1. The document discusses multi-factor authentication and provides a taxonomy for categorizing different authentication methods. 2. It analyzes authentication methods and categories them as something you are (biometrics), something you know, something you have, something you do (behaviors), and context. 3. The document provides guidance on how to evaluate the strength of different authentication methods and factors, noting that combining factors from different categories increases security more than combining factors from the same category.

1. Multi-factor Authentication
Methods Taxonomy
Some Thoughts
Abbie Barbir, Ph.D
Co-chair OASIS Trust Elevation TC
https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=trust-el

2. Authentication Strength
• (entity) authentication: A process used to achieve sufficient confidence in the binding
between the entity and the presented identity.
• What is Authentication Strength (or Trust in the Authentication Step)?
– Measures how difficult it is for imposter to masquerade as the legitimate user
– Authentication strength is often more formally expressed as a "level of assurance“
(ITU X.1254 and ISO 29115 (Based on NIST 800 63))
• Two-factor authentication (TFA, T-FA or 2FA) is an approach to authentication which
requires the presentation of two or more factors from the same or different category
• Multi-factor authentication uses more that 2 factors from different categories
• Analysis
– Overall objective is to elevate Trust in the Authentication step
– Established terms, such as “2FA" are no longer precise enough to guide
technology decisions
– Choosing the method or methods appropriate for the needs of securing the
enterprise using appropriate comparisons of different vendors' products and
services require a more granular taxonomy
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3. How to determine the "Best" Authentication Method
Use Needs and Constraints to Determine
• Authentication strength
• indicated by the level of risk
• Total Cost of Ownership
• Constrained by budget
• Ease of use
• universally desirable
• Other constraints
• consistency and control of the endpoint
important
Source Gartner
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4. Elevating Trust in Authentication Strength Level
• Increasing the strength of authentication can be done by adding factors from
the same or different kinds of authentication categories that don’t have the
same vulnerabilities.
• There are five categories of authentication methods
• who you are,
• what you know,
• what you have,
• what you typically do and
• the context.
• What you typically do consists of behavioral habits that are independent of
physical biometric attributes.
• Context includes, “but is not limited” to, location, time, party, prior
relationship, social relationship and source.
• Authentication assurance or elevation can be within the classic four X.1254
ITU-T LoA (ISO 29115 (NIST 800-063))
• Adding factors from different categories can increase strength only if the
overall set of vulnerabilities is reduced.
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5. Authentication Categories
Who You Are what you what you what you
Biometric know have Do Context
Physical • User Name and • One Time
Password (UN/PW), Password • Browsing • Location;
Biometric A passphrase, a PIN (OTP) patterns Time of
• immutable and unique • Time of access;
• Very often used • Smart card
• Facial recognition alone or in access • Subscriber
• X.509 and
• Iris Scan combinations with
PKI • Type of identity
KBA methods. device module (SIM)
• Retinal Scan • Rarely used
• Knowledge Based • Used in • Frequency of
• Fingerprint Palm Scan Authentication alone Combinatio access;
• Voice (KBA) • Used in n with other • Source and
• Liveliness biometric • Static KBA combination methods
with UN/PW endpoint
factors include: •
• Dynamic and a PIN identity
• Pulse. KBA attributes such
CAPTCHA; etc
as
Behavioral • Used in
Biometric Combination
• based on person’s of other
physical behavioural methods
activity patterns
• Keyboard
signature
• Voice Mostly used to provide
Secondary Attributes
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6. How to Evaluate Authentication Strength
1. Two aspects to consider
• Method's resistance to attack
– how difficult is it for an attacker to directly compromise or undermine the authentication
method (without the user's knowing collusion)
• Method resistance to wilful misuse
– how difficult is it for a user to deliberately allow others to share his account?
2. Authentication Strength
• Measures how hard it is for another person to masquerade as the legitimate user
– Authentication may be undermined by two kinds of attacks:
– Masquerade attacks, in which an attacker is (by some means) able to corroborate a falsely
claimed digital identity and, thus, log in as a legitimate user.
– Session hijacking attacks, such as a man-in-the browser attack, which take control of or
parasitize an already-authenticated session after a legitimate user's claimed digital identity
has been corroborated.
Session hijacking attacks bypass authentication and, thus, can succeed no matter how strong
the authentication method is.
There is always a need for fraud detection, misuse monitoring
and other compensating controls in order to elevate trust . Source : Gartner
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7. How to Evaluate Authentication Strength
• Combining two or more authentication methods can potentially increases
authentication strength, compared with using either one.
– For example, passwords are vulnerable to key logging
– adding a second, partial password entered via drop down menu may
reduce vulnerability to this attack.
• Point of Caution
– Each type of authentication attribute has a set of overlapping and
intrinsic vulnerabilities with other attributes
– A combination of two attributes of the same type tends to share many of
vulnerabilities
– It is a big mistake to assume that strong authentication always result
when combining multiple authentication attributes/factors.
Only by combining attributes of different kinds (that is, different factors)
with different (non-overlapping) sets of vulnerabilities is there a significant
increase in resistance to attack and, thus, in authentication strength
7 Source: OASIS, ITU, NIST, Gartner

