Authentication Method Reference Values
draft-ietf-oauth-amr-values-08

Abstract

The amr (Authentication Methods References) claim is defined and registered in the IANA "JSON Web Token Claims" registry but no standard Authentication Method Reference values are currently defined. This specification establishes a registry for Authentication Method Reference values and defines an initial set of Authentication Method Reference values.

Status of This Memo

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This Internet-Draft will expire on September 14, 2017.

Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. Requirements Notation and Conventions
• 1.2. Terminology
• 2. Authentication Method Reference Values
• 3. Relationship to "acr" (Authentication Context Class Reference)
• 4. Privacy Considerations
• 5. Security Considerations
• 6. IANA Considerations
• 6.1. Authentication Method Reference Values Registry
• 6.1.1. Registration Template
• 6.1.2. Initial Registry Contents
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Appendix A. Examples
• Appendix B. Acknowledgements
• Appendix C. Document History
• Authors' Addresses

1. Introduction

The amr (Authentication Methods References) claim is defined and registered in the IANA "JSON Web Token Claims" registry [IANA.JWT.Claims] but no standard Authentication Method Reference values are currently defined. This specification establishes a registry for Authentication Method Reference values and defines an initial set of Authentication Method Reference values.

For context, the amr (Authentication Methods References) claim is defined by Section 2 of the OpenID Connect Core 1.0 specification [OpenID.Core] as follows:

amr

OPTIONAL. Authentication Methods References. JSON array of strings that are identifiers for authentication methods used in the authentication. For instance, values might indicate that both password and OTP authentication methods were used. The definition of particular values to be used in the amr Claim is beyond the scope of this specification. Parties using this claim will need to agree upon the meanings of the values used, which may be context-specific. The amr value is an array of case sensitive strings.

Each amr value typically provides an identifier for a family of closely-related authentication methods. For example, the otp identifier intentionally covers both time-based and HMAC-based OTPs. Many relying parties will be content to know that an OTP has been used in addition to a password; the distinction between which kind of OTP was used is not useful to them. Thus, there's a single identifier that can be satisfied in two or more nearly equivalent ways.

Similarly, there's a whole range of nuances between different fingerprint matching algorithms. They differ in false positive and false negative rates over different population samples and also differ based on the kind and model of fingerprint sensor used. Like the OTP case, many relying parties will be content to know that a fingerprint match mas made, without delving into and differentiating based on every aspect of the implementation of fingerprint capture and match. The fpt identifier accomplishes this.

Ultimately, the relying party is depending upon the identity provider to do reasonable things. If it does not trust the identity provider to do so, it has no business using it. The amr value lets the identity provider signal to the relying party additional information about what it did, for the cases in which that information is useful to the relying party.

The amr values defined by this specification are not intended to be an exhaustive set covering all use cases. Additional values can and will be added to the registry by other specifications. Rather, the values defined herein are an intentionally small set that are already actually being used in practice.

The values defined by this specification only make distinctions that are known to be useful to relying parties. Slicing things more finely than would be used in practice would actually hurt interop, rather than helping it, because it would force relying parties to recognize that several or many different values actually mean the same thing to them.

For context, while the claim values registered pertain to authentication, note that OAuth 2.0 [RFC6749] is designed for resource authorization and cannot be used for authentication without employing appropriate extensions, such as those defined by OpenID Connect Core 1.0 [OpenID.Core]. The existence of the amr claim and values for it should not be taken as encouragement to try to use OAuth 2.0 for authentication without employing extensions enabling secure authentication to be performed.

When used with OpenID Connect, if the identity provider supplies an amr claim in the ID Token resulting from a successful authentication, the relying party can inspect the values returned and thereby learn details about how the authentication was performed. For instance, the relying party might learn that only a password was used or it might learn that iris recognition was used in combination with a hardware-secured key. Whether amr values are provided and which values are understood by what parties are both beyond the scope of this specification. The OpenID Connect MODRNA Authentication Profile 1.0 [OpenID.MODRNA] is one example of an application context that uses amr values defined by this specification.

1.1. Requirements Notation and Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

1.2. Terminology

This specification uses the terms defined by JSON Web Token (JWT) [JWT] and OpenID Connect Core 1.0 [OpenID.Core].

