Secure Channel Injection and Anonymous Proofs of Account Ownership

We introduce secure channel injection (SCI) protocols, which allow one party to insert a private message into another party’s encrypted communications. We construct an efficient SCI protocol for communications delivered over TLS, and use it to realize anonymous proofs of account ownership for SMTP servers. This allows [email protected] to prove ownership of some email address @mail.com, without revealing “alice” to the verifier. We show experimentally that our system works with standard email server implementations as well as Gmail. We go on to extend our basic SCI protocol to realize a “blind” certificate authority: the account holder can obtain a valid X.509 certificate binding [email protected] to her public key, if it can prove ownership of some email address @mail.com. The authority never learns which email account is used.