Internet Engineering Task Force (IETF)
Request for Comments: 8485
Category: Standards Track
ISSN: 2070-1721

J. Richer, Ed.
Bespoke Engineering
L. Johansson
Swedish University Network
October 2018

Vectors of Trust

Abstract

This document defines a mechanism for describing and signaling several aspects of a digital identity transaction and its participants. These aspects are used to determine the amount of trust to be placed in that transaction.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8485.

Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Identity Model  . . . . . . . . . . . . . . . . . . . . .   5
     1.4.  Component Architecture  . . . . . . . . . . . . . . . . .   6
   2.  Component Dimension Definitions . . . . . . . . . . . . . . .   6
     2.1.  Identity Proofing (P) . . . . . . . . . . . . . . . . . .   7
     2.2.  Primary Credential Usage (C)  . . . . . . . . . . . . . .   8
     2.3.  Primary Credential Management (M) . . . . . . . . . . . .   8
     2.4.  Assertion Presentation (A)  . . . . . . . . . . . . . . .   8
   3.  Communicating Vector Values to RPs  . . . . . . . . . . . . .   9
     3.1.  On-the-Wire Representation  . . . . . . . . . . . . . . .  10
     3.2.  In OpenID Connect . . . . . . . . . . . . . . . . . . . .  11
   4.  Requesting Vector Values  . . . . . . . . . . . . . . . . . .  11
     4.1.  In OpenID Connect . . . . . . . . . . . . . . . . . . . .  12
   5.  Trustmarks  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Defining New Vector Values  . . . . . . . . . . . . . . . . .  13
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Vector of Trust Components Registry . . . . . . . . . . .  14
       7.1.1.  Registration Template . . . . . . . . . . . . . . . .  14
       7.1.2.  Initial Registry Contents . . . . . . . . . . . . . .  15
     7.2.  Addition to the OAuth Parameters Registry . . . . . . . .  15
     7.3.  Additions to JWT Claims Registry  . . . . . . . . . . . .  16
     7.4.  Additions to OAuth Token Introspection Response . . . . .  16
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  17
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Vectors of Trust Default Component Value Definitions  19
     A.1.  Identity Proofing . . . . . . . . . . . . . . . . . . . .  19
     A.2.  Primary Credential Usage  . . . . . . . . . . . . . . . .  20
     A.3.  Primary Credential Management . . . . . . . . . . . . . .  20
     A.4.  Assertion Presentation  . . . . . . . . . . . . . . . . .  21
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1. Introduction

Methods for measuring trust in digital identity transactions have historically fallen into two main categories: either all measurements are combined into a single scalar value or trust decisions are calculated locally based on a detailed set of attribute metadata. This document defines a method of conveying trust information that is more expressive than a single value but less complex than comprehensive attribute metadata.

Prior to the third edition [SP-800-63-3] published in 2017, NIST Special Publication 800-63 [SP-800-63-2] used a single scalar measurement of trust called a Level of Assurance (LoA). An LoA can be used to compare different transactions within a system at a coarse level. For instance, an LoA4 transaction is generally considered more trusted (across all measured categories) than an LoA2 transaction. The LoA for a given transaction is computed by the Identity Provider (IdP) and is consumed by a Relying Party (RP). Since the trust measurement is a simple numeric value, it's trivial for RPs to process and compare. However, since each LoA encompasses many different aspects of a transaction, it can't express many real- world situations. For instance, an anonymous user account might have a very strong credential, such as would be common of a whistle-blower or political dissident. Despite the strong credential, the lack of identity proofing would make any transactions conducted by the account to fall into a low LoA. Furthermore, different use cases and domains require subtly different definitions for their LoA categories, and one group's LoA2 is not equivalent or even comparable to another group's LoA2.

