wdiff rfc9190v1.txt rfc9190.txt

Internet Engineering Task Force (IETF) J. Preuß Mattsson
Request for Comments: 9190 M. Sethi
Updates: 5216 Ericsson
Category: Standards Track January February 2022
ISSN: 2070-1721

Using

EAP-TLS 1.3: Using the Extensible Authentication Protocol with TLS 1.3 (EAP-TLS 1.3)

Abstract

The Extensible Authentication Protocol (EAP), defined in RFC 3748,
provides a standard mechanism for support of multiple authentication
methods. This document specifies the use of EAP-Transport Layer
Security (EAP-TLS) EAP-TLS with TLS 1.3
while remaining backwards compatible with existing implementations of
EAP-TLS. TLS 1.3 provides significantly improved security and
privacy, and reduced latency when compared to earlier versions of
TLS. EAP-TLS with TLS 1.3 (EAP-TLS 1.3) further improves security
and privacy by always providing forward secrecy, never disclosing the
peer identity, and by mandating use of revocation checking when
compared to EAP-TLS with earlier versions of TLS. This document also
provides guidance on authentication, authorization, and resumption
for EAP-TLS in general (regardless of the underlying TLS version
used). This document updates RFC 5216.

Status of This Memo

This is an Internet Standards Track document.

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). Further information on
Internet Standards is available in 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/rfc9190.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Requirements and Terminology
2. Protocol Overview
2.1. Overview of the EAP-TLS Conversation
2.1.1. Authentication
2.1.2. Ticket Establishment
2.1.3. Resumption
2.1.4. Termination
2.1.5. No Peer Authentication
2.1.6. Hello Retry Request
2.1.7. Identity
2.1.8. Privacy
2.1.9. Fragmentation
2.2. Identity Verification
2.3. Key Hierarchy
2.4. Parameter Negotiation and Compliance Requirements
2.5. EAP State Machines
3. Detailed Description of the EAP-TLS Protocol
4. IANA Considerations
5. Security Considerations
5.1. Security Claims
5.2. Peer and Server Identities
5.3. Certificate Validation
5.4. Certificate Revocation
5.5. Packet Modification Attacks
5.6. Authorization
5.7. Resumption
5.8. Privacy Considerations
5.9. Pervasive Monitoring
5.10. Discovered Vulnerabilities
5.11. Cross-Protocol Attacks
6. References
6.1. Normative References
6.2. Informative references
Appendix A. Updated References
Acknowledgments
Contributors
Authors' Addresses

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) EAP-TLS [RFC5216] specifies an EAP authentication method
with certificate-based mutual authentication utilizing the TLS
handshake protocol for cryptographic algorithms and protocol version
negotiation and establishment of shared secret keying material. EAP-TLS EAP-
TLS is widely supported for authentication and key establishment in
IEEE 802.11 [IEEE-802.11] (Wi-Fi) and IEEE 802.1AE [IEEE-802.1AE]
(MACsec) networks using IEEE 802.1X [IEEE-802.1X] and it's the
default mechanism for certificate-
based certificate-based authentication in 3GPP 5G
[TS.33.501] and MulteFire [MulteFire] networks. Many other EAP
methods such as the Flexible Authentication via Secure Tunneling Extensible Authentication Protocol (EAP-FAST)
method (EAP-
FAST) [RFC4851], the EAP Tunneled Transport Layer Security (EAP-
TTLS) Authenticated Protocol (EAP-TTLS)
[RFC5281], the Tunnel Extensible Authentication Protocol (TEAP)
[RFC7170], and as well as vendor-specific EAP methods such as the
Protected Extensible Authentication Protocol (PEAP) [PEAP] [PEAP], depend on
TLS and EAP-TLS.

EAP-TLS [RFC5216] references TLS 1.0 [RFC2246] and TLS 1.1 [RFC4346]
but can also work with TLS 1.2 [RFC5246]. TLS 1.0 and 1.1 are
formally deprecated and prohibited from being negotiated or used
[RFC8996]. Weaknesses found in TLS 1.2 as well as new requirements
for security, privacy, and reduced latency have led to the
specification of TLS 1.3 [RFC8446], which obsoletes TLS 1.2
[RFC5246]. TLS 1.3 is in large part a complete remodeling of the TLS
handshake protocol including a different message flow, different
handshake messages, different key schedule, different cipher suites,
different resumption mechanism, different privacy protection, and
different record padding. This means that significant parts of the
normative text in the previous EAP-TLS specification [RFC5216] are
not applicable to EAP-TLS with TLS 1.3. Therefore, aspects such as
resumption, privacy handling, and key derivation need to be
appropriately addressed for EAP-TLS with TLS 1.3.

This document updates [RFC5216] to define how to use EAP-TLS with TLS
1.3. When older TLS versions are negotiated, RFC 5216 applies to
maintain backwards compatibility. However, this document does
provide additional guidance on authentication, authorization, and
resumption for EAP-TLS regardless of the underlying TLS version used.
This document only describes differences compared to [RFC5216]. When
EAP-TLS is used with TLS 1.3, some references are updated as
specified in Appendix A. All message flows are example message flows
specific to TLS 1.3 and do not apply to TLS 1.2. Since EAP-TLS
couples the TLS handshake state machine with the EAP state machine,
it is possible that new versions of TLS will cause incompatibilities
that introduce failures or security issues if they are not carefully
integrated into the EAP-TLS protocol. Therefore, implementations
MUST limit the maximum TLS version they use to 1.3, unless later
versions are explicitly enabled by the administrator.

This document specifies EAP-TLS 1.3 and does not specify how other
TLS-based EAP methods use TLS 1.3. The specification for how other
TLS-based EAP methods use TLS 1.3 is left to other documents such as
[TLS-EAP-TYPES].

In addition to the improved security and privacy offered by TLS 1.3,
there are other significant benefits of using EAP-TLS with TLS 1.3.
Privacy, which in EAP-TLS means that no information about the
underlying peer identity is disclosed, is mandatory and achieved
without any additional round trips. Revocation checking is mandatory
and simplified with Online Certificate Status Protocol (OCSP)
stapling, and TLS 1.3 introduces more possibilities to reduce
fragmentation when compared to earlier versions of TLS.

1.1. Requirements and 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-TLS [RFC5216] and TLS [RFC8446]. The term EAP-TLS peer is
used for the entity acting as EAP peer and TLS client. The term EAP-
TLS server is used for the entity acting as EAP server and TLS
server.

This document follows the terminology from [TLS-bis] where the master
secret is renamed to the main secret and the exporter_master_secret
is renamed to the exporter_secret.

2. Protocol Overview

2.1. Overview of the EAP-TLS Conversation

This section updates Section 2.1 of [RFC5216] by amending it in
accordance with the following discussion.

If the TLS implementation correctly implements TLS version
negotiation, EAP-TLS will automatically leverage that capability.
The EAP-TLS implementation needs to know which version of TLS was
negotiated to correctly support EAP-TLS 1.3 as well as to maintain
backward compatibility with EAP-TLS 1.2.

TLS 1.3 changes both the message flow and the handshake messages
compared to earlier versions of TLS. Therefore, much of Section 2.1
of [RFC5216] does not apply for TLS 1.3. Except for Sections 2.2 and
5.7, this update applies only when TLS 1.3 is negotiated. When TLS
1.2 is negotiated, then [RFC5216] applies.

