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Multi-factor authentication (MFA) is a method of confirming a user's claimed identity in which a user is granted access only after successfully presenting 2 to 3 pieces of evidence (or factors) to an authentication mechanism: knowledge (something they and only they know), possession (something they and only they have), and inherence (something they and only they are).
Two-factor authentication (also known as 2FA) is a type (subset) of multi-factor authentication. It is a method of confirming a user's claimed identity by utilizing a combination of two different factors: 1) something they know, 2) something they have, or 3) something they are.
A good example of two-factor authentication is the withdrawing of money from a ATM; only the correct combination of a bank card (something that the user possesses) and a PIN (personal identification number, something that the user knows) allows the transaction to be carried out.
Two-step verification or two-step authentication is a method of confirming a user's claimed identity by utilizing something they know (password) and a second factor other than something they have or something they are. An example of a second step is the user repeating back something that was sent to them through an out-of-band mechanism. Or the second step might be a 6 digit number generated by an app that is common to the user and the authentication system. 
- 1 Authentication factors
- 2 Mobile phone two-step authentication
- 3 Legislation and regulation
- 4 Security
- 5 Implementation considerations
- 6 Examples
- 7 See also
- 8 References
- 9 Further reading
- 10 External links
The use of multiple authentication factors to prove one's identity is based on the premise that an unauthorized actor is unlikely to be able to supply the factors required for access. If, in an authentication attempt, at least one of the components is missing or supplied incorrectly, the user's identity is not established with sufficient certainty and access to the asset (e.g., a building, or data) being protected by multi-factor authentication then remains blocked. The authentication factors of a multi-factor authentication scheme may include:
- some physical object in the possession of the user, such as a USB stick with a secret token, a bank card, a key, etc.
- some secret known to the user, such as a password, PIN, TAN, etc.
- some physical characteristic of the user (biometrics), such as a fingerprint, eye iris, voice, typing speed, pattern in key press intervals, etc.
Knowledge factors are the most commonly used form of authentication. In this form, the user is required to prove knowledge of a secret in order to authenticate.
A password is a secret word or string of characters that is used for user authentication. This is the most commonly used mechanism of authentication. Many multi-factor authentication techniques rely on password as one factor of authentication. Variations include both longer ones formed from multiple words (a passphrase) and the shorter, purely numeric, personal identification number (PIN) commonly used for ATM access. Traditionally, passwords are expected to be memorized.
Many secret questions such as "Where were you born?" are poor examples of a knowledge factor because they may be known to a wide group of people, or be able to be researched.
Possession factors ("something the user and only the user has") have been used for authentication for centuries, in the form of a key to a lock. The basic principle is that the key embodies a secret which is shared between the lock and the key, and the same principle underlies possession factor authentication in computer systems. A security token is an example of a possession factor.
Disconnected tokens have no connections to the client computer. They typically use a built-in screen to display the generated authentication data, which is manually typed in by the user.
Connected tokens are devices that are physically connected to the computer to be used. Those devices transmit data automatically. There are a number of different types, including card readers, wireless tags and USB tokens.
These are factors associated with the user, and are usually bio-metric methods, including fingerprint readers, retina scanners or voice recognition.
Mobile phone two-step authentication
Mobile phone's apps, SMS, direct push notifications, etc are not considered 'something you have' and thus are not considered in the multi-factor/two-factor equation. Mobile phone two-step authentication is more secure than a the single factor password protection, but suffer some security concerns. Phones can be cloned and apps can be running on several phones, SMS can be read by cell phone maintenance personnel. Not least, cell phones can be compromised in general, meaning the phone is no longer something you and only you have.
The major drawback of authentication performed including something that the user possesses is that the physical token (the USB stick, the bank card, the key or similar) must be carried around by the user, practically at all times. Loss and theft are a risk. Many organizations forbid USB and electronic devices being carried in or out owing to malware and data theft risks, and most important machines do not have USB ports for the same reason. Physical tokens usually do not scale, typically requiring a new token for each new account and system. There are also costs involved in procuring and subsequently replacing tokens of this kind. In addition, there are inherent conflicts and unavoidable trade-offs between usability and security.
Mobile phone two-step authentication, where devices such as mobile phones and smartphones, was developed to provide an alternative method that would avoid such issues. To authenticate themselves, people can use their personal access codes to the device (i.e. something that only the individual user knows) plus a one-time-valid, dynamic passcode, typically consisting of 4 to 6 digits. The passcode can be sent to their mobile device by SMS or push notification or can be generated by a one-time-passcode-generator (app). In all three cases, the advantage of using a mobile phone is that there is no need for an additional dedicated token as users tend to carry their mobile devices around at all times.
SMS is the most broadly adopted multi-factor authentication method to date for consumer-facing accounts, though in 2016 and 2017 respectively, both Google and Apple have started offering user two-step authentication with push notification as an alternative method, although this method is not Out-of-band.   Despite its popularity, SMS verification has been condemned as a form of authentication by the NIST and has been publicly criticized by many security advocates. As such, SMS verification is no longer considered verification.
Security of the mobile-delivered security tokens fully depends on the mobile operator's operational security and can be easily breached by wiretapping or SIM cloning by national security agencies.
Advantages of mobile phone two-step authentication
- No additional tokens are necessary because it uses mobile devices that are (usually) carried all the time.
- As they are constantly changed, dynamically generated passcodes are safer to use than fixed (static) log-in information.
- Depending on the solution, passcodes that have been used are automatically replaced in order to ensure that a valid code is always available; acute transmission/reception problems do not therefore prevent logins.
