wdiff rfc9813.original rfc9813.txt

RADEXT Working Group

Internet Engineering Task Force (IETF) A. DeKok
Internet-Draft
Request for Comments: 9813 InkBridge Networks
Intended status:
BCP: 243 June 2025
Category: Best Current Practice 21 January 2025
Expires: 25 July 2025
ISSN: 2070-1721

Operational Considerations for RADIUS and TLS-PSK
draft-ietf-radext-tls-psk-12 TLS Pre-Shared Key (TLS-PSK)

Abstract

This document provides implementation and operational considerations
for using TLS-PSK TLS Pre-Shared Keys (TLS-PSKs) with RADIUS/TLS (RFC6614) (RFC 6614)
and RADIUS/DTLS
(RFC7360). (RFC 7360). The purpose of the document is to help
smooth the operational transition from the use of the RADIUS/UDP to
RADIUS/TLS.

About This Document

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

Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-radext-tls-psk/.

Discussion of this document takes place on the RADEXT Working Group
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Source for this draft and an issue tracker can be found at
https://github.com/radext-wg/radext-tls-psk.git.

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This Internet-Draft will expire on 25 July 2025.
https://www.rfc-editor.org/info/rfc9813.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Justification of PSK. . . . . . . . . . . . . . . . . . . . . 4 PSK
4. General Discussion of PSKs and PSK Identities . . . . . . . . 5
4.1. Guidance for PSKs . . . . . . . . . . . . . . . . . . . . 5
4.1.1. PSK Requirements . . . . . . . . . . . . . . . . . . 5
4.1.2. Usability Guidance . . . . . . . . . . . . . . . . . 7
4.1.3. Interaction between Between PSKs and RADIUS Shared Secrets . 8
4.2. PSK Identities . . . . . . . . . . . . . . . . . . . . . 9
4.2.1. Security of PSK Identities . . . . . . . . . . . . . 11
4.3. PSK and PSK Identity Sharing . . . . . . . . . . . . . . 13
4.4. PSK Lifetimes . . . . . . . . . . . . . . . . . . . . . . 13
5. Guidance for RADIUS Clients . . . . . . . . . . . . . . . . . 14
5.1. PSK Identities . . . . . . . . . . . . . . . . . . . . . 14
5.1.1. PSK Identity Requirements . . . . . . . . . . . . . . 14
5.1.2. Usability Guidance . . . . . . . . . . . . . . . . . 15
6. Guidance for RADIUS Servers . . . . . . . . . . . . . . . . . 15
6.1. Current Practices . . . . . . . . . . . . . . . . . . . . 16
6.2. Practices for TLS-PSK . . . . . . . . . . . . . . . . . . 16
6.2.1. IP Filtering . . . . . . . . . . . . . . . . . . . . 17
6.2.2. PSK Authentication . . . . . . . . . . . . . . . . . 19
6.2.3. Resumption . . . . . . . . . . . . . . . . . . . . . 20
6.2.4. Interaction with other Other TLS authentication methods . . 21 Authentication Methods
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 22
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
12.1.
10.1. Normative References . . . . . . . . . . . . . . . . . . 23
12.2.
10.2. Informative References . . . . . . . . . . . . . . . . . 24
Acknowledgments
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 25

1. Introduction

The previous specifications "Transport Layer Security (TLS)
Encryption for RADIUS" [RFC6614] and "Datagram Transport Layer
Security (DTLS) as a Transport Layer for RADIUS" [RFC7360] defined
how (D)TLS can be used as a transport protocol for RADIUS. However,
those documents do not provide guidance for using TLS-PSK TLS Pre-Shared Keys
(TLS-PSKs) with RADIUS. This document provides that missing
guidance, and gives implementation and operational considerations.

To clearly distinguish the various secrets and keys, this document
uses "shared secret" to mean "RADIUS shared secret", and Pre-Shared "Pre-Shared
Key (PSK) (PSK)" to mean secret "secret keys which that are used with TLS-PSK. TLS-PSK".

The purpose of the document is to help smooth the operational
transition from the use of the insecure RADIUS/UDP to the use of the
much more secure RADIUS/TLS. While using PSKs is often less
preferable to using public / or private keys, the operational model of
PSKs follows the legacy RADIUS "shared secret" model. As such, it
can be easier for implementers and operators to transition to TLS
when that transition is offered as a series of small changes.

The intent for

TLS-PSK is intended to be used in networks where the addresses of client
clients and server servers are known, as with RADIUS/UDP. This situation is
similar to the use-case use case of RADIUS/UDP with shared secrets. TLS-
PSK TLS-PSK
is not suitable for situations where clients dynamically discover
servers, as there is no way for the client to dynamically determine
which PSK should be used with a new server (or vice versa). In
contrast, [RFC7585] dynamic discovery [RFC7585] allows for a client or server
to authenticate a previously unknown server or client, as the parties
can be issued a certificate by a known Certification Authority (CA).

TLS-PSKs have the same issue of symmetric information between client
and server: both parties know the secret key. A client could, in
theory, pretend to be a server. In contrast, certificates are
asymmetric, where it is impossible for the parties to assume the
others
other's identity. Further discussion of this topic is contained in
[]{#sharing}.
Section 4.3.

Unless it is explicitly called out that a recommendation applies to
TLS alone or to DTLS alone, each recommendation applies to both TLS and DTLS.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

External PSK
A PSK (along with a related PSK Identity) which that is administratively
configured. That is, one which that is external to
TLS, TLS and is not
created by the TLS subsystem.

