Secure Electronic Transaction (SET) SMU CSE 5349/7349

Secure Electronic Transaction (SET) SMU CSE 5349/7349

Credit Cards on the Internet Problem: communicate credit card and purchasing data securely to gain consumer trust – Authentication of buyer and merchant – Confidential transmissions Systems vary by – – – – – SMU Type of public-key encryption Type of symmetric encryption Message digest algorithm Number of parties having private keys Number of parties having certificates CSE 5349/7349

Credit Card Protocols SSL 1 or 2 parties have private keys TLS (Transport Layer Security) – IETF version of SSL i KP (IBM) SEPP (Secure Encryption Payment Protocol) – MasterCard, IBM, Netscape STT (Secure Transaction Technology) – VISA, Microsoft SET (Secure Electronic Transactions) – MasterCard, VISA all parties have certificates SMU CSE 5349/7349 OBSOLETE VERY SLOW ACCEPTANCE

Secure Electronic Transaction (SET) Developed by Visa and MasterCard Designed to protect credit card transactions Confidentiality: all messages encrypted Trust: all parties must have digital certificates Privacy: information made available only when and where necessary SMU CSE 5349/7349

Participants in the SET System SMU CSE 5349/7349

SET Business Requirements Provide confidentiality of payment and ordering information Ensure the integrity of all transmitted data Provide authentication that a cardholder is a legitimate user of a credit card account Provide authentication that a merchant can accept credit card transactions through its relationship with a financial institution SMU CSE 5349/7349

SET Business Requirements (cont’d) Ensure the use of the best security practices and system design techniques to protect all legitimate parties in an electronic commerce transaction Create a protocol that neither depends on transport security mechanisms nor prevents their use Facilitate and encourage interoperability among software and network providers SMU CSE 5349/7349

SET Transactions SMU CSE 5349/7349

SET Transactions The customer opens an account with a card issuer. – MasterCard, Visa, etc. The customer receives a X.509 V3 certificate signed by a bank. – X.509 V3 A merchant who accepts a certain brand of card must possess two X.509 V3 certificates. – One for signing & one for key exchange The customer places an order for a product or service with a merchant. The merchant sends a copy of its certificate for verification. SMU CSE 5349/7349

SET Transactions The customer sends order and payment information to the merchant. The merchant requests payment authorization from the payment gateway prior to shipment. The merchant confirms order to the customer. The merchant provides the goods or service to the customer. The merchant requests payment from the payment gateway. SMU CSE 5349/7349

Key Technologies of SET Confidentiality of information: DES Integrity of data: RSA digital signatures with SHA-1 hash codes Cardholder account authentication: X.509v3 digital certificates with RSA signatures Merchant authentication: X.509v3 digital certificates with RSA signatures Privacy: separation of order and payment information using dual signatures SMU CSE 5349/7349

Dual Signatures Links two messages securely but allows only one party to read each. MESSAGE 1 MESSAGE 2 HASH 1 & 2 WITH SHA DIGEST 1 DIGEST 2 CONCATENATE DIGESTS TOGETHER HASH WITH SHA TO CREATE NEW DIGEST NEW DIGEST ENCRYPT NEW DIGEST WITH SIGNER’S PRIVATE KEY PRIVATE KEY DUAL SIGNATURE SMU CSE 5349/7349

Dual Signature for SET Concept: Link Two Messages Intended for Two Different Receivers: – Order Information (OI): Customer to Merchant – Payment Information (PI): Customer to Bank Goal: Limit Information to A “Need-to-Know” Basis: – Merchant does not need credit card number. – Bank does not need details of customer order. – Afford the customer extra protection in terms of privacy by keeping these items separate. This link is needed to prove that payment is intended for this order and not some other one. SMU CSE 5349/7349

Why Dual Signature? Suppose that customers send the merchant two messages: The signed order information (OI). The signed payment information (PI). In addition, the merchant passes the payment information (PI) to the bank. If the merchant can capture another order information (OI) from this customer, the merchant could claim this order goes with the payment information (PI) rather than the original. SMU CSE 5349/7349

Dual Signature Operation The operation for dual signature is as follows: – Take the hash (SHA-1) of the payment and order information. – These two hash values are concatenated [H(PI) H(OI)] and then the result is hashed. – Customer encrypts the final hash with a private key creating the dual signature. DS EKRC [ H(H(PI) H(OI)) ] SMU CSE 5349/7349