- The option to specify a maximum permitted number of incorrect entries reduces the risk of attacks by unauthorized persons.
- It is user friendly.
Disadvantages of mobile phone two-step authentication
- The mobile phone must be carried by the user, charged, and kept in range of a cellular network whenever authentication might be necessary. If the phone is unable to display messages, such as if it becomes damaged or shuts down for an update or due to temperature extremes (e.g. winter exposure), access is often impossible without backup plans.
- The user must share their personal mobile number with the provider, reducing personal privacy and potentially allowing spam.
- The user may be charged by their mobile carrier for messaging fees.
- Text messages to mobile phones using SMS are insecure and can be intercepted. The token can thus be stolen and used by third parties.
- Text messages may not be delivered instantly, adding additional delays to the authentication process.
- Account recovery typically bypasses mobile phone two-factor authentication.
- Modern smart phones are used both for browsing email and for receiving SMS. Email is usually always logged in. So if the phone is lost or stolen, all accounts for which the email is the key can be hacked as the phone can receive the second factor. So smart phones combine the two factors into one factor.
- Mobile phones can be stolen, potentially allowing the thief to gain access into the user's accounts.
- SIM cloning gives hackers access to mobile phone connections. Social engineering attacks against mobile operator companies resulted in handing over duplicate SIM cards to criminals.
Legislation and regulation
The Payment Card Industry (PCI) Data Security Standard, requirement 8.3, requires the use of MFA for all remote network access that originates from outside the network to a Card Data Environment (CDE). Beginning with PCI-DSS version 3.2, the use of MFA is required for all administrative access to the CDE, even if the user is within a trusted network.
Details for authentication in the USA are defined with the Homeland Security Presidential Directive 12 (HSPD-12).
Existing authentication methodologies involve the explained three types of basic "factors". Authentication methods that depend on more than one factor are more difficult to compromise than single-factor methods.
IT regulatory standards for access to Federal Government systems require the use of multi-factor authentication to access sensitive IT resources, for example when logging on to network devices to perform administrative tasks and when accessing any computer using a privileged login.
In 2005, the United States' Federal Financial Institutions Examination Council issued guidance for financial institutions recommending financial institutions conduct risk-based assessments, evaluate customer awareness programs, and develop security measures to reliably authenticate customers remotely accessing online financial services, officially recommending the use of authentication methods that depend on more than one factor (specifically, what a user knows, has, and is) to determine the user's identity. In response to the publication, numerous authentication vendors began improperly promoting challenge-questions, secret images, and other knowledge-based methods as "multi-factor" authentication. Due to the resulting confusion and widespread adoption of such methods, on August 15, 2006, the FFIEC published supplemental guidelines—which states that by definition, a "true" multi-factor authentication system must use distinct instances of the three factors of authentication it had defined, and not just use multiple instances of a single factor.
According to proponents, multi-factor authentication could drastically reduce the incidence of online identity theft and other online fraud, because the victim's password would no longer be enough to give a thief permanent access to their information. However, many multi-factor authentication approaches remain vulnerable to phishing, man-in-the-browser, and man-in-the-middle attacks.
Multi-factor authentication may be ineffective against modern threats, like ATM skimming, phishing, and malware.
In May 2017 O2 Telefónica, a German mobile service provider, confirmed that cybercriminals had exploited SS7 vulnerabilities to bypass SMS based two-step authentication to do unauthorized withdrawals from users bank accounts. The criminals first infected the account holder's computers in an attempt to steal their bank account credentials and phone numbers. Then the attackers purchased access to a fake telecom provider and set-up a redirect for the victim's phone number to a handset controlled by them. Finally the attackers logged into victims' online bank accounts and requested for the money on the accounts to be withdrawn to accounts owned by the criminals. SMS passcodes were routed to phone numbers controlled by the attackers and the criminals transferred the money out.
Many multi-factor authentication products require users to deploy client software to make multi-factor authentication systems work. Some vendors have created separate installation packages for network login, Web access credentials and VPN connection credentials. For such products, there may be four or five different software packages to push down to the client PC in order to make use of the token or smart card. This translates to four or five packages on which version control has to be performed, and four or five packages to check for conflicts with business applications. If access can be operated using web pages, it is possible to limit the overheads outlined above to a single application. With other multi-factor authentication solutions, such as "virtual" tokens and some hardware token products, no software must be installed by end users.
There are drawbacks to multi-factor authentication that are keeping many approaches from becoming widespread. Some consumers have difficulty keeping track of a hardware token or USB plug. Many consumers do not have the technical skills needed to install a client-side software certificate by themselves. Generally, multi-factor solutions require additional investment for implementation and costs for maintenance. Most hardware token-based systems are proprietary and some vendors charge an annual fee per user. Deployment of hardware tokens is logistically challenging. Hardware tokens may get damaged or lost and issuance of tokens in large industries such as banking or even within large enterprises needs to be managed. In addition to deployment costs, multi-factor authentication often carries significant additional support costs. A 2008 survey of over 120 U.S. credit unions by the Credit Union Journal reported on the support costs associated with two-factor authentication. In their report, software certificates and software toolbar approaches were reported to have the highest support costs.
Several popular web services employ multi-factor authentication, usually as an optional feature that is deactivated by default. 
- Two-factor authentication
- Many Internet services (among them: Google, Amazon AWS) use open Time-based One-time Password Algorithm (TOTP) to support two-step authentication
- Comparison of authentication solutions
- Identity management
- Mutual authentication
- Reliance authentication
- Strong authentication
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- Two Step Verification Complete Guide
- Microsoft Windows Lock Screen Integration with Smart Card from RjR Innovations