TLS 1.3 introduces several new handshake messages including
HelloRetryRequest, NewSessionTicket, and KeyUpdate. In general,
these messages will be handled by the underlying TLS libraries and
are not visible to EAP-TLS; however, there are a few things to note:

* The HelloRetryRequest is used by the server to reject the
parameters offered in the ClientHello and suggest new parameters.
When this message is encountered, it will increase the number of
round trips used by the protocol.

* The NewSessionTicket message is used to convey resumption
information and is covered in Sections 2.1.2 and 2.1.3.

* The KeyUpdate message is used to update the traffic keys used on a
TLS connection. EAP-TLS does not encrypt significant amounts of
data so this functionality is not needed. Implementations SHOULD
NOT send this message; however, some TLS libraries may
automatically generate and process this message.

* Early Data MUST NOT be used in EAP-TLS. EAP-TLS servers MUST NOT
send an early_data extension and clients MUST NOT send an
EndOfEarlyData message.

* Post-handshake authentication MUST NOT be used in EAP-TLS.
Clients MUST NOT send a "post_handshake_auth" extension and
Servers MUST NOT request post-handshake client authentication.

After receiving an EAP-Request packet with EAP-Type=EAP-TLS as
described in [RFC5216], the conversation will continue with the TLS
handshake protocol encapsulated in the data fields of EAP-Response
and EAP-Request packets. When EAP-TLS is used with TLS version 1.3,
the formatting and processing of the TLS handshake SHALL be done as
specified in version 1.3 of TLS. This update only lists additional
and different requirements, restrictions, and processing compared to
[RFC8446] and [RFC5216].

2.1.1. Authentication

This section updates Section 2.1.1 of [RFC5216] by amending it in
accordance with the following discussion.

The EAP-TLS server MUST authenticate with a certificate and SHOULD
require the EAP-TLS peer to authenticate with a certificate.
Certificates can be of any type supported by TLS including raw public
keys. Pre-Shared Key (PSK) authentication SHALL NOT be used except
for resumption. The full handshake in EAP-TLS with TLS 1.3 always
provides forward secrecy by exchange of ephemeral "key_share"
extensions in the ClientHello and ServerHello (e.g., containing
Ephemeral Elliptic Curve Diffie-Hellman (ECDHE) public keys).
SessionID is deprecated in TLS 1.3; see Sections 4.1.2 and 4.1.3 of
[RFC8446]. TLS 1.3 introduced early application data that like all
application data (other than the protected success indication
described below) is not used in EAP-TLS; see Section 4.2.10 of
[RFC8446] for additional information on the "early_data" extension.
Resumption is handled as described in Section 2.1.3. As a protected
success indication [RFC3748], the EAP-TLS server always sends TLS
application data 0x00; see Section 2.5. Note that a TLS
implementation MAY not allow the EAP-TLS layer to control in which
order things are sent and the application data MAY therefore be sent
before a NewSessionTicket. TLS application data 0x00 is therefore to
be interpreted as success after the EAP-Request that contains TLS
application data 0x00. After the EAP-TLS server has sent an EAP-
Request containing the TLS application data 0x00 and received an EAP-
Response packet of EAP-Type=EAP-TLS and no data, the EAP-TLS server
sends EAP-Success.

Figure 1 shows an example message flow for a successful EAP-TLS full
handshake with mutual authentication (and neither HelloRetryRequest
nor post-handshake messages are sent).

EAP-TLS Peer EAP-TLS Server

EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
TLS CertificateRequest,
TLS Certificate,
TLS CertificateVerify,
<-------- TLS Finished)
EAP-Response/
EAP-Type=EAP-TLS
(TLS Certificate,
TLS CertificateVerify,
TLS Finished) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Application Data 0x00)
EAP-Response/
EAP-Type=EAP-TLS -------->
<-------- EAP-Success

Figure 1: EAP-TLS Mutual Authentication

2.1.2. Ticket Establishment

This is a new section when compared to [RFC5216].

To enable resumption when using EAP-TLS with TLS 1.3, the EAP-TLS
server MUST send one or more post-handshake NewSessionTicket messages
(each associated with a PSK, a PSK identity, a ticket lifetime, and
other parameters) in the initial authentication. Note that TLS 1.3
[RFC8446] limits the ticket lifetime to a maximum of 604800 seconds
(7 days) and EAP-TLS servers MUST respect this upper limit when
issuing tickets. The NewSessionTicket is sent after the EAP-TLS
server has received the client Finished message in the initial
authentication. The NewSessionTicket can be sent in the same flight
as the TLS server Finished or later. The PSK associated with the
ticket depends on the client Finished and cannot be pre-computed (so
as to be sent in the same flight as the TLS server Finished) in
handshakes with client authentication. The NewSessionTicket message
MUST NOT include an "early_data" extension. If the "early_data"
extension is received, then it MUST be ignored. Servers should take
into account that fewer NewSessionTickets will likely be needed in
EAP-TLS than in the usual HTTPS connection scenario. In most cases,
a single NewSessionTicket will be sufficient. A mechanism by which
clients can specify the desired number of tickets needed for future
connections is defined in [TICKET-REQUESTS].

Figure 2 shows an example message flow for a successful EAP-TLS full
handshake with mutual authentication and ticket establishment of a
single ticket.

EAP-TLS Peer EAP-TLS Server

EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
TLS CertificateRequest,
TLS Certificate,
TLS CertificateVerify,
<-------- TLS Finished)
EAP-Response/
EAP-Type=EAP-TLS
(TLS Certificate,
TLS CertificateVerify,
TLS Finished) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS NewSessionTicket,
<-------- (TLS Application Data 0x00)
EAP-Response/
EAP-Type=EAP-TLS -------->
<-------- EAP-Success

Figure 2: EAP-TLS Ticket Establishment

2.1.3. Resumption

This section updates Section 2.1.2 of [RFC5216] by amending it in
accordance with the following discussion.

EAP-TLS is typically used with client authentication and typically
fragments the TLS flights into a large number of EAP requests EAP-requests and EAP
responses.
EAP-responses. Resumption significantly reduces the number of round
trips and enables the EAP-TLS server to omit database lookups needed
during a full handshake with client authentication. TLS 1.3 replaces
the session resumption mechanisms in earlier versions of TLS with a
new PSK exchange. When EAP-TLS is used with TLS version 1.3, EAP-TLS
SHALL use a resumption mechanism compatible with version 1.3 of TLS.

For TLS 1.3, resumption is described in Section 2.2 of [RFC8446]. If
the client has received a NewSessionTicket message from the EAP-TLS
server, the client can use the PSK identity associated with the
ticket to negotiate the use of the associated PSK. If the EAP-TLS
server accepts it, then the resumed session has been deemed to be
authenticated and securely associated with the prior authentication
or resumption. It is up to the EAP-TLS peer to use resumption, but
it is RECOMMENDED that the EAP-TLS peer use resumption if it has a
valid ticket that has not been used before. It is left to the EAP-
TLS server whether to accept resumption, but it is RECOMMENDED that
the EAP-TLS server accept resumption if the ticket that was issued is
still valid. However, the EAP-TLS server MAY choose to require a
full handshake. In the case a full handshake is required, the
negotiation proceeds as if the session was a new authentication, and
the resumption attempt is ignored. The requirements of Sections
2.1.1 and 2.1.2 then apply in their entirety. As described in
Appendix C.4 of [RFC8446], reuse of a ticket allows passive observers
to correlate different connections. EAP-TLS peers and EAP-TLS
servers SHOULD follow the client tracking preventions in Appendix C.4
of [RFC8446].

