Purchase Request of SET

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 Purchase Request

 Before the Purchase Request exchange begins, the cardholder has completed browsing, selecting, and ordering. The end of this preliminary phase occurs when the merchant sends a completed order form to the customer. All of the preceding occurs without the use of SET. The purchase request exchange consists of four messages: 

a) Initiate Request. 

b)Initiate Response. 

c) Purchase Request. 

d) Purchase Response.


a) 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(timestamp) used to ensure timeliness.

b) Initiate Response

-  Merchant Generates a Response 

  • Signs with Private Signature Key.
  •  transaction ID for Purchase Transaction 
  • Merchant’s Signature Certificate 
  • Payment Gateway’s Key Exchange Certificate 
  • the nonce from the customer 
  • another nonce for the customer to return in the next message

c) Purchase Request. 
  •  Cardholder Verifies Two Certificates(merchant and gateway) Using Their CAs and Creates the OI and PI. 
  • First SET Message Includes:
-  Purchase-related Information,
-  Order-related Information 
-  Cardholder Certificate
  1.  Purchase Request – Customer
  2.  Purchase Request – Merchant

  1.  Purchase Request – Customer
shikhadeep.com.np

Purchase Request – Customer 
The cardholder prepares the Purchase Request message and generates a one-time symmetric encryption key, Ks. 
The message includes the following: 
1. Purchase-related information. This information will be forwarded to the payment gateway by the merchant and consists of
 - The PI - The dual signature, calculated over the PI and OI, signed with the customer's private signature key 
- The OI message digest (OIMD) The (OIMD) is needed for the payment gateway to verify the dual signature, as explained previously. All of these items are encrypted with Ks. The final item is 
- The digital envelope. This is formed by encrypting Ks with the payment gateway's public key exchange key. It is called a digital envelope because this envelope must be opened (decrypted) before the other items listed previously can be read. The value of Ks is not made available to the merchant. Therefore, the merchant cannot read any of this payment-related information.


2. Purchase Request – Merchant
purchase request of SET

Purchase Request – Merchant 
When the merchant receives the Purchase Request message, it performs the following actions 

1. Verifies the cardholder certificates by means of its Certification Authority (CA) signatures. 

2. Verifies the dual signature using the customer's public signature key. This ensures that the order has not been tampered with in transit and that it was signed using the cardholder's private signature key.

 3. Processes the order and forwards the payment information to the payment gateway for authorization (described later). 

4. Sends a purchase response to the cardholder.


                                 OR,

Merchant Verifies Purchase Request
 ♦ When the merchant receives the Purchase Request message, it performs the following actions:
 – Verify the cardholder certificates by means of their CA signatures.
 – Verifies the dual signature using the customer’s public key signature.
 – Processes the order and forwards the payment information to the payment gateway for authorization. – Sends a purchase response to the cardholder.


d)  Purchase Response Message 
  • The message that Acknowledges the Order and References Corresponding Transaction Number 
  • Response 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


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