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{{Short description|One-time password used in banking}}
A '''transaction authentication number''' ('''TAN''') is used by some [[online banking]] services as a form of ''single use'' [[one-time password]]s to authorize [[financial transaction]]s. TANs are a second layer of security above and beyond the traditional single-password [[authentication]].▼
{{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]].
TANs provide additional security because they act as a form of
==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 bank creates a set of unique TANs for the user. 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
# 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.
== 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 attack]]s, including phishing attacks where the attacker tricks the user into logging into a forged copy of the bank's website and [[Man-in-the-browser|man-in-the-browser attacks]]<ref name="symantec">Candid Wüest, Symantec Global Security Response Team [https://www.iriss.ie/iriss/Candid_Wueest.pdf ''Current Advances in Banking Trojans?''] {{Webarchive|url=https://web.archive.org/web/20140425003333/https://www.iriss.ie/iriss/Candid_Wueest.pdf |date=2014-04-25 }} iriss.ie, Irish Reporting and Information Security Service, December 2, 2012 (PDF; 1,9 MB)</ref> which allow the attacker to secretly swap the transaction details in the background of the PC as well as to conceal the actual transactions carried out by the attacker in the online account overview.<ref>[http://winfuture.de/news,59152.html ''Katusha: LKA zerschlägt Ring von Online-Betrügern''] WinFuture.de, October 29, 2010</ref>
Therefore, in 2012 the [[European Network and Information Security Agency|European Union Agency for Network and Information Security]] advised all banks to consider the PC systems of their users being infected by [[malware]] by default and use security processes where the user can cross-check the transaction data against manipulations like for example (provided the security of the mobile phone holds up) [[#Mobile_TAN_.28mTAN.29|mTAN]] or smartcard readers with
== Indexed TAN with CAPTCHA (iTANplus) ==
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=
== 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=
{{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=
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>
== pushTAN ==
pushTAN is an [[Mobile application software|app]]-based TAN scheme by German Sparkassen banking group reducing some of the shortcomings of the [[#Mobile TAN (mTAN)|mTAN]] scheme. It eliminates the cost of SMS messages and is not susceptible to SIM card fraud, since the messages are sent via a special text-messaging application to the user's smartphone using an encrypted Internet connection. Just like mTAN, the scheme allows the user to cross-check the transaction details against hidden manipulations carried out by [[Trojan horse (computing)|Trojans]] on the user's PC by including the actual transaction details the bank received in the pushTAN message. Although analogous to using mTAN with a smartphone, there is the risk of a parallel malware infection of PC and smartphone. To reduce this risk the pushTAN app ceases to function if the mobile device is [[Rooting (Android OS)|rooted]] or
== TAN generators ==
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However, the TAN generated is not tied to the details of a specific transaction. Because the TAN is valid for any transaction submitted with it, it does not protect against [[phishing]] attacks where the TAN is directly used by the attacker, or against [[man-in-the-middle attack]]s.
=== ChipTAN / Sm@rt-TAN / CardTAN ===
[[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/
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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An additional advantage of this scheme is that because the TAN generator is generic, requiring a card to be inserted, it can be used with multiple accounts across different banks, and losing the generator is not a security risk because the security-critical data is stored on the bank card.
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
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|
==See also==
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