DS Verification by Merchant The merchant has the public key of the customer obtained from the customer’s certificate. Now, the merchant can compute two values: H(PIMD H(OI)) DKUC[DS] Should be equal! SMU CSE 5349/7349

DS Verification by Bank The bank is in possession of DS, PI, the message digest for OI (OIMD), and the customer’s public key, then the bank can compute the following: H(H(PI) OIMD) DKUC [ DS ] SMU CSE 5349/7349

What did we accomplish? The merchant has received OI and verified the signature. The bank has received PI and verified the signature. The customer has linked the OI and PI and can prove the linkage. SMU CSE 5349/7349

SET Supported Transactions card holder registration merchant registration purchase notification purchase request payment authorization authorization reversal payment capture certificate query credit reversal purchase inquiry SMU CSE 5349/7349 sale transaction capture reversal

Purchase Request Browsing, Selecting, and Ordering is Done Purchasing Involves 4 Messages: – Initiate Request – Initiate Response – Purchase Request – Purchase Response SMU CSE 5349/7349

Purchase Request: Initiate Request Basic Requirements: – Cardholder Must Have Copy of Certificates for Merchant and Payment Gateway Customer Requests the Certificates in the Initiate Request Message to Merchant – Brand of Credit Card – ID Assigned to this Request/response pair by customer – Nonce SMU CSE 5349/7349

Purchase Request: Initiate Response Merchant Generates a Response – Signs with Private Signature Key – Include Customer Nonce – Include Merchant Nonce (Returned in Next Message) – Transaction ID for Purchase Transaction In Addition – Merchant’s Signature Certificate – Payment Gateway’s Key Exchange Certificate SMU CSE 5349/7349

Purchase Request: Purchase Request Cardholder Verifies Two Certificates Using Their CAs and Creates the OI and PI. Message Includes: – Purchase-related Information – Order-related Information – Cardholder Certificate SMU CSE 5349/7349

Purchase Request The cardholder generates a one-time symmetric encryption key, KS, SMU CSE 5349/7349

Merchant Verifies Purchase Request When the merchant receives the Purchase Request message, it performs the following actions: – Verify the cardholder certificates by means of its CA signatures. – Verifies the dual signature using the customer’s public key signature. SMU CSE 5349/7349

Merchant Verification (cont’d) – Processes the order and forwards the payment information to the payment gateway for authorization. – Sends a purchase response to the cardholder. SMU CSE 5349/7349

Purchase Response Message Message that Acknowledges the Order and References Corresponding Transaction Number Block is – Signed by Merchant Using its Private Key – Block and Signature Are Sent to Customer Along with Merchant’s Signature Certificate Upon Reception – Verifies Merchant Certificate – Verifies Signature on Response Block – Takes the Appropriate Action SMU CSE 5349/7349

Payment Process The payment process is broken down into two steps: – Payment authorization – Payment capture SMU CSE 5349/7349

Payment Authorization The merchant sends an authorization request message to the payment gateway consisting of the following: – Purchase-related information PI Dual signature calculated over the PI & OI and signed with customer’s private key. The OI message digest (OIMD) The digital envelop – Authorization-related information – Certificates SMU CSE 5349/7349

Payment Authorization (cont’d) – Authorization-related information An authorization block including: – A transaction ID – Signed with merchant’s private key – Encrypted one-time session key – Certificates Cardholder’s signature key certificate Merchant’s signature key certificate Merchant’s key exchange certificate SMU CSE 5349/7349

Payment: Payment Gateway Verify All Certificates Decrypt Authorization Block Digital Envelope to Obtain Symmetric Key and Decrypt Block Verify Merchant Signature on Authorization Block Decrypt Payment Block Digital Envelope to Obtain Symmetric Key and Decrypt Block Verify Dual Signature on Payment Block Verify Received Transaction ID Received from Merchant Matches PI Received from Customer Request and Receive Issuer Authorization SMU CSE 5349/7349

Authorization Response Authorization Response Message – Authorization-related Information – Capture Token Information – Certificate SMU CSE 5349/7349

SET Overhead Simple purchase transaction: Four messages between merchant and customer Two messages between merchant and payment gateway 6 digital signatures 9 RSA encryption/decryption cycles 4 DES encryption/decryption cycles 4 certificate verifications Scaling: Multiple servers need copies of all certificates SMU CSE 5349/7349