Cardholder to Merchant SET Protocol I have some doubts regarding the SET protocol. Let say the Alice payment information runs through a sha-1 hash function which is a one-way algorithm, irreversible to create a message digest. How do bob verify that the message is really from alice since he can't see the content of the message which bob shouldn't see in the first place?

Based on my understanding, it has something to do with public digital signature and comparing it with the message digest but I don't get it. Can anyone explain the concept to me in layman terms.
In SET, Bob the merchant doesnt need to know message is from Alice the cardholder. Bob needs to get paid for the order received. So Bob proceeds on response from Acquirer (running a SET payment gateway) telling him this particular cardholder (whatever the name) has sufficient funds in his account. This would likely mean funds were reserved, according to policy of issuing bank.

Yes, a "dual signature" was introduced in SET, meaning message to be signed is actually just two hashes, one is the order and the other is payment information. Merchant is supposed to receive the order and the hash of payment information, unless gateway would open more details.