Attribute-Based Access Control (ABAC) systems used by RPs may need to know details about a user's attributes, such as how recently the attribute data was verified and by whom. Attribute metadata systems are capable of expressing extremely fine-grained detail about the transaction. However, this approach requires the IdP to collect, store, and transmit all of this attribute data for the RP's consumption. The RP must process this data, which may be prohibitive for trivial security decisions.

The Vectors of Trust (VoT) approach proposed in this document seeks a balance between these two alternatives by allowing expression of multiple aspects of an identity transaction (including but not limited to identity proofing, credential strength, credential management, and assertion strength), without requiring full attribute metadata descriptions. This method of measurement gives more actionable data and expressiveness than an LoA, but it is still relatively easy for the RP to process. It is anticipated that VoT can be used alongside more detailed attribute metadata systems, such as the one proposed by NISITIR 8112 [NISTIR-8112]. The RP can use the vector value for most basic decisions but be able to query the IdP for additional attribute metadata where needed. Furthermore, for RPs that do not have a need for the vector's more fine-grained detail, it is anticipated that some trust frameworks will provide a simple mapping between certain sets of vector values to LoAs. In such systems, an RP is given a choice of how much detail to request from the IdP in order to process a given transaction.

This document defines a data model for these vectors and an on-the- wire format for conveying them between parties. The values of the vectors defined by the data model are anchored in a trust definition. This document also provides guidance for defining values for use in conveying this information, including four component categories and guidance on defining values within those categories. Additionally, this document defines a general-purpose set of component values in an appendix (Appendix A) for use cases that do not need something more specific.

1.1. Requirements Language

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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

1.2. Terminology

   Identity Federation:  A protocol in which an Identity Provider (IdP)
   
      asserts a user's identity information to an RP.  through the use
      of a cryptographic assertion or other verifiable mechanism, or a
      system implementing such a protocol.  It is also referred to
      simply as "federation".
   
   Identity Provider (IdP):  A system that manages identity information
      and is able to assert this information across the network through
      an identity API.
   
   Identity Subject:  The individual (user) engaging in the identity
      transaction, that is, being identified by the identity provider to
      the RP.
   
   Identity Proofing:  The process of verifying and validating that a
      set of identity attributes belongs to a real-world identity
      subject.
   
   Primary Credential:  The means used by the identity subject to
      authenticate to the identity provider.
   
   Federated Credential:  The assertion presented by the IdP to the RP
      across the network to authenticate the user.
   
   Relying Party (RP):  A system that consumes identity information from
      an IdP for the purposes of authenticating the user.
   
   Trust Framework:  A document containing business rules and legal
      clauses that defines how different parties in an identity
      transaction may act.
   
   Trustmark:  A URL referencing a specific trust framework and its
      definition of vector components and vector component values.
   
   Trustmark Provider:  Defines the trust framework referenced by its
      trustmark and can verify that a given system (such as an identity
      provider) is both capable of asserting and allowed to assert the
      vector component values it is claiming.
   
   Vector:  A multi-part data structure, which is used here for
      conveying information about an authentication transaction.
   
   Vector Component:  One of several constituent parts that make up a
      vector, indicating a category of information.
   
   Vector Component Value:  One of the values applied to a vector
      component within a vector.

1.3. Identity Model

This document assumes the following model for identity based on identity federation technologies:

The identity subject (also known as the user) is associated with an identity provider that acts as a trusted third party on behalf of the user, with regard to an RP by making identity assertions about the user to the RP.

The real-world individual represented by the identity subject is in possession of a primary credential bound to the identity subject by the identity provider (or an agent thereof) in such a way that the binding between the credential and the real-world user is a representation of the identity proofing process performed by the identity provider (or an agent thereof) to verify the identity of the real-world individual. This information is carried across the network as part of an identity assertion presented to the RP during the authentication transaction.

1.4. Component Architecture

The term "Vectors of Trust" is inspired by the mathematical construct of a vector, which is defined as an item composed of multiple independent values.