Resumption PSK
A PSK (along with a related PSK Identity) which that is created by the
TLS subsystem and/or application, for use with resumption.

3. Justification of PSK. PSK

TLS deployments usually rely on certificates in most common uses.
However, we recognize that it may be difficult to fully upgrade
client implementations to allow for certificates to be used with
RADIUS/TLS and RADIUS/DTLS. These upgrades involve not only
implementing TLS, but can also require significant changes to
administration interfaces and application programming interfaces
(APIs) in order to fully support certificates.

For example, unlike shared secrets, certificates expire. This
expiration means that a working system using TLS can suddenly stop
working. Managing this expiration can require additional
notification APIs on RADIUS clients and servers which that were previously
not required when shared secrets were used.

Certificates also require the use of certification authorities (CAs), (CAs)
and chains of certificates. RADIUS implementations using TLS
therefore have to track not just a small shared secret, but also
potentially many large certificates. The use of TLS-PSK can
therefore provide a simpler upgrade path for implementations to
transition from RADIUS shared secrets to TLS.

In terms of ongoing maintenance, it is generally simpler to maintain
servers than clients. For one, there are many fewer servers than
clients. Servers are also typically less resource constrained, and
often run on general-purpose operating systems, where maintenance can
be automated using widely-available widely available tools.

In contrast, clients are often numerous, resource constrained, and
are more
likely to be closed or proprietary systems with limited interfaces.
As a result, it can be difficult to update these clients when a root
CA expires. The use of TLS-PSK in such an environment may therefore
offer management efficiencies.

4. General Discussion of PSKs and PSK Identities

Before we define any RADIUS-specific use of PSKs, we must first
review the current standards for PSKs, and give general advice on
PSKs and PSK Identities.

The requirements in this section apply to both client and server
implementations which that use TLS-PSK. Client-specific and server-
specific issues are discussed in more detail later in this document.

4.1. Guidance for PSKs

We first give requirements for creating and managing PSKs, followed
by usability guidance, and then a discussion of RADIUS shared secrets
and their interaction with PSKs.

4.1.1. PSK Requirements

Reuse of a PSK in multiple versions of TLS (e.g., TLS 1.2 and TLS
1.3) is considered unsafe ([RFC8446], (see [RFC8446], Appendix E.7). Where TLS
1.3 binds the PSK to a particular key derivation function, function (KDF), TLS
1.2 does not. This binding means that it is possible to use the same
PSK in different hashes, leading to the potential for attacking the
PSK by comparing the hash outputs. While there are no known
insecurities, these uses are not known to be secure, and should
therefore be avoided. For this reason, an implementation MUST NOT
use the same PSK for TLS 1.3 and for earlier versions of TLS. The
exact manner in which this requirement is enforced is implementation-specific. implementation-
specific. One possibility is to have two different PSKs. Another
possibility is to forbid the use of TLS versions less than TLS 1.3

[RFC9258] adds a key derivation function (KDF) KDF to the import interface of (D)TLS 1.3, which
binds the externally provided PSK to the protocol version. That
process is preferred to any TOFU trust-on-first-use (TOFU) mechanism. In
particular, that document:

| ... describes a mechanism for importing PSKs derived from external
| PSKs by including the target KDF, (D)TLS protocol version, and an
| optional context string to ensure uniqueness. This process yields
| a set of candidate PSKs, each of which are bound to a target KDF
| and protocol, that are separate from those used in (D)TLS 1.2 and
| prior versions. This expands what would normally have been a
| single PSK and identity into a set of PSKs and identities.

An implementation MUST NOT use the same PSK for TLS 1.3 and for
earlier versions of TLS. This requirement prevents reuse of a PSK
with multiple TLS versions, which prevents the attacks discussed in
[RFC8446], Appendix E.7. The exact manner in which this requirement
is enforced is implementation-specific. One possibility is to have
two different PSKs. Another possibility is to forbid the use of TLS
versions less than TLS 1.3.

Implementations MUST follow the directions of [RFC9257], Section 6
for the use of external PSKs in TLS. That document provides
extremely useful guidance on generating and using PSKs.

Implementations MUST support PSKs of at least 32 octets in length,
and SHOULD support PSKs of 64 octets or more. As the PSKs are
generally hashed before being used in TLS, the useful entropy of a
PSK is limited by the size of the hash output. This output may be
256, 384, or 512 bits in length. Nevertheless, it is good practice
for implementations to allow entry of PSKs of more than 64 octets, as
the PSK may be in a form other than bare binary data.
Implementations which that limit the PSK to a maximum of 64 octets are
likely to use PSKs which that have much less than 512 bits of entropy.
That is, a PSK with high entropy may be expanded via some construct
(e.g., base32 as in the example below) in order to make it easier for
people to interact with. Where 512 bits of entropy are input to an
encoding construct, the output may be larger than 64 octets.

Implementations MUST require that PSKs be at least 16 octets in
length. That is, short PSKs MUST NOT be permitted to be used, and
PSKs MUST be random. The strength of the PSK is not determined by
the length of the PSK, but instead by the number of bits of entropy
which
that it contains. People are not good at creating data with high
entropy, so a source of cryptographically secure random numbers MUST
be used.

Where user passwords are generally intended to be remembered and
entered by people on a regular basis, PSKs are intended to be entered
once, and then automatically saved in a system configuration. As
such, due to the limited entropy of passwords, they are not
acceptable for use with TLS-PSK, and would only be acceptable for use
with a password-authenticated key exchange (PAKE) TLS method
[RFC8492]. Implementations MUST therefore support entry and storage
of PSKs as undistinguished octets.

