Internet Engineering Task Force (IETF)                          M. Sethi
Request for Comments: 9191                             J. Preuß Mattsson
Category: Informational                                         Ericsson
ISSN: 2070-1721                                                S. Turner
                                                                   sn3rd
                                                           February 2022


        Handling Large Certificates and Long Certificate Chains
                        in TLS-Based EAP Methods

Abstract

   The Extensible Authentication Protocol (EAP), defined in RFC 3748,
   provides a standard mechanism for support of multiple authentication
   methods.  EAP-TLS and other TLS-based EAP methods are widely deployed
   and used for network access authentication.  Large certificates and
   long certificate chains combined with authenticators that drop an EAP
   session after only 40 - 50 round trips is a major deployment problem.
   This document looks at this problem in detail and describes the
   potential solutions available.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see 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/rfc9191.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   to this document.  Code Components extracted from this document must
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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Experience with Deployments
   4.  Handling of Large Certificates and Long Certificate Chains
     4.1.  Updating Certificates and Certificate Chains
       4.1.1.  Guidelines for Certificates
       4.1.2.  Pre-distributing and Omitting CA Certificates
       4.1.3.  Using Fewer Intermediate Certificates
     4.2.  Updating TLS and EAP-TLS Code
       4.2.1.  URLs for Client Certificates
       4.2.2.  Caching Certificates
       4.2.3.  Compressing Certificates
       4.2.4.  Compact TLS 1.3
       4.2.5.  Suppressing Intermediate Certificates
       4.2.6.  Raw Public Keys
       4.2.7.  New Certificate Types and Compression Algorithms
     4.3.  Updating Authenticators
   5.  IANA Considerations
   6.  Security Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The Extensible Authentication Protocol (EAP), defined in [RFC3748],
   provides a standard mechanism for support of multiple authentication
   methods.  EAP-TLS [RFC5216] [RFC9190] relies on TLS [RFC8446] to
   provide strong mutual authentication with certificates [RFC5280] and
   is widely deployed and often used for network access authentication.
   There are also many other standardized TLS-based EAP methods such as
   Flexible Authentication via Secure Tunneling (EAP-FAST) [RFC4851],
   Tunneled Transport Layer Security (EAP-TTLS) [RFC5281], the Tunnel
   Extensible Authentication Protocol (TEAP) [RFC7170], as well as
   several vendor-specific EAP methods such as the Protected Extensible
   Authentication Protocol (PEAP) [PEAP].

   Certificates in EAP deployments can be relatively large, and the
   certificate chains can be long.  Unlike the use of TLS on the web,
   where typically only the TLS server is authenticated, EAP-TLS
   deployments typically authenticate both the EAP peer and the EAP
   server.  Also, from deployment experience, EAP peers typically have
   longer certificate chains than servers.  This is because EAP peers
   often follow organizational hierarchies and tend to have many
   intermediate certificates.  Thus, EAP-TLS authentication usually
   involves exchange of significantly more octets than when TLS is used
   as part of HTTPS.

   Section 3.1 of [RFC3748] states that EAP implementations can assume a
   Maximum Transmission Unit (MTU) of at least 1020 octets from lower
   layers.  The EAP fragment size in typical deployments is just 1020 -
   1500 octets (since the maximum Ethernet frame size is ~ 1500 bytes).
   Thus, EAP-TLS authentication needs to be fragmented into many smaller
   packets for transportation over the lower layers.  Such fragmentation
   not only can negatively affect the latency, but also results in other
   challenges.  For example, some EAP authenticator (e.g., an access
   point) implementations will drop an EAP session if it has not
   finished after 40 - 50 round trips.  This is a major problem and
   means that, in many situations, the EAP peer cannot perform network
   access authentication even though both the sides have valid
   credentials for successful authentication and key derivation.

   Not all EAP deployments are constrained by the MTU of the lower
   layer.  For example, some implementations support EAP over Ethernet
   "jumbo" frames that can easily allow very large EAP packets.  Larger
   packets will naturally help lower the number of round trips required
   for successful EAP-TLS authentication.  However, deployment
   experience has shown that these jumbo frames are not always
   implemented correctly.  Additionally, EAP fragment size is also
   restricted by protocols such as RADIUS [RFC2865], which are
   responsible for transporting EAP messages between an authenticator
   and an EAP server.  RADIUS can generally transport only about 4000
   octets of EAP in a single message (the maximum length of a RADIUS
   packet is restricted to 4096 octets in [RFC2865]).