An important goal for this work is to balance the need for simplicity (particularly on the part of the RP) with the need for expressiveness. As such, this vector construct is designed to be composable and extensible.

The vector is constructed of orthogonal components, such that no aspect of a component overlaps an aspect of another component, as much as is possible.

2. Component Dimension Definitions

This specification defines four orthogonal components: identity proofing, primary credential usage, primary credential management, and assertion presentation.

This specification also defines values for each of these components to be used in the absence of a more specific trust framework in Appendix A. It is expected that trust frameworks will provide context, semantics, and mapping to legal statutes and business rules for each value in each component.

Consequently, a particular vector value can only be compared with vectors defined in the context of a specific trust framework. The RP MUST understand and take into account the trust framework context in which a vector is being expressed in order to process a vector correctly.

Each component is identified by a demarcator consisting of a single uppercase ASCII letter in the range "[A-Z]". The demarcator SHOULD reflect the category with which it is associated in a natural manner. Demarcators for components MUST be registered as described in Section 7. It is anticipated that trust framework definitions will use this registry to define specialized components, but it is RECOMMENDED that trust frameworks reuse existing components categories wherever possible. The same demarcator MUST NOT be used for two different dimensions, and different trust frameworks SHOULD use the same demarcator for similar information. It is further anticipated that there will be relatively few component dimensions over time, and this specification defines four general-purpose categories in this section. Note that since the processing for all vector values is contextual to a trust framework, the exact semantics of interpreting a component will vary based on the trust framework in use.

The value for a given component within a vector of trust is defined by its demarcator character followed by a single digit or lowercase ASCII letter in the range "[0-9a-z]". Categories that have a natural ordering SHOULD prefer digits, with larger digits indicating stronger assertions than smaller digits. Categories that do not have a natural ordering, or that can have an ambiguous ordering, SHOULD prefer letters. Note that while letters could also imply order, they can also more naturally be used mnemonically. Trust frameworks MAY use any possible values within a category without the need for them to be contiguous.

Categories MAY use both letters and digits simultaneously. For example, a category could define "0" as meaning "no statement is made" while using letters such as "a", "b", and "c" for normal values to indicate specific options. Another system could have an ordered base set of digits with additional details provided by letters.

Each component MAY be repeated with multiple different values within a single vector, representing the logical AND of the values (see Section 3.1 for details). The same component and value combination MUST NOT be repeated within a single vector. For example, a vector could contain both "P1" and "Pa" but not two instances of "P1". A trust framework MAY define additional restrictions on combinations of values.

Regardless of the type of value, the RP MUST NOT assume that the values assigned to each component of a vector have inherent ordinal or subsumptive properties when compared to the same or other components in the vector space without specific knowledge of the trust framework in use. In other words, "1" is always different from "2", but it is dangerous to assume that "2" is always better than "1" or that "2" satisfies all the requirements of "1".

2.1. Identity Proofing (P)

The identity proofing dimension defines, overall, how strongly the set of identity attributes have been verified and vetted. In other words, this dimension describes how likely it is that a given digital identity transaction corresponds to a particular (real-world) identity subject. For example, did the user have to provide documentation to a trusted party to prove their legal name and address, or were they able to self-assert such values?

This dimension uses the "P" demarcator, such as "P0", "P1", etc. Most definitions of identity proofing will have a natural ordering, as more or less stringent proofing can be applied to an individual being granted an account. In such cases, it is RECOMMENDED that a digit be used for this component and that only a single value be allowed to be communicated in a transaction.

2.2. Primary Credential Usage (C)

The primary credential usage dimension defines how strongly the primary credential can be verified by the IdP. In other words, how easily that credential could be spoofed or stolen. For example, did the user log in with a password, a biometric, a cryptographic hardware device, or some combination of the above?

This dimension uses the "C" demarcator, such as "Ca", "Cb", etc. Most definitions of credential usage will not have an overall natural ordering, as there may be several equivalent classes described within a trust framework. In such cases, it is RECOMMENDED that a letter be used for this component and that multiple distinct credential usage factors be allowed to be communicated simultaneously, such as when multi-factor authentication is used.

