave is shaped like a circle, with the entrance in one side and the magic door blocking the opposite side.

• Victor (verifier) will pay Peggy for the secret, but not until he's sure that she really knows it. Peggy says she'll tell him the secret, but not until she receives the money.

• Zero knowledge – need a method by which Peggy proves to Victor that she knows the word without telling it to him

• Solution– Victor waits outside the cave, Peggy goes in. Label the left and right paths

from the entrance A and B. She takes either A or B at random (Victor does not know which)

– Victor enters the cave and shouts the path (A or B at random) on which she must return

– Peggy returns along the path chosen by Victor, opening the door if it was not the path on which she had entered the cave

– If Peggy did not know the word, there is a 50% chance she can return on the correct path; repeat the above many times, Peggy’s chance of successfully returning becomes negligible

(assume chance of Peggy guessing the word is negligible)– If Peggy returns correctly each time, this proves she knows the word

enter

AB

door

24

Agenda


• Zero Knowledge



25

Oblivious Transfer

m = 10 or 1

• Alice transfers a secret bit m to Bob with probability ½ such that – Bob knows whether or not he receives m– Alice doesn’t know if m transferred to Bob

26

Oblivious Transfer – Example Method• Alice oblivious transfer to Bob• Alice

– picks 2 random primes p,q, set n = pq– encrypt message m using n (such as with RSA)– c = resulting ciphertext– sends n and c to Bob

• Bob – picks a Z*n at random– sends w = a2 mod n to Alice

• Alice – computes square roots of w: S = {x,-x,y,-y}– picks one of the four at random, s S and sends to Bob (probability s = a

or –a is ½)• Bob

– if s ≠ a, -a: Bob can factor n and obtain m (and will know he has m)– Won’t walk through why – see Rabin cryptosystem

• Bob obtains m with probability ½ and Alice doesn’t know result

27

Oblivious Transfer – Example Method

• Works if Bob selects a at random (doesn’t cheat)• Not known if Bob gains any advantage (can

cheat) if intentionally selects a specific a

28

Oblivious Transfer – Application

• Alice and Bob will each sign a contract only if the other also signs it

• Idea– If names are of equal length, could sign a letter at a

time, alternating• But someone must go last and could abort – not complete

last letter– Sign small fragment at a time – bit, pixel

• Don’t know who will be last• If one stops, both are approximately at same point

– But one could send garbage in a fragment• Oblivious transfer solves the problem

29

Oblivious Transfer – Application

• Alice and Bob will each sign a contract only if the other also signs it

• Idea– If names are of equal length, could sign a letter at a

time, alternating• But someone must go last and could abort – not complete

last letter– Sign small fragment at a time – bit, pixel

• Don’t know who will be last• If one stops, both are approximately at same point

– But one could send garbage in a fragment• Oblivious transfer solves the problem

30

Oblivious Transfer – Application• Alice and Bob each

– create 2 signatures, pick 2 random keys and encrypt each signature (such as with a block cipher)

• Alice– LA = “Alice, this is my signature of the left half of the contract”– RA = “Alice, this is my signature of the right half of the contract”– Keys KLA, KRA– CLA = EKLA(LA), CRA = EKRA(RA)

• Bob– LB = “Bob, this is my signature of the left half of the contract”– RB = “Bob, this is my signature of the right half of the contract”– Keys KLB, KRB– CLB = EKLB(LB), CRB = EKRB(RB)

• Contract is considered signed only if Alice and Bob each have both halves of other’s signature

31

Oblivious Transfer – Application• Alice sends one of KLA, KRA to Bob using oblivious

transfer• Bob sends one of KLB, KRB to Alice using oblivious

transfer• Alice and Bob exchange bits of both keys, one bit at a

time from each key, in order, until all bits are sent.• If Alice sees a mistake in key bits received, Alice aborts

– Likewise if Bob sees a mistake• Bob does not know if Alice has KBL or KBR, so he

cannot risk sending an incorrect value for either key– Likewise for Alice

• Must exchange bits simultaneously; otherwise, last one could flip last bit

32

Agenda


• Zero Knowledge



Some slides are modified from a presentation by Juan Garay, Bell Labs

33

Secure MPC

• A set of parties with private inputs wish to compute some joint function of their inputs.

