Dual Signature of SET

 Dual Signature of SET/SET’s Dual Signature 

The purpose of the dual signature is to link two messages that are intended for two different recipients. In this case, the customer wants to send the order information (OI) to the merchant and the payment information (PI) to the bank. The merchant doesn't need to know the customer's credit card number, and the bank doesn't need to know the details of the customer's order. The customer is afforded extra protection in terms of privacy by keeping these two items separate. However, the two items must be linked in a way that can be used to resolve disputes if necessary. The link is needed so that the customer can prove that this payment is intended for this order and not for some other goods or services.

To see the need for the link, suppose that the customer sends the merchant two messages: a signed OI and a signed PI, and the merchant passes the PI to the bank. If the merchant can capture another OI from this customer, the merchant could claim that this OI goes with the PI, rather than the original OI. The linkage prevents this. Figure 2 shows the use of a dual signature to meet this requirement.

                                          Figure 2 Construction of dual signature.

The customer takes the hash (using SHA-1) of the PI and the hash of the OI. These two hashes are then concatenated and the hash of the result is taken. Finally, the customer encrypts the final hash with his or her private signature key, creating the dual signature. The operation can be summarized as shown in Figure 3, where KRc is the customer's private signature key:

                                                               Figure 3

Now suppose that the merchant is in possession of the dual signature (DS), the OI, and the message digest for the PI (PIMD). The merchant also has the public key of the customer, taken from the customer's certificate. Then the merchant can compute the two quantities shown in Figure 4, where KUc is the customer's public signature key:

                                                                         Figure 4

If these two quantities are equal, the merchant has verified the signature. Similarly, if the bank is in possession of DS, PI, the message digest for OI (OIMD), and the customer's public key, the bank can compute the following (see Figure 5):

                                                                   Figure 5

Again, if these two quantities are equal, the bank has verified the signature. In summary,

1. The merchant has received OI and verified the signature.
2. The bank has received PI and verified the signature.
3. The customer has linked the OI and PI and can prove the linkage.

For example, suppose the merchant wants to substitute another OI in this transaction, to its advantage. It would then have to find another OI whose hash matches the existing OIMD. With SHA-1, this is deemed not to be feasible. Thus, the merchant cannot link another OI with this PI.

                                              

                                                  OR,

SET’s Dual Signature 

• The purpose of the dual signature is to link two messages that are going to different recipients.

 ◦ Order Information (OI): Customer to Merchant

 ◦ Payment Information (PI): Customer to Bank 

• The customer needs to send OI and PI to the merchant and bank respectively. 

• The merchant does not need to know the customer's credit card number. 

• The bank does not need to know what the customer is buying. 

• however, the two items must be linked in a way that can be used to resolve disputes if necessary.

 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!

 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 ]

The goal of the dual signature

• Goal: Limit Information to A “Need-to-Know” Basis:

 – Merchant does not need a credit card number.

 – Bank does not need details of customer orders

. – 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. SET’s Dual Signature

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.