2.3. Primary Credential Management (M)

The primary credential management dimension conveys information about the expected lifecycle of the primary credential in use, including its binding, rotation, and revocation. In other words, the use and strength of policies, practices, and security controls used in managing the credential at the IdP and its binding to the intended individual. For example, can the user bring their own cryptographic device or is one provided by the IdP?

This dimension uses the "M" demarcator, such as "Ma", "Mb", etc. Most definitions of credential management will not have an overall natural ordering, though there can be preference and comparison between values in some circumstances. In such cases, it is RECOMMENDED that a letter be used for this component and that multiple distinct values be allowed to be communicated simultaneously.

2.4. Assertion Presentation (A)

The assertion presentation dimension defines how well the given digital identity can be communicated across the network without information leaking to unintended parties and without spoofing. In other words, this dimension describes how likely it is that a given digital identity was asserted by a given identity provider for the identity subject of a given transaction. While this information is largely already known by the RP as a side effect of processing an identity assertion in a federation protocol, this dimension is still very useful when the RP requests a login (see Section 4) and when describing the capabilities of an IdP. This value also allows the RP to detect when an assertion is presented in a manner it was not intended for, as may be the case with an attack.

This dimension uses the "A" demarcator, such as "Aa", "Ab", etc. Most definitions of assertion presentation will not have an overall natural ordering. In such cases, it is RECOMMENDED that a letter be used for this component and that multiple values be allowed to be communicated simultaneously.

3. Communicating Vector Values to RPs

A vector of trust is designed to be used in the context of an identity and authentication transaction, providing information about the context of a federated credential. The vector therefore needs to be able to be communicated in the context of the federated credential in a way that is strongly bound to the assertion representing the federated credential.

This vector has several requirements for use.

All applicable vector components and values need to be combined into a single vector.

The vector can be communicated across the wire unbroken and untransformed.

All vector components need to remain individually available, not "collapsed" into a single value.

The vector needs to be protected in transit.

The vector needs to be cryptographically bound to the assertion that it is describing.

The vector needs to be interpreted in the context of a specific trust framework definition identified by a trustmark URL.

These requirements lead us to defining a simple string-based representation of the vector that can be incorporated within a number of different locations and protocols without further encoding.

3.1. On-the-Wire Representation

The vector MUST be represented as a period-separated ('.') list of vector components. A vector component type can occur multiple times within a single vector, but a specific value of a vector component cannot occur more than once in a single vector. That is, while "Cc.Cd" is a valid vector, "Cc.Cc" is not. Multiple values for a component are considered a logical AND of the values.

Vector component values MAY appear in any order within a vector, and the RP MUST consider different orderings of the same vector equivalent during processing. For example, "P1.Cc.Cd.Aa", "Aa.Cc.Cd.P1", "Cd.P1.Cc.Aa", and "Aa.P1.Cd.Cc" are all considered equivalent to each other.

Possible vector components MAY be omitted from a vector. No holding space is left for an omitted vector component. If a vector component is omitted, the vector is making no claim for that component. No default values are assumed for a missing component category.

Vector values MUST be communicated along with a trustmark URL (see Section 5) to give the components and component values context. The trustmark MUST be cryptographically bound to the vector value, such as the two values being carried together in a signed assertion. A vector value without context is unprocessable, and vectors defined in different contexts are not directly comparable as whole values. Different trust frameworks MAY reuse component definitions (including their values), but processing of such cross-context values is outside the scope of this specification.

