advanced cryptographic protocols lecture 12 supakorn kungpisdan [email protected] nete46301

Advanced Cryptographic Protocols

Lecture 12

Supakorn [email protected]

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Outlines

• Payment Protocols• Account-based Payment Protocols• Token-based Payment Protocols

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• Primitive transactions– Payment– Value Subtraction– Value Claim

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Value claimValue claimValue Value

subtractionsubtraction

PaymentPayment

ClientClient MerchantMerchant

Payment GatewayPayment Gateway

IssuerIssuer AcquirerAcquirer

Electronic Payment Model

E-Payment Model (cont.)

Payment, Value substraction

Value subtraction, Value claim

Value claim, Value subtraction

Payment, Value substraction

Payment Gateway (PG)

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Account-based Account-based Payment SystemsPayment Systems

Electronic Payment SystemsElectronic Payment Systems

Token-based Token-based Payment SystemsPayment Systems

Credit card, Electronic Checks Micropayment, Electronic Cash

Types of Payment Systems


Types of Payment Systems (cont.)• Account-based payment systems

– Represented by the transfer between accounts– Credit-card, debit-card, or electronic check– PayPal, Amex’s PayFlow, SET, iKP, NetChex– Requires payment authorization from banks in every

transaction– Suitable for high-value transactions

• Token-based payment systems– Represented by electronic money– Micropayment, electronic coins, or electronic cash– CyberCash, PayWord, Millicent– No payment authorization required in every transaction– Suitable for low-value transactions

Outlines


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Outlines


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Micropayment

• Represented by electronic money• Lightweight compared to credit-card

payment in terms of cryptographic operations

• No payment authorization required in every transaction

• Suitable for Low-value payment transactions e.g. from 1 cents to 2 dollars

• Prepaid and Postpaid micropayment

2. PayWord Certificate

1 Certificaterequest

4. Certificate verification, first payment

5. Repeated payment

3. Generate coins

6. Redeem coins

7. Billing

ClientClient

BankBank

MerchantMerchant

PayWord

PayWord (cont.)• Postpaid micropayment protocol based on

public-key operations. • Three parties are involved in the system:

client, merchant, and bank.• The bank issues the client a PayWord

certificate containing an authorized amount CL

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PayWord (cont.)

1. Client generates a set of coins {c0, ..., cn}, where n = CL, which is specific to the merchant.

2. The set of coins is generated as follows:ci= h(ci+1), where i = 1, ..., n

3. In the first payment, the client sends the merchant a commitment:

commitment = {certificate, c0}

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PayWord (cont.)

4. In each payment, the client sends the coin ci to the merchant.

5. The merchant can infer the value of the coin i by applying a number of hash operations to ci as follows: c0 = hi(ci).

6. At the end of the month, the merchant sends the highest value of ci together with the commitment (containing c0) to the bank.

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PayFair

Prepaid coupon

1 22

33

4

55

6

Request amount

Money Deduct

Coupon verification

Coupon verification

ClientClient

MerchantMerchant

BankBank

Verification response

PaymentPayment


PayFair• A prepaid micropayment protocol which deploys

symmetric-key operations and hash functions.

• The bank returns the message containing a payment token {N, RN}SK, RN = random number, N = serial number

• The client generates a set of coins wi, i = 0, …, n

wi = h(wi+1)

PayFair (cont.)

• For each payment,

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Limitations of PayWord & PayFairPayWord, • High Computation at the client due to asymmetric

cryptographic operation• Payment information (price of goods) is revealed to other

parties.

PayFair,• The prepaid coupon is merchant-specific when used.• The client has to contact the bank for issuing a new prepaid

coupon every time she runs out of credits.• The bank is able to impersonate as any client to make a

payment to a merchant.• The client cannot refund un-used coins and coupons.

Kungpisdan’s Approach

• Lightweight protocol• Provide a general-purposed prepaid card: able to

make payments to many merchant.• Extend validity period of a prepaid coupon• Enhance the ability to identify the originator of

the message (to prevent impersonation).• All private information must be kept secret.• Offer the ability to refund and cancel prepaid card

Setup Protocol

2. Bank coupon

1 bank coupon request

Deductmoney

ClientClient

BankBank

3. Create coins

Merchant coupon for M1

Remaining bank coupon

MerchantM1

Bank coupon

Payment Initialization Protocol

4. Coupon verification

6. Updated bank coupon

ClientClient

Merchant

BankBank

5. M1’s coupon

Merchant coupon for M1M1


Deposit to M1’s account

Payment Protocol

6

Client

Merchant

BankBank

PaymentMerchant coupon for M1

M1

Extra Credit Request Protocol


8. Authorization request

Merchant

Bank

9. Approval

7. Extra credit request M1

ClientClient



Requested value

Coupon Cancellation Protocol

12. Cancellation response

11. Bank coupon cancellation request

Merchant

Bank

M1

Client


Deposit to the client’s account

Coin Return Protocol


14. Authorization request

Merchant

Bank

15. Approval

13. Coin return request M1

ClientClient

Remaining M1’s coupon

Requested value


Deduct From M1’s account

Extension to Postpaid Micropayment

2. Bank coupon

1 Certificaterequest

4. Bank coupon verification, first payment

5. Repeated payment

3. Generate coins

6. Redeem coins

7. Billing

Client

Bank

Merchant

Question?

NextGroup discussion

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Group Discussion• Work in a group of 6 students• Design an Internet bill payment protocol

based on any cryptographic operations– Symmetric encryption, Public-key encryption,

Hash functions and MACs

• Must have the following process:– Client and merchant (biller) registration– The client can add a merchant– The client can make a payment to the merchant

• Give a demo by the end of the class

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advanced cryptographic protocols lecture 12 supakorn kungpisdan [email protected] nete46301

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