• Parties wish to preserve some security properties. E.g., privacy and correctness.– Example: secure election protocol

• Security must be preserved in the face of adversarial behavior by some of the participants

34

Secure MPC

Multi-party computation (MPC) [Goldreich-Micali-Wigderson 87] :

– n parties {P1, P2, …, Pn}: each Pi holds a private input xi

– One public function f (x1,x2,…,xn)

– All want to learn y = f (x1,x2,…,xn) (Correctness)

– Nobody wants to disclose his private input (Privacy)

2-party computation (2PC) [Yao 82, Yao 86] : n=2

Studied for a long time. Focus has been security.

35

Secure MPC

s s1

s2

s4

s5

s3

sn

Alice has a secret key, s, (such as a key to a system)

Afraid she may lose s

Want to give is to someone, but don’t trust anyone with entire secret

Share secret among n people: s = s1 s2 s3 s4 s5 … sn

36

Secure MPC

s s1

s2

s4

s5

s3

sn

X

• Suppose need more flexibility– ith person loses si or is malicious

• n people– Any subset of size t can recover s

– Any subset of size < t cannot recover any information about s

37

Secure MPC

• Use polynomials

• Finite field F (such as some Z*p for prime p)

• F(x) = a0x0 + a1x1 + a2x2 + … atxt

• Coefficients a0, a1, a2 … at F

• t+1 terms (degree is t)

38

Secure MPC

• Polynomials properties:

• Interpolation: given t+1 distinct points, (x1,y1), (x2,y2) … (xt+1,yt+1) , can find a0, a1, a2 ,… at

• Secrecy: if have only t (or fewer) distinct points, can’t determine anything about a0

39

Secure MPC

• Coefficients– Pick a1, a2 .. at at random

– set a0 = s

• Each person is associated with a distinct point– Person i assigned distinct xi

– Set si = f(xi) : (xi,si) is point for person i

40

Secure MPC

• Polynomial construction requires honesty– Entity choosing ai’s, xi’s is dishonest

– Collusion – person 2 may give (x2,s2) to person 10

• Verifiable secret sharing– Each person can verify piece received is a proper

piece– Allows for O(log n) colluders– Based on factoring [Chor, Goldwasser, Micali,

Awerbuch]

41

Instances of 2PC

• Authentication– Parties: 1 server, 1 client.– Function : if (server.passwd == client.passwd), then return “succeed,” else return “fail.”

• On-line Bidding– Parties: 1 seller, 1 buyer.– Function: if (seller.price <= buyer.price), then

return (seller.price + buyer.price)/2, else return “no transaction.”

– Intuition: In NYSE, the trading price is between the ask (selling) price and bid (buying) price.

42

Instances of MPC

• Auctions– Parties: 1 auctioneer, (n-1) bidders.– Function: Many possibilities (e.g., Vickrey*).

• Consider a secure auction (with secret bids):– An adversary may wish to learn the bids of all parties – to prevent this, requires privacy– An adversary may wish to win with a lower bid than the highest – to prevent this, requires correctness

*sealed-bid, bidders submit written bids without knowing the bid of the others. Highest bidder wins, but pays second-highest bid. Intent: bidders bid true value.

43

MPC Protocols

Consider 2PC (MPC is similar). Two parties: P0 and P1.Encode function as a Boolean circuit of ANDs and XORs

Bits on each wire are shared using XOR

Per-gate evaluation:

(Shared) Inputs: x = x0 x1 , y = y0 y1

XOR: No interaction needed

( x y ) = ( x0 y0 ) ( x1 y1 )

AND: More complex (needs interaction)

Each party reveals his shares

44

MPC – Elections

• m voters: v1, v2, … vm

• ith voter inputs xi

• Result: function, f, of xi’s• Required properties

– Only authorized voters can vote– Each can only vote once– Each vote is secret– No vote can be duplicated by another voter– Vote tally correctly computed– Anyone can check the tally is correct– Fault tolerate protocol: works if some number of “bad” parties– Cannot coerce a voter into revealing how he/she voted (no vote-buying)