For example, the vector "P1.Cc.Ab" translates to "pseudonymous, proof of shared key, signed browser-passed verified assertion, and no claim made toward credential management" in the context of this specification's definitions (see Appendix A). A different vector "Cb.Mc.Cd.Ac" translates to "known device, full proofing required for credential issuance and rotation, cryptographic proof of possession of a shared key, signed back-channel verified assertion, and no claim made toward identity proofing" in the same context. Since no claim is made here for identity proofing, no specific value can be assumed by the RP. Note that this doesn't mean the user wasn't proofed at all: it's possible that the user was fully proofed to the highest capabilities within the trust framework, but here the IdP is not making any specific claim about proofing to the RP, perhaps to protect the user's privacy.

3.2. In OpenID Connect

In OpenID Connect [OpenID], the IdP MUST send the vector as a string within the "vot" (vector of trust) claim in the ID token. The trustmark (see Section 5) that applies to this vector MUST be sent as a URL in the "vtm" (vector trust mark) claim to provide context to the vector.

The "vot" and "vtm" claims are interpreted by the RP to apply to the entire identity transaction and not necessarily to any one attribute specifically.

For example, assume that for the given trustmark, the body of an ID token claiming "pseudonymous, proof of shared key, signed back- channel verified token, and no claim made toward credential management" could look like this JSON object [RFC8259] payload of the ID token.

   {
       "iss": "https://idp.example.com/",
       "sub": "jondoe1234",
       "vot": "P1.Cc.Ac",
       "vtm": "https://example.org/vot-trust-framework"
   }

The body of the ID token is signed and optionally encrypted using JSON Object Signing and Encryption (JOSE), as per the OpenID Connect specification. By putting the "vot" and "vtm" values inside the ID token, the vector and its context are strongly bound to the federated credential represented by the ID token.

Vector values MAY be returned in a token introspection [RFC7662] response describing the ID token or access token issued during an OpenID Connect transaction using the same claims.

4. Requesting Vector Values

In some identity protocols, the RP can request that particular vector values be used for a given identity transaction. An RP can describe the particular vector component values it desires the IdP assert for a given identity transaction by using the same syntax as defined in Section 3.1. Processing and fulfillment of these requests are in the purview of the IdP, and details are outside the scope of this specification.

Future specifications MAY define alternative ways for an RP to request vector values from an IdP.

4.1. In OpenID Connect

In OpenID Connect [OpenID], the client MAY request a partial set of acceptable VoT values with the "vtr" (vector of trust) claim request as part of the request object. The value of this field is a JSON array of strings [RFC8259], each string identifying an acceptable set of vector components. The component values within each vector are ANDed together while the separate vectors are ORed together. For example, a list of vectors in the form '["P1.Cb.Cc.Ab", "Ce.Ab"]' is stating that either the full set of "P1 AND Cb AND Cc AND Ab" simultaneously OR the full set of "Ce AND Ab" simultaneously are acceptable to this RP for this transaction.

Vector request values MAY omit components, indicating that any value is acceptable for that component category, including omission of that component in the response vector.

The mechanism by which the IdP processes the "vtr" and maps that to the authentication transaction are out of scope of this specification.

5. Trustmarks

A trustmark is an HTTPS URL that references a specific set of vector values as defined by a trust framework. This URL MUST point to a human-readable document that describes what components and values are valid, how they are used together, and what practices the component values represent within the trust framework. The contents of the trustmark URL MUST be reachable by the operators or implementors of the RP. The URL MUST be stable over time for a given trust framework to allow RPs to process incoming vectors in a consistent fashion. New versions of a trust framework that require different processing rules MUST use a different trustmark URL.

   For example, <https://www.rfc-editor.org/info/rfc8485> is used as the
   trustmark to reference the values defined in Appendix A.

The process of a trustmark provider determining the ability of a particular IdP to correctly assert values from a given trust framework is outside the scope of this specification. Determining how an RP should apply the values of a given vector to the RP's processing is outside the scope of this specification.