8. How to Evaluate Authentication Strength
• Not any MFA method is stronger than an authentication method
based on a single authentication factor/attribute.
• For example, a biometric authentication method using heart
beat is stronger than a password + OTP
• For some type of attacks, a 2FA method might not be stronger
than one of its components if used alone.
– For example, a "fly-phishing" attack that captures and
immediately use an OTP will be equally successful whether
the OTP token was PIN-protected or not.
• Some issues to consider
• How Unique is the credential
• Level Trust of Binding of credential to entity
Source: NIST, Gartner
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9. Evaluating Authentication Strength “Take Away”
• Counting Factors is not enough to evaluate authentication strength
Source: Gartner
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10. Authentication Process Threats
• Online guessing • Eavesdropping
• Phishing • Replay
• Pharming • Session hijack
• Man-in-the-middle
Threat Resistance per Assurance Level
Source: ITU-T, NIST
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11. Example Calculating Overall Authentication
Assurance Level (LOA 3)
• Overall authentication assurance level is based on the weakest link of the
assurance levels for each components
• For instance, to achieve an overall assurance level of 3:
– The registration and identity proofing process shall, at a minimum, use
Level 3 processes or higher.
– Token (or combination of tokens) used shall have an assurance level of 3
or higher.
– The binding between the identity proofing and the token(s), if proofing
is done separately from token issuance, shall be established at level 3.
– The authentication protocols used shall have a Level 3 assurance level
or higher.
– The token and credential management processes shall use a Level 3
assurance level or higher
– Authentication assertions (if used) shall have a Level 3 assurance or
higher
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12. Tokens
• A Token is something that the Claimant possesses and controls (typically a
cryptographic module or password) that is used to authenticate the
Claimant’s identity
• Single-factor Token –
• A token that uses one of the three factors to achieve authentication.
• For example, a password is something you know.
• There are no additional factors required to activate the token, so this
is considered single factor.
• Multi-factor Token –
• A token that uses two or more factors to achieve authentication.
• For example, a private key on a smart card that is activated via PIN
is a multi-factor token.
• The smart card is something you have, and something you know
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(the PIN) is required to activate the token.

13. Tokens “Token Style”
1. Memorized Secret Token
– A secret shared between the Subscriber and the CSP
2. Pre-registered Knowledge Token
– A series of responses to a set of prompts or challenges
3. Look-up Secret Token
– A physical or electronic token that stores a set of secrets shared between
the Claimant and the CSP. The token authenticator is the secret(s)
identified by the prompt. Look-up secret tokens are something you have.
4. Out of Band Token
– A physical token that is uniquely addressable and can receive a Verifier-
selected secret for one-time use. The device is possessed and controlled
by the Claimant and supports private communication19 over a channel
that is separate from the primary channel for e-authentication.
5. Single-factor (SF) One-Time Password (OTP) Device
– A hardware device that supports the spontaneous generation of one-time
passwords
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14. Tokens “Token Style”
6. Single-factor (SF) Cryptographic Device
– A hardware device that performs cryptographic operations on input provided to the device.
This device does not require activation through a second factor of authentication
7. Multi-factor (MF) Software Cryptographic Token
– A cryptographic key is stored on disk or some other “soft” media and requires activation
through a second factor of authentication.
8. Multi-factor (MF) One-Time Password (OTP) Device
– A hardware device that generates one-time passwords for use in authentication and which
requires activation through a second factor of authentication
9. Multi-factor (MF) Cryptographic Device
– A hardware device that contains a protected cryptographic key that requires activation through
a second authentication factor. Authentication is accomplished by proving possession of the
device and control of the key. The token authenticator is highly dependent on the specific
cryptographic device and protocol, but it is generally some type of signed message. For
example, in TLS, there is a “certificate verify” message. The MF Cryptographic device is
something you have, and it may be activated by either something you know or something you
are.
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15. Token Threats
Source: NIST, ITU-T
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16. Token Threat Mitigation Strategies
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17. NIST: Assurance Levels for Multi-Token E-Authentication
Schemes
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