TOC 
DraftD. Recordon
 Six Apart
 M. Jones
 Microsoft
 J. Bufu, Ed.
 Independent
 J. Daugherty, Ed.
 JanRain
 N. Sakimura
 NRI
 October 15, 2008


OpenID Provider Authentication Policy Extension 1.0 - Draft 6

Abstract

This extension to the OpenID Authentication protocol provides a mechanism by which a Relying Party can request that particular authentication policies be applied by the OpenID Provider when authenticating an End User. This extension also provides a mechanism by which an OpenID Provider may inform a Relying Party which authentication policies were used. Thus a Relying Party can request that the End User authenticate, for example, using a phishing-resistant or multi-factor authentication method.

This extension also provides a mechanism by which a Relying Party can request that the OpenID Provider communicate the levels of authentication used, as defined within one or more sets of requested custom Assurance Levels, and for the OpenID Provider to communicate the levels used.

This extension is not intended to provide all information regarding the quality of an OpenID Authentication assertion. Rather, it is designed to be balanced with information the Relying Party already has with regard to the OpenID Provider and the level of trust it places in it. If additional information is needed about processes such as new End User enrollment on the OpenID Provider, such information should either be transmitted out-of-band or in other extensions such as OpenID Attribute Exchange. Other aspects (e.g. security characteristics, credential provisioning, etc) could be dealt with in the future.

This extension is optional, though its use is certainly recommended. This extension can be used with OpenID Authentication versions 1.1 and 2.0.

While none of the information transmitted using this extension can be verified by the Relying Party using technology alone, this does not limit the utility of this extension. Because there is no trust model specified by OpenID, Relying Parties must decide for themselves which Providers are trustworthy; likewise, RPs can decide whether to trust authentication policy claims from such OpenID Providers as well. As with other OpenID extensions, it is the Relying Party's responsibility to implement policy relative to the OpenID Provider's response.



Table of Contents

1.  Definitions
    1.1.  Requirements Notation
    1.2.  Conventions
    1.3.  Terminology
2.  Extension Overview
3.  Advertising Supported Authentication Policies
4.  Defined Authentication Policies
    4.1.  Custom Assurance Level Name Spaces
5.  Authentication Protocol
    5.1.  Request Parameters
    5.2.  Response Parameters
6.  Security Considerations
    6.1.  NIST Assurance Levels
Appendix A.  Examples
Appendix A.1.  Authentication Method Classifications
Appendix B.  Acknowledgements
7.  Normative References
§  Authors' Addresses




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1.  Definitions



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1.1.  Requirements Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] (Bradner, B., “Key words for use in RFCs to Indicate Requirement Levels,” 1997.) .



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1.2.  Conventions

Throughout this document, values are quoted to indicate that they are to be taken literally. When using these values in protocol messages, the quotes MUST NOT be used as part of the value.

All OpenID 2.0 messages that contain a Provider Authentication Policy Extension (PAPE) element MUST contain the following extension namespace declaration, as specified in the Extensions section of [OpenIDAuthentication2.0] (specs@openid.net, “OpenID Authentication 2.0,” 2007.) .

openid.ns.<alias>=http://specs.openid.net/extensions/pape/1.0

The actual extension namespace alias should be determined on a per-message basis by the party composing the messages, in such a manner as to avoid conflicts between multiple extensions. For the purposes of this document and when constructing OpenID 1.1 messages, the extension namespace alias SHALL be "pape".

Additionally, this specification uses name spaces for the custom authentication level identification. It is in the form of

openid.pape.auth_level.ns.<cust>=http://some.authlevel.uri

The actual extension namespace alias should be determined on a per-message basis by the party composing the messages, in such a manner as to avoid conflicts between multiple extensions. For the purposes of this document and when constructing OpenID 1.1 messages, the one custom authentication level identification extension namespace defined by this specification is "nist". Others may also be defined and used by implementations, for example, "jisa".



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1.3.  Terminology

The following terms are defined in [OpenIDAuthentication2.0] (specs@openid.net, “OpenID Authentication 2.0,” 2007.) :

Authentication Method:
An Authentication Method is a single mechanism by which the End User authenticated to their OpenID Provider, for example, a password or a hardware credential.
Authentication Policy:
An Authentication Policy is a plain-text description of requirements that dictate which Authentication Methods can be used by an End User when authenticating to their OpenID Provider. An Authentication Policy is defined by a URI which must be previously agreed upon by one or more OPs and RPs.



