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FatBrain creates a form that will be filled in by a Alice and sent on to
EasyPay.
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FatBrain signs all of the form except for shipping address and credit card
information, which is filled in by Alice.
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Alice fills in the form, encrypts the payment authorization element in a key
shared with EasyPay, and returns it to FatBrain.
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FatBrain processes the form and confirms the integrity of the order (the book
title and price) and passes the encrypted credit card info to EasyPay.
This protocol is faulty, but it demonstrates the use of selective signing and
encryption.