Internet Engineering Task Force (IETF) R. Housley
Request for Comments: 9257
Category: Informational J. Hoyland
ISSN: 2070-1721 Cloudflare Ltd.
C. A. Wood
Guidance for External Pre-Shared Key (PSK) Usage in TLS
This document provides usage guidance for external Pre-Shared Keys
(PSKs) in Transport Layer Security (TLS) 1.3 as defined in RFC 8446
It lists TLS security properties provided by PSKs under certain
assumptions, then it demonstrates how violations of these assumptions
lead to attacks. Advice for applications to help meet these
assumptions is provided. This document also discusses PSK use cases
and provisioning processes. Finally, it lists the privacy and
security properties that are not provided by TLS 1.3 when external
PSKs are used.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9257
Copyright (c) 2022 IETF Trust and the persons identified as the
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) in effect on the date of
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Table of Contents 1.
Conventions and Definitions 3.
PSK Security Properties 4.1.
Shared PSKs 4.2.
PSK Entropy 5.
External PSKs in Practice 5.1.
Use Cases 5.2.
Provisioning Examples 5.3.
Provisioning Constraints 6.
Recommendations for External PSK Usage 6.1.
Stack Interfaces 6.1.1.
PSK Identity Encoding and Comparison 6.1.2.
PSK Identity Collisions 7.
Privacy Considerations 8.
Security Considerations 9.
IANA Considerations 10.
Normative References 10.2.
This document provides guidance on the use of external Pre-Shared
Keys (PSKs) in Transport Layer Security (TLS) 1.3 [RFC8446
guidance also applies to Datagram TLS (DTLS) 1.3 [RFC9147
Compact TLS 1.3 [CTLS]. For readability, this document uses the term
"TLS" to refer to all such versions.
External PSKs are symmetric secret keys provided to the TLS protocol
implementation as external inputs. External PSKs are provisioned out
This document lists TLS security properties provided by PSKs under
certain assumptions and demonstrates how violations of these
assumptions lead to attacks. This document discusses PSK use cases,
provisioning processes, and TLS stack implementation support in the
context of these assumptions. This document also provides advice for
applications in various use cases to help meet these assumptions.
There are many resources that provide guidance for password
generation and verification aimed towards improving security.
However, there is no such equivalent for external PSKs in TLS. This
document aims to reduce that gap.
2. Conventions and Definitions
The key words "MUST
", "MUST NOT
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in
BCP 14 [RFC2119
] when, and only when, they appear in all
capitals, as shown here.
For purposes of this document, a "logical node" is a computing
presence that other parties can interact with via the TLS protocol.
A logical node could potentially be realized with multiple physical
instances operating under common administrative control, e.g., a
server farm. An "endpoint" is a client or server participating in a
4. PSK Security Properties
The use of a previously established PSK allows TLS nodes to
authenticate the endpoint identities. It also offers other benefits,
including resistance to attacks in the presence of quantum computers;
see Section 4.2
for related discussion. However, these keys do not
provide privacy protection of endpoint identities, nor do they
provide non-repudiation (one endpoint in a connection can deny the
conversation); see Section 7
for related discussion.
PSK authentication security implicitly assumes one fundamental
property: each PSK is known to exactly one client and one server and
they never switch roles. If this assumption is violated, then the
security properties of TLS are severely weakened as discussed below.
4.1. Shared PSKs
As discussed in Section 5.1
, to demonstrate their attack, [AASS19]
describes scenarios where multiple clients or multiple servers share
a PSK. If this is done naively by having all members share a common
key, then TLS authenticates only group membership, and the security
of the overall system is inherently rather brittle. There are a
number of obvious weaknesses here: 1.
Any group member can impersonate any other group member. 2.
If a PSK is combined with the result of a fresh ephemeral key
exchange, then compromise of a group member that knows the
resulting shared secret will enable the attacker to passively
read traffic (and actively modify it). 3.
If a PSK is not combined with the result of a fresh ephemeral key
exchange, then compromise of any group member allows the attacker
to passively read all traffic (and actively modify it), including
Additionally, a malicious non-member can reroute handshakes between
honest group members to connect them in unintended ways, as described
below. Note that a partial mitigation for this class of attack is
available: each group member includes the Server Name Indication
(SNI) extension [RFC6066
] and terminates the connection on mismatch
between the presented SNI value and the receiving member's known
identity. See [Selfie] for details.
To illustrate the rerouting attack, consider three peers, A, B, and
C, who all know the PSK. The attack proceeds as follows: 1.
A sends a ClientHello to B. 2.
The attacker intercepts the message and redirects it to C. 3.
C responds with a second flight (ServerHello, ...) to A. 4.
A sends a Finished message to B. A has completed the handshake,
ostensibly with B.