   This document looks at related work and potential tools available for
   overcoming the deployment challenges induced by large certificates
   and long certificate chains.  It then discusses the solutions
   available to overcome these challenges.  Many of the solutions
   require TLS 1.3 [RFC8446].  The IETF has standardized EAP-TLS 1.3
   [RFC9190] and is working on specifications such as [TLS-EAP-TYPES]
   for how other TLS-based EAP methods use TLS 1.3.

2.  Terminology

   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.

   Readers are expected to be familiar with the terms and concepts used
   in EAP [RFC3748], EAP-TLS [RFC5216], and TLS [RFC8446].  In
   particular, this document frequently uses the following terms as they
   have been defined in [RFC5216]:

   Authenticator:  The entity initiating EAP authentication.  Typically
         implemented as part of a network switch or a wireless access
         point.

   EAP peer:  The entity that responds to the authenticator.  In
         [IEEE-802.1X], this entity is known as the supplicant.  In EAP-
         TLS, the EAP peer implements the TLS client role.

   EAP server:  The entity that terminates the EAP authentication method
         with the peer.  In the case where no backend authentication
         server is used, the EAP server is part of the authenticator.
         In the case where the authenticator operates in pass-through
         mode, the EAP server is located on the backend authentication
         server.  In EAP-TLS, the EAP server implements the TLS server
         role.

   The document additionally uses the terms "trust anchor" and
   "certification path" defined in [RFC5280].

3.  Experience with Deployments

   As stated earlier, the EAP fragment size in typical deployments is
   just 1020 - 1500 octets.  A certificate can, however, be large for a
   number of reasons:

   *  It can have a long Subject Alternative Name field.

   *  It can have long Public Key and Signature fields.

   *  It can contain multiple object identifiers (OIDs) that indicate
      the permitted uses of the certificate as noted in Section 5.3 of
      [RFC5216].  Most implementations verify the presence of these OIDs
      for successful authentication.

   *  It can contain multiple organization naming fields to reflect the
      multiple group memberships of a user (in a client certificate).

   A certificate chain (called a certification path in [RFC5280]) in
   EAP-TLS can commonly have 2 - 6 intermediate certificates between the
   end-entity certificate and the trust anchor.

   The size of certificates (and certificate chains) may also increase
   manyfold in the future with the introduction of post-quantum
   cryptography.  For example, lattice-based cryptography would have
   public keys of approximately 1000 bytes and signatures of
   approximately 2000 bytes.

   Many access point implementations drop EAP sessions that do not
   complete within 40 - 50 round trips.  This means that if the chain is
   larger than ~ 60 kilobytes, EAP-TLS authentication cannot complete
   successfully in most deployments.

4.  Handling of Large Certificates and Long Certificate Chains

   This section discusses some possible alternatives for overcoming the
   challenge of large certificates and long certificate chains in EAP-
   TLS authentication.  Section 4.1 considers recommendations that
   require an update of the certificates or certificate chains used for
   EAP-TLS authentication without requiring changes to the existing EAP-
   TLS code base.  It also provides some guidelines that should be
   followed when issuing certificates for use with EAP-TLS.  Section 4.2
   considers recommendations that rely on updates to the EAP-TLS
   implementations and can be deployed with existing certificates.
   Finally, Section 4.3 briefly discusses what could be done to update
   or reconfigure authenticators when it is infeasible to replace
   deployed components giving a solution that can be deployed without
   changes to existing certificates or code.