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2.  Extension Overview

  1. As part of the [Yadis] (Miller, J., Ed., “Yadis Specification 1.0,” 2005.) Discovery process, OpenID Providers can optionally add supported authentication policies to an End User's XRDS document. This aids Relying Parties in choosing between multiple listed OPs depending on authentication policy requirements.
  2. The Relying Party includes parameters in the OpenID Authentication request describing its preferences for authentication policy for the current assertion.
  3. The OpenID Provider processes the PAPE request, prompting the End User to fulfill the requested policies during the authentication process.
  4. As part of the OpenID Provider's response to the Relying Party, the OP includes PAPE information around the End User's authentication. An OP MAY include this response information even if not requested by the RP.
  5. When processing the OpenID Provider's response, the Relying Party takes the PAPE information into account when determining if the End User should be sent through additional verification steps or if the OpenID login process cannot proceed due to not meeting policy requirements.



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3.  Advertising Supported Authentication Policies

Via the use of [Yadis] (Miller, J., Ed., “Yadis Specification 1.0,” 2005.) within OpenID, Relying Parties are able to discover OpenID Provider service information in an automated fashion. This is used within OpenID Authentication for a RP to discover what version of the protocol each OP listed supports as well as any extensions, such as this one, that are supported. To aide in the process of a Relying Party selecting which OP they wish to interact with, it is STRONGLY RECOMMENDED that the following information be added to the End User's XRDS document. An OP may choose to advertise both custom levels and supported polices in the same <xrd:Service>. An OP should only advertise the authentication policies and custom assurance level namespaces that it supports.

When advertising supported policies, each policy URI MUST be added as the value of an <xrd:Type> element of an OpenID <xrd:Service> element in an XRDS document.

Example:

<xrd>
  <Service>
    <Type>http://specs.openid.net/auth/2.0/signon</Type>
    <Type>
  http://schemas.openid.net/pape/policies/2007/06/phishing-resistant
    </Type>
    <URI>https://example.com/server</URI>
  </Service>
</xrd>

When advertising supported custom Assurance Level name spaces, each name space URI MUST be added as the value of an <xrd:Type> element of an OpenID <xrd:Service> element in an XRDS document.

Example:

<xrd>
  <Service>
    <Type>http://specs.openid.net/auth/2.0/signon</Type>
    <Type>
  http://csrc.nist.gov/publications/nistpubs/800-63/SP800-63V1_0_2.pdf
    </Type>
    <URI>https://example.com/server</URI>
  </Service>
</xrd>


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4.  Defined Authentication Policies

The following are defined policies and policy identifiers describing how the End User may authenticate to an OP. Additional policies can be specified elsewhere and used without making changes to this document. The policies described below are designed to be a starting point to cover the most common use-cases. Additional polices can be found at http://schemas.openid.net/pape/policies/.

When multiple policies are listed in the Relying Party's request, it is up to the OpenID Provider to satisfy as many of the policies as it can. This might mean that the OP needs to understand the relationship between policies (such as if one encompasses another or if one is stronger than another). This also means that when the RP processes the OP's response, it will have to make its own determinations as to if its requirements were met. For instance, if the RP requested Multi-Factor Authentication and the OP authentication employed Multi-Factor Physical Authentication, it is recommended that the OP include both policies in the response.



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4.1.  Custom Assurance Level Name Spaces

Custom Assurance Levels are optional. The namespaces may be defined by various parties, such as country or industry specific standards bodies, or other groups or individuals.

The namespace URI should be chosen with care to be unambiguous when used as a <xrd:Type> element to advertise the namespaces supported by the OP.

The custom Assurance Level namespace should define the meaning of the strings that are returned by the OP in the openid.pape.auth_level.<cust> element.



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5.  Authentication Protocol



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5.1.  Request Parameters

The following parameters MUST be included during an OpenID Authentication request (specs@openid.net, “OpenID Authentication 2.0,” 2007.) [OpenIDAuthentication2.0] by the Relying Party that uses this extension unless marked as optional.



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5.2.  Response Parameters

In response to a Relying Party's request, the following parameters MUST be included in the OpenID Authentication Response. All response parameters MUST be included in the signature of the Authentication Response. It is RECOMMENDED that an OP supporting this extension include the following parameters even if not requested by the Relying Party.

All response parameters MUST describe the End User's current session with the OpenID Provider.



