Transaction authentication number: Difference between revisions

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{{Short description|One-time password used in banking}}

{{Other uses|TAN (disambiguation){{!}}Tan}}

A '''transaction authentication number''' ('''TAN''') 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]].

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==Classic TAN==

TANs often function as follows:

# The bank creates a set of unique TANs for the user.<ref>{{Cite web |title=Transaction Authentication Number (TAN) |url=https://fraud.net/d/transaction-authentication-number/ |access-date=2023-12-14 |website=Fraud.net |language=en-US}}</ref> Typically, there are 50 TANs printed on a list, enough to last half a year for a normal user; each TAN being six or eight characters long.

# The user picks up the list from the nearest bank branch (presenting a [[passport]], an [[ID card]] or similar document) or is sent the TAN list through mail.

# The password (PIN) is mailed separately.

# To log on to his/hertheir account, the user must enter user name (often the account number) and password ([[Personal identification number|PIN]]). This may give access to account information but the ability to process transactions is disabled.

# To perform a transaction, the user enters the request and authorizes the transaction by entering an unused TAN. The bank verifies the TAN submitted against the list of TANs they issued to the user. If it is a match, the transaction is processed. If it is not a match, the transaction is rejected.

# The TAN has now been used and will not be recognized for any further transactions.

# If the TAN list is compromised, the user may cancel it by notifying the bank.

However, as any TAN can be used for any transaction, TANs are still prone to [[phishing attacks]] where the victim is tricked into providing both password/PIN and one or several TANs. Further, they provide no protection against [[man-in-the-middle attack]]s, (where an attacker intercepts the transmission of the TAN, and uses it for a forged transaction, such as when the client system becomes compromised by some form of [[Antivirus software|malware]] that enables a [[Hacker (computer security)|malicious user]]. Although the remaining TANs are uncompromised and can be used safely, users are generally advised to take further action, as soon as possible.

Especially when the client system becomes compromised by some form of [[Antivirus software|malware]] that enables a [[Hacker (computer security)|malicious user]], the possibility of an unauthorized transaction is high. Although the remaining TANs are uncompromised and can be used safely, users are generally advised to take an appropriate action, as soon as possible.

== Indexed TAN (iTAN) ==

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Prior to entering the iTAN, the user is presented a [[CAPTCHA]], which in the background also shows the transaction data and data deemed unknown to a potential attacker, such as the user's birthdate. This is intended to make it hard (but not impossible) for an attacker to forge the CAPTCHA.

This variant of the iTAN is method used by some German banks adds a [[CAPTCHA]] to reduce the risk of man-in-the-middle attacks.<ref>{{cite web|url=http://www.heise.de/newsticker/meldung/98025|title=Verbessertes iTAN-Verfahren soll vor Manipulationen durch Trojaner schützen|author=heise online|date=2007-10-26|language=Germande}}</ref> Some Chinese banks have also deployed a TAN method similar to iTANplus. A recent study shows that these CAPTCHA-based TAN schemes are not secure against more advanced automated attacks.<ref>{{Cite conference | first = Shujun | last = Li |author2=Syed Amier Haider Shah |author3=Muhammad Asad Usman Khan |author4=Syed Ali Khayam |author5=Ahmad-Reza Sadeghi |author6=Roland Schmitz | title = Breaking e-Banking CAPTCHAs | booktitlebook-title = Proceedings of 26th Annual Computer Security Applications Conference (ACSAC 2010) | pages = 171–180 | publisher = ACM | year = 2010 | ___location = New York, NY, USA | url = http://www.acsac.org/2010/openconf/modules/request.php?module=oc_program&action=summary.php&id=53 | doi = 10.1145/1920261.1920288 }}</ref>

== Mobile TAN (mTAN) ==

mTANs are used by banks in Austria, Bulgaria, Czech Republic, Germany, Hungary, Malaysia, the Netherlands, Poland, Russia, Singapore, South Africa, Spain, Switzerland and some in New Zealand, Australia, UK, and Ukraine. When the user initiates a transaction, a TAN is generated by the bank and sent to the user's mobile phone by [[SMS]]. The SMS may also include transaction data, allowing the user to verify that the transaction has not been modified in transmission to the bank.

However, the security of this scheme depends on the security of the mobile phone system. In South Africa, where SMS-delivered TAN codes are common, a new attack has appeared: SIM Swap Fraud. A common attack vector is for the attacker to [[Identity theft|impersonate]] the victim, and obtain a replacement [[SIM card]] for the victim's phone from the [[mobile network operator]]. The victim's user name and password are obtained by other means (such as [[keylogging]] or [[phishing]]). In-between obtaining the cloned/replacement SIM and the victim noticing their phone no longer works, the attacker can transfer/extract the victim's funds from their accounts.<ref>[http://www.iol.co.za/news/south-africa/victim-s-sim-swop-fraud-nightmare-1.385531 ''Victim's SIM swop fraud nightmare''] iol.co.za, Independent Online, January 12, 2008</ref> In 2016 a [https://theantisocialengineer.com/sim-swap-fraud-porting-your-digital-life-in-minutes/ study was conducted on SIM Swap Fraud] by a [[Social engineering (security)|social engineer]], revealing weaknesses in issuing porting numbers.

