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Identity for E2E-Secure Communications

Document Type Active Internet-Draft (individual)
Authors Richard Barnes , Rohan Mahy
Last updated 2023-10-23
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More Instant Messaging Interoperability                        R. Barnes
Internet-Draft                                                     Cisco
Intended status: Informational                                   R. Mahy
Expires: 25 April 2024                                              Wire
                                                         23 October 2023

                 Identity for E2E-Secure Communications


   End-to-end (E2E) security is a critical property for modern user
   communications systems.  E2E security protects users' communications
   from tampering or inspection by intermediaries that are involved in
   delivering those communcations from one logical endpoint to another.
   In addition to the much-discussed E2E encryption systems, true E2E
   security requires an identity mechanism that prevents the
   communications provider from impersonating participants in a session,
   as a way to gain access to the session.  This document describes a
   high-level architecture for E2E identity, identifying the critical
   mechanisms that need to be specified.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at

   Discussion of this document takes place on the More Instant Messaging
   Interoperability Working Group mailing list (,
   which is archived at
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   Source for this draft and an issue tracker can be found at

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 25 April 2024.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Operational Context and Assumptions . . . . . . . . . . . . .   4
   4.  An Architecture for E2E Identity  . . . . . . . . . . . . . .   6
     4.1.  Issuance  . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Presentation  . . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Verification  . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Instantiations  . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Manual Verification of Key Fingerprints . . . . . . . . .   9
     5.2.  X.509 . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.3.  Verifiable Credentials  . . . . . . . . . . . . . . . . .  11
   6.  Requirements for Interoperable Identity . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

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1.  Introduction

   End-to-end (E2E) security protects users' communications from
   tampering or inspection by intermediaries that are involved in
   delivering those communcations from one logical endpoint to another.
   Almost all user-to-user communications systems today involve
   application-level intermediaries, such as message queues or media
   servers.  In this context, "hop-by-hop" security refers to the
   security properties of the channel between the client and
   application-level intermediary, despite the fact that this channel
   may transit a number of network-level intermediaries.  "End-to-end"
   security refers to security properties of the communications between
   one end client and another.

   Given the ubiquity of application-level intermediation, E2E security
   is a critical property for modern user communications systems.  E2E
   security is typically implemented with two separate mechanisms:

   *  *E2E encryption*, which establishes keys among a group of
      communicating clients, and authenticates them at a cryptographic
      level, and

   *  *E2E identity*, which associates non-cryptographic attributes to
      the cryptographic representation of clients in the E2E encryption

   Broadly speaking, E2E encryption protects against passive attacks by
   intermediaries.  E2E identity protects against active attacks such as
   impersonation attacks.  Both layers are required to attain a complete
   notion of E2E security.

   An overview of identity considerations for messaging systems is
   provided in [I-D.mahy-mimi-identity].  In this document, we describe
   a concrete framework for E2E identity, drawing on some initial
   deployment experience, and highlighting the mechanisms that need to
   be defined for an interoperable solution.

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

   We use the following terms below:

   Client:  The hardware or software used by a user to interact with an

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      E2E-secure communications system.

   Communications provider:  The system of intermediaries that connects
      communicating clients.

   Identity authority:  An entity that is trusted to make statements
      about the identity attributes associated to clients.

   Session:  An E2E-secure interaction among clients, e.g., an MLS group

   Credential:  An object issued by an identity authority that
      associates identity attributes with a client's public key.

3.  Operational Context and Assumptions

   The context in which E2E identity is implement is shown in Figure 1.
   It involves the following actors:

   *  A number of *clients* participating in an E2E-encrypted session

      -  A *presenting client* that is claiming to represent certain
         identity attributes

      -  *Verifying clients* that authenticate the claimed identity

   *  One or more *communications providers* that facilitates
      communications among the clients

   *  An *identity authority* that asserts identity attributes of the
      presenting client, and which is trusted by the verifying client to
      make such assertions

   Note that in most settings, each client will act as both a presenting
   client and a verifying client, authenticating itself and verifying
   the other clients in the session or group.  Each client could use a
   different identity authority for its identity attributes.