We also incorporate by reference the requirements of [RFC7360],
Section 10.2 when using PSKs.

It may be tempting for servers to implement a "trust on first use"
(TOFU) TOFU policy with
respect to clients. Such behavior is NOT RECOMMENDED. When servers
receive a connection from an unknown client, they SHOULD log the PSK
Identity, source IP address, and any other information which that may be
relevant. An administrator can then later look at the logs and
determine the appropriate action to take.

4.1.2. Usability Guidance

PSKs are in their purest form are opaque tokens, represented as an
undistinguished series of octets. Where PSKs are expected to be
managed automatically by scripted methods, this format is acceptable.
However, in some cases it is necessary for administrators to share
PSKs, in which case humanly readable human-readable formats may be useful.
Implementations SHOULD support entering PSKs as both binary data, data and
via a humanly readable human-readable form such as hex encoding.

Implementations SHOULD use a humanly readable human-readable form of PKSs PSKs for
interfaces which that are intended to be used by people, and SHOULD allow
for binary data to be entered via an application programming
interface (API). Implementations SHOULD also allow for PSKs to be
displayed in the above-mentioned hex encoding, encoding mentioned above, so that administrators
can manually verify that a particular PSK is being used.

When using PSKs, administrators SHOULD use PSKs of at least 24
octets, octets
that are generated using a source of cryptographically secure random
numbers. Implementers needing a secure random number generator
should see [RFC8937] for for further guidance. PSKs are not passwords,
and administrators should not try to manually create PSKs.

In order to guide implementers and administrators, we give example
commands below which that generate random PSKs from a locally secure
source. While some commands may not work on some systems systems, one of the
commands should succeed. The intent here is to document a concise
and simple example of creating PSKs which that are both secure, secure and
humanly human-
manageable. This document does not mandate that the PSKs follow this format,
format or any other format.

openssl rand -base64 16

dd if=/dev/urandom bs=1 count=16 | base64

dd if=/dev/urandom bs=1 count=16 | base32

dd if=/dev/urandom bs=1 count=16 | (hexdump -ve '/1 "%02x"' && echo)

Only one of the above commands should be run, run; there is no need to run
all of them. Each command reads 128 bits (16 octets) of random data
from a secure source, and encodes it as printable / and readable ASCII.
This form of PSK will be accepted by any implementation which that supports
at least 32 octets for PSKs. Larger PSKs can be generated by
changing the "16" number to a larger value. The above derivation
assumes that the random source returns one bit of entropy for every
bit of randomness which that is returned. Sources failing that assumption
are NOT RECOMMENDED.

4.1.3. Interaction between Between PSKs and RADIUS Shared Secrets

Any shared secret used for RADIUS/UDP or RADIUS/TLS MUST NOT be used
for TLS-PSK.

It is RECOMMENDED that RADIUS clients and servers track all used
shared secrets and PSKs, and then verify that the following
requirements all hold true:

* no shared secret is used for more than one RADIUS client

* no PSK is used for more than one RADIUS client

* no shared secret is used as a PSK

Note that the shared secret of "radsec" given in [RFC6614] can be
used across multiple clients, as that value is mandated by the
specification. The intention here is to recommend best practices for
administrators who enter site-local shared secrets.

There may be use-cases use cases for using one shared secret across multiple
RADIUS clients. There may similarly be use-cases use cases for sharing a PSK
across multiple RADIUS clients. Details of the possible attacks on
reused PSKs are given in [RFC9257], Section 4.1.

There are no known use-cases use cases for using a PSK as a shared secret, or
vice-versa.
vice versa.

Implementations MUST reject configuration attempts that try to use
the same value for the PSK and shared secret. To prevent
administrative errors, implementations should not have interfaces which
that confuse PSKs and shared secrets, secrets or which that allow both PSKs and
shared secrets to be entered at the same time. There is too much of
a temptation for administrators to enter the same value in both
fields, which would violate the limitations given above. Similarly,
using a "shared secret" field as a way for administrators to enter
PSKs is bad practice. The PSK entry fields need to be labeled as
being related to PSKs, and not to shared secrets.

4.2. PSK Identities

[RFC4279], Section 5.1 requires that PSK Identities be encoded in
UTF-8 format. However, [RFC8446], Section 4.2.11 describes the "Pre-
Shared Key Extension" and defines the ticket as an opaque string:
"opaque identity<1..2^16-1>;". This PSK is then used in [RFC8446],
Section 4.6.1 for resumption.

These definitions appear to be in conflict. This conflict is
addressed in [RFC9257], Section 6.1.1, which discusses requirements
for encoding and comparison of PSK Identities. Systems MUST follow
the directions of [RFC9257], Section 6.1.1 when using or comparing
PSK Identities for RADIUS/TLS. Implementations MUST follow the
recommendations of [RFC8265] for handling PSK Identity strings.

In general, implementers should allow for external PSK Identities to
follow [RFC4279] and be UTF-8, while PSK Identities provisioned as
part of resumption are automatically provisioned, and therefore
follow [RFC8446].

Note that the PSK Identity is sent in the clear, and is therefore
visible to attackers. Where privacy is desired, the PSK Identity
could be either an opaque token generated cryptographically, or
perhaps in the form of a Network Access Identifier (NAI) [RFC7542],
where the "user" portion is an opaque token. For example, an NAI
could be "68092112@example.com". If the attacker already knows that
the client is associated with "example.com", then using that domain
name in the PSK Identity offers no additional information. In
contrast, the "user" portion needs to be both unique to the client
and private, so using an opaque token there is a more secure approach.

