Transaction Authentication Number - Indexed TAN (iTAN)

Indexed TAN (iTAN)

Indexed TANs reduce the risk of phishing. To authorize a transaction, the user is not asked to use an arbitrary TAN from the list but to enter a specific TAN as identified by a sequence number (index). As the index is randomly chosen by the bank, an arbitrary TAN acquired by an attacker is usually worthless.

However, iTANs are still susceptible to man-in-the-middle attacks, including phishing attacks where the attacker tricks the user into logging in into a forged copy of the bank's website.