It is RECOMMENDED to use Network Access Identifiers (NAIs) with the
same realm during resumption and the original full handshake. This
requirement allows EAP packets to be routed to the same destination
as the original full handshake. If this recommendation is not
followed, resumption is likely impossible. When NAI reuse can be
done without privacy implications, it is RECOMMENDED to use the same
NAI in the resumption as was used in the original full handshake
[RFC7542]. For example, the NAI @realm can safely be reused since it
does not provide any specific information to associate a user's
resumption attempt with the original full handshake. However,
reusing the NAI P2ZIM2F+OEVAO21nNWg2bVpgNnU=@realm enables an on-path
attacker to associate a resumption attempt with the original full
handshake. The TLS PSK identity is typically derived by the TLS
implementation and may be an opaque blob without a routable realm.
The TLS PSK identity on its own is therefore unsuitable as an NAI in
the Identity Response.

Figure 3 shows an example message flow for a subsequent successful
EAP-TLS resumption handshake where both sides authenticate via a PSK
provisioned via an earlier NewSessionTicket and where the server
provisions a single new ticket.

EAP-TLS Peer EAP-TLS Server

EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello
+ pre_shared_key) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
<-------- TLS Finished,
TLS NewSessionTicket)
EAP-Response/
EAP-Type=EAP-TLS
(TLS Finished) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Application Data 0x00)
EAP-Response/
EAP-Type=EAP-TLS -------->
<-------- EAP-Success

Figure 3: EAP-TLS Resumption

As specified in Section 2.2 of [RFC8446], the EAP-TLS peer SHOULD
supply a "key_share" extension when attempting resumption, which
allows the EAP-TLS server to potentially decline resumption and fall
back to a full handshake. If the EAP-TLS peer did not supply a
"key_share" extension when attempting resumption, the EAP-TLS server
needs to send a HelloRetryRequest to signal that additional
information is needed to complete the handshake, and the EAP-TLS peer
needs to send a second ClientHello containing that information.
Providing a "key_share" and using the "psk_dhe_ke" pre-shared key
exchange mode is also important in order to limit the impact of a key
compromise. When using "psk_dhe_ke", TLS 1.3 provides forward
secrecy meaning that compromise of the PSK used for resumption does
not compromise any earlier connections. The "psk_dh_ke" key exchange
mode MUST be used for resumption unless the deployment has a local
requirement to allow configuration of other mechanisms.

2.1.4. Termination

This section updates Section 2.1.3 of [RFC5216] by amending it in
accordance with the following discussion.

TLS 1.3 changes both the message flow and the handshake messages
compared to earlier versions of TLS. Therefore, some normative text
in Section 2.1.3 of [RFC5216] does not apply for TLS 1.3. The two
paragraphs below replace the corresponding paragraphs in
Section 2.1.3 of [RFC5216] when EAP-TLS is used with TLS 1.3. The
other paragraphs in Section 2.1.3 of [RFC5216] still apply with the
exception that SessionID is deprecated.

If the EAP-TLS peer authenticates successfully, the EAP-TLS server
MUST send an EAP-Request packet with EAP-Type=EAP-TLS containing
TLS records conforming to the version of TLS used. The message
flow ends with a protected success indication from the EAP-TLS
server, followed by an EAP-Response packet of EAP-Type=EAP-TLS and
no data from the EAP-TLS peer, followed by EAP-Success from the
server.

If the EAP-TLS server authenticates successfully, the EAP-TLS peer
MUST send an EAP-Response message with EAP-Type=EAP-TLS containing
TLS records conforming to the version of TLS used.

Figures 4, 5, and 6 illustrate message flows in several cases where
the EAP-TLS peer or EAP-TLS server sends a TLS Error alert message.
In earlier versions of TLS, error alerts could be warnings or fatal.
In TLS 1.3, error alerts are always fatal and the only alerts sent at
warning level are "close_notify" and "user_canceled", both of which
indicate that the connection is not going to continue normally; see
[RFC8446].

In TLS 1.3 [RFC8446], error alerts are not mandatory to send after a
fatal error condition. Failure to send TLS Error alerts means that
the peer or server would have no way of determining what went wrong.
EAP-TLS 1.3 strengthens this requirement. Whenever an implementation
encounters a fatal error condition, it MUST send an appropriate TLS
Error alert.

Figure 4 shows an example message flow where the EAP-TLS server
rejects the ClientHello with an error alert. The EAP-TLS server can
also partly reject the ClientHello with a HelloRetryRequest; see
Section 2.1.6.

EAP-TLS Peer EAP-TLS Server

Figure 4: EAP-TLS Server Rejection of ClientHello

Figure 5 shows an example message flow where EAP-TLS server
authentication is unsuccessful and the EAP-TLS peer sends a TLS Error
alert.

EAP-TLS Peer EAP-TLS Server

EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
TLS CertificateRequest,
TLS Certificate,
TLS CertificateVerify,
<-------- TLS Finished)
EAP-Response/
EAP-Type=EAP-TLS
(TLS Error Alert)
-------->
<-------- EAP-Failure

Figure 5: EAP-TLS Unsuccessful EAP-TLS Server Authentication

Figure 6 shows an example message flow where the EAP-TLS server
authenticates to the EAP-TLS peer successfully, but the EAP-TLS peer
fails to authenticate to the EAP-TLS server and the server sends a
TLS Error alert.

EAP-TLS Peer EAP-TLS Server

EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
TLS CertificateRequest,
TLS Certificate,
TLS CertificateVerify,
<-------- TLS Finished)
EAP-Response/
EAP-Type=EAP-TLS
(TLS Certificate,
TLS CertificateVerify,
TLS Finished) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Error Alert)
EAP-Response/
EAP-Type=EAP-TLS -------->
<-------- EAP-Failure

Figure 6: EAP-TLS Unsuccessful Client Authentication

2.1.5. No Peer Authentication

This is a new section when compared to [RFC5216].

Figure 7 shows an example message flow for a successful EAP-TLS full
handshake without peer authentication (e.g., emergency services, as
described in [RFC7406]).

EAP-TLS Peer EAP-TLS Server

EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
TLS Certificate,
TLS CertificateVerify,
<-------- TLS Finished)
EAP-Response/
EAP-Type=EAP-TLS
(TLS Finished) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Application Data 0x00)
EAP-Response/
EAP-Type=EAP-TLS -------->
<-------- EAP-Success

Figure 7: EAP-TLS without Peer Authentication

2.1.6. Hello Retry Request

This is a new section when compared to [RFC5216].

As defined in TLS 1.3 [RFC8446], EAP-TLS servers can send a
HelloRetryRequest message in response to a ClientHello if the EAP-TLS
server finds an acceptable set of parameters but the initial
ClientHello does not contain all the needed information to continue
the handshake. One use case is if the EAP-TLS server does not
support the groups in the "key_share" extension (or there is no
"key_share" extension) but supports one of the groups in the
"supported_groups" extension. In this case, the client should send a
new ClientHello with a "key_share" that the EAP-TLS server supports.

Figure 8 shows an example message flow for a successful EAP-TLS full
handshake with mutual authentication and HelloRetryRequest. Note the
extra round trip as a result of the HelloRetryRequest.