In 2014, a weakness in the [[Signalling System No. 7]] used for SMS transmission was published, which allows interception of messages. It was demonstrated by Tobias Engel during the 31st [[Chaos Communication Congress]].<ref>{{cite web|title=31C3: Mobilfunk-Protokoll SS7 offen wie ein Scheunentor|url=https://www.heise.de/newsticker/meldung/31C3-Mobilfunk-Protokoll-SS7-offen-wie-ein-Scheunentor-2506892.html|date=2014-12-28|language=Germande}}</ref>. At the beginning of 2017, this weakness was used successfully in Germany to intercept SMS and fraudulently redirect fund transfers.<ref>

{{cite web| url=https://www.heise.de/newsticker/meldung/Deutsche-Bankkonten-ueber-UMTS-Sicherheitsluecken-ausgeraeumt-3702194.html| title=Deutsche Bankkonten über UMTS-Sicherheitslücken ausgeräumt| author=Fabian A. Scherschel| date=2017-05-03|language=Germande}}</ref>.

Also the rise of [[smartphone]]s led to malware attacks trying to simultaneously infect the PC and the mobile phone as well to break the mTAN scheme.<ref>[http://news.techworld.com/security/3415014/eurograbber-sms-trojan-steals-36-million-from-online-banks/ ''Eurograbber SMS Trojan steals €36 million from online banks''] techworld.com, December 5, 2012</ref>

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[[File:SmartTAN optic-Gadget.jpg|thumb|ChipTAN generator (optical version) with bank card attached. The two white arrows mark the borders of the barcode on the computer screen.]]

ChipTAN is a TAN scheme used by many German and Austrian banks.<ref>[https://www.postbank.de/privatkunden/pk_chiptanservices/banking-und-brokerage/chiptan.html Postbank chipTAN comfort] official page of Postbank, Retrieved on April 10, 2014.</ref><ref>[http://www.sparkasse.de/privatkunden/sicherheit-im-internet/chipTAN.html chipTAN: Listen werden überflüssig] official page of Sparkasse, Retrieved on April 10, 2014.</ref><ref>[http://www.raiffeisen.at/cardtan Die cardTAN] official page of Raiffeisen Bankengruppe Österreich, Retrieved on April 10, 2014.</ref> It is known as ChipTAN or Sm@rt-TAN<ref>{{Cite web|url=https://www.vr-banking-app.de/smart-tan.html|title=Sm@rt-TAN|website=www.vr-banking-app.de|language=de|access-date=2018-10-10}}</ref> in Germany and as CardTAN in Austria, whereas cardTAN is a technically independent standard.<ref>[http://ebankingsicherheit.at/die-neue-cardtan Die neue cardTAN] ebankingsicherheit.at, Gemalto N.V., Retrieved on October 22, 2014.</ref>

A ChipTAN generator is not tied to a particular account; instead, the user must insert their [[bank card]] during use. The TAN generated is specific to the bank card as well as to the current transaction details. There are two variants: In the older variant, the transaction details (at least amount and account number) must be entered manually. {{anchor|Flicker code}}In the modern variant, the user enters the transaction online, then the TAN generator reads the transaction details via a flickering [[barcode]] on the computer screen (using [[photodetector]]s). It then shows the transaction details on its own screen to the user for confirmation before generating the TAN.

As it is independent hardware, coupled only by a simple communication channel, the TAN generator is not susceptible to attack from the user's computer. Even if the computer is subverted by a [[Trojan horse (computing)|Trojan]], or if a [[man-in-the-middle attack]] occurs, the TAN generated is only valid for the transaction confirmed by the user on the screen of the TAN generator, therefore modifying a transaction retroactively would cause the TAN to be invalid.

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While it offers protection from technical manipulation, the ChipTAN scheme is still vulnerable to [[social engineering (security)|social engineering]]. Attackers have tried to persuade the users themselves to authorize a transfer under a pretext, for example by claiming that the bank required a "test transfer" or that a company had falsely transferred money to the user's account and they should "send it back".<ref name="symantec"/><ref>[http://www.trusteer.com/blog/tatanga-attack-exposes-chiptan-weaknesses ''Tatanga Attack Exposes chipTAN Weaknesses''] trusteer.com, September 4, 2012</ref> Users should therefore never confirm bank transfers they have not initiated themselves.

ChipTAN is also used to secure batch transfers (''Sammelüberweisungen''). However, this method offers significantly less security than the one for individual transfers. In case of a batch transfer the TAN generator will only show the number and total amount of all transfers combined – thus for batch transfers there is little protection from manipulation by a Trojan.<ref>{{cite web|title=chipTAN-Verfahren / Was wird im TAN-Generator angezeigt?|url=https://www.sparkasse-neckartal-odenwald.de/pdf/download/anzeige_tan_generator.pdf|publisher=Sparkasse Neckartal-Odenwald|accessdateaccess-date=1 December 2014|date=June 2013|quote=SEPA-Sammelüberweisung, Inhalt: mehr als 1 Posten. Anzeige 1: Summe, Anzeige 2: Anzahl Posten}}</ref> This vulnerability was reported by RedTeam Pentesting in November 2009.<ref>{{cite web|title=Man-in-the-Middle Attacks against the chipTAN comfort Online Banking System|url=https://www.redteam-pentesting.de/en/publications/mitm-chiptan-comfort/-man-in-the-middle-attacks-against-the-chiptan-comfort-online-banking-system|publisher=RedTeam Pentesting GmbH|accessdateaccess-date=1 December 2014}}</ref> In response, as a mitigation, some banks changed their batch transfer handling so that batch transfers containing only a single record are treated as individual transfers.

==See also==