Implementations MUST support PSK Identities of 128 octets, and SHOULD
support longer PSK Identities. We note that while TLS provides for
PSK Identities of up to 2^16-1 octets in length, there are few
practical uses for extremely long PSK Identities.

It is up to administrators and implementations as to how they
differentiate external PSK Identities from session resumption PSK
Identities used in TLS 1.3 session tickets. While [RFC9257],
Section 6.1.2 suggests the identities should be unique, it offers no
concrete steps for how this differentiation may be done.

One approach could be to have externally provisioned PSK Identities
contain an NAI such as what is described above, while session
resumption PSK Identities contain large blobs of opaque, encrypted,
and authenticated text. It should then be relatively straightforward
to differentiate the two types of identities. One is UTF-8, the
other is not. One is unauthenticated, the other is authenticated.

Servers MUST assign and/or track session resumption PSK Identities in
a way which that facilities the ability to distinguish those identities
from externally configured PSK Identities, and which that enables them to
both find and validate the session resumption PSK. See
{}(#resumption) Section 6.2.3
below for more discussion of issues around resumption.

A sample validation flow for TLS-PSK Identities could be performed
via the following steps:

1. PSK Identities provided via an administration interface are
enforced to be only UTF-8 on both client and server.

2. The client treats session tickets received from the server as
opaque blobs.

3. When the server issues session tickets for resumption, the server
ensures that they are not valid UTF-8.

4. One way to do this is to use stateless resumption with a forced
non-UTF-8 key_name per [RFC5077], Section 4, such as by setting
one octet to 0x00.

5. When receiving TLS, the server receives a Client-Hello containing
a PSK, and checks if the identity is valid UTF-8. UTF-8:

5.1. If yes, it searches for a pre-configured preconfigured client which that matches
that identity.

5.1.1. If the identity is found, it authenticates the
client via PSK.

5.1.2. else Else, the identity is invalid, and the server
closes the connection.

5.2

5.2. If the identity is not UTF-8, not, try resumption, which is usually be handled by a TLS
library.

5.2.1

5.2.1. If the TLS library verifies the session ticket,
then resumption has happened, and the connection is
established.

5.2.2. else Else, the server ignores the session ticket, and
performs a normal TLS handshake with a certificate.

This validation flow is only suggested. Other validation methods are
possible.

4.2.1. Security of PSK Identities

We note that the PSK Identity is a field created by the connecting
client. Since the client is untrusted until both the identity and
PSK have been verified, both of those fields MUST be treated as
untrusted. That is, a well-formed PSK Identity is likely to be in
UTF-8 format, due to the requirements of [RFC4279], Section 5.1.
However, implementations MUST support managing PSK Identities as a
set of undistinguished octets.

It is not safe to use a raw PSK Identity to look up a corresponding
PSK. The PSK may come from an untrusted source, source and may contain
invalid or malicious data. For example, the identity may have
incorrect UTF-8 format; or it may contain data which that forms an
injection attack for SQL, LDAP, REST Lightweight Directory Access Protocol
(LDAP), Representational State Transfer (REST), or shell meta
characters; or it may contain embedded NUL octets which that are
incompatible with APIs
which that expect NUL terminated strings. The
identity may also be up to 65535 octets long.

As such, implementations MUST validate the identity prior to it being
used as a lookup key. When the identity is passed to an external API
(e.g., database lookup), implementations MUST either escape any
characters in the identity which that are invalid for that API, or else
reject the identity entirely. The exact form of any escaping depends
on the API, and we cannot document all possible methods here.
However, a few basic validation rules are suggested, as outlined
below. Any identity which that is rejected by these validation rules MUST
cause the server to close the TLS connection.

The suggested validation rules for identities used outside of
resumption are as follows:

* Identities MUST be checked to see if they have been provisioned as
a resumption PSK. If so, then the session can be resumed, subject
to any policies around resumption.

* Identities longer than a fixed maximum SHOULD be rejected. The
limit is implementation dependent, but SHOULD NOT be less than
128, and SHOULD NOT be more than 1024. There is no purpose to
allowing extremely long identities, and allowing them does little
more than complicate implementations.

* Identities configured by administrators SHOULD be in UTF-8 format,
and SHOULD be in the [RFC7542] NAI format. format from [RFC7542]. While [RFC8446],
Section 4.2.11 defines the PSK Identity as "opaque
identity<1..2^16-1>", it is useful for administrators to manage
humanly-readable
human-readable identities in a recognizable format.

This suggestion makes it easier to distinguish TLS-PSK Identities
from TLS 1.3 resumption identities. It also allows
implementations to more easily filter out unexpected or bad
identities, and then to close inappropriate TLS connections.

It is RECOMMENDED that implementations extend these rules with any
additional validation which are that is found to be useful. For example,
implementations and/or deployments could both generate PSK Identities
in a particular format for passing to client systems, and then also
verify that any received identity matches that format. For example,
a site could generate PSK Identities which that are composed of characters
in the local language. The site could then reject identities which that
contain characters from other languages, even if those characters are
valid UTF-8.

The purpose of these rules is to help administrators and implementers
more easily manage systems using TLS-PSK, while also minimizing
complexity and protecting from potential attackers attackers' traffic. The
rules follow a principle of "discard bad traffic quickly", which
helps to improve system stability and performance.

4.3. PSK and PSK Identity Sharing

While administrators may desire to share PSKs and/or PSK Identities
across multiple systems, such usage is NOT RECOMMENDED. Details of
the possible attacks on reused PSKs are given in [RFC9257],
Section 4.1.