EAP-TLS Peer EAP-TLS Server

EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS HelloRetryRequest)
<--------
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
TLS CertificateRequest,
TLS Certificate,
TLS CertificateVerify,
TLS Finished)
EAP-Response/
EAP-Type=EAP-TLS
(TLS Certificate,
TLS CertificateVerify,
TLS Finished) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Application Data 0x00)
EAP-Response/
EAP-Type=EAP-TLS -------->
<-------- EAP-Success

Figure 8: EAP-TLS with Hello Retry Request

2.1.7. Identity

This is a new section when compared to [RFC5216].

It is RECOMMENDED to use anonymous NAIs [RFC7542] in the Identity
Response as such identities are routable and privacy-friendly. While
opaque blobs are allowed by [RFC3748], such identities are NOT
RECOMMENDED as they are not routable and should only be considered in
local deployments where the EAP-TLS peer, EAP authenticator, and EAP-
TLS server all belong to the same network. Many client certificates
contain an identity such as an email address, which is already in NAI
format. When the client certificate contains an NAI as subject name
or alternative subject name, an anonymous NAI SHOULD be derived from
the NAI in the certificate; see Section 2.1.8. More details on
identities are described in Sections 2.1.3, 2.1.8, 2.2, and 5.8.

2.1.8. Privacy

This section updates Section 2.1.4 of [RFC5216] by amending it in
accordance with the following discussion.

EAP-TLS 1.3 significantly improves privacy when compared to earlier
versions of EAP-TLS. EAP-TLS 1.3 forbids cipher suites without
confidentiality, which means that TLS 1.3 is always encrypting large
parts of the TLS handshake including the certificate messages.

EAP-TLS peer and server implementations supporting TLS 1.3 MUST
support anonymous Network Access Identifiers (NAIs) (Section 2.4 of
[RFC7542]) and a
[RFC7542]). A client supporting TLS 1.3 MUST NOT send its username
(or any other permanent identifiers) in cleartext in the Identity Response.
Response (or any message used instead of the Identity Response).
Following [RFC7542], it is RECOMMENDED to omit the username (i.e.,
the NAI is @realm), but other constructions such as a fixed username
(e.g., anonymous@realm) or an encrypted username (e.g.,
xCZINCPTK5+7y81CrSYbPg+RKPE3OTrYLn4AQc4AC2U=@realm) are allowed.
Note that the NAI MUST be a UTF-8 string as defined by the grammar in
Section 2.2 of [RFC7542].

The HelloRequest message used for privacy in EAP-TLS 1.2 does not
exist in TLS 1.3 but as the certificate messages in TLS 1.3 are
encrypted, there is no need to send an empty certificate_list and
perform a second handshake for privacy (as needed by EAP-TLS with
earlier versions of TLS). When EAP-TLS is used with TLS version 1.3,
the EAP-TLS peer and EAP-TLS server SHALL follow the processing
specified by version 1.3 of TLS. This means that the EAP-TLS peer
only sends an empty certificate_list if it does not have an
appropriate certificate to send, and the EAP-TLS server MAY treat an
empty certificate_list as a terminal condition.

EAP-TLS with TLS 1.3 is always used with privacy. This does not add
any extra round trips and the message flow with privacy is just the
normal message flow as shown in Figure 1.

2.1.9. Fragmentation

This section updates Section 2.1.5 of [RFC5216] by amending it in
accordance with the following discussion.

Including ContentType (1 byte), ProtocolVersion (2 bytes), and length
(2 bytes) headers, a single TLS record may be up to 16645 octets in
length. EAP-TLS fragmentation support is provided through addition
of a flags octet within the EAP-Response and EAP-Request packets, as
well as a (conditional) TLS Message Length field of four octets.
Implementations MUST NOT set the L bit in unfragmented messages, but
they MUST accept unfragmented messages with and without the L bit
set.

Some EAP implementations and access networks may limit the number of
EAP packet exchanges that can be handled. To avoid fragmentation, it
is RECOMMENDED to keep the sizes of EAP-TLS peer, EAP-TLS server, and
trust anchor certificates small and the length of the certificate
chains short. In addition, it is RECOMMENDED to use mechanisms that
reduce the sizes of Certificate messages. For a detailed discussion
on reducing message sizes to prevent fragmentation, see [RFC9191].

2.2. Identity Verification

This section updates replaces Section 2.2 of [RFC5216] by amending it in
accordance with the following
discussion. The guidance in this section is relevant for EAP-TLS in
general (regardless of the underlying TLS version used).

The EAP peer identity provided in the EAP-Response/Identity is not
authenticated by EAP-TLS. Unauthenticated information MUST NOT be
used for accounting purposes or to give authorization. The
authenticator and the EAP-TLS server MAY examine the identity
presented in EAP-Response/Identity for purposes such as routing and
EAP method selection. EAP-TLS servers MAY reject conversations if
the identity does not match their policy. Note that this also
applies to resumption; see Sections 2.1.3, 5.6, and 5.7.

The EAP server identity in the TLS server certificate is typically a
fully qualified domain name (FQDN) in the SubjectAltName (SAN)
extension. Since EAP-TLS deployments may use more than one EAP
server, each with a different certificate, EAP peer implementations
SHOULD allow for the configuration of one or more trusted root
certificates (CA certificate) to authenticate the server certificate
and one or more server names to match against the SubjectAltName
(SAN) extension in the server certificate. If any of the configured
names match any of the names in the SAN extension, then the name
check passes. To simplify name matching, an EAP-TLS deployment can
assign a name to represent an authorized EAP server and EAP Server
certificates can include this name in the list of SANs for each
certificate that represents an EAP-TLS server. If server name
matching is not used, then it degrades the confidence that the EAP
server with which it is interacting is authoritative for the given
network. If name matching is not used with a public root CA, then
effectively any server can obtain a certificate that will be trusted
for EAP authentication by the Peer. peer. While this guidance to verify
domain names is new, and was not mentioned in [RFC5216], it has been
widely implemented in EAP-TLS peers. As such, it is believed that
this section contains minimal new interoperability or implementation
requirements on EAP-TLS peers and can be applied to earlier versions
of TLS.

The process of configuring a root CA certificate and a server name is
non-trivial; therefore, automated methods of provisioning are
RECOMMENDED. For example, the eduroam federation [RFC7593] provides
a Configuration Assistant Tool (CAT) to automate the configuration
process. In the absence of a trusted root CA certificate (user
configured or system-wide), EAP peers MAY implement a trust on first
use (TOFU) mechanism where the peer trusts and stores the server
certificate during the first connection attempt. The EAP peer
ensures that the server presents the same stored certificate on
subsequent interactions. Use of a TOFU mechanism does not allow for
the server certificate to change without out-of-band validation of
the certificate and is therefore not suitable for many deployments
including ones where multiple EAP servers are deployed for high
availability. TOFU mechanisms increase the susceptibility to traffic
interception attacks and should only be used if there are adequate
controls in place to mitigate this risk.

2.3. Key Hierarchy

This section updates Section 2.3 of [RFC5216] by replacing it in
accordance with the following discussion.

TLS 1.3 replaces the TLS pseudorandom function (PRF) used in earlier
versions of TLS with the HMAC-based Key Derivation Function (HKDF)
and completely changes the key schedule. The key hierarchies shown
in Section 2.3 of [RFC5216] are therefore not correct when EAP-TLS is
used with TLS version 1.3. For TLS 1.3 the key schedule is described
in Section 7.1 of [RFC8446].

