In the NACHA Certificate Authority Interoperability pilot we helped Bank of America deploy the directory infrastructure necessary to support a public key infrastructure based on the four-corner model . We developed software to exchange Certificate Revocation Lists (CRLs) with other participating banks such as Citibank, Mellon Bank and Zions Bank. As part of this pilot we also developed software that enabled merchants to obtain from their bank, signature verification and certificate path validation services that would guarantee the non-repudiability of digitally signed customer purchase orders. Our merchant software served as the backend to SAIC's secure commerce application. To support interbank online certificate status, we developed a prototype implementation of the Online Certificate Status Protocol (OCSP) that allowed participating banks to get real time status of public key certificates. We demonstrated interoperability of our OCSP implementation with Certco and Verisign OCSP responders. When combined, these software elements enabled banks, merchants, and customers to participate in a 4-corner model of trust.