Internet Engineering Task Force (IETF) M. Sethi
Request for Comments: 9191
J. Preuß Mattsson
Category: Informational Ericsson
ISSN: 2070-1721 S. Turner
Handling Large Certificates and Long Certificate Chains
in TLS-Based EAP Methods
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 (c) 2022 IETF Trust and the persons identified as the
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) in effect on the date of
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Table of Contents 1.
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.
Normative References 7.2.
The Extensible Authentication Protocol (EAP), defined in [RFC3748
provides a standard mechanism for support of multiple authentication
methods. EAP-TLS [RFC5216
] relies on TLS [RFC8446
provide strong mutual authentication with certificates [RFC5280
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
] and is working on specifications such as [TLS-EAP-TYPES]
for how other TLS-based EAP methods use TLS 1.3.
The key words "MUST
", "MUST NOT
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in
BCP 14 [RFC2119
] 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
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
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
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
]. 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
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)
] 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
]. 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
- 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
* 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),
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
* 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
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
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
] 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
] 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
The Authority Information Access (AIA) extension specified in
] 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
] 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)
] 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)
] 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
Specific security considerations of the referenced documents apply
when they are taken into use.
7.1. Normative References
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748
, DOI 10.17487/RFC3748
, June 2004,
] Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou, "The
Flexible Authentication via Secure Tunneling Extensible
Authentication Protocol Method (EAP-FAST)", RFC 4851
, May 2007,
] 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
] 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,
] Funk, P. and S. Blake-Wilson, "Extensible Authentication
Protocol Tunneled Transport Layer Security Authenticated
Protocol Version 0 (EAP-TTLSv0)", RFC 5281
, August 2008,
] 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,
] 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
] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446
, DOI 10.17487/RFC8446
, August 2018,
] 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,
7.2. Informative References
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,
[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,
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,
[PEAP] Microsoft Corporation, "[MS-PEAP]: Protected Extensible
Authentication Protocol (PEAP)", June 2021.
] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", RFC 2865
, DOI 10.17487/RFC2865
, June 2000,
] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246
, August 2008,
] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066
, January 2011,
] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090
, February 2011,
] 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
] Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", RFC 7924
, July 2016,
] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032
, January 2017,
] 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
] 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
, August 2018,
] Ghedini, A. and V. Vasiliev, "TLS Certificate
Compression", RFC 8879
, DOI 10.17487/RFC8879
] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949
, December 2020,
[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,
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/
[TLS-SIC] Thomson, M., "Suppressing Intermediate Certificates in
TLS", Work in Progress, Internet-Draft, draft-thomson-tls-
sic-00, 27 March 2019,
This document is a result of several useful discussions with Alan
DeKok, Bernard Aboba, Jari Arkko, Jouni Malinen, Darshak Thakore, and
John Preuß Mattsson