The use of the transaction number (TAN) is a common security procedure in home banking or online banking. The transaction number is a 6- digit random number that is given to the bank customer as a list by his financial institution or financial service provider.
Before the transaction, the customer authorizes himself by his personal identification number( PIN) and carries out his transactions with the transaction numbers known only to him. Each individual transaction between the customer and the financial institution, be it a bank transfer or a share purchase, is carried out using a unique transaction number that can only be used once. This number loses its validity after the transaction.
An improvement in TAN security is achieved with the indexed transaction number( iTAN). With indexed TANs, the bank does not ask for an arbitrary transaction number, but for a specific transaction number on the TAN list, which is always linked to a specific transaction.