System and method for cross directory authentication in a Public Key Infrastructure. A first directory is configured to query a second directory when receiving queries regarding signature certificates from a second enterprise PKI. The first directory is part of a first enterprise PKI, and the second directory is part of the second enterprise PKI. Access to a first enterprise PKI server is attempted by a user. The user presents a signature certificate from the second enterprise PKI to the server for authentication. A query is sent to the first directory from the server to determine if the user is allowed access to the server. A query is sent to the second directory from the first directory to determine if the user is a member of the second enterprise PKI. The server approves access to the server if the user is a member of the second enterprise PKI.