4.1.  Updating Certificates and Certificate Chains

   Many IETF protocols now use elliptic curve cryptography (ECC)
   [RFC6090] for the underlying cryptographic operations.  The use of
   ECC can reduce the size of certificates and signatures.  For example,
   at a 128-bit security level, the size of a public key with
   traditional RSA is about 384 bytes, while the size of a public key
   with ECC is only 32-64 bytes.  Similarly, the size of a digital
   signature with traditional RSA is 384 bytes, while the size is only
   64 bytes with the elliptic curve digital signature algorithm (ECDSA)
   and the Edwards-curve digital signature algorithm (EdDSA) [RFC8032].
   Using certificates that use ECC can reduce the number of messages in
   EAP-TLS authentication, which can alleviate the problem of
   authenticators dropping an EAP session because of too many round
   trips.  In the absence of a standard application profile specifying
   otherwise, TLS 1.3 [RFC8446] requires implementations to support ECC.
   New cipher suites that use ECC are also specified for TLS 1.2
   [RFC8422].  Using ECC-based cipher suites with existing code can
   significantly reduce the number of messages in a single EAP session.

4.1.1.  Guidelines for Certificates

   The general guideline of keeping the certificate size small by not
   populating fields with excessive information can help avert the
   problems of failed EAP-TLS authentication.  More specific
   recommendations for certificates used with EAP-TLS are as follows:

   *  Object Identifier (OID) is an ASN.1 data type that defines unique
      identifiers for objects.  The OID's ASN.1 value, which is a string
      of integers, is then used to name objects to which they relate.
      The Distinguished Encoding Rules (DER) specify that the first two
      integers always occupy one octet and subsequent integers are
      base-128 encoded in the fewest possible octets.  OIDs are used
      lavishly in X.509 certificates [RFC5280] and while not all can be
      avoided, e.g., OIDs for extensions or algorithms and their
      associate parameters, some are well within the certificate
      issuer's control:

      -  Each naming attribute in a DN (Distinguished Name) has one.
         DNs are used in the issuer and subject fields as well as
         numerous extensions.  A shallower name will be smaller, e.g.,
         C=FI, O=Example, SN=B0A123499EFC as against C=FI, O=Example,
         OU=Division 1, SOPN=Southern Finland, CN=Coolest IoT Gadget
         Ever, and SN=B0A123499EFC.

      -  Every certificate policy (and qualifier) and any mappings to
         another policy uses identifiers.  Consider carefully what
         policies apply.

   *  DirectoryString and GeneralName types are used extensively to name
      things, e.g., the DN naming attribute O= (the organizational
      naming attribute) DirectoryString includes "Example" for the
      Example organization and uniformResourceIdentifier can be used to
      indicate the location of the Certificate Revocation List (CRL),
      e.g., "http://crl.example.com/sfig2s1-128.crl", in the CRL
      Distribution Point extension.  For these particular examples, each
      character is a single byte.  For some non-ASCII character strings,
      characters can be several bytes.  Obviously, the names need to be
      unique, but there is more than one way to accomplish this without
      long strings.  This is especially true if the names are not meant
      to be meaningful to users.

   *  Extensions are necessary to comply with [RFC5280], but the vast
      majority are optional.  Include only those that are necessary to
      operate.

   *  As stated earlier, certificate chains of the EAP peer often follow
      organizational hierarchies.  In such cases, information in
      intermediate certificates (such as postal addresses) do not
      provide any additional value and they can be shortened (for
      example, only including the department name instead of the full
      postal address).

4.1.2.  Pre-distributing and Omitting CA Certificates

   The TLS Certificate message conveys the sending endpoint's
   certificate chain.  TLS allows endpoints to reduce the size of the
   Certificate message by omitting certificates that the other endpoint
   is known to possess.  When using TLS 1.3, all certificates that
   specify a trust anchor known by the other endpoint may be omitted
   (see Section 4.4.2 of [RFC8446]).  When using TLS 1.2 or earlier,
   only the self-signed certificate that specifies the root certificate
   authority may be omitted (see Section 7.4.2 of [RFC5246]).
   Therefore, updating TLS implementations to version 1.3 can help to
   significantly reduce the number of messages exchanged for EAP-TLS
   authentication.  The omitted certificates need to be pre-distributed
   independently of TLS, and the TLS implementations need to be
   configured to omit these pre-distributed certificates.

4.1.3.  Using Fewer Intermediate Certificates

   The EAP peer certificate chain does not have to mirror the
   organizational hierarchy.  For successful EAP-TLS authentication,
   certificate chains SHOULD NOT contain more than 4 intermediate
   certificates.

