Authentication system using one-time passwords

A system for authenticating a user located at a requesting node to a resource such as a host application located at an authenticating node using one-time passwords that change pseudorandomly with each request for authentication. At the requesting node a non-time-dependent value is generated from nonsecret information identifying the user and the host application, using a secret encryption key shared with the authenticating node. The non-time-dependent value is combined with a time-dependent value to generate a composite value that is encrypted to produce an authentication parameter. The authentication parameter is reversibly transformed into an alphanumeric character string that is transmitted as a one-time password to the authenticating node. At the authenticating node the received password is transformed back into the corresponding authentication parameter, which is decrypted to regenerate the composite value. The non-time-dependent value is replicated at the authenticating node using the same nonsecret information and encryption key shared with the requesting node. The locally generated non-time-dependent value is combined with the regenerated composite value to regenerate the time-dependent value. The user is authenticated if the regenerated time-dependent value is within a predetermined range of a time-dependent value that is locally generated at the authenticating node. Passwords found not to have a corresponding authentication parameter are trivially rejected by the authenticating node to avoid unnecessary processing steps. A encryption system for transforming short blocks into encrypted blocks of the same length is also disclosed.