Abstract: The invention is directed to a secure data transmission system and method for use in connection with potentially untrusted computer systems and data communication networks. The method involves transmission of sensitive data, such as authentication credentials, between at least two entities (for example, client and server systems and zero or more trusted token systems). This method utilizes symmetric encryption, shared secrets, and data strings composed of pseudo-random characters (also known as “tokens”) to authenticate entities to other entities and to securely transmit data between entities.