   We assume that the E2E encryption for the session is provided by
   means of an E2E encryption protocol such as DoubleRatchet [signal] or
   MLS [I-D.ietf-mls-protocol], in which each participant is
   cryptographically authenticated by means of a digital signature key

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   The phrase "identity attributes" above is deliberately broad, to
   encompass any attribute of the client that is not directly
   cryptographcally verifiable.  This includes technical identifiers
   such as DNS names or URLs, but also user-meaningful identifers such
   as given or family names, or names of organizations or roles.

                                                   | Verifying |
                                              +--->|  Client   |
                                              |    +-----------+
                         +----------------+   |
   +------------+        | Communications |   |    +-----------+
   | Presenting |        |   Provider(s)  |   |    | Verifying |
   |   Client   +-----------------------------+--->|  Client   |
   +-----+------+        |                |        +-----+-----+
         |               +----------------+              |
         |                                               |
         |                                               |
         |                  +-----------+                |
         +------------------+ Identity  +----------------+
                            | Authority |

               Figure 1: Operational context for E2E identity

   In this context, the goal of E2E identity is to protect the
   authenticity of the binding between the presenting client's public
   key (which represents them in the E2E encryption protocol) and their
   identity attributes, against attack by the communications provider.
   Only a client that legitimately represents the claimed attributes
   should be able to cause a verifying client to associate those
   attributes with the first client.  More succinctly, E2E identity
   protects against impersonation attacks by the communications

   The architecture described here achieves this protection by means of
   role separation between communications provider and the identity
   authority.  The communications provider is untrusted, in the same way
   that network attackers are untrusted in the traditional Internet
   Threat Model [RFC3552].  The identity authority is trusted to
   correctly assert bindings between identity attributes and public
   keys.  This includes verifying that presenting clients control the
   corresponding public keys, to avoid Unknown Key Share attacks
   analogous to those in [RFC8844].

   There are other techniques that can reduce the trust that is placed
   in the identity authority, for example CONIKS [coniks] or various
   self-sovereign identity approaches.  These approaches have not been

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   deployed at scale in the same way authority-based approaches have,
   and they can be layered on top of a authority-based scheme.
   (Certificate Transparency was deployed many years after the Web PKI
   [RFC9162]).  Thus this document focuses on an authority-based system
   for E2E identity, as a baseline to which these other approaches might
   later be added.

   A secondary goal is to minimize the amount of information about the
   clients and sessions that is exposed to the identity authority.  The
   identity authority should not learn which communications providers or
   verifying clients a presenting client is interacting with.

   Verifying clients use ultimately authenticated identity attributes to
   make policy decisions as to the security state of the meeting.  For
   example, a verifying client may have attempted to reach the holder of
   the SIP URI, and would regard the session as
   compromised if they were not actually connected to that entity.  Or a
   verifying client might wish to require that all participants in a
   session belong to a given organization.  E2E identity assures that
   these policies are evaluated on correct inputs.

4.  An Architecture for E2E Identity

   Figure 2 shows the three critical steps in a system E2E identity:

   1.  *Issuance:* An identity authority provides the presenting client
       with a credential associating identity attributes to a public

   2.  *Presentation:* The presenting client provides its credential to
       the verifying client, along with proof that the presenting client
       controls the corresponding private key.

   3.  *Verification:* The verifying client verifies that the credential
       was issued by a trusted identity authority, and verifies the
       presenting clients proof of control of the corresponding private
       key.  The verifying client may verify by communicating with the
       identity authority, or autonomously using previously configured
       information about the identity authority.

   If all three of these steps is successfully completed, with the
   security properties described below, then the verifying client can
   safely associate the identity attributes in the credential with the
   presenting client.