Implementations MUST support the ability to configure a unique PSK
and PSK Identity for each possible client-server relationship. This
configuration allows administrators desiring security to use unique
PSKs for each such relationship. This configuration is also
compatible with the practice of administrators who wish to re-use reuse PSKs
and PSK Identities where local policies permit.

Implementations SHOULD warn administrators if the same PSK Identity
and/or PSK is used for multiple client-server relationships.

4.4. PSK Lifetimes

Unfortunately, [RFC9257] offers no guidance on PSK lifetimes other
than to note in Section 4.2 that:

| Forward security may be achieved by using a PSK-DH mode or by
| using PSKs with short lifetimes.

It is RECOMMENDED that PSKs be rotated regularly. We offer no
additional guidance on how often this process should occur. Changing
PSKs has a non-zero cost. It is therefore up to administrators to
determine how best to balance the cost of changing the PSK against
the cost of a potential PSK compromise.

TLS-PSK MUST use modes such as PSK-DH or ECDHE_PSK [RFC5489] which that
provide forward secrecy. Failure to use such modes would mean that
compromise of a PSK would allow an attacker to decrypt all sessions
which
that had used that PSK.

As the PSKs are looked up by identity, the PSK Identity MUST also be
changed when the PSK changes.

Servers SHOULD track when a connection was last received for a
particular PSK Identity, and SHOULD automatically invalidate
credentials when a client has not connected for an extended period of
time. This process helps to mitigate the issue of credentials being
leaked when a device is stolen or discarded.

5. Guidance for RADIUS Clients

Client implementations MUST allow the use of a pre-shared key (PSK)
for RADIUS/TLS. The client implementation can then expose a user
interface flag which is "TLS yes / no", and then also fields which that ask
for the PSK Identity and PSK itself.

For TLS 1.3, Implementations implementations MUST support the "psk_dhe_ke" Pre-Shared
Key Exchange Mode in TLS 1.3 as discussed in [RFC8446], Section 4.2.9
and in [RFC9257], Section 6. Implementations MUST implement the
recommended cipher suites in [RFC9325], Section 4.2 for TLS 1.2, 1.2 and
in [RFC8446], Section 9.1 for TLS 1.3. In order to future-proof
these recommendations, we give the following recommendations: recommendations.

* Implementations SHOULD use the "Recommended" cipher suites listed
in the IANA "TLS Cipher Suites" registry, registry:

- for For TLS 1.3, the use the "psk_dhe_ke" PSK key exchange mode, mode.

- for For TLS 1.2 and earlier, use cipher suites which that require
ephemeral keying.

If a client initiated a connection using a PSK with TLS 1.3 by
including the pre-shared key extension, it MUST close the connection
if the server did not also select the pre-shared key to continue the
handshake.

5.1. PSK Identities

This section offers advice on both requirements for PSK Identities, Identities
and on usability.

5.1.1. PSK Identity Requirements

[RFC6614] is silent on the subject of PSK Identities, which is an
issue that we correct here. Guidance is required on the use of PSK
Identities, as the need to manage identities associated with PSK PSKs is
a new requirement for NAS both Network Access Server (NAS) management interfaces,
interfaces and is a new
requirement for RADIUS servers.

RADIUS systems implementing TLS-PSK MUST support identities as per
[RFC4279], Section 5.3, 5.3 and MUST enable configuring TLS-PSK Identities
in management interfaces as per [RFC4279], Section 5.4.

The historic methods of signing RADIUS packets have not yet been
broken, but they are believed to be much less secure than modern TLS.
Therefore, when a RADIUS shared secret is used to sign RADIUS/UDP or
RADIUS/TCP packets, that shared secret MUST NOT be used with TLS-PSK.
If the secrets were to be reused, then an attack on historic RADIUS
cryptography could be trivially leveraged to decrypt TLS-PSK
sessions.

With TLS-PSK, RADIUS/TLS clients MUST permit the configuration of a
RADIUS server IP address or host name, because dynamic server lookups
[RFC7585] can only be used if servers use certificates.

5.1.2. Usability Guidance

In order to prevent confusion between shared secrets and TLS-PSKs,
management interfaces and APIs need to label PSK fields as "PSK" or
"TLS-PSK", rather than as "shared secret".

6. Guidance for RADIUS Servers

In order to support clients with TLS-PSK, server implementations MUST
allow the use of a pre-shared key (TLS-PSK) for RADIUS/TLS.

Systems which that act as both client and server at the same time MUST NOT
share or reuse PSK Identities between incoming and outgoing
connections. Doing so would open up the systems to attack, as
discussed in [RFC9257], Section 4.1.

* Implementations SHOULD use the "Recommended" cipher suites listed
in the IANA "TLS Cipher Suites" registry, registry:

- for For TLS 1.3, use the "psk_dhe_ke" PSK key exchange mode, mode.

- for For TLS 1.2 and earlier, use cipher suites which that require
ephemeral keying.

The following section(s) describe guidance for RADIUS server
implementations and deployments. We first give an overview of
current practices, and then extend and/or replace those practices for
TLS-PSK.

6.1. Current Practices

RADIUS identifies clients by source IP address ([RFC2865] (see [RFC2865] and
[RFC6613]) or by client certificate ([RFC6614] (see [RFC6614] and [RFC7585]).
Neither of these approaches work for TLS-PSK. This section describes
current practices and mandates behavior for servers which that use TLS-
PSK. TLS-PSK.

