VarietyCash: A Multi-purpose Electronic Payment System

By M. Bellare, J. Garay, C. Jutla, M. Yung --- 1998

By Liang Li ChrisMarch 29th

Introduction

• What is Electronic Payment

System?−Essential Component of Electronic

Commerce

• Widely Used in the World

• Then … Secure???

Issues Concerned

• Anonymity −Payment Untraceable to a third party

• Account-based or Account-less−Account-based is more expensive in order to

maintain the accounts

• Network versus card-based−Transactions across the network

• Atomicity−Fair and Robust in the sense of network

failure

System Proposed

• Trust-Based Anonymity

• Account-less

• Network-Based & Card-Based

• Atomicity

System Proposed - More

• What is new here?−Coins and Tokens authenticated under

an issuer master key−Spent coins can be erased (Improve the

scalability)−Combine symmetric-key cryptography

for performance with public-key systems

Cryptographic Primitives Used

Model & System Architecture

Security & Design Goals

• Protocol Security−Safety guarantees, no adversaries can

compromise or spoil the system

• Internal Security−Withstand insider attacks, no cheaters

• Network Security−Prevent attacks through “break-ins”. Need

careful design of interface to the external network

• User Security−Protect its own database (E-coin)

Parties & Roles

• Participant− A generic name for a party involved in the system

• Issuer− Party who issues the coins

• Coin Holder *− A party who holds the money

• Payee− A party who is willing to accept coins as payment

• Bank− A number of banks will be involved in moving funds due to

conversions between electronic and real money

• Certification Authority− The Party who can certify the public keys of the participants

Coin-Holder

• Coin Purchaser− Purchase coins from the issuer

• Redeemer− Turn coins into real money

• Payer− The party who pays for the good/service

• Refresher− Get new coins for old

• Changer− Make changes

Others

• Register−Register a public key at the issuer

• Enroller−Enroll for a particular role such as coin

purchaser or merchant

E-coin

• The most basic component in the

whole system

• An object consisting of a unique

identifier (coin ID), amount, expiry

date and an authenticating

cryptographic tag

E-coin

•Unique

Identifier

•Value (Amount)

•Expiry Date

•Authenticating

Cryptographic

Tag

Fig 1. Structure of an un-encrypted coin

E-coin - More

•Unique Identifier

•Value (Amount)

•Expiry Date

•Authenticating

Cryptographic

Tag

•Search Tag

•Coin StatusFig 2. Encrypted coin in the Issuer’s database

Protection from Forgery Coin

• Tag is computed in protected,

tamper-proof hardware

• Tag computing algorithm is strong

• Coin Database is protected

Coin Purchase Request

• Take form of a list of denominations−For example, (2, $2.50), (1, $1.25), (3,

$2) means 2 coins of value $2.50, 1 coin of value $1.25, 3 coins of value $2. The total sum is 12.25

Process & Protocols

Operations Involved

• Registration – Register ID and Get own PK• Enrollment – A Participant enrolls for a role• Coin Purchase * - Buy E-coin from Issuer• Payment * - Deal Transaction• Change – Make Changes from Issuer• Redeem – Get Real Money• Refresh – Keep freshness of E-coins• Refund – Ask for E-coins if network failure

Coin Purchase ProtocolMore Detail

• Design Requirement−Valid Transaction go through−Cannot get coins free−No false debit

Coin Purchase ProtocolMore Detail

•Terminology

•Overview

Coin Purchase Protocol• How Does ACH Work?

• 1.A company/individual (Receiver) authorizes a company/individual (Originator) to initiate a transaction to their financial institution account.

• 2.The Originator prepares information about the transactions that are to be automated for its customers or employees and passes it along to an Originating Depository Financial Institution (ODFI). 

•

• 3.The ODFI collects ACH transactions from participating companies, consolidates  the information and presents it to the ACH Operator. (The ODFI may retain entries for its own account holders) 

•

• 4.ACH Operator processes transaction files from submitting ODFIs and distributes it to Receiving Depository Financial Institutions (RDFls).

•

• 5.The RDFI receives entries for its customer accounts and posts entries on the settlement date. Transactions are also reported on account statements

Coin Purchase ProtocolMore Detail

•Coin Request

Coin Purchase ProtocolMore Detail

•Execution

•Issuance

Coin Purchase Protocol

• Message Integrity−Plaintext Awareness Encryption

Scheme• Correct Decryption convinces the decryptor

that the transmitter knows the plaintext encrypted

• Prevent from tampering with a ciphertext but no authentication guaranteed

Payment Protocol - More Detail

• Design Requirement−Valid Payment go through−Accepted payments are valid−Payment is paid to the correct party−No double spending

Payment Protocol - More Detail

•Overview

Payment Protocol - More Detail

•Terminology

Payment Protocol - More Detail

•Invoice

•Send Coins

Payment Protocol - More Detail

•Validation Request

•Issuance

Payment Protocol - More Detail

•Receipt

More Applications Used

• Integrate with Card Cash

• Card-Based System−Pseudo-anonymous−Offline−Non-circulating

Integrate with Card Based System

•Load Protocol−On-line Account-based load

•Terminology

Integrate with Card Based System

• Protocol Flows

Blind Signature

• A signature scheme that the signer

signs it with no idea of what the

content is

• Properties−Cannot prove that he signed it in that

particular protocol−The signature is valid

• Use cut-and-choose technical

Blind Signature

• General Process1. Takes a document and multiplies it by

a random value which is called a blinding factor.

2. Send the blinded document3. Sign the blinded document4. Divide out the blinding factor, leaving

the original document signed

Implementation

• Scenario− There is a group of counterintelligence

agents. They want to the counterintelligence agency sign a document for them for diplomatic immunity. Even the counterintelligence agency have no idea of who they are. The document should insert agent’s cover name that each agent has a list of them.

− ?What will he do then?

Protocol

• Assumption− The signature function and

multiplication are commutative

• Parties− ALICE – Agency’s Large Intelligent

Computing Engine− BOB – Bogota Operations Branch

Protocol - More

1. BOB prepares n documents each using a different cover name giving himself diplomatic immunity

2. BOB blinds each of these documents with a different blinding factor

3. BOB sends n documents to ALICE

4. ALICE chooses n-1 documents at random and ask ZBOB for the blinding factor for each of them

Protocol - More

5. BOB sends ALICE the appropriate blinding factor

6. ALICE opens n-1 documents, makes sure they are correct --- and not pension authorizations

7. ALICE signs the remaining document and sends it to BOB

8. The agent removes the blinding factor and reads the new cover name in the document. It is “James Bond”

Attack on Blind Signature

• Probability Guess− Choose n/2 documents rather than n-1

one

• Tricky Twin Document− Choose 2 different blinder factor so

that transform 2 different document into the same blinded document.

?Questions?

Thank you

Acknowledgement

• Prof. C. Lynch

• http://www.achnetwork.com/introtoach.html

• Applied Cryptography – Bruce Shneier

• Blind Signature for Untraceable Payments

• --- 2005 Mar 29th

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