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   +------------+                                  +-----------+
   | Presenting |         2. Presentation          | Verifying |
   |   Client   +--------------------------------->|  Client   |
   +-----+------+                                  +-----+-----+
         ^                                               ^
         |                                               |
         |                                               |
         |                  +-----------+                |
         +------------------+ Identity  +<---------------+
            1. Issuance     | Authority | 3. Verification

          Figure 2: A three-step process for assuring E2E identity

4.1.  Issuance

   The issuance process is similar to existing widely-deployed processes
   for issuing public-key credentials, such as ACME [RFC8555].  This
   process results in the presenting client holding a credential that
   associates a public key to a set of identity attributes.  So the
   issuance process must assure the identity authority of two things
   about the presenting client:

   *  That it controls the private key corresponding to the public key

   *  That it legitimately represents the identity attributes

   In addition, to prevent unknown key share attacks (UKS), the issuance
   process must verify these properties jointly -- that the entity that
   controls the private key is the same as the entity that holds the
   identity attributes.

   These assurances are typically provided by having the presenting
   client create a signature with the private key over an object that
   reflects the identity attributes.  In ACME, the client sends a
   Certificate Signing Request [RFC2986], which contains both the
   desired identity attributes and a signature by the client's private

4.2.  Presentation

   The presentation process accomplishes two things:

   *  It conveys the presenting client's credential to the verifying

   *  It proves to the verifying client that the presenting client holds
      the corresponding private key.

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   The latter function ties the presentation process to the E2E
   encryption protocol being used.  The credential used for E2E identity
   authenticates the key pair that represents a client in the E2E
   encryption protocol.  The E2E encryption protocol will thus already
   include mechanisms for the presenting client to prove they hold the
   private key corresponding to the credential.

   If the E2E encryption protocol can also deliver the credential (as
   opposed to passing it in some other way), then in addition to
   simplifying application architecture, the E2E encryption protocol can
   provide some additional security benefits.  E2E encryption protocols
   where the presenting client signs the credential being presented are
   generally immune to unknown key share attacks, even if there is a UKS
   vulnerability in the underlying issuance process.

   As a concrete example: In the MLS protocol mentioned above, each
   client presents its credential in a LeafNode structure that is signed
   with the client's private key.  This structure is conveyed to the
   other participants in an MLS-based session when the presenting client
   joins the session.  This provides the other participants with both
   the credential and a signature over the credential (and the other
   contents of the LeafNode) which acts as a proof of possession of the
   private key.

4.3.  Verification

   A credential will typically be an object signed by the identity
   authority, so the verifying client will only need to know the
   identity authority's public key in order to verify that the
   credential was authentically issued by the identity authority.
   Credentials will typically also have some notion of expiration, e.g.,
   the notBefore/notAfter fields in X.509 or the nbf/exp fields in JWT
   [RFC5280] [RFC7519].  Verification at this level is simple, fast, and

   In some cases, though, it is necessary to have a notion of revocation
   of credentials.  Here revocation of a credential means that the
   credential will no longer be accepted by verifying clients, even
   though the credential itself is otherwise valid (e.g., its signature
   is valid and it has not expired).  Since the credential itself cannot
   reflect revocation information, a verifying client needs to get
   revocation information from the identity authority independently.

   Several design patterns for revocation have been explored in the PKI

   1.  Short-lived certificates with no revocation checking

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   2.  Online Certificate Status Protocol (OCSP) checks by clients

   3.  OCSP stapling and the "TLS Feature" extension [RFC6066] [RFC633]

   4.  Certificate Revocation Lists (CRLs) fetched by clients [RFC5280]

   5.  CRLs fetched by vendors and redistributed to clients [crlite]

   The Web PKI has generally settled on central CRL fetching (5), by the
   following process of elimination:

   *  Short-lived certificates are unacceptable in settings where
      clients might be offline for longer than the revocation checking

   *  OCSP has bad performance properties and leaks lots of information
      to the identity authority.