When EAP-TLS is used with TLS version 1.3, the Key_Material and
Method-Id SHALL be derived from the exporter_secret using the TLS
exporter interface [RFC5705] (for TLS 1.3, this is defined in
Section 7.5 of [RFC8446]). Type is the value of the EAP Type field
defined in Section 2 of [RFC3748]. For EAP-TLS, the Type field has
value 0x0D.

Type = 0x0D
Key_Material = TLS-Exporter("EXPORTER_EAP_TLS_Key_Material",
Type, 128)
Method-Id = TLS-Exporter("EXPORTER_EAP_TLS_Method-Id",
Type, 64)
Session-Id = Type || Method-Id

The Master Session Key (MSK) MSK and Extended Master Session Key (EMSK) EMSK are derived from the Key_Material in the same manner
as with EAP-TLS [RFC5216], Section 2.3. The definitions are repeated
below for simplicity:

MSK = Key_Material(0, 63)
EMSK = Key_Material(64, 127)

Other TLS-based EAP methods can use the TLS exporter in a similar
fashion; see [TLS-EAP-TYPES].

[RFC5247] deprecates the use of an Initialization Vector (IV). Thus,
RECV-IV and SEND-IV are not exported in EAP-TLS with TLS 1.3. As
noted in [RFC5247], lower layers use the MSK in a lower-layer-
dependent manner. EAP-TLS with TLS 1.3 exports the MSK and does not
specify how it is used by lower layers.

Note that the key derivation MUST use the length values given above.
While in TLS 1.2 and earlier it was possible to truncate the output
by requesting less data from the TLS-Exporter function, this practice
is not possible with TLS 1.3. If an implementation intends to use
only a part of the output of the TLS-Exporter function, then it MUST
ask for the full output and then only use the desired part. Failure
to do so will result in incorrect values being calculated for the
above keying material.

By using the TLS exporter, EAP-TLS can use any TLS 1.3 implementation
that provides a public API for the exporter. Note that when TLS 1.2
is used with the EAP-TLS exporter [RFC5705] it generates the same key
material as in EAP-TLS [RFC5216].

2.4. Parameter Negotiation and Compliance Requirements

This section updates Section 2.4 of [RFC5216] by amending it in
accordance with the following discussion.

TLS 1.3 cipher suites are defined differently than in earlier
versions of TLS (see Appendix B.4 of [RFC8446]), and the cipher
suites discussed in Section 2.4 of [RFC5216] can therefore not be
used when EAP-TLS is used with TLS version 1.3.

When EAP-TLS is used with TLS version 1.3, the EAP-TLS peers and EAP-
TLS servers MUST comply with the compliance requirements (mandatory-
to-implement cipher suites, signature algorithms, key exchange
algorithms, extensions, etc.) defined in Section 9 of [RFC8446]. In
EAP-TLS with TLS 1.3, only cipher suites with confidentiality SHALL
be supported.

While EAP-TLS does not protect any application data except for the
0x00 byte that serves as protected success indication, the negotiated
cipher suites and algorithms MAY be used to secure data as done in
other TLS-based EAP methods.

2.5. EAP State Machines

This is a new section when compared to [RFC5216] and only applies to
TLS 1.3. [RFC4137] offers a proposed state machine for EAP.

TLS 1.3 [RFC8446] introduces post-handshake messages. These post-
handshake messages use the handshake content type and can be sent
after the main handshake. Examples of post-handshake messages are
NewSessionTicket, which is used for resumption and KeyUpdate, which
is not useful and not expected in EAP-TLS. After sending TLS
Finished, the EAP-TLS server may send any number of post-handshake
messages in one or more EAP-Requests.

To provide a protected success result indication and to decrease the
uncertainty for the EAP-TLS peer, the following procedure MUST be
followed:

When an EAP-TLS server has successfully processed the TLS client
Finished and sent its last handshake message (Finished or a post-
handshake message), it sends an encrypted TLS record with application
data 0x00. The encrypted TLS record with application data 0x00 is a
protected success result indication, as defined in [RFC3748]. After
sending an EAP-Request that contains the protected success result
indication, the EAP-TLS server must not send any more EAP-Requests
and may only send an EAP-Success. The EAP-TLS server MUST NOT send
an encrypted TLS record with application data 0x00 alert before it has
successfully processed the client Finished and sent its last
handshake message.

TLS Error alerts SHOULD be considered a failure result indication, as
defined in [RFC3748]. Implementations following [RFC4137] set the
alternate indication of failure variable altReject after sending or
receiving an error alert. After sending or receiving a TLS Error
alert, the EAP-TLS server may only send an EAP-Failure. Protected
TLS Error alerts are protected failure result indications, and
unprotected TLS Error alerts are not.

The keying material can be derived after the TLS server Finished has
been sent or received. Implementations following [RFC4137] can then
set the eapKeyData and aaaEapKeyData variables.

The keying material can be made available to lower layers and the
authenticator after the authenticated success result indication has
been sent or received. Implementations following [RFC4137] can set
the eapKeyAvailable and aaaEapKeyAvailable variables.

3. Detailed Description of the EAP-TLS Protocol

There are no updates to Section 3 of [RFC5216].

4. IANA Considerations

This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to EAP-TLS
1.3 in accordance with [RFC8126].

Per this document, IANA has added the following labels to the "TLS
Exporter Labels" registry defined by [RFC5705]. These labels are
used in derivation of Key_Material and Method-Id as defined in
Section 2.3:

+===============================+=========+=============+======+
| Value | DTLS-OK | Recommended | Note |
+===============================+=========+=============+======+
| EXPORTER_EAP_TLS_Key_Material | N | Y | |
+-------------------------------+---------+-------------+------+
| EXPORTER_EAP_TLS_Method-Id | N | Y | |
+-------------------------------+---------+-------------+------+

Table 1: TLS Exporter Labels

5. Security Considerations

The security considerations of TLS 1.3 [RFC8446] apply to EAP-TLS
1.3.

5.1. Security Claims

Using EAP-TLS with TLS 1.3 does not change the security claims for
EAP-TLS as given in Section 5.1 of [RFC5216]. However, it
strengthens several of the claims as described in the following
updates to the notes given in Section 5.1 of [RFC5216].

[1] Mutual authentication: By mandating revocation checking of
certificates, the authentication in EAP-TLS with TLS 1.3 is
stronger as authentication with revoked certificates will always
fail.

[2] Confidentiality: The TLS 1.3 handshake offers much better
confidentiality than earlier versions of TLS. EAP-TLS with TLS
1.3 mandates use of cipher suites that ensure confidentiality.
TLS 1.3 also encrypts certificates and some of the extensions.
When using EAP-TLS with TLS 1.3, the use of privacy is mandatory
and does not cause any additional round trips.

[3] Cryptographic strength: TLS 1.3 only defines strong algorithms
without major weaknesses and EAP-TLS with TLS 1.3 always provides
forward secrecy; see [RFC8446]. Weak algorithms such as 3DES,
CBC mode, RC4, SHA-1, MD5, P-192, and RSA-1024 have not been
registered for use in TLS 1.3.

[4] Cryptographic negotiation: The TLS layer handles the negotiation
of cryptographic parameters. When EAP-TLS is used with TLS 1.3,
EAP-TLS inherits the cryptographic negotiation of the AEAD
algorithm, HKDF hash algorithm, key exchange groups, and
signature algorithm; see Section 4.1.1 of [RFC8446].

5.2. Peer and Server Identities

No updates to Section 5.2 of [RFC5216]. Note that Section 2.2 has
additional discussion on identities.