6. Defining New Vector Values

Vectors of Trust is meant to be a flexible and reusable framework for communicating authentication data between networked parties in an identity federation protocol. However, the exact nature of the information needed depends on the parties requiring the information and the relationship between them. While this document does define a usable default set of values in Appendix A, it is anticipated that many situations will require an extension of this specification for their own use.

Component categories such as those defined in Section 2 are intended to be general-purpose and reusable in a variety of trust frameworks. Extension specifications SHOULD reuse existing category definitions where possible. Extensions MAY create additional categories where needed by using the registry defined in Section 7. The registry encourages reuse and discovery of existing categories across different trust frameworks. For example, the "P" category in another framework SHOULD be used for identity proofing and related information.

The values of components such as those defined in Appendix A are intended to be contextual to the defining trust document. While this specification's component values are intended to be general-purpose and extensions MAY reuse the values and their definitions, trust frameworks MUST define all allowable values. As these values are always interpreted in the context of a trustmark, these values are not recorded in a central registry. Consequently, a P1" value from one framework and a "P1" value from another framework could have very different interpretations depending on their contextual trust framework documents, even though in both cases the "P" component is used for identity proofing in some fashion.

Trust frameworks that implement this specification SHOULD choose either a numerical ordering or a group category approach to component values as described in Section 2, though combinations of both types MAY be used. Trust frameworks MUST specify whether multiple values are allowed for each category, and while any component category is generally allowed to have multiple distinct values, a specific definition of a set of values in an extension MAY limit a given component category to a single value per transaction. It is RECOMMENDED that trust frameworks use a "0" value to indicate an empty or null condition for a given category (for example, no proofing being done or no authentication token being used).

All trust frameworks that extend and implement this specification MUST be referenced by a unique trustmark URL (see Section 5) to allow RPs to differentiate between different trust frameworks.

7. IANA Considerations

This specification creates one registry and registers several values into existing registries.

7.1. Vector of Trust Components Registry

This specification establishes the "Vectors of Trust Components" registry.

Component demarcators are registered by the Specification Required policy documented in [RFC8126].

Criteria that should be applied by the designated experts includes determining whether the proposed registration is distinct enough from existing entries to warrant registration, whether it is likely to be of general applicability, and whether the registration description is clear. Since all vector processing is contextual to a trust framework, component demarcators that do not meet these criteria can still be used in trust frameworks. The registry contains vector components that are believed to have general applicability that can be used as well.

Registration requests sent to the vot@ietf.org mailing list for review should use an appropriate subject (e.g., "Request to register Vector of Trust Component name: example"). The designated expert(s) will provide review within a two-week period and 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. IANA must only accept registry updates from the designated expert(s) and should direct all requests for registration to the vot@ietf.org mailing list. If the designated experts do not respond within the designated period, IANA should contact the IESG for guidance.

7.1.1. Registration Template

Demarcator Symbol:

      An uppercase ASCII letter in the range [A-Z] representing this
      component (e.g., "X").

Description:: Brief description of the component (e.g., "Example description").

Change Controller:: For IETF-stream RFCs, state "IESG". For other documents, give the name of the responsible party.

Specification document(s):: Reference to the document(s) that specifies the vector component, preferably including a URL that can be used to retrieve a copy of the document(s). An indication of the relevant sections may also be included but is not required.

7.1.2. Initial Registry Contents

The "Vector of Trust Components" registry contains the definitions of vector components and their associated demarcators.

o Demarcator Symbol: P
o Description: Identity proofing
o Change Controller: IESG
o Specification document(s): [RFC8485]

o Demarcator Symbol: C
o Description: Primary credential usage
o Change Controller: IESG
o Specification document(s): [RFC8485]

o Demarcator Symbol: M
o Description: Primary credential management
o Change Controller: IESG
o Specification document(s): [RFC8485]

o Demarcator Symbol: A
o Description: Assertion presentation
o Change Controller: IESG
o Specification document(s): [RFC8485]

7.2. Addition to the OAuth Parameters Registry

This specification adds the following value to the "OAuth Parameters" registry established by [RFC6749].