A RADIUS/UDP server is typically configured with a set of information
per client, which includes at least the source IP address and shared
secret. When the server receives a RADIUS/UDP packet, it looks up
the source IP address, finds a client definition, and therefore the
shared secret. The packet is then authenticated (or not) using that
shared secret.

That is, the IP address is treated as the clients identity, and the
shared secret is used to prove the clients authenticity and shared
trust. The set of clients forms a logical database "client table", table"
with the IP address as the key.

A server may be configured with additional site-local policies
associated with that client. For example, a client may be marked up
as being a WiFi Wi-Fi Access Point, or a VPN concentrator, etc. Different
clients may be permitted to send different kinds of requests, where
some may send Accounting-Request packets, and other clients may not
send accounting packets.

6.2. Practices for TLS-PSK

We define practices for TLS-PSK by analogy with the RADIUS/UDP use-
case, use
case and by extending the additional policies associated with the
client. The PSK Identity replaces the source IP address as the
client identifier. The PSK replaces the shared secret as proof of
client authenticity and shared trust. However, systems implementing
RADIUS/TLS [RFC6614] and RADIUS/DTLS [RFC7360] MUST still use the
shared secret as discussed in those specifications. Any PSK is only
used by the TLS layer, layer and has no effect on the RADIUS data which that is
being transported. That is, the RADIUS data transported in a TLS
tunnel is the same no matter if client authentication is done via PSK
or by client certificates. The encoding of the RADIUS data is
entirely unaffected by the use (or not) of PSKs and client
certificates.

In order to securely support dynamic source IP addresses for clients,
we also require that servers limit clients based on a network range.
The alternative would be to suggest that RADIUS servers allow any
source IP address to connect and try TLS-PSK, which could be a
security risk. When RADIUS servers do no source IP address
filtering, it is easier for attackers to send malicious traffic to
the server. An issue with a TLS library or even a TCP/IP stack could
permit the attacker to gain unwarranted access. In contrast, when IP
address filtering is done, attackers generally must first gain access
to a secure network before attacking the RADIUS server.

Even where [RFC7585] dynamic discovery [RFC7585] is not used, the use of TLS-
PSK across unrelated organizations requires that those organizations
share PSKs. Such sharing makes it easier for a client to impersonate
a server, and vice versa. In contrast, when certificates are used,
such impersonations are impossible. It is therefore NOT RECOMMENDED
to use TLS-PSK across organizational boundaries.

When TLS-PSK is used in an environment where both client and server
are part of the same organization, then impersonations only affect
that organization. As TLS offers significant advantages over RADIUS/
UDP, it is RECOMMENDED that organizations use RADIUS/TLS with TLS-PSK
to replace RADIUS/UDP for all systems managed within the same
organization. While such systems are generally located inside of
private networks, there are no known security issues with using TLS-
PSK for RADIUS/TLS connections across the public Internet.

If a client system is compromised, its complete configuration is
exposed to the attacker. Exposing a client certificate means that
the attacker can pretend to be the client. In contrast, exposing a
PSK means that the attacker can not cannot only pretend to be the client, but
can also pretend to be the server.

The main benefit of TLS-PSK, therefore, is that its operational
processes are similar to that used for managing RADIUS/UDP, while
gaining the increased security of TLS. However, it is still
beneficial for servers to perform IP address filtering, in order to
further limit their exposure to attacks.

6.2.1. IP Filtering

A server supporting this specification MUST perform IP address
filtering on incoming connections. There are few reasons for a
server to have a default configuration which that allows connections from
any source IP address.

A TLS-PSK server MUST be configurable with a set of "allowed" network
ranges from which clients are permitted to connect. Any connection
from outside of the allowed range(s) MUST be rejected before any PSK
Identity is checked. It is RECOMMENDED that servers support IP
address filtering even when TLS-PSK is not used.

The "allowed" network ranges could be implemented as a global list,
or one or more network ranges could be tied to a client or clients.
The intent here is to allow connections to be filtered by source IP
address,
address and to allow clients to be limited to a subset of network
addresses. The exact method and representation of that filtering is
up to an implementation.

Conceptually, the set of IP addresses and ranges, along with
permitted clients and their credentials forms credentials, form a logical "client
table" which that the server uses to both filter and authenticate clients.
The client table should contain information such as allowed network
ranges, PSK Identity and associated PSK, credentials for another TLS
authentication method, or flags which that indicate that the server should
require a client certificate.

Once a server receives a connection, it checks the source IP address
against the list of all allowed IP addresses or ranges in the client
table. If none match, the connection MUST be rejected. That is, the
connection MUST be from an authorized source IP address.

Once a connection has been established, the server MUST NOT process
any application data inside of the TLS tunnel until the client has
been authenticated. Instead, the server normally receives a TLS-PSK
Identity from the client. The server then uses this identity to look
up the client in the client table. If there is no matching client,
the server MUST close the connection. The server then also checks if
this client definition allows this particular source IP address. If
the source IP address is not allowed, the server MUST close the
connection.

Where the server does not receive TLS-PSK from the client, it
proceeds with another authentication method such as client
certificates. Such requirements are discussed elsewhere, most
notably in [RFC6614] and [RFC7360].

An implementation may perform two independent IP address lookups.
First, lookups:
first to check if the connection is allowed at all, and second to
check if the connection is authorized for this particular client.
One or both checks may be used by a particular implementation. The
two sets of IP addresses can overlap, and implementations SHOULD
support that capability.

Depending on the implementation, one or more clients may share a list
of allowed network ranges. Alternately, the allowed network ranges
for two clients can overlap only partially, or not at all. All of
these possibilities MUST be supported by the server implementation.

