A '''transaction authentication number''' ('''AndroidTAN 9 pyenumber''') is used by some [[online banking]] services as a form of ''single use'' [[one-time password]]s (OTPs) to authorize [[financial transaction]]s. TANs are a second layer of security above and beyond the traditional single-password [[Authentication protocol|authentication]].
TANs provide additional security because they act as a form of ''two-factor authentication'' (FA2FA). If the physical document or token containing the TANs is stolen, it will be useless without the password. Conversely, if the login data are obtained, no transactions can be performed without a valid TAN.