5.3. Certificate Validation

No updates to Section 5.3 of [RFC5216]. In addition to Section 5.3
of [RFC5216], guidance on server certificate validation can be found
in [RFC6125].

5.4. Certificate Revocation

This section updates Section 5.4 of [RFC5216] by amending it in
accordance with the following discussion.

There are a number of reasons (e.g., key compromise, CA compromise,
privilege withdrawn, etc.) why EAP-TLS peer, EAP-TLS server, or sub-
CA certificates have to be revoked before their expiry date.
Revocation of the EAP-TLS server's certificate is complicated by the
fact that the EAP-TLS peer may not have Internet connectivity until
authentication completes.

When EAP-TLS is used with TLS 1.3, the revocation status of all the
certificates in the certificate chains MUST be checked (except the
trust anchor). An implementation may use the Certificate Revocation
List (CRL), Online Certificate Status Protocol (OSCP), or other
standardized/proprietary methods for revocation checking. Examples
of proprietary methods are non-standard formats for distribution of
revocation lists as well as certificates with very short lifetime.

EAP-TLS servers supporting TLS 1.3 MUST implement Certificate Status
Requests (OCSP stapling) as specified in [RFC6066] and
Section 4.4.2.1 of [RFC8446]. It is RECOMMENDED that EAP-TLS peers
and EAP-TLS servers use OCSP stapling for verifying the status of the
EAP-TLS server's certificate chain. When an EAP-TLS peer uses
Certificate Status Requests to check the revocation status of the
EAP-TLS server's certificate chain, it MUST treat a CertificateEntry
(except
(but not the trust anchor) without a valid CertificateStatus
extension as invalid and abort the handshake with an appropriate
alert. The OCSP status handling in TLS 1.3 is different from earlier
versions of TLS; see Section 4.4.2.1 of [RFC8446]. In TLS 1.3, the
OCSP information is carried in the CertificateEntry containing the
associated certificate instead of a separate CertificateStatus
message as in [RFC6066]. This enables sending OCSP information for
all certificates in the certificate chain (except the trust anchor).

To enable revocation checking in situations where EAP-TLS peers do
not implement or use OCSP stapling, and where network connectivity is
not available prior to authentication completion, EAP-TLS peer
implementations MUST also support checking for certificate revocation
after authentication completes and network connectivity is available.
An EAP peer implementation SHOULD NOT trust the network (and any
services) until it has verified the revocation status of the server
certificate after receiving network connectivity. An EAP peer MUST
use a secure transport to verify the revocation status of the server
certificate. An EAP peer SHOULD NOT send any other traffic before
revocation checking for the server certificate is complete.

5.5. Packet Modification Attacks

This section updates Section 5.5 of [RFC5216] by amending it in
accordance with the following discussion.

As described in [RFC3748] and Section 5.5 of [RFC5216], the only
information that is integrity and replay protected in EAP-TLS are the
parts of the TLS Data that TLS protects. All other information in
the EAP-TLS message exchange including EAP-Request and EAP-Response
headers, the identity in the Identity Response, EAP-TLS packet header
fields, Type, Flags, EAP-Success, and EAP-Failure can be modified,
spoofed, or replayed.

Protected TLS Error alerts are protected failure result indications
and enable the EAP-TLS peer and EAP-TLS server to determine that the
failure result was not spoofed by an attacker. Protected failure
result indications provide integrity and replay protection but MAY be
unauthenticated. Protected failure results do not significantly
improve availability as TLS 1.3 treats most malformed data as a fatal
error.

5.6. Authorization

This is a new section when compared to [RFC5216]. The guidance in
this section is relevant for EAP-TLS in general (regardless of the
underlying TLS version used).

EAP servers will usually require the EAP peer to provide a valid
certificate and will fail the connection if one is not provided.
Some deployments may permit no peer authentication for some or all
connections. When peer authentication is not used, EAP-TLS server
implementations MUST take care to limit network access appropriately
for unauthenticated peers, and implementations MUST use resumption
with caution to ensure that a resumed session is not granted more
privilege than was intended for the original session. An example of
limiting network access would be to invoke a vendor's walled garden
or quarantine network functionality.

EAP-TLS is typically encapsulated in other protocols such as PPP
[RFC1661], RADIUS [RFC2865], Diameter [RFC6733], or the Protocol for
Carrying Authentication for Network Access (PANA) [RFC5191]. The
encapsulating protocols can also provide additional, non-EAP
information to an EAP-TLS server. This information can include, but
is not limited to, information about the authenticator, information
about the EAP-TLS peer, or information about the protocol layers
above or below EAP (MAC addresses, IP addresses, port numbers, Wi-Fi
Service Set Identifiers (SSIDs), etc.). EAP-TLS servers implementing
EAP-TLS inside those protocols can make policy decisions and enforce
authorization based on a combination of information from the EAP-TLS
exchange and non-EAP information.

As noted in Section 2.2, the identity presented in EAP-Response/
Identity is not authenticated by EAP-TLS and is therefore trivial for
an attacker to forge, modify, or replay. Authorization and
accounting MUST be based on authenticated information such as
information in the certificate or the PSK identity and cached data
provisioned for resumption as described in Section 5.7. Note that
the requirements for Network Access Identifiers (NAIs) specified in
Section 4 of [RFC7542] still apply and MUST be followed.

EAP-TLS servers MAY reject conversations based on non-EAP information
provided by the encapsulating protocol, for example if the MAC
address of the authenticator does not match the expected policy.

In addition to allowing configuration of one or more trusted root
certificates (CA certificate) to authenticate the server certificate
and one or more server names to match against the SubjectAltName
(SAN) extension, EAP peer implementations MAY allow binding the
configured acceptable SAN to a specific CA (or CAs) that should have
issued the server certificate to prevent attacks from rogue or
compromised CAs.

5.7. Resumption

This is a new section when compared to [RFC5216]. The guidance in
this section is relevant for EAP-TLS in general (regardless of the
underlying TLS version used).

There are a number of security issues related to resumption that are
not described in [RFC5216]. The problems, guidelines, and
requirements in this section therefore apply to EAP-TLS when it is
used with any version of TLS.

When resumption occurs, it is based on cached information at the TLS
layer. To perform resumption securely, the EAP-TLS peer and EAP-TLS
server need to be able to securely retrieve authorization information
such as certificate chains from the initial full handshake. This
document uses the term "cached data" to describe such information.
Authorization during resumption MUST be based on such cached data.
The EAP-TLS peer and EAP-TLS server MAY perform fresh revocation
checks on the cached certificate data. Any security policies for
authorization MUST be followed also for resumption. The certificates
may have been revoked since the initial full handshake and the
authorizations of the other party may have been reduced. If the
cached revocation data is not sufficiently current, the EAP-TLS peer
or EAP-TLS server MAY force a full TLS handshake.

There are two ways to retrieve the cached data from the original full
handshake. The first method is that the EAP-TLS server and client
cache the information locally. The cached information is identified
by an identifier. For TLS versions before 1.3, the identifier can be
the session ID; for TLS 1.3, the identifier is the PSK identity. The
second method for retrieving cached information is via [RFC5077] or
[RFC8446], where the EAP-TLS server avoids storing information
locally and instead encapsulates the information into a ticket that
is sent to the client for storage. This ticket is encrypted using a
key that only the EAP-TLS server knows. Note that the client still
needs to cache the original handshake information locally and will
obtain it while determining the session ID or PSK identity to use for
resumption. However, the EAP-TLS server is able to decrypt the
ticket or PSK to obtain the original handshake information.

