The Internet Keyed Payments Protocol (iKP) is an architecture for secure payments involving three or more parties [BGH95]. Developed at IBM's T.J. Watson Research Center and Zurich Research Laboratory, the protocol defines transactions of a ``credit card'' nature, where a buyer and seller interact with a third party ``acquirer,'' such as a credit-card system or a bank, to authorize transactions. The protocol is based on public-key cryptography.
iKP is no longer widely in use, however it is the current foundation for SET (see Question 4.2.3).
Additional information on iKP is available from http://www.zurich.ibm.com/security/past-projects/ecommerce/iKP.html.