   Administrators responsible for deployments using TLS-based EAP
   methods can examine the certificate chains and make rough
   calculations about the number of round trips required for successful
   authentication.  For example, dividing the total size of all the
   certificates in the peer and server certificate chain (in bytes) by
   1020 bytes will indicate the number of round trips required.  If this
   number exceeds 50, then administrators can expect failures with many
   common authenticator implementations.

4.2.  Updating TLS and EAP-TLS Code

   This section discusses how the fragmentation problem can be avoided
   by updating the underlying TLS or EAP-TLS implementation.  Note that
   in some cases, the new feature may already be implemented in the
   underlying library and simply needs to be enabled.

4.2.1.  URLs for Client Certificates

   [RFC6066] defines the "client_certificate_url" extension, which
   allows TLS clients to send a sequence of Uniform Resource Locators
   (URLs) instead of the client certificate chain.  URLs can refer to a
   single certificate or a certificate chain.  Using this extension can
   curtail the amount of fragmentation in EAP deployments thereby
   allowing EAP sessions to successfully complete.

4.2.2.  Caching Certificates

   The TLS Cached Information Extension [RFC7924] specifies an extension
   where a server can exclude transmission of certificate information
   cached in an earlier TLS handshake.  The client and the server would
   first execute the full TLS handshake.  The client would then cache
   the certificate provided by the server.  When the TLS client later
   connects to the same TLS server without using session resumption, it
   can attach the "cached_info" extension to the ClientHello message.
   This would allow the client to indicate that it has cached the
   certificate.  The client would also include a fingerprint of the
   server certificate chain.  If the server's certificate has not
   changed, then the server does not need to send its certificate and
   the corresponding certificate chain again.  In case information has
   changed, which can be seen from the fingerprint provided by the
   client, the certificate payload is transmitted to the client to allow
   the client to update the cache.  The extension, however, necessitates
   a successful full handshake before any caching.  This extension can
   be useful when, for example, a successful authentication between an
   EAP peer and EAP server has occurred in the home network.  If
   authenticators in a roaming network are stricter at dropping long EAP
   sessions, an EAP peer can use the Cached Information Extension to
   reduce the total number of messages.

   However, if all authenticators drop the EAP session for a given EAP
   peer and EAP server combination, a successful full handshake is not
   possible.  An option in such a scenario would be to cache validated
   certificate chains even if the EAP-TLS exchange fails, but such
   caching is currently not specified in [RFC7924].

4.2.3.  Compressing Certificates

   The TLS Working Group has standardized an extension for TLS 1.3
   [RFC8879] that allows compression of certificates and certificate
   chains during full handshakes.  The client can indicate support for
   compressed server certificates by including this extension in the
   ClientHello message.  Similarly, the server can indicate support for
   compression of client certificates by including this extension in the
   CertificateRequest message.  While such an extension can alleviate
   the problem of excessive fragmentation in EAP-TLS, it can only be
   used with TLS version 1.3 and higher.  Deployments that rely on older
   versions of TLS cannot benefit from this extension.

4.2.4.  Compact TLS 1.3

   [cTLS] defines a "compact" version of TLS 1.3 and reduces the message
   size of the protocol by removing obsolete material and using more
   efficient encoding.  It also defines a compression profile with which
   either side can define a dictionary of "known certificates".  Thus,
   cTLS could provide another mechanism for EAP-TLS deployments to
   reduce the size of messages and avoid excessive fragmentation.

4.2.5.  Suppressing Intermediate Certificates

   For a client that has all intermediate certificates in the
   certificate chain, having the server send intermediates in the TLS
   handshake increases the size of the handshake unnecessarily.
   [TLS-SIC] proposes an extension for TLS 1.3 that allows a TLS client
   that has access to the complete set of published intermediate
   certificates to inform servers of this fact so that the server can
   avoid sending intermediates, reducing the size of the TLS handshake.
   The mechanism is intended to be complementary with certificate
   compression.