The EAP-TLS server or EAP client MUST cache data during the initial
full handshake sufficient to allow authorization decisions to be made
during resumption. If cached data cannot be retrieved securely,
resumption MUST NOT be done.

The above requirements also apply if the EAP-TLS server expects some
system to perform accounting for the session. Since accounting must
be tied to an authenticated identity, and resumption does not supply
such an identity, accounting is impossible without access to cached
data. Therefore, systems that expect to perform accounting for the
session SHOULD cache an identifier that can be used in subsequent
accounting.

As suggested in [RFC8446], EAP-TLS peers MUST NOT store resumption
PSKs or tickets (and associated cached data) for longer than 604800
seconds (7 days) regardless of the PSK or ticket lifetime. The EAP-
TLS peer MAY delete them earlier based on local policy. The cached
data MAY also be removed on the EAP-TLS server or EAP-TLS peer if any
certificate in the certificate chain has been revoked or has expired.
In all such cases, an attempt at resumption results in a full TLS
handshake instead.

Information from the EAP-TLS exchange (e.g., the identity provided in
EAP-Response/Identity) as well as non-EAP information (e.g., IP
addresses) may change between the initial full handshake and
resumption. This change creates a "time-of-check time-of-use"
(TOCTOU) security vulnerability. A malicious or compromised user
could supply one set of data during the initial authentication, and a
different set of data during resumption, potentially allowing them to
obtain access that they should not have.

If any authorization, accounting, or policy decisions were made with
information that has changed between the initial full handshake and
resumption, and if change may lead to a different decision, such
decisions MUST be reevaluated. It is RECOMMENDED that authorization,
accounting, and policy decisions are reevaluated based on the
information given in the resumption. EAP-TLS servers MAY reject
resumption where the information supplied during resumption does not
match the information supplied during the original authentication.
If a safe decision is not possible, EAP-TLS servers SHOULD reject the
resumption and continue with a full handshake.

Sections 2.2 and 4.2.11 of [RFC8446] provide security considerations
for TLS 1.3 resumption.

5.8. Privacy Considerations

This is a new section when compared to [RFC5216].

TLS 1.3 offers much better privacy than earlier versions of TLS as
discussed in Section 2.1.8. In this section, we only discuss the
privacy properties of EAP-TLS with TLS 1.3. For privacy properties
of TLS 1.3 itself, see [RFC8446].

EAP-TLS sends the standard TLS 1.3 handshake messages encapsulated in
EAP packets. Additionally, the EAP-TLS peer sends an identity in the
first EAP-Response. The other fields in the EAP-TLS Request and the
EAP-TLS Response packets do not contain any cleartext privacy-
sensitive information.

Tracking of users by eavesdropping on Identity Responses or
certificates is a well-known problem in many EAP methods. When EAP-
TLS is used with TLS 1.3, all certificates are encrypted, and the
username part of the Identity Response is not revealed (e.g., using
anonymous NAIs). Note that even though all certificates are
encrypted, the server's identity is only protected against passive
attackers while the client's identity is protected against both
passive and active attackers. As with other EAP methods, even when
privacy-friendly identifiers or EAP tunneling is used, the domain
name (i.e., the realm) in the NAI is still typically visible. How
much privacy-sensitive information the domain name leaks is highly
dependent on how many other users are using the same domain name in
the particular access network. If all EAP-TLS peers have the same
domain, no additional information is leaked. If a domain name is
used by a small subset of the EAP-TLS peers, it may aid an attacker
in tracking or identifying the user.

Without padding, information about the size of the client certificate
is leaked from the size of the EAP-TLS packets. The EAP-TLS packets
sizes may therefore leak information that can be used to track or
identify the user. If all client certificates have the same length,
no information is leaked. EAP-TLS peers SHOULD use record padding;
see Section 5.4 of [RFC8446] to reduce information leakage of
certificate sizes.

If anonymous NAIs are not used, the privacy-friendly identifiers need
to be generated with care. The identities MUST be generated in a
cryptographically secure way so that it is computationally infeasible
for an attacker to differentiate two identities belonging to the same
user from two identities belonging to different users in the same
realm. This can be achieved, for instance, by using random or
pseudo-random usernames such as random byte strings or ciphertexts
and only using the pseudo-random usernames a single time. Note that
the privacy-friendly usernames also MUST NOT include substrings that
can be used to relate the identity to a specific user. Similarly,
privacy-friendly usernames MUST NOT be formed by a fixed mapping that
stays the same across multiple different authentications.

An EAP-TLS peer with a policy allowing communication with EAP-TLS
servers supporting only TLS 1.2 without privacy and with a static RSA
key exchange is vulnerable to disclosure of the EAP-TLS peer
username. An active attacker can in this case make the EAP-TLS peer
believe that an EAP-TLS server supporting TLS 1.3 only supports TLS
1.2 without privacy. The attacker can simply impersonate the EAP-TLS
server and negotiate TLS 1.2 with static RSA key exchange and send a
TLS alert message when the EAP-TLS peer tries to use privacy by
sending an empty certificate message. Since the attacker
(impersonating the EAP-TLS server) does not provide a proof-of-
possession of the private key until the Finished message when a
static RSA key exchange is used, an EAP-TLS peer may inadvertently
disclose its identity (username) to an attacker. Therefore, it is
RECOMMENDED for EAP-TLS peers to not use EAP-TLS with TLS 1.2 and
static RSA-based cipher suites without privacy. This implies that an
EAP-TLS peer SHOULD NOT continue the EAP authentication attempt if a
TLS 1.2 EAP-TLS server sends an EAP-TLS/Request with a TLS alert
message in response to an empty certificate message from the peer.

5.9. Pervasive Monitoring

This is a new section when compared to [RFC5216].

Pervasive monitoring refers to widespread surveillance of users. In
the context of EAP-TLS, pervasive monitoring attacks can target EAP-
TLS peer devices for tracking them (and their users) as and when they join a
network. By encrypting more information, mandating the use of
privacy, and always providing forward secrecy, EAP-TLS with TLS 1.3
offers much better protection against pervasive monitoring. In
addition to the privacy attacks discussed above, surveillance on a
large scale may enable tracking of a user over a wide geographical
area and across different access networks. Using information from
EAP-TLS together with information gathered from other protocols
increases the risk of identifying individual users.

In TLS 1.3, the post-handshake key update mechanism provides forward
secrecy for the traffic secrets. EAP-TLS 1.3 does not provide a
similar mechanism for MSK and EMSK. Implementation using the
exported MSK and EMSK can achieve forward secrecy by frequently
deriving new keys in a similar way as described in Section 7.2 of
[RFC8446].

5.10. Discovered Vulnerabilities

This is a new section when compared to [RFC5216].

Over the years, there have been several serious attacks on earlier
versions of Transport Layer Security (TLS), including attacks on its
most commonly used ciphers and modes of operation. [RFC7457]
summarizes the attacks that were known at the time of publishing, and
BCP 195 [RFC7525] [RFC8996] provides recommendations and requirements
for improving the security of deployed services that use TLS.
However, many of the attacks are less serious for EAP-TLS as EAP-TLS
only uses the TLS handshake and does not protect any application
data. EAP-TLS implementations MUST mitigate known attacks. EAP-TLS
implementations need to monitor and follow new EAP- and TLS-related
security guidance and requirements such as [RFC8447] and [RFC9155].