   *  OCSP stapling with the "must-staple" extension is equivalent to
      short-lived certificates.

   *  CRL fetches by clients are either highly inefficient or require
      complicated caching schemes that are better done centrally.

   These design patterns and arguments also apply, mutatis mutandis, in
   the context of E2E identity.  Thus, revocation mechanisms developed
   for E2E identity should be oriented toward centralized CRL fetching,
   but accounting for an important difference -- that the client vendor
   is often the communications service provider, and thus an untrusted
   actor.  This means that clients will need to verify the authenticity
   of revocation information, and mechanisms such as CRLite [crlite]
   will not work.  Rather than having the vendor compress an
   uncompressed representation (as in CRLite), the revocation data
   provided by the identity authority will have to already be compact.

5.  Instantiations

   There are a few deployed and emerging models for E2E identity that
   can be viewed as instantiations of the above architecture.  In this
   section, we examine a few example cases.

5.1.  Manual Verification of Key Fingerprints

   E2E-encrypted messaging apps used by billions of users today rely on
   users manually comparing each others' key fingerprints for their only
   E2E identity assurance.  This can be viewed as a degenerate case of
   the above architecture:

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   *  The credential is only the presenting client's public key, without
      identity attributes.

   *  Each user acts as their own identity authority.

   *  Issuance is done by the user's client generating a key pair.

   *  Presentation comprises the E2E encryption protocol's proof of
      possession of the presenting client's private key.

   *  Verification is the manual comparison of key fingerprints, the
      user of the verifying client confirming with the user of the
      presenting client that they have the same view of the presenting
      client's public key.

   This case is degenerate in the sense that it does not actually
   authenticate any identity attributes; the only non-cryptographic
   attributes attested are those claimed by the presenting client in the
   verification interaction.  This system is thus vulnerable to UKS
   attacks when the user of the presenting client abuses their position
   as a trusted identity authority [signal-uks].

5.2.  X.509

   X.509 and related PKI technologies are a widely used instantiation of
   authority-based authentication, and map naturally in to this

   *  The credentials are certificates.

   *  The identity authorities are certificate authorities (CAs).

   *  Issuance is done via issuance protocols such as ACME or EST
      [RFC8555] [RFC7030].

   *  Several key exchange protocols contain the required mechanics for
      presentation, e.g., the X509Credential mechanism in MLS.

   *  Verification follows the process in [RFC5280], with revocation
      checking done via one of the several mechanisms discussed in
      Section 4.3.

   This scheme works well for cases where a PKI is available with
   authorities that will attest to the required idenitty attributes, and
   where the operational context allows for certificates to be
   provisioned to clients.  Multiple E2E-secure communications products
   today use this scheme.

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5.3.  Verifiable Credentials

   Certificates and PKI protocols tend to be a bad fit for
   authenticating user identities.  Systems like SAML [saml] and OpenID
   Connect [oidc] are more commonly used for user identity, but only
   produce bearer tokens, not the public key credentials required for
   E2E identity -- using bearer tokens for E2E identity would allow the
   verifying client to impersonate the presenting client!  Likewise,
   because the verifier needs to check a bearer tokens validity directly
   with the issuer, the identity authority learns every verifier to whom
   a client authenticates.

   More recently, there has been work to apply the W3C Verifiable
   Credentials (VC) framework to this problem [].  The
   VC model aligns well conceptually with the above architecture, and
   some of the required protocols are in development:

   *  Credentials would be verifiable credentials or verifiable

   *  The identity authorities would be Issuers in the VC model.
      (Likewise, the presenting client would be a Holder and the
      verifying client a Verifier.)

   *  The issuance process here corresponds to the issuance interaction
      in the VC model, for example using OpenID for Verifiable
      Credential Issuance [openid-4-vci]

   *  The presentation process here corresponds to the presentation
      interaction in the VC model, for example using an integration with
      the E2E encryption protocol analogous to the X509Credential
      integration in MLS mentioned above.