• Meeting all requirements tricky, especially last• In real life – system, logistical problems main issues as opposed to

protocols

45

MPC –Digital Cash

• Required properties– Prevent forgery– Prevent or detect duplication– Preserve customer’s anonymity

• Practical: – operationally feasible (ex. no large single

database of all issued digital cash)

46

Digital Cash

• 3 protocols– Withdrawal – user can obtain a digital coin– Payment – user buys goods from vendor using digital

coin– Deposit – vendor gives digital coin to bank to be

credited to the vendors account• Notation:

– U = user– B = Bank– V = vendor– D = digital coin of $100– SKB{x} = signature of B on x

47

Digital Cash - Withdrawal

• U notifies B wants to withdraw D

• B gives D to UD = SKB{I am a $100 bill, #4527}

• U checks signature– accepts D if it is valid– else rejects D

• Bank deducts D from U’s account if U accepts D

48

Digital Cash - Payment

• U pays V with D

• V checks signature– accepts D if it is valid– else rejects D

49

Digital Cash - Deposit

• V gives D to B

• B checks signature– accepts D if it is valid and credits V’s account– else rejects D

50

Digital Cash – Do Properties Hold?

• Prevent forgery– ok, infeasible under basic assumptions of

signature schemes

• Prevent or detect duplication– Very easy to duplicate coins, double spend

• Preserve customer’s anonymity– No anonymity – know U and where D was

spent

51

Digital Cash – Fix Properties

• Blind signature– U presents D to B– B signs D without seeing its contents (i.e.

can’t associate with U)• Analogy: U covers check with carbon paper and

seals both inside an envelope. Bank signs outside of envelop

– But how does B know D is not fake?

52


• RSA Blind signature – Have key pair (e,n) (d,n)– U picks random r mod n– U computes D’ = D – re mod n– presents D’ to B– B signs D’: s’ = (D’)d mod n (= Md (re)d = Mdr mod n )– B deducts D from U’s account – U computes signature on M: s = Md mod n by dividing s’ by r

• Now anonymity: no link between U and D• But

– B can be tricked into signing fake D– D can be duplicated and double spent

53


• One denomination or one public key per denomination, feasible – not many denominations

• Probability method– U makes up 100 D’s– Blind signature on all 100 and gives all to B– B signs one, requires U to unblind the rest (reveal r’s)– U has 1/100 chance of successfully cheating– U spends 1 remaining D, anonymous

54


• Now have anonymity and no (or small) chance of cheating

• How to prevent double spending?

• B has database of all D, records D’s as they are returned spent from V– not too practical – large database, V must wait

for B to ok each D at time of purchase

55


• What if just detect double spending?• Random identity string (RIS)

– Different for every payment of D– Only U can create a valid RIS– Two different RIS’s on same D allows B to retrieve

U’s name

• If B receives two identical D’s with different RIS values, U cheated

• If B receives two different D’s with same RIS values, V cheated

56


• H = hash

• U creates 100 D’s– Di = (I’m a $100 bill, #4527i,yi,1y’i,1,yi,2, y’i,2, …

yi,k,y’i,k)

– where

yi,j = H(xi,j); y’i,j = H(x’i,j)

xi,j and x’i,j are randomly chosen but such that xi,j x’i,j = U’s name i,j

57


• Withdrawal – U blinds each Di, get D’i– B has U unblind all but one D’i and reveals

appropriate xi,j and x’i,j – B checks for each

yi,j = H(xi,j); y’i,j = H(x’i,j)

xi,j x’i,j = U’s name

– B signs remaining blind D’i and gives to U

58

Digital Cash – Fix Properties• Payment

– U gives D’i to V– V checks B’s signature on D’i – V creates a challenge: random bit string b1,b2, … bk

– If bj = 0, U reveals xi,j , else reveals x’i,j – V checks yi,j = H(xi,j) or y’i,j = H(x’i,j) and accepts if yes, else

rejects• Probability in a different payment that same RIS is

produced is 2-k because V creates challenge at random• Only U can produce valid RIS – H is computationally

infeasible to invert• Two different RIS values on same D’I leaks U’s name:

have xi,j and x’i,j for some j (due two different challenges)