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6.  Security Considerations

As to commonly accepted security practices, it should be noted that the overall strength of any authentication is only as strong as its weakest step. It is thus recommended that provisioning of phishing-resistant and other credentials stronger than shared secrets should be accomplished using methods that are at least as strong as the credential being provisioned. By counter-example, allowing people to retrieve a phishing-resistant credential using only a phishable shared secret negates much of the value provided by the phishing-resistant credential itself. Similarly, sometimes using a phishing-resistant method when a phishable method continues to sometimes be employed may still enable phishing attacks to compromise the OpenID.

OpenID Providers need to make smart decisions as to how to describe the authentication performed with respect to that requested by the Relying Party. For example, if the RP were to request phishing-resistant authentication it may or may not make sense for the OP to actually tell it that the End User did in fact perform phishing-resistant, physical multi-factor authentication. Likewise, an OP MAY choose to respond with a level or levels used for the particular authentication even in some cases where this information was explicitly requested.

OPs SHOULD attempt to use the authentication policies requested by the RP and the reply SHOULD minimally contain at least the subset of the requested policies that applied to the authentication performed. The OP MAY also choose to return additional policies that applied to the authentication performed, even if not requested.

If the RP requested that an authentication level or levels be returned and the OP supports those level types, then the OP SHOULD return the actual level values for those types, if available.



 TOC 

6.1.  NIST Assurance Levels

National Institute of Standards and Technology (NIST) in Special Publication 800-63 (Burr, W., Dodson, D., and W. Polk, Ed., “Electronic Authentication Guideline,” April 2006.) [NIST_SP800‑63] defines a set of Assurance Levels from 1 to 4. These may be returned by the OP to the RP to communicate which NIST level the identity proofing, authentication method, and policies employed by the OP when authenticating the End User corresponds to.

Value: Integer value between 0 and 4 inclusive.

Note: Level 0 is not an assurance level defined by NIST, but rather SHOULD be used to signify that the OP recognizes the parameter and the End User authentication did not meet the requirements of Level 1. See Appendix A.1.2 (NIST Authentication Mechanism Levels) for high-level example classifications of authentication methods within the defined levels. Authentication using a long-lived browser cookie, for instance, is one example where the use of "level 0" is appropriate. Authentications with level 0 should never be used to authorize access to any resource of any monetary value whatsoever. The previous sentence should not be construed as implying that any of the other levels are recommended or appropriate for accessing resources with monetary value either without the Relying Party doing an appropriate risk assessment of the particular OpenID provider asserting them and their issuance and authentication procedures as they apply to the particular online interaction in question.

Depending on the particular use case being satisfied by the authentication response and PAPE information, the OpenID Provider will have to make a decision, ideally with the consent of the End User, as if it will include the "openid.pape.auth_level.nist" parameter. This information is designed to give Relying Parties more information around the strength of credentials used without actually disclosing the specific credential type. Disclosing the specific credential type can be considered a potential privacy or security risk.

It is RECOMMENDED that this parameter always be included in the response from the OP. This holds true even in cases where the End User authentication does not meet one of the defined Authentication Policies. For example, if the End User is authenticating using a password via HTTPS there is still value to the RP in knowing if the strength of the Password corresponds to the entropy requirements laid out by Level 1 or 2 or that it does not even meet the minimum requirement for the lowest level. With that said, discretion needs to be used by OP's as conveying that one of their End User's has a weak password to an "un-trustworthy" RP would not generally be considered a good idea.



 TOC 

Appendix A.  Examples



 TOC 

Appendix A.1.  Authentication Method Classifications

This non-normative section illustrates classification of various common authentication methods and their respective conformance within the defined policies and levels.



 TOC 

Appendix A.1.1.  Authentication Policy Examples

This table provides examples of common authentication technologies and their mapping to the Authentication Policies defined in Section 4 (Defined Authentication Policies) .

MethodPhishing-ResistantMulti-FactorPhysical Multi-Factor
Password via HTTPS      
Visual secret via HTTPS      
PIN and digital certificate via HTTPS X X  
PIN and "soft" OTP token via HTTPS   X  
PIN and "hard" OTP token via HTTPS   X X
PIN and "hard" crypto token via HTTPS X X X
Information Card via HTTPS X X  



 TOC 

Appendix A.1.2.  NIST Authentication Mechanism Levels

This section is designed to highlight the Authentication Mechanism Levels described in [NIST_SP800‑63] (Burr, W., Dodson, D., and W. Polk, Ed., “Electronic Authentication Guideline,” April 2006.) . All normative and authoritative text can be found in [NIST_SP800‑63] (Burr, W., Dodson, D., and W. Polk, Ed., “Electronic Authentication Guideline,” April 2006.) . Note that assurance level is not only comprised of Authentication Mechanism employed but also the nature of the identity proofing performed. The overall assurance level is determined as a combination of these factors.