o Name: vtr
o Description: Vector of Trust request
o Parameter usage location: authorization request, token request o Change Controller: IESG
o Reference: [RFC8485]

7.3. Additions to JWT Claims Registry

This specification adds the following values to the "JSON Web Token Claims" registry established by [RFC7519].

o Claim name: vot
o Claim Description: Vector of Trust value
o Change Controller: IESG
o Reference: [RFC8485]

o Claim name: vtm
o Claim Description: Vector of Trust trustmark URL
o Change Controller: IESG
o Reference: [RFC8485]

7.4. Additions to OAuth Token Introspection Response

This specification adds the following values to the "OAuth Token Introspection Response" registry established by [RFC7662].

o Name: vot
o Description: Vector of Trust value
o Change Controller: IESG
o Reference: [RFC8485]

o Name: vtm
o Description: Vector of Trust trustmark URL
o Change Controller: IESG
o Reference: [RFC8485]

8. Security Considerations

The vector of trust value needs to be cryptographically protected in transit between parties, such as by using TLS as described in [BCP195]. The vector of trust value must be associated with a trustmark by the RP processing the vector. A signed OpenID Connect ID Token or a similarly signed assertion from another protocol would fulfill this requirement by carrying both the vector value and the trustmark URL as claims.

The vector value is always associated with a trustmark and needs to be interpreted by the RP in the context of the trust framework defined by that trustmark. Different trust frameworks can apply different interpretations to the same component value, much as was the case with LoA. Therefore, an RP interpreting a component value in the wrong context could mistakenly accept or reject a request. In order to avoid this mistake, RPs need to reject vectors that are defined in trust frameworks that they do not understand how to interpret properly.

The VoT framework provides a mechanism for describing and conveying trust information. It does not define any policies for an IdP determining which vector component values apply to a given transaction, nor does it define any policies for applying the values of a vector to an RP's security decision process. These policies and associated practices are to be agreed upon by the IdP and RP, and they should be expressed in detail in an associated human-readable trust framework document available at the trustmark URL.

9. Privacy Considerations

By design, vector of trust values contain information about the user's authentication and associations that can be made thereto. Therefore, all aspects of a vector of trust contain potentially privacy-sensitive information and must be guarded as such. Even in the absence of specific attributes about a user, knowledge that the user has been highly proofed or issued a strong token could provide more information about the user than was intended. It is recommended that IdPs send and RPs request only the information necessary for their use case in order to prevent inadvertent information disclosure.

10. References

10.1. Normative References

   [OpenID]   Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0", November 2014,
              <http://openid.net/specs/openid-connect-core-1_0.html>.
   
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.
   
   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.
   
   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.
   
   [RFC7662]  Richer, J., Ed., "OAuth 2.0 Token Introspection",
              RFC 7662, DOI 10.17487/RFC7662, October 2015,
              <https://www.rfc-editor.org/info/rfc7662>.
   
   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.
   
   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.
   
   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

10.2. Informative References

   [BCP195]   Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, May 2015,
              <https://www.rfc-editor.org/info/bcp195>.

[NISTIR-8112]

              National Institute of Standards and Technology, "A
              Proposed Schema for Evaluating Federated Attributes", NIST
              Internal Report 8112, DOI 10.6028/NIST.IR.8112, January
              2018, <https://nvlpubs.nist.gov/nistpubs/ir/2018/
              NIST.IR.8112.pdf>.

[SP-800-63-2]

              National Institute of Standards and Technology,
              "Electronic Authentication Guideline", NIST Special
              Publication SP 800-63-2, DOI 10.6028/NIST.SP.800-63-2,
              August 2013,
              <https://dx.doi.org/10.6028/NIST.SP.800-63-2>.

[SP-800-63-3]

              National Institute of Standards and Technology, "Digital
              Identity Guideline", NIST Special Publication SP 800-63-3,
              DOI 10.6028/NIST.SP.800-63-3, June 2017,
              <https://doi.org/10.6028/NIST.SP.800-63-3>.