2. Authentication Method Reference Values

The following is a list of Authentication Method Reference values defined by this specification:

face

Biometric authentication [RFC4949] using facial recognition

fpt

Biometric authentication [RFC4949] using a fingerprint

geo

Use of geolocation information for authentication, such as that provided by [W3C.REC-geolocation-API-20161108]

hwk

Proof-of-possession (PoP) of a hardware-secured key. See Appendix C of [RFC4211] for a discussion on PoP.

iris

Biometric authentication [RFC4949] using an iris scan

kba

Knowledge-based authentication [NIST.800-63-2] [ISO29115]

mca

Multiple-channel authentication [MCA]. The authentication involves communication over more than one distinct communication channel. For instance, a multiple-channel authentication might involve both entering information into a workstation's browser and providing information on a telephone call to a pre-registered number.

mfa

Multiple-factor authentication [NIST.800-63-2] [ISO29115]. When this is present, specific authentication methods used may also be included.

otp

One-time password [RFC4949]. One-time password specifications that this authentication method applies to include [RFC4226] and [RFC6238].

pin

Personal Identification Number (PIN) [RFC4949] or pattern (not restricted to containing only numbers) that a user enters to unlock a key on the device. This mechanism should have a way to deter an attacker from obtaining the PIN by trying repeated guesses.

pwd

Password-based authentication [RFC4949]

rba

Risk-based authentication [JECM]

retina

Biometric authentication [RFC4949] using a retina scan

sc

Smart card [RFC4949]

sms

Confirmation using SMS [SMS] text message to the user at a registered number

swk

Proof-of-possession (PoP) of a software-secured key. See Appendix C of [RFC4211] for a discussion on PoP.

tel

Confirmation by telephone call to the user at a registered number. This authentication technique is sometimes also referred to as "call back" [RFC4949].

user

User presence test. Evidence that the end-user is present and interacting with the device. This is sometimes also referred to as "test of user presence" [W3C.WD-webauthn-20170216].

vbm

Biometric authentication [RFC4949] using a voiceprint

wia

Windows integrated authentication [MSDN]

3. Relationship to "acr" (Authentication Context Class Reference)

The acr (Authentication Context Class Reference) claim and acr_values request parameter are related to the amr (Authentication Methods References) claim, but with important differences. An Authentication Context Class specifies a set of business rules that authentications are being requested to satisfy. These rules can often be satisfied by using a number of different specific authentication methods, either singly or in combination. Interactions using acr_values request that the specified Authentication Context Classes be used and that the result should contain an acr claim saying which Authentication Context Class was satisfied. The acr claim in the reply states that the business rules for the class were satisfied -- not how they were satisfied.

In contrast, interactions using the amr claim make statements about the particular authentication methods that were used. This tends to be more brittle than using acr, since the authentication methods that may be appropriate for a given authentication will vary over time, both because of the evolution of attacks on existing methods and the deployment of new authentication methods.

4. Privacy Considerations

The list of amr claim values returned in an ID Token reveals information about the way that the end-user authenticated to the identity provider. In some cases, this information may have privacy implications.

While this specification defines identifiers for particular kinds of credentials, it does not define how these credentials are stored or protected. For instance, ensuring the security and privacy of biometric credentials that are referenced by some of the defined Authentication Method Reference values is beyond the scope of this specification.

5. Security Considerations

The security considerations in OpenID Connect Core 1.0 [OpenID.Core] and OAuth 2.0 [RFC6749] and the OAuth 2.0 Threat Model [RFC6819] apply to applications using this specification.

As described in Section 3, taking a dependence upon particular authentication methods may result in brittle systems since the authentication methods that may be appropriate for a given authentication will vary over time.

6. IANA Considerations

6.1. Authentication Method Reference Values Registry

This specification establishes the IANA "Authentication Method Reference Values" registry for amr claim array element values. The registry records the Authentication Method Reference value and a reference to the specification that defines it. This specification registers the Authentication Method Reference values defined in Section 2.

Values are registered on an Expert Review [RFC5226] basis after a three-week review period on the jwt-reg-review@ietf.org mailing list, on the advice of one or more Designated Experts. To increase potential interoperability, the experts are requested to encourage registrants to provide the location of a publicly-accessible specification defining the values being registered, so that their intended usage can be more easily understood.

Registration requests sent to the mailing list for review should use an appropriate subject (e.g., "Request to register Authentication Method Reference value: otp").

Within the review period, the Designated Experts will either approve or deny the registration request, communicating this decision to the review list and IANA. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful. Registration requests that are undetermined for a period longer than 21 days can be brought to the IESG's attention (using the iesg@ietf.org mailing list) for resolution.

IANA must only accept registry updates from the Designated Experts and should direct all requests for registration to the review mailing list.

It is suggested that the same Designated Experts evaluate these registration requests as those who evaluate registration requests for the IANA "JSON Web Token Claims" registry [IANA.JWT.Claims].

Criteria that should be applied by the Designated Experts includes determining whether the proposed registration duplicates existing functionality, whether it is likely to be of general applicability or whether it is useful only for a single application, whether the value is actually being used, and whether the registration description is clear.