This table is republished from page 39 of [NIST_SP800‑63] (Burr, W., Dodson, D., and W. Polk, Ed., “Electronic Authentication Guideline,” April 2006.) .

Token TypeLevel 1Level 2Level 3Level 4
Hard crypto token X X X X
One-time password device X X X  
Soft crypto token X X X  
Passwords & PINs X X    

This table is republished from page 39 of [NIST_SP800‑63] (Burr, W., Dodson, D., and W. Polk, Ed., “Electronic Authentication Guideline,” April 2006.) .

Protect AgainstLevel 1Level 2Level 3Level 4
On-line guessing X X X X
Replay X X X X
Eavesdropper   X X X
Verifier impersonation     X X
Man-in-the-middle     X X
Session hijacking       X

The following table illustrates the minimum number of factors required at each Authentication Mechanism Level.

LevelFactors
1 1
2 1
3 2
4 2

In all cases, implementing a commonly accepted nonce and cross-site scripting protection when entering authentication credentials is required to satisfy all four Authentication Mechanism Levels. All examples below assume this requirement is met.

It should be noted that NIST Authentication Mechanism Levels 1 and 2 have differing password entropy requirements. When working with passwords, you should refer to the [NIST_SP800‑63] (Burr, W., Dodson, D., and W. Polk, Ed., “Electronic Authentication Guideline,” April 2006.) specification for more details. All examples below assume the password meets these requirements.

This table provides examples of common authentication technologies and their mapping to NIST Authentication Mechanism Levels, please be aware that there are details not represented in these examples that may bear on the resulting Authentication Mechanism Level.

MethodLevel 1Level 2Level 3Level 4
Password via HTTP Yes, if challenge-response      
Password via HTTPS Yes Yes    
PIN and Digital Certificate via HTTPS Yes Yes Yes  
PIN and "soft" OTP token via HTTPS Yes Yes Yes  
PIN and "hard" OTP token via HTTPS Yes Yes Yes  
PIN and "hard" crypto token via HTTPS Yes Yes Yes Yes, if FIPS 140-2 Level 2 crypto and Level 3 physical



 TOC 

Appendix B.  Acknowledgements

The authors would like to thank Barry Ferg, Ben Laurie, Dick Hardt, Drummond Reed, George Fletcher, Kim Cameron, Arun Nanda, Allen Tom, Tatsuki Sakushima, Nate Klingstein, Gary Krall, and John Bradley for their feedback when drafting this specification. David Recordon would also like to acknowledge VeriSign who employed him during the original authoring of this specification.



 TOC 

7. Normative References

[NIST_SP800-63] Burr, W., Dodson, D., and W. Polk, Ed., “Electronic Authentication Guideline,” April 2006.
[OpenIDAuthentication2.0] specs@openid.net, “OpenID Authentication 2.0,” 2007 (TXT, HTML).
[RFC2119] Bradner, B., “Key words for use in RFCs to Indicate Requirement Levels,” RFC 2119, 1997.
[RFC3339] Klyne, G. and C. Newman, “Date and Time on the Internet: Timestamps,” RFC 3339.
[Yadis] Miller, J., Ed., “Yadis Specification 1.0,” 2005 (PDF, ODT).


 TOC 

Authors' Addresses

  David Recordon
  Six Apart, Ltd.
  548 4th Street
  San Francisco, CA 94107
  USA
Email:  david@sixapart.com
URI:  http://www.sixapart.com/
  
  Michael B. Jones
  Microsoft Corporation
  One Microsoft Way, Building 40/5138
  Redmond, WA 98052
  USA
Email:  mbj@microsoft.com
URI:  http://www.microsoft.com/
  
  Johnny Bufu (editor)
  Independent
Email:  johnny.bufu@gmail.com
URI: 
  
  Jonathan Daugherty (editor)
  JanRain
  5331 SW Macadam Ave. #375
  Portland, OR 97239
  USA
Email:  cygnus@janrain.com
URI:  http://janrain.com/
  
  Nat Sakimura
  Nomura Research Institute, Ltd.
  Marunouchi Kitaguchi Building, 1-6-5 Marunouchi
  Chiyoda-ku, Tokyo 100-0005
  Japan
Email:  n-sakimura@nri.co.jp
URI:  http://www.nri.co.jp/