   *  The verification process here corresponds to VC verification,
      using a mechanism such as [StatusList2021] for revocation.

   A VC-based model for E2E identity is clearly still incomplete, but
   given the good conceptual alignment and potential for a better fit
   with user identity than PKI, it seems like a promising candidate for
   further development.

6.  Requirements for Interoperable Identity

   The MIMI working group is focused on establishing interoperability
   among messaging systems.  In order to have E2E identity protections
   in an interoperable context, the interoperating parties will need to
   agree on the answers to a few questions:

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   *  What E2E encryption protocol is being used?

   *  How are credentials integrated with the E2E encryption protocol?

      -  How is a credential verified against a participant public key?

      -  What mechanisms for revocation are used (if any)?

      -  How are credentials associated to the encryption protocol?

   *  What types of credentials are clients expected to be able to

   *  Which identity providers are trusted?

   Most of these questions are addressed at the presentation and
   verification phases in the above architecture.  The interoperability
   considerations around issuance are different: For issuance, there
   does not need to be a common solution across the population of
   clients, only between a client and the authority that issues its
   credential.  Nonetheless, having a common, interoperable issuance
   interface is still valuable, since it simplifies integration between
   clients and authorities.

7.  Security Considerations

   This document describes a scheme for authentication in E2E security
   contexts.  Security requirements are described in Section 3, a
   general architecture in Section 4, and some candidate instantiations
   of the architecture in Section 5.

8.  IANA Considerations

   This document has no IANA actions.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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9.2.  Informative References

   [coniks]   Melara, M., Blankstein, A., Bonneau, J., Felten, E., and
              M. Freedman, "CONIKS: Bringing Key Transparency to End
              Users", August 2015,

   [crlite]   Larisch, J., Choffnes, D., Levin, D., Maggs, B., Mislove,
              A., and C. Wilson, "CRLite: A Scalable System for Pushing
              All TLS Revocations to All Browsers", 2017 IEEE Symposium
              on Security and Privacy (SP), DOI 10.1109/sp.2017.17, May
              2017, <>.

              Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
              Omara, E., and K. Cohn-Gordon, "The Messaging Layer
              Security (MLS) Protocol", Work in Progress, Internet-
              Draft, draft-ietf-mls-protocol-20, 27 March 2023,

              Mahy, R., "More Instant Messaging Interoperability (MIMI)
              Identity Concepts", Work in Progress, Internet-Draft,
              draft-mahy-mimi-identity-02, 10 July 2023,

   [oidc]     "OpenID Connect Core 1.0", n.d.,

              "OpenID for Verifiable Credential Issuance", n.d.,

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,

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   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [RFC633]   McKenzie, A., "IMP/TIP preventive maintenance schedule",
              RFC 633, DOI 10.17487/RFC0633, March 1974,

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,

   [RFC8844]  Thomson, M. and E. Rescorla, "Unknown Key-Share Attacks on
              Uses of TLS with the Session Description Protocol (SDP)",
              RFC 8844, DOI 10.17487/RFC8844, January 2021,

   [RFC9162]  Laurie, B., Messeri, E., and R. Stradling, "Certificate
              Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
              December 2021, <>.

   [saml]     "Assertions and Protocols for the OASIS Security Assertion
              Markup Language (SAML) V2.0", n.d., <https://www.oasis-

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   [signal]   Perrin, T. and M. Marlinspike, "The Double Ratchet
              Algorithm", 20 November 2016,

              Cohn-Gordon, K., Cremers, C., Dowling, B., Garratt, L.,
              and D. Stebila, "A Formal Security Analysis of the Signal
              Messaging Protocol", 2017 IEEE European Symposium on
              Security and Privacy (EuroS&P),
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Authors' Addresses

   Richard Barnes

   Rohan Mahy

Barnes & Mahy             Expires 25 April 2024                [Page 15]