A transaction authentication system uses a computer network and mobile telephone network to authenticate a user. The user initiates a transaction and provides an identity token, such as the mobile telephone number. The identity token is used by an authentication server to initiate the issuance of a new temporary identity for the corresponding mobile device. The new temporary identity is forwarded from the mobile device to the authentication server which issues a security code if there is a match between the new temporary identities. The security code is forwarded to a transaction server which relays it to the authentication server. If the forwarded security code matches the generated security code, the transaction is permitted to continue.