6.1.1. Registration Template

Authentication Method Reference Name:

The name requested (e.g., "otp") for the authentication method or family of closely-related authentication methods. Because a core goal of this specification is for the resulting representations to be compact, it is RECOMMENDED that the name be short -- that is, not to exceed 8 characters without a compelling reason to do so. To facilitate interoperability, the name must use only printable ASCII characters excluding double quote ('"') and backslash ('\') (the Unicode characters with code points U+0021, U+0023 through U+005B, and U+005D through U+007E). This name is case sensitive. Names may not match other registered names in a case-insensitive manner unless the Designated Experts state that there is a compelling reason to allow an exception.

Authentication Method Reference Description:

Brief description of the Authentication Method Reference (e.g., "One-time password").

Change Controller:

For Standards Track RFCs, state "IESG". For others, give the name of the responsible party. Other details (e.g., postal address, email address, home page URI) may also be included.

Specification Document(s):

Reference to the document or documents that specify the parameter, preferably including URIs that can be used to retrieve copies of the documents. An indication of the relevant sections may also be included but is not required.

6.1.2. Initial Registry Contents

• Authentication Method Reference Name: face
• Authentication Method Reference Description: Facial recognition
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: fpt
• Authentication Method Reference Description: Fingerprint biometric
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: geo
• Authentication Method Reference Description: Geolocation
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: hwk
• Authentication Method Reference Description: Proof-of-possession of a hardware-secured key
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: iris
• Authentication Method Reference Description: Iris scan biometric
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: kba
• Authentication Method Reference Description: Knowledge-based authentication
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: mca
• Authentication Method Reference Description: Multiple-channel authentication
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: mfa
• Authentication Method Reference Description: Multiple-factor authentication
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: otp
• Authentication Method Reference Description: One-time password
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: pin
• Authentication Method Reference Description: Personal Identification Number or pattern
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: pwd
• Authentication Method Reference Description: Password-based authentication
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: rba
• Authentication Method Reference Description: Risk-based authentication
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: retina
• Authentication Method Reference Description: Retina scan biometric
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: sc
• Authentication Method Reference Description: Smart card
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: sms
• Authentication Method Reference Description: Confirmation using SMS
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: swk
• Authentication Method Reference Description: Proof-of-possession of a software-secured key
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: tel
• Authentication Method Reference Description: Confirmation by telephone call
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: user
• Authentication Method Reference Description: User presence test
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: vbm
• Authentication Method Reference Description: Voice biometric
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

• Authentication Method Reference Name: wia
• Authentication Method Reference Description: Windows integrated authentication
• Change Controller: IESG
• Specification Document(s): Section 2 of [[ this specification ]]

7. References

7.1. Normative References

7.2. Informative References

Appendix A. Examples

  "amr": ["iris"]

In some cases, the amr claim value returned may contain a single Authentication Method Reference value. For example, the following amr claim value indicates that the authentication performed used an iris scan biometric:

  "amr": ["pwd", "kba"]

In other cases, the amr claim value returned may contain multiple Authentication Method Reference values. For example, the following amr claim value indicates that the authentication performed used a password and knowledge-based authentication:

Appendix B. Acknowledgements

Caleb Baker participated in specifying the original set of amr values. Jari Arkko, John Bradley, Ben Campbell, Brian Campbell, William Denniss, Linda Dunbar, Stephen Farrell, Paul Kyzivat, Elaine Newton, James Manger, Catherine Meadows, Alexey Melnikov, Kathleen Moriarty, Nat Sakimura, and Mike Schwartz provided reviews of the specification.

Appendix C. Document History

[[ to be removed by the RFC editor before publication as an RFC ]]

-08

• Added text in the IANA Registration Template saying that names can be for families of closely-related authentication methods, as suggested by Stephen Farrell.

-07

• Clarified that the values are intended to provide identifiers for families of closely-related authentication methods.
• Updated the MODRNA Authentication Profile reference.

-06

• Addressed IESG comments. Identifiers are now restricted to using only printable JSON-friendly ASCII characters. Additional references to documentation relevant to specific AMR values were added.

-05

• Specified characters allowed in amr values, reusing the IANA Considerations language on this topic from RFC 7638.

-04

• Added examples with single and multiple values.
• Clarified that the actual credentials referenced are not part of this specification to avoid additional privacy concerns for biometric data.
• Clarified that the OAuth 2.0 Threat Model [RFC6819] applies to applications using this specification.

-03

• Addressed shepherd comments.

-02

• Addressed working group last call comments.

-01

• Distinguished between retina and iris biometrics.
• Expanded the introduction to provide additional context to readers.
• Referenced the OpenID Connect MODRNA Authentication Profile 1.0 specification, which uses amr values defined by this specification.

-00

• Created the initial working group draft from draft-jones-oauth-amr-values-05 with no normative changes.

Authors' Addresses