59


• Deposit – V gives D’i, s’, RIS to B

– B verifies signature, checks if already returned

– If already in database, B compares RIS values

– If different, U double spent– If equal, V trying to deposit twice

60

Other MPC Applications

• Database Query: – Alice has a string q; Bob has a database of strings – Alice wants to know whether there exists a string ti in

Bob's database that matches q (exact or close) – Privacy:– Bob cannot know Alice's q or the response – Alice cannot know the database contents except for

what can be known from the query result

61


• Profile Matching– Alice has a database of known hacker's behaviors – Bob has a hacker's behavior from a recent break-in– How can Bob check if his hacker is in Alice’s

database while• Not disclosing the hacker's actual behavior to Alice – doing

so that might disclose the vulnerability in his system. • Not obtaining contents of Alice’s database – it contains

confidential information.

62


• Companies want to cooperate in preventing intrusions into their networks. – Need to share data patterns, but this is

sensitive information

• Real data for IDS/security research– None exists– Outdated/now irrelevant MIT Lincoln

Laboratory IDS Evaluation Data Set 1998-2000

63


• In general, many database applications– Simple queries– Determining intersection– Sharing of patterns without revealing actual

content

64

Backup

65

On-line Bidding: Definition of Security

• Correctness: seller.output = buyer.output = f (seller.price, buyer.price)

• Privacy: The transcript carries no additional information about seller.price and buyer.price.

seller buyer (seller.price) (buyer.price)

(seller.output) (buyer.output)} transcript

66

“Privacy” is a little tricky…On-line Bidding Function

if (seller.price <= buyer.price), then return (seller.price + buyer.price)/2, else return “no transaction.”

• If seller.price ≤ buyer.price, then both parties can learn each other’s private input.

• If seller.price > buyer.price, then both parties should learn nothing more than this fact.

• Privacy: Each party should only learn whatever can be inferred from the output (which can be a lot sometimes).

67

Fair Secure Multi-Party Computation (FMPC)

Security is about absolute information gain.“At the end of the protocol, each party learns y (and anything inferable from y).”

Parties P1, P2, …, Pn (some corrupted), each holding private input xi, wish to compute y = f(x1, x2,…, xn) privately and correctly.

Fairness is about relative information gain.“At the end of the protocol, either all parties learn y,or no party learns anything.”

Important in MPC; crucial in some applications(e.g., two-party contract signing).

68

Security vs. Fairness

• The problem of secure MPC/2PC is well-studied and well-understood.

• The problem of fair MPC/2PC less developed • Security and fairness – different concepts

– Fair without being secure

69

Security Fairness

On-line Bidding Functionif (seller.price <= buyer.price), then return (seller.price + buyer.price)/2else return “no transaction.”

E.g., in an unfair on-line bidding protocol, the seller may learn the output (and thus buyer.price) before the buyer learns anything.

70

Cheating with Unfair ProtocolsA cheating seller: 1. Initiate protocol with price x (originally $999,999).2. Run until getting the output (buyer hasn’t got the output yet).3. if (output == “no transaction”), then abort (e.g., announce

“network failure”), set x x-1, and repeat.

A cheating seller can:– find out the buyer’s price (destroys privacy) and– achieve maximum profit (destroys correctness)

(the actual function computed is {return buyer.price})

The lack of fairness completely voids the security!

71

Fairness: Positive Results

n parties, t corrupted:

• t n/3 — possible with p2p channels – computational [GMW87] – information-theoretic [BGW88, CCD88]

• n/3 t n/2 — possible with broadcast channel – computational [GMW87]

– information-theoretic [RB89]

72

Unfortunately…

• Fairness is impossible with corrupted majority (t n/2):

Intuition (2 parties): Party sending the last message may abort early.

• Consequently, many security definitions do not consider fairness, or only consider partial fairness [BG90, BL91, FGHHS02, GL02].

73

Fairness After the Impossibility Result

We still need (some form of) fairness, so “tweak” model/definition:

“Gradual Release” approach (tweak the definition) [Blum83, D95, BN00,…]

• No trusted party needed. • Parties take turns releasing info’ “little-by-little.”• Still somewhat unfair, but we can quantify and control the amount of “unfairness.”