For example, a RADIUS server could be configured to be accept
connections from a source network of 192.0.2.0/24 or 2001:DB8::/32.
The server could therefore discard any TLS connection request which that
comes from a source IP address outside of that network. In that
case, there is no need to examine the PSK Identity or to find the
client definition. Instead, the IP source filtering policy would
deny the connection before any TLS communication had been performed.

As some clients may have dynamic IP addresses, it is possible for a one
PSK Identity to appear at different source IP addresses over time.
In addition, as there may be many clients behind one NAT gateway,
there may be multiple RADIUS clients using one public IP address.
RADIUS servers MUST support multiple PSK Identifiers at one source IP
address.

That is, a server needs to support multiple different clients within
one network range, multiple clients behind a NAT, and one client
having different IP addresses over time. All of those use-cases use cases are
common and necessary.

The following section describes these requirements in more detail.

6.2.2. PSK Authentication

Once the source IP address has been verified to be allowed for this
particular client, the server authenticates the TLS connection via
the PSK taken from the client definition. If the PSK is verified,
the server then accepts the connection, connection and proceeds with RADIUS/TLS
as per [RFC6614].

If the PSK is not verified, then the server MUST close the
connection. While TLS provides for fallback to other authentication
methods such as client certificates, there is no reason for a client
to be configured simultaneously with multiple authentication methods.

A client MUST use only one authentication method for TLS. An
authentication method is either TLS-PSK, client certificates, or some
other method supported by TLS.

That is, client configuration is relatively simple: use a particular
set of credentials to authenticate to a particular server. While
clients may support multiple servers and fail-over or load-balancing,
that configuration is generally orthogonal to the choice of which
credentials to use.

6.2.3. Resumption

It is NOT RECOMMENDED that servers enable resumption for sessions
which
that use TLS-PSK. There are few practical benefits to supporting
resumption,
resumption and many complexities.

However, some systems will need to support both TLS-PSK, TLS-PSK and other
TLS-based authentication methods such as certificates, while also
supporting session resumption. It is therefore vital for servers to
be able to distinguish the use-case use case of TLS-PSK with pre-configured preconfigured
identities from TLS-PSK which that is being used for resumptions.

The above discussion of PSK Identities is complicated by the use of
PSKs for resumption in TLS 1.3. A server which that receives a PSK
Identity via TLS typically cannot query the TLS layer to see if this
identity is for a resumed session (which is possibly for another TLS
authentication method), or is instead a static pre-provisioned
identity. This confusion complicates server implementations.

One way for a server to tell the difference between the two kinds of
identities is via construction. Identities used for resumption can
be constructed via a fixed format, such as what is recommended by
[RFC5077], Section 4. A static pre-provisioned identity could be in
the format of an NAI, as given in [RFC7542]. An implementation could
therefore examine the incoming identity, identity and determine from the
identity alone what kind of authentication was being performed.

An alternative way for a server to distinguish the two kinds of
identities is to maintain two tables. One table would contain static
identities, as the logical client table described above. Another
table could be the table of identities handed out for resumption.
The server would then look up any PSK Identity in one table, and if
it is not found, query the other one. An Either an identity would be
found in a table, in which case it can be authenticated. Or, authenticated, or the
identity would not be found in either table, in which case it is
unknown, and the server MUST close the connection.

As suggested in [RFC8446], TLS-PSK peers MUST NOT store resumption
PSKs or tickets (and associated cached data) for longer than 604800
seconds (7 days) days), regardless of the PSK or ticket lifetime.

Since resumption in TLS 1.3 uses PSK Identies Identities and keys, it is NOT
RECOMMENDED to permit resumption of sessions when TLS-PSK is used.
The use of resumption offers additional complexity with minimal
addition benefit.
additional benefits.

Where resumption is allowed with TLS-PSK, systems MUST cache data
during the initial full handshake sufficient sufficiently enough to allow
authorization decisions to be made during resumption. If the cached
data cannot be retrieved securely, resumption MUST NOT be done.
Instead, the system MUST perform a full handshake.

The data which that needs to be cached is typically information such as the
original PSK Identity, along with any policies associated with that
identity.

Information from the original TLS exchange (e.g., the original PSK
Identity) as well as related information (e.g., source 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 client could supply one
set of data during the initial authentication, authentication and a different set of
data during resumption, potentially allowing them to obtain access
that they should not have.

If any authorization or policy decisions were made with information
that has changed between the initial full handshake and resumption,
and if change changes may lead to a different decision, such decisions MUST
be reevaluated. Systems MUST also reevaluate authorization and
policy decisions during resumption, based on the information given in
the new connection. Servers MAY refuse to perform resumption where
the information supplied during resumption does not match the
information supplied during the original authentication. If a safe
decision is not possible, servers MUST instead continue with a full
handshake.

6.2.4. Interaction with other Other TLS authentication methods Authentication Methods

When a server supports both TLS-PSK and client certificates, it MUST
be able to accept authenticated connections from clients which that may use
either type of credentials, perhaps even from the same source IP
address and at the same time. That is, servers are required to both
authenticate the client, client and also to filter clients by source IP
address. These checks both have to match in order for a client to be
accepted.

7. Privacy Considerations

We make no changes over to [RFC6614] and [RFC7360].

8. Security Considerations

The primary focus of this document is addressing security
considerations for RADIUS.

Previous specifications discuss security considerations for TLS-PSK
in detail. We refer the reader to [RFC8446], Appendix E.7, E.7 of [RFC8446],
[RFC9257], and [RFC9258]. Those documents are newer than [RFC6614]
and [RFC7360], andtherefore and therefore raise issues which that were not considered
during the initial design of RADIUS/TLS and RADIUS/DTLS.