5. The attacker redirects the Finished message to C. C has
completed the handshake with A.
In this attack, peer authentication is not provided. Also, if C
supports a weaker set of ciphersuites than B, cryptographic algorithm
downgrade attacks might be possible. This rerouting is a type of
identity misbinding attack [Krawczyk] [Sethi]. Selfie attack
[Selfie] is a special case of the rerouting attack against a group
member that can act as both a TLS server and a client. In the Selfie
attack, a malicious non-member reroutes a connection from the client
to the server on the same endpoint.
Finally, in addition to these weaknesses, sharing a PSK across nodes
may negatively affect deployments. For example, revocation of
individual group members is not possible without establishing a new
PSK for all of the members that have not been revoked.
4.2. PSK Entropy
Entropy properties of external PSKs may also affect TLS security
properties. For example, if a high-entropy PSK is used, then PSK-
only key establishment modes provide expected security properties for
TLS, including establishment of the same session keys between peers,
secrecy of session keys, peer authentication, and downgrade
protection. See Appendix E.1 of [RFC8446
] for an explanation of
these properties. However, these modes lack forward security.
Forward security may be achieved by using a PSK-DH mode or by using
PSKs with short lifetimes.
In contrast, if a low-entropy PSK is used, then PSK-only key
establishment modes are subject to passive exhaustive search attacks,
which will reveal the traffic keys. PSK-DH modes are subject to
active attacks in which the attacker impersonates one side. The
exhaustive search phase of these attacks can be mounted offline if
the attacker captures a single handshake using the PSK, but those
attacks will not lead to compromise of the traffic keys for that
connection because those also depend on the Diffie-Hellman (DH)
exchange. Low-entropy keys are only secure against active attack if
a Password-Authenticated Key Exchange (PAKE) is used with TLS. At
the time of writing, the Crypto Forum Research Group (CFRG) is
working on specifying recommended PAKEs (see [CPACE] and [OPAQUE] for
the symmetric and asymmetric cases, respectively).5.
External PSKs in Practice
PSK ciphersuites were first specified for TLS in 2005. PSKs are now
an integral part of the TLS 1.3 specification [RFC8446
]. TLS 1.3
also uses PSKs for session resumption. It distinguishes these
resumption PSKs from external PSKs that have been provisioned out of
band. This section describes known use cases and provisioning
processes for external PSKs with TLS.
5.1. Use Cases
This section lists some example use cases where pairwise external
PSKs (i.e., external PSKs that are shared between only one server and
one client) have been used for authentication in TLS. There was no
attempt to prioritize the examples in any particular order.
* Device-to-device communication with out-of-band synchronized keys.
PSKs provisioned out of band for communicating with known
identities, wherein the identity to use is discovered via a
different online protocol.
* Intra-data-center communication. Machine-to-machine communication
within a single data center or Point of Presence (PoP) may use
externally provisioned PSKs; this is primarily for the purpose of
supporting TLS connections with early data. See Section 8
considerations when using early data with external PSKs.
* Certificateless server-to-server communication. Machine-to-
machine communication may use externally provisioned PSKs; this is
primarily for the purposes of establishing TLS connections without
requiring the overhead of provisioning and managing PKI
* Internet of Things (IoT) and devices with limited computational
] defines TLS and DTLS profiles for
resource-constrained devices and suggests the use of PSK
ciphersuites for compliant devices. The Open Mobile Alliance
Lightweight Machine-to-Machine (LwM2M) Technical Specification
[LwM2M] states that LwM2M servers MUST
support the PSK mode of
* Securing RADIUS [RFC2865
] with TLS. PSK ciphersuites are optional
for this use case, as specified in [RFC6614
* 3GPP server-to-user equipment authentication. The Generic
Authentication Architecture (GAA) defined by 3GPP mentions that
TLS PSK ciphersuites can be used between server and user equipment
for authentication [GAA].
* Smart Cards. The German electronic Identity (eID) card supports
authentication of a card holder to online services with TLS PSK
* Quantum resistance. Some deployments may use PSKs (or combine
them with certificate-based authentication as described in
]) because of the protection they provide against quantum
There are also use cases where PSKs are shared between more than two
entities. Some examples below (as noted by Akhmetzyanova, et al.
* Group chats. In this use case, group participants may be
provisioned an external PSK out of band for establishing
authenticated connections with other members of the group.
* IoT and devices with limited computational capabilities. Many PSK
provisioning examples are possible in this use case. For example,
in a given setting, IoT devices may all share the same PSK and use
it to communicate with a central server (one key for n devices),
have their own key for communicating with a central server (n keys
for n devices), or have pairwise keys for communicating with each
other (n^2 keys for n devices).
5.2. Provisioning Examples
The exact provisioning process depends on the system requirements and
threat model. Whenever possible, avoid sharing a PSK between nodes;
however, sharing a PSK among several nodes is sometimes unavoidable.