“Optimistic” approach (tweak the model) [M97, ASW98, CC00,…]• Adds a trusted party as an arbiter in case of dispute.• Needs to be (constantly) available.

74

The Gradual Release Approach

• Reasonably studied– Initial idea by [Blum 83]– Subsequent work: […,Damgard 95, Boneh-Naor 00,

Garay-Pomerance 03, Pinkas 03,…]

• Not quite well-understood– Ad hoc security notions– Limited general constructions (only 2PC)– Few practical constructions

75

Security and Fairness

A typical gradual release protocol (e.g., [BN00, GP03, P03]) consists of two phases:

1. Computation phase: “Normal” computation.2. Revealing phase: Each Pi gradually reveals a “secret” si ;

then each Pi computes the result y from s1, s2,…, sn.

76

Observation on Existing MPC Protocols

Many (unfair) MPC protocols (e.g., [GMW87, CDN01, CLOS02]) share the same structure:

Sharing phase: Parties share data among themselves (simple sharing, or (n, t) threshold sharing)

Evaluation phase: “Gate-by-gate” evaluation (all intermediate data are shared or “blinded”)

Revealing phase: Each party reveal its secret share (all parties learn the result from the shares)

Unfai

r!

Honest parties reveal their secrets, and corrupted parties abort (and learn the result).

77

FCPFO : Commit-Prove-Fair-Open

• Commit phase: Every party Pi commits to a value xi. • Prove phase: Every party Pi proves a relation about xi. • Open phase: Open x1, x2,…, xn simultaneously.

Using FCPFO, the revealing phase becomes fair, and so does theMPC protocol.

Simultaneous opening guarantees fairness — either all parties learn all the committed values, or nobody learns anything.

78

Time-lines: Towards realizing FCPFO

• A time-line: An array of numbers (head, …, tail).• Time-line commitments:

– TL-Commit(x) = (head, tail· x)– Perfect binding.– Hiding (2k steps to compute tail from head).– Gradual opening: Each accelerator cuts the number of steps

by half.

…head tail

accelerator 1 accelerator 2 accelerator k

79

A time-line, mathematically [BN00,GJ02,GP03]

• N: “safe Blum modulus,” N = p·q, where p, q, (p-1)/2, (q-1)/2 are all primes.

• g a random element in ZN*.

• head = g, tail = g22k

g22kg g22k-1

g2(2k-1+2k-2) …

accelerator 1 accelerator 2 …

80

A time-line, mathematically (cont’d)

g22kg g22k-1

g2(2k-1+2k-2) …

accelerator 1 accelerator 2 …

• Can move forward m positions by doing m squarings• Knowing (N), one can compute G[i] efficiently, for any i.• Hard to move backward, not knowing factorization of N• inefficient to move forward (step-by-step) point “far away” is “unknown”…

G[i] = g22i

81

Fair exchange using time-lines• START: Alice has a, Bob has b.• COMMIT:

– Alice sends TL-Commit(a) to Bob, – Bob sends TL-Commit(b) to Alice.

• OPEN: Take turns to gradually open the commitments.

Bob

Alice

82

Fair exchange using time-lines (cont’d)

• ABORT: If Bob aborts and force-opens in t steps, Alice can do it as well in 2t steps.

Bob

Alice

t

2t

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Realizing FCPFO using time-lines

• Setup: A “master” time-line T = N; g; G[j], j=1,…,k in CRS.• Commit: Each party Pi : Derives a time-line Ti= N; gi; Gi[j]; TL-commits to xi : (gi; Gi[k]· xi), • Prove: Standard ZK proof.• Open: In round m, each party Pi reveals Gi[m] with ZK proof; if any party aborts, enter panic mode. Panic mode: Depends on current round m…

• If (k-m) is “large,” then abort. (A does not have enough time to force-open.)• If (k-m) is “small,” then force-open. (A has enough time to force-open as well.)

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Putting things together…

Plug FCPFO into existing MPC protocols Fair MPC protocols

introduction to practical cryptography forward key security zero knowledge oblivious transfer...

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