   The Authority Information Access (AIA) extension specified in
   [RFC5280] can be used with end-entity and CA certificates to access
   information about the issuer of the certificate in which the
   extension appears.  For example, it can be used to provide the
   address of the Online Certificate Status Protocol (OCSP) responder
   from where revocation status of the certificate (in which the
   extension appears) can be checked.  It can also be used to obtain the
   issuer certificate.  Thus, the AIA extension can reduce the size of
   the certificate chain by only including a pointer to the issuer
   certificate instead of including the entire issuer certificate.
   However, it requires the side receiving the certificate containing
   the extension to have network connectivity (unless the information is
   already cached locally).  Naturally, such indirection cannot be used
   for the server certificate (since EAP peers in most deployments do
   not have network connectivity before authentication and typically do
   not maintain an up-to-date local cache of issuer certificates).

4.2.6.  Raw Public Keys

   [RFC7250] defines a new certificate type and TLS extensions to enable
   the use of raw public keys for authentication.  Raw public keys use
   only a subset of information found in typical certificates and are
   therefore much smaller in size.  However, raw public keys require an
   out-of-band mechanism to bind the public key with the entity
   presenting the key.  Using raw public keys will obviously avoid the
   fragmentation problems resulting from large certificates and long
   certificate chains.  Deployments can consider their use as long as an
   appropriate out-of-band mechanism for binding public keys with
   identifiers is in place.  Naturally, deployments will also need to
   consider the challenges of revocation and key rotation with the use
   of raw public keys.

4.2.7.  New Certificate Types and Compression Algorithms

   There is ongoing work to specify new certificate types that are
   smaller than traditional X.509 certificates.  For example,
   [CBOR-CERT] defines a Concise Binary Object Representation (CBOR)
   [RFC8949] encoding of X.509 Certificates.  The CBOR encoding can be
   used to compress existing X.509 certificates or for natively signed
   CBOR certificates.  [TLS-CWT] registers a new TLS Certificate type
   that would enable TLS implementations to use CBOR Web Tokens (CWTs)
   [RFC8392] as certificates.  While these are early initiatives, future
   EAP-TLS deployments can consider the use of these new certificate
   types and compression algorithms to avoid large message sizes.

4.3.  Updating Authenticators

   There are several legitimate reasons that authenticators may want to
   limit the number of packets / octets / round trips that can be sent.
   The main reason has been to work around issues where the EAP peer and
   EAP server end up in an infinite loop ACKing their messages.  Another
   reason is that unlimited communication from an unauthenticated device
   using EAP could provide a channel for inappropriate bulk data
   transfer.  A third reason is to prevent denial-of-service attacks.

   Updating the millions of already deployed access points and switches
   is in many cases not realistic.  Vendors may be out of business or no
   longer supporting the products and administrators may have lost the
   login information to the devices.  For practical purposes, the EAP
   infrastructure is ossified for the time being.

   Vendors making new authenticators should consider increasing the
   number of round trips allowed to 100 before denying the EAP
   authentication to complete.  Based on the size of the certificates
   and certificate chains currently deployed, such an increase would
   likely ensure that peers and servers can complete EAP-TLS
   authentication.  At the same time, administrators responsible for EAP
   deployments should ensure that this 100 round-trip limit is not
   exceeded in practice.

5.  IANA Considerations

   This document has no IANA actions.

6.  Security Considerations

   Updating implementations to TLS version 1.3 allows omitting all
   certificates with a trust anchor known by the other endpoint.  TLS
   1.3 additionally provides improved security, privacy, and reduced
   latency for EAP-TLS [RFC9190].

   Security considerations when compressing certificates are specified
   in [RFC8879].

   Specific security considerations of the referenced documents apply
   when they are taken into use.

7.  References

7.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, Ed., "Extensible Authentication Protocol
              (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
              <https://www.rfc-editor.org/info/rfc3748>.

   [RFC4851]  Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou, "The
              Flexible Authentication via Secure Tunneling Extensible
              Authentication Protocol Method (EAP-FAST)", RFC 4851,
              DOI 10.17487/RFC4851, May 2007,
              <https://www.rfc-editor.org/info/rfc4851>.

   [RFC5216]  Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
              Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216,
              March 2008, <https://www.rfc-editor.org/info/rfc5216>.

   [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,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5281]  Funk, P. and S. Blake-Wilson, "Extensible Authentication
              Protocol Tunneled Transport Layer Security Authenticated
              Protocol Version 0 (EAP-TTLSv0)", RFC 5281,
              DOI 10.17487/RFC5281, August 2008,
              <https://www.rfc-editor.org/info/rfc5281>.

