Network Working Group                                           M. Sethi
Internet-Draft                                               J. Mattsson
Intended status: Informational                                  Ericsson
Expires: September 6, 2020                                     S. Turner
                                                           March 5, 2020

        Handling Large Certificates and Long Certificate Chains
                        in TLS-based EAP 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 memo looks at the this problem in detail and describes the
   potential solutions available.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 6, 2020.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Experience with Deployments . . . . . . . . . . . . . . . . .   4
   4.  Handling of Large Certificates and Long Certificate Chains  .   4
     4.1.  Updating Certificates and Certificate Chains  . . . . . .   5
       4.1.1.  Guidelines for certificates . . . . . . . . . . . . .   5
     4.2.  Updating TLS and EAP-TLS Code . . . . . . . . . . . . . .   6
       4.2.1.  Pre-distributing and Omitting CA Certificates . . . .   6
       4.2.2.  Caching Certificates  . . . . . . . . . . . . . . . .   7
       4.2.3.  Compressing Certificates  . . . . . . . . . . . . . .   7
       4.2.4.  Suppressing Intermediate Certificates . . . . . . . .   8
       4.2.5.  Using Fewer Intermediate Certificates . . . . . . . .   8
     4.3.  Updating Authenticators . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The Extensible Authentication Protocol (EAP), defined in [RFC3748],
   provides a standard mechanism for support of multiple authentication
   methods.  EAP-Transport Layer Security (EAP-TLS) [RFC5216]
   [I-D.ietf-emu-eap-tls13] 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 TLS-based EAP methods, such as FAST [RFC4851], TTLS
   [RFC5281], TEAP [RFC7170], and possibly many vendor specific EAP

   TLS certificates are often relatively large, and the certificate
   chains are often long.  Unlike the use of TLS on the web, where
   typically only the TLS server is authenticated; EAP-TLS deployments
   typically authenticates 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

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   certificates.  Thus, EAP-TLS authentication usually involve
   significantly more octets than when TLS is used as part of HTTPS.

   Section 3.1 of [RFC3748] states that EAP implementations can assume a
   MTU of at least 1020 octets from lower layers.  The EAP fragment size
   in typical deployments is just 1020 - 1500 octets.  Thus, EAP-TLS
   authentication needs to be fragmented into many smaller packets for
   transportation over the lower layers.  Such fragmentation can not
   only negatively affect the latency, but also results in other
   challenges.  For example, many EAP authenticator (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.

   This memo 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.

2.  Terminology

   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.

   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]:

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

   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

3.  Experience with Deployments

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

   o  Long Subject Alternative Name field.

   o  Long Public Key and Signature fields.

   o  Can contain multiple object identifiers (OID) 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.

   o  Multiple user groups in the certificate.

   The certificate chain can typically include 2 - 6 certificates to the

   Most common access point implementations drop EAP sessions that do
   not complete within 50 round-trips.  This means that if the chain is
   larger than ~ 60 kB, 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.  In Section 4.1 we look at recommendations that
   require an update of the certificates or certificate chains that are
   used for EAP-TLS authentication without requiring changes to the

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   existing EAP-TLS code base.  We also provide some guidelines when
   issuing certificates for use with EAP-TLS.  In Section 4.2 we look at
   recommendations that rely on updates to the EAP-TLS implementations
   which can be deployed with existing certificates.  In Section 4.3 we
   shortly discuss the solution to update or reconfigure authenticator
   which can be deployed without changes to existing certificates or
   EAP-TLS 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 public keys with traditional
   RSA is about 384 bytes, while the size of public keys with ECC is
   only 32-64 bytes.  Similarly, the size of digital signatures with
   traditional RSA is 384 bytes, while the size is only 64 bytes with
   elliptic curve digital signature algorithm (ECDSA) and 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.  TLS 1.3
   [RFC8446] requires implementations to support ECC.  New cipher suites
   that use ECC are also specified for TLS 1.2 [RFC5289].  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

