A payment authentication service authenticates the identity of a payer during online transactions. The authentication service allows a card issuer to verify a cardholder's identity using a variety of authentication methods, such as with the use of tokens. Authenticating the identity of a cardholder during an online transaction involves querying an access control server to determine if a cardholder is enrolled in the payment authentication service, requesting a password from the cardholder, verifying the password, and notifying a merchant whether the cardholder's authenticity has been verified. Systems for implementing the authentication service in which a cardholder uses a mobile device capable of transmitting messages via the Internet are described. Systems for implementing the authentication service in which a cardholder uses a mobile device capable of transmitting messages through voice and messaging channels is also described.