Using TLS-PSK across the wider Internet for RADIUS can have different
security considerations than for other protocols. For example, if
TLS-PSK was for client/server communication with HTTPS, then having a
PSK be exposed or broken could affect one users user's traffic. In
contrast, RADIUS contains credentials and personally identifiable
information (PII) for many users. As a result, an attacker being
able to see inside of a TLS-PSK connection for RADIUS would result in
substantial amounts of PII being leaked, possibly including
passwords.

When modes providing forward secrecy are used (e.g. (e.g., ECDHE_PSK as
seen in [RFC5489] and [RFC8442]), such attacks are limited to future
sessions, and historical sessions are still secure.

9. IANA Considerations

There are

This document has no IANA considerations in this document.

RFC Editor: This section may be removed before final publication. actions.

10. Acknowledgments

Thanks to the many reviewers in the RADEXT working group for positive
feedback.

11. Changelog

* 00 - initial version

* 01 - update examples

12. References

12.1.

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

[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/rfc/rfc2865>.
<https://www.rfc-editor.org/info/rfc2865>.

[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, DOI 10.17487/RFC4279, December 2005,
<https://www.rfc-editor.org/rfc/rfc4279>.
<https://www.rfc-editor.org/info/rfc4279>.

[RFC6614] Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
"Transport Layer Security (TLS) Encryption for RADIUS",
RFC 6614, DOI 10.17487/RFC6614, May 2012,
<https://www.rfc-editor.org/rfc/rfc6614>.
<https://www.rfc-editor.org/info/rfc6614>.

[RFC7360] DeKok, A., "Datagram Transport Layer Security (DTLS) as a
Transport Layer for RADIUS", RFC 7360,
DOI 10.17487/RFC7360, September 2014,
<https://www.rfc-editor.org/rfc/rfc7360>.
<https://www.rfc-editor.org/info/rfc7360>.

[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/rfc/rfc8174>. <https://www.rfc-editor.org/info/rfc8174>.

[RFC8265] Saint-Andre, P. and A. Melnikov, "Preparation,
Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", RFC 8265,
DOI 10.17487/RFC8265, October 2017,
<https://www.rfc-editor.org/rfc/rfc8265>.
<https://www.rfc-editor.org/info/rfc8265>.

[RFC9257] Housley, R., Hoyland, J., Sethi, M., and C. A. Wood,
"Guidance for External Pre-Shared Key (PSK) Usage in TLS",
RFC 9257, DOI 10.17487/RFC9257, July 2022,
<https://www.rfc-editor.org/rfc/rfc9257>.
<https://www.rfc-editor.org/info/rfc9257>.

[RFC9325] Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
2022, <https://www.rfc-editor.org/rfc/rfc9325>.

12.2. <https://www.rfc-editor.org/info/rfc9325>.

10.2. Informative References

[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/rfc/rfc5077>. <https://www.rfc-editor.org/info/rfc5077>.

[RFC5489] Badra, M. and I. Hajjeh, "ECDHE_PSK Cipher Suites for
Transport Layer Security (TLS)", RFC 5489,
DOI 10.17487/RFC5489, March 2009,
<https://www.rfc-editor.org/rfc/rfc5489>.
<https://www.rfc-editor.org/info/rfc5489>.

[RFC6613] DeKok, A., "RADIUS over TCP", RFC 6613,
DOI 10.17487/RFC6613, May 2012,
<https://www.rfc-editor.org/rfc/rfc6613>.
<https://www.rfc-editor.org/info/rfc6613>.

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

[RFC7585] Winter, S. and M. McCauley, "Dynamic Peer Discovery for
RADIUS/TLS and RADIUS/DTLS Based on the Network Access
Identifier (NAI)", RFC 7585, DOI 10.17487/RFC7585, October
2015, <https://www.rfc-editor.org/rfc/rfc7585>. <https://www.rfc-editor.org/info/rfc7585>.

[RFC8442] Mattsson, J. and D. Migault, "ECDHE_PSK with AES-GCM and
AES-CCM Cipher Suites for TLS 1.2 and DTLS 1.2", RFC 8442,
DOI 10.17487/RFC8442, September 2018,
<https://www.rfc-editor.org/rfc/rfc8442>.
<https://www.rfc-editor.org/info/rfc8442>.

[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/rfc/rfc8446>.
<https://www.rfc-editor.org/info/rfc8446>.

[RFC8492] Harkins, D., Ed., "Secure Password Ciphersuites for
Transport Layer Security (TLS)", RFC 8492,
DOI 10.17487/RFC8492, February 2019,
<https://www.rfc-editor.org/rfc/rfc8492>.
<https://www.rfc-editor.org/info/rfc8492>.

[RFC8937] Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N.,
and C. Wood, "Randomness Improvements for Security
Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020,
<https://www.rfc-editor.org/rfc/rfc8937>.
<https://www.rfc-editor.org/info/rfc8937>.

[RFC9258] Benjamin, D. and C. A. Wood, "Importing External Pre-
Shared Keys (PSKs) for TLS 1.3", RFC 9258,
DOI 10.17487/RFC9258, July 2022,
<https://www.rfc-editor.org/rfc/rfc9258>.
<https://www.rfc-editor.org/info/rfc9258>.

Acknowledgments

Thanks to the many reviewers in the RADEXT Working Group for positive
feedback.

Author's Address

Alan DeKok
InkBridge Networks
Email: alan.dekok@inkbridge.io