   [RFC7170]  Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna,
              "Tunnel Extensible Authentication Protocol (TEAP) Version
              1", RFC 7170, DOI 10.17487/RFC7170, May 2014,
              <https://www.rfc-editor.org/info/rfc7170>.

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC9190]  Preuß Mattsson, J. and M. Sethi, "EAP-TLS 1.3: Using the
              Extensible Authentication Protocol with TLS 1.3",
              RFC 9190, DOI 10.17487/RFC9190, February 2022,
              <https://www.rfc-editor.org/rfc/rfc9190>.

7.2.  Informative References

   [CBOR-CERT]
              Raza, S., Höglund, J., Selander, G., Preuß Mattsson, J.,
              and M. Furuhed, "CBOR Encoded X.509 Certificates (C509
              Certificates)", Work in Progress, Internet-Draft, draft-
              mattsson-cose-cbor-cert-compress-08, 22 February 2021,
              <https://datatracker.ietf.org/doc/html/draft-mattsson-
              cose-cbor-cert-compress-08>.

   [cTLS]     Rescorla, E., Barnes, R., and H. Tschofenig, "Compact TLS
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              ctls-04, 25 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              ctls-04>.

   [IEEE-802.1X]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              NNetworks--Port-Based Network Access Control",
              DOI 10.1109/IEEESTD.2020.9018454, IEEE Standard 802.1X-
              2020, February 2020,
              <https://doi.org/10.1109/IEEESTD.2020.9018454>.

   [PEAP]     Microsoft Corporation, "[MS-PEAP]: Protected Extensible
              Authentication Protocol (PEAP)", June 2021.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, DOI 10.17487/RFC2865, June 2000,
              <https://www.rfc-editor.org/info/rfc2865>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090,
              DOI 10.17487/RFC6090, February 2011,
              <https://www.rfc-editor.org/info/rfc6090>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <https://www.rfc-editor.org/info/rfc7250>.

   [RFC7924]  Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", RFC 7924,
              DOI 10.17487/RFC7924, July 2016,
              <https://www.rfc-editor.org/info/rfc7924>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

   [RFC8422]  Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
              Curve Cryptography (ECC) Cipher Suites for Transport Layer
              Security (TLS) Versions 1.2 and Earlier", RFC 8422,
              DOI 10.17487/RFC8422, August 2018,
              <https://www.rfc-editor.org/info/rfc8422>.

   [RFC8879]  Ghedini, A. and V. Vasiliev, "TLS Certificate
              Compression", RFC 8879, DOI 10.17487/RFC8879, December
              2020, <https://www.rfc-editor.org/info/rfc8879>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

   [TLS-CWT]  Tschofenig, H. and M. Brossard, "Using CBOR Web Tokens
              (CWTs) in Transport Layer Security (TLS) and Datagram
              Transport Layer Security (DTLS)", Work in Progress,
              Internet-Draft, draft-tschofenig-tls-cwt-02, 13 July 2020,
              <https://datatracker.ietf.org/doc/html/draft-tschofenig-
              tls-cwt-02>.

   [TLS-EAP-TYPES]
              DeKok, A., "TLS-based EAP types and TLS 1.3", Work in
              Progress, Internet-Draft, draft-ietf-emu-tls-eap-types-04,
              22 January 2022, <https://datatracker.ietf.org/doc/html/
              draft-ietf-emu-tls-eap-types-04>.

   [TLS-SIC]  Thomson, M., "Suppressing Intermediate Certificates in
              TLS", Work in Progress, Internet-Draft, draft-thomson-tls-
              sic-00, 27 March 2019,
              <https://datatracker.ietf.org/doc/html/draft-thomson-tls-
              sic-00>.

Acknowledgements

   This document is a result of several useful discussions with Alan
   DeKok, Bernard Aboba, Jari Arkko, Jouni Malinen, Darshak Thakore, and
   Hannes Tschofening.

Authors' Addresses

   Mohit Sethi
   Ericsson
   FI-02420 Jorvas
   Finland

   Email: mohit@iki.fi


   John Preuß Mattsson
   Ericsson
   Kista
   Sweden

   Email: john.mattsson@ericsson.com


   Sean Turner
   sn3rd

   Email: sean@sn3rd.com