A protocol for establishing the authenticity of a client to a server in an electronic transaction by encrypting a certificate with a key known only to the client and the server. The trust of the server, if necessary, can be established by a public key protocol. The client generates and sends over a communications channel a message containing at least a part of a certificate encrypted with the server's public key or a secret session key. The server receives and processes the message to recover at least part of the certificate, verifies and accepts it as proof of the client's authenticity.