Appendix A. Vectors of Trust Default Component Value Definitions

The following general-purpose component definitions MAY be used when a more specific set is unavailable. This document defines a trust framework for these component values. The trustmark URL of this trust framework is <https://www.rfc-editor.org/info/rfc8485>. All normative requirements following in this section apply to this trust framework alone.

Other trust frameworks that extend and implement this specification SHOULD define their own component values as described in Section 6. Where possible, extensions MAY reuse specific values and definitions as listed here, but those specific values MUST be relisted.

A.1. Identity Proofing

The identity proofing component of this vector definition represents the level of scrutiny applied to the identity subject during the proofing process. Higher levels are largely subsumptive of lower levels, such that "P2" fulfills requirements for "P1", etc. Multiple distinct values from this category MUST NOT be used in a single transaction.

   P0:  No proofing is done, and data is not guaranteed to be persistent
   
        across sessions
   
   P1:  Attributes are self-asserted but consistent over time,
        potentially pseudonymous
   
   P2:  Identity has been proofed either in person or remotely using
        trusted mechanisms (such as social proofing)
   
   P3:  There is a binding relationship between the identity provider
        and the identified party (such as signed/notarized documents and
        employment records)

A.2. Primary Credential Usage

The primary credential usage component of this vector definition represents distinct categories of primary credential that MAY be used together in a single transaction. Multiple distinct values from this category MAY be used in a single transaction.

   C0:  No credential is used / anonymous public service
   
   Ca:  Simple session HTTP cookies (with nothing else)
   
   Cb:  Known device, such as those indicated through device posture or
        device management systems
   
   Cc:  Shared secret, such as a username and password combination
   
   Cd:  Cryptographic proof of key possession using shared key
   
   Ce:  Cryptographic proof of key possession using asymmetric key
   
   Cf:  Sealed hardware token / keys stored in a trusted platform module
   
   Cg:  Locally verified biometric

A.3. Primary Credential Management

The primary credential management component of this vector definition represents distinct categories of management that MAY be considered separately or together in a single transaction. Many trust framework deployments MAY use a single value for this component as a baseline for all transactions and thereby omit it. Multiple distinct values from this category MAY be used in a single transaction.

   Ma:  Self-asserted primary credentials (user chooses their own
        credentials and must rotate or revoke them manually) / no
        additional verification for primary credential issuance or
        rotation
   
   Mb:  Remote issuance and rotation / use of backup recover credentials
        (such as email verification) / deletion on user request
   
   Mc:  Full proofing required for each issuance and rotation /
        revocation on suspicious activity

A.4. Assertion Presentation

The assertion presentation component of this vector definition represents distinct categories of assertion that are RECOMMENDED to be used in a subsumptive manner but MAY be used together. Multiple distinct values from this category MAY be used in a single transaction.

   Aa:  No protection / unsigned bearer identifier (such as an HTTP
        session cookie in a web browser)
   
   Ab:  Signed and verifiable assertion, passed through the user agent
        (web browser)
   
   Ac:  Signed and verifiable assertion, passed through a back channel
   
   Ad:  Assertion encrypted to the RP's key

Acknowledgements

The authors would like to thank the members of the Vectors of Trust mailing list in the IETF for discussion and feedback on the concept and document, as well as the members of the ISOC Trust and Identity team for their support. In particular, the authors would like to thank Paul Grassi, Jim Fenton, Sarah Squire, Benjamin Kaduk, John Bradley, and Karen O'Donoghue.

Authors' Addresses

Justin Richer (editor)
Bespoke Engineering

   Email: ietf@justin.richer.org

Leif Johansson
Swedish University Network
Thulegatan 11
Stockholm
Sweden

   Email: leifj@sunet.se
   URI:   http://www.sunet.se