5.11. Cross-Protocol Attacks

This is a new section when compared to [RFC5216].

Allowing the same certificate to be used in multiple protocols can
potentially allow an attacker to authenticate via one protocol and
then "resume" that session in another protocol. Section 2.2 suggests
that certificates typically have one or more FQDNs in the SAN
extension. However, those fields are for EAP validation only and do
not indicate that the certificates are suitable for use on the WWW with HTTPS or
other protocol servers protocols on the named host.

Section 2.1.3 suggests that authorization rules should be reapplied
on resumption but does not mandate this behavior. As a result, this
cross-protocol resumption could allow the attacker to bypass
authorization policies and to obtain undesired access to secured
systems. Along with making sure that appropriate authorization
information is available and used during resumption, using different
certificates and resumption caches for different protocols is
RECOMMENDED to help keep different protocol usages separate.

6. References

6.1. 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>.

[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>.

[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.

[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>.

[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,
<https://www.rfc-editor.org/info/rfc6960>.

[RFC7542] DeKok, A., "The Network Access Identifier", RFC 7542,
DOI 10.17487/RFC7542, May 2015,
<https://www.rfc-editor.org/info/rfc7542>.

[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>.

6.2. Informative references

[IEEE-802.11]
IEEE, "IEEE Standard for Information technology-
Telecommunications and information exchange between
systems Local and metropolitan area networks-Specific
requirements - Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications", IEEE
Std. 802.11-2020, DOI 10.1109/IEEESTD.2016.7786995,
February 2021,
<https://doi.org/10.1109/IEEESTD.2016.7786995>.

[IEEE-802.1AE]
IEEE, "IEEE Standard for Local and metropolitan area
networks -- Media Access Control (MAC) Security", IEEE
Std. 802.1AE-2018, DOI 10.1109/IEEESTD.2018.8585421,
December 2018,
<https://doi.org/10.1109/IEEESTD.2018.8585421>.

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

[MulteFire]
MulteFire Alliance, "MulteFire Release 1.1 Specification",
2019.

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

[RFC1661] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)",
STD 51, RFC 1661, DOI 10.17487/RFC1661, July 1994,
<https://www.rfc-editor.org/info/rfc1661>.

[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, DOI 10.17487/RFC2246, January 1999,
<https://www.rfc-editor.org/info/rfc2246>.

[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560,
DOI 10.17487/RFC2560, June 1999,
<https://www.rfc-editor.org/info/rfc2560>.

[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>.

[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
DOI 10.17487/RFC3280, April 2002,
<https://www.rfc-editor.org/info/rfc3280>.

[RFC4137] Vollbrecht, J., Eronen, P., Petroni, N., and Y. Ohba,
"State Machines for Extensible Authentication Protocol
(EAP) Peer and Authenticator", RFC 4137,
DOI 10.17487/RFC4137, August 2005,
<https://www.rfc-editor.org/info/rfc4137>.

[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282,
DOI 10.17487/RFC4282, December 2005,
<https://www.rfc-editor.org/info/rfc4282>.

[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346,
DOI 10.17487/RFC4346, April 2006,
<https://www.rfc-editor.org/info/rfc4346>.

[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>.

[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>.

[RFC5191] Forsberg, D., Ohba, Y., Ed., Patil, B., Tschofenig, H.,
and A. Yegin, "Protocol for Carrying Authentication for
Network Access (PANA)", RFC 5191, DOI 10.17487/RFC5191,
May 2008, <https://www.rfc-editor.org/info/rfc5191>.

[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>.

[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management Framework",
RFC 5247, DOI 10.17487/RFC5247, August 2008,
<https://www.rfc-editor.org/info/rfc5247>.

[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>.

[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.

[RFC6733] Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
Ed., "Diameter Base Protocol", RFC 6733,
DOI 10.17487/RFC6733, October 2012,
<https://www.rfc-editor.org/info/rfc6733>.

[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>.

[RFC7406] Schulzrinne, H., McCann, S., Bajko, G., Tschofenig, H.,
and D. Kroeselberg, "Extensions to the Emergency Services
Architecture for Dealing With Unauthenticated and
Unauthorized Devices", RFC 7406, DOI 10.17487/RFC7406,
December 2014, <https://www.rfc-editor.org/info/rfc7406>.

[RFC7457] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Known Attacks on Transport Layer Security (TLS) and
Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457,
February 2015, <https://www.rfc-editor.org/info/rfc7457>.

[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.

[RFC7593] Wierenga, K., Winter, S., and T. Wolniewicz, "The eduroam
Architecture for Network Roaming", RFC 7593,
DOI 10.17487/RFC7593, September 2015,
<https://www.rfc-editor.org/info/rfc7593>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/info/rfc8447>.

[RFC8996] Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
<https://www.rfc-editor.org/info/rfc8996>.

[RFC9155] Velvindron, L., Moriarty, K., and A. Ghedini, "Deprecating
MD5 and SHA-1 Signature Hashes in TLS 1.2 and DTLS 1.2",
RFC 9155, DOI 10.17487/RFC9155, December 2021,
<https://www.rfc-editor.org/info/rfc9155>.

[RFC9191] Sethi, M., Preuß Mattsson, J., and S. Turner, "Handling
Large Certificates and Long Certificate Chains in TLS-
Based EAP Methods", RFC 9191, DOI 10.17487/RFC9191,
January
February 2022, <https://www.rfc-editor.org/rfc/rfc9191>.

[TICKET-REQUESTS]
Pauly, T., Schinazi, D., and C. A. Wood, "TLS Ticket
Requests", Work in Progress, Internet-Draft, draft-ietf-
tls-ticketrequests-07, 3 December 2020,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
ticketrequests-07>.

[TLS-bis] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", Work in Progress, Internet-Draft, draft-
ietf-tls-rfc8446bis-03, 25 October 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
rfc8446bis-03>.

[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,
21 January 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-emu-tls-eap-types-04>.

[TS.33.501]
3GPP, "Security architecture and procedures for 5G
System",
system", Release 17, TS 33.501, September 2021. January 2022.

Appendix A. Updated References

The following references in [RFC5216] are updated as specified below
when EAP-TLS is used with TLS 1.3.

* All references to [RFC2560] are updated to refer to [RFC6960].

* All references to [RFC3280] are updated to refer to [RFC5280].
References to Section 4.2.1.13 of [RFC3280] are updated to refer
to Section 4.2.1.12 of [RFC5280].

* All references to [RFC4282] are updated to refer to [RFC7542].
References to Section 2.1 of [RFC4282] are updated to refer to
Section 2.2 of [RFC7542].

Acknowledgments

The authors want to thank Bernard Aboba, Jari Arkko, Terry Burton,
Alan DeKok, Ari Keränen, Benjamin Kaduk, Jouni Malinen, Oleg Pekar,
Eric Rescorla, Jim Schaad, Joseph Salowey, Martin Thomson, Vesa
Torvinen, Hannes Tschofenig, and Heikki Vatiainen for comments and
suggestions on this document. Special thanks to the Document
Shepherd Joseph Salowey.

Contributors

Alan DeKok, FreeRADIUS

Authors' Addresses

John Preuß Mattsson
Ericsson
SE-164 40 Stockholm Kista
Sweden

Email: john.mattsson@ericsson.com

Mohit Sethi
Ericsson
FI-02420 Jorvas
Finland

Email: mohit@piuha.net mohit@iki.fi