   This section provides some recommendations for certificates used for
   EAP-TLS authentication:

   o  Object Identifiers (OIDs) is 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 DER length for the 1st two integers is always one octet and
      subsequent integers are base 128-encoded in the fewest possible
      octets.  OIDs are used lavishly in X.509 certificates 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 (Directory Name) has one.  DNs
         used in the issuer and subject fields as well as numerous
         extensions.  A shallower naming will be smaller, e.g., C=FI,
         O=Example, SN=B0A123499EFC vs C=FI, O=Example, OU=Division 1,
         SOPN=Southern Finland, CN=Coolest IoT Gadget Ever,

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

   o  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 CRL, e.g., "
      sfig2s1-128.crl", in the CRL Distribution Point extension.  For
      these particular examples, each character is a byte.  For some
      non-ASCII character strings in the DN, characters can be multi-
      byte.  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

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

   o  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).

4.2.  Updating TLS and EAP-TLS Code

4.2.1.  Pre-distributing and Omitting CA Certificates

   The TLS Certificate message conveys the sending endpoint's
   certificate chain.  TLS allows endpoints to reduce the sizes of the
   Certificate messages 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 implementation need to be configured to omit the pre-
   distributed certificates.

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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, when a successful authentication between an
   EAP peer and EAP server has occurred in the home network.  If
   authenticators in a roaming network are more strict 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 this is
   currently not allowed according to [RFC7924].

4.2.3.  Compressing Certificates

   The TLS working group is also working on an extension for TLS 1.3
   [I-D.ietf-tls-certificate-compression] 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.

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4.2.4.  Suppressing Intermediate Certificates

   For a client that has all intermediates, having the server send
   intermediates in the TLS handshake increases the size of the
   handshake unnecessarily.  The TLS working group is working on an
   extension for TLS 1.3 [I-D.thomson-tls-sic] 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

4.2.5.  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 2-4 intermediate

   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 by 1020 will
   indicate the minimum number of round trips required.  If this number
   exceeds 50, then, administrators can expect failures with many common
   authenticator implementations.

4.3.  Updating Authenticators

   There are several legitimate reasons that authenticators may want to
   limit the number of round-trips/packets/octets 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
   second reason is that unlimited communication from an unauthenticated
   device as EAP could otherwise be use for 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 do
   no longer support 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.  At the same time, administrators

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   responsible for EAP deployments should ensure that this 100 roundtrip
   limit is not exceeded in practice.

5.  IANA Considerations

   This memo includes no request to IANA.

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 [I-D.ietf-emu-eap-tls13].

   When compressing certificates, the underlying compression algorithm
   MUST output the same data that was provided as input by.  After
   decompression, the Certificate message MUST be processed as if it
   were encoded without being compressed.  Additional security
   considerations when compressing certificates are specified in

   As noted in [I-D.thomson-tls-sic], suppressing intermediate
   certificates creates an unencrypted signal that might be used to
   identify which clients believe that they have all intermediates.
   This might also allow more effective fingerprinting and tracking of

7.  References

7.1.  Normative References

              Mattsson, J. and M. Sethi, "Using EAP-TLS with TLS 1.3",
              draft-ietf-emu-eap-tls13-08 (work in progress), December

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

   [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,

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   [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,

   [RFC5216]  Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
              Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216,
              March 2008, <>.

   [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,

   [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,

   [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,

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

7.2.  Informative References

              Ghedini, A. and V. Vasiliev, "TLS Certificate
              Compression", draft-ietf-tls-certificate-compression-10
              (work in progress), January 2020.

              Thomson, M., "Suppressing Intermediate Certificates in
              TLS", draft-thomson-tls-sic-00 (work in progress), March

              Institute of Electrical and Electronics Engineers, "IEEE
              Standard for Local and metropolitan area networks -- Port-
              Based Network Access Control", IEEE Standard 802.1X-2010 ,
              February 2010.

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   [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,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC5289]  Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
              256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
              DOI 10.17487/RFC5289, August 2008,

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090,
              DOI 10.17487/RFC6090, February 2011,

   [RFC7924]  Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", RFC 7924,
              DOI 10.17487/RFC7924, July 2016,

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,


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

Authors' Addresses

   Mohit Sethi
   Jorvas  02420


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   John Mattsson


   Sean Turner


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