When PSK sharing happens, other accommodations SHOULD
be used as
discussed in Section 6
Examples of PSK provisioning processes are included below.
* Many industrial protocols assume that PSKs are distributed and
assigned manually via one of the following approaches: (1) typing
the PSK into the devices or (2) using a trust-on-first-use (TOFU)
approach with a device completely unprotected before the first
login took place. Many devices have a very limited UI. For
example, they may only have a numeric keypad or even fewer
buttons. When the TOFU approach is not suitable, entering the key
would require typing it on a constrained UI.
* Some devices provision PSKs via an out-of-band, cloud-based
* Some secrets may be baked into hardware or software device
components. Moreover, when this is done at manufacturing time,
secrets may be printed on labels or included in a Bill of
Materials for ease of scanning or import.
5.3. Provisioning Constraints
PSK provisioning systems are often constrained in application-
specific ways. For example, although one goal of provisioning is to
ensure that each pair of nodes has a unique key pair, some systems do
not want to distribute pairwise shared keys to achieve this. As
another example, some systems require the provisioning process to
embed application-specific information in either PSKs or their
identities. Identities may sometimes need to be routable, as is
currently under discussion for [EAP-TLS-PSK].
6. Recommendations for External PSK Usage
Recommended requirements for applications using external PSKs are as
Each PSK SHOULD
be derived from at least 128 bits of entropy, MUST
be at least 128 bits long, and SHOULD
be combined with an
ephemeral key exchange, e.g., by using the "psk_dhe_ke" Pre-
Shared Key Exchange Mode in TLS 1.3 for forward secrecy. As
discussed in Section 4
, low-entropy PSKs (i.e., those derived
from less than 128 bits of entropy) are subject to attack and SHOULD
be avoided. If only low-entropy keys are available, then
key establishment mechanisms such as PAKE that mitigate the risk
of offline dictionary attacks SHOULD
be employed. Note that no
such mechanisms have yet been standardized, and further that
these mechanisms will not necessarily follow the same
architecture as the process for incorporating external PSKs
described in [RFC9258
Unless other accommodations are made to mitigate the risks of
PSKs known to a group, each PSK MUST
be restricted in its use to
at most two logical nodes: one logical node in a TLS client role
and one logical node in a TLS server role. (The two logical
be the same, in different roles.) Two acceptable
accommodations are described in [RFC9258
]: (1) exchanging client
and server identifiers over the TLS connection after the
handshake and (2) incorporating identifiers for both the client
and the server into the context string for an external PSK
use external PSK importers [RFC9258
configuring PSKs for a client-server pair when applicable.
Importers make provisioning external PSKs easier and less error-
prone by deriving a unique, imported PSK from the external PSK
for each key derivation function a node supports. See the
Security Considerations of [RFC9258
] for more information. 4.
Where possible, the main PSK (that which is fed into the
be deleted after the imported keys have been
generated. This prevents an attacker from bootstrapping a
compromise of one node into the ability to attack connections
between any node; otherwise, the attacker can recover the main
key and then re-run the importer itself.
6.1. Stack Interfaces
Most major TLS implementations support external PSKs. Stacks
supporting external PSKs provide interfaces that applications may use
when configuring PSKs for individual connections. Details about some
existing stacks at the time of writing are below.
* OpenSSL and BoringSSL: Applications can specify support for
external PSKs via distinct ciphersuites in TLS 1.2 and below.
Also, they can then configure callbacks that are invoked for PSK
selection during the handshake. These callbacks must provide a
PSK identity and key. The exact format of the callback depends on
the negotiated TLS protocol version, with new callback functions
added specifically to OpenSSL for TLS 1.3 [RFC8446
] PSK support.
The PSK length is validated to be between 1-256 bytes (inclusive).
The PSK identity may be up to 128 bytes long.
* mbedTLS: Client applications configure PSKs before creating a
connection by providing the PSK identity and value inline.
Servers must implement callbacks similar to that of OpenSSL. Both
PSK identity and key lengths may be between 1-16 bytes long
* gnuTLS: Applications configure PSK values as either raw byte
strings or hexadecimal strings. The PSK identity and key size are
* wolfSSL: Applications configure PSKs with callbacks similar to
6.1.1. PSK Identity Encoding and Comparison Section 5.1
] mandates that the PSK identity should be
first converted to a character string and then encoded to octets
using UTF-8. This was done to avoid interoperability problems
(especially when the identity is configured by human users). On the
other hand, [RFC7925
] advises implementations against assuming any
structured format for PSK identities and recommends byte-by-byte
comparison for any operation. When PSK identities are configured
manually, it is important to be aware that visually identical strings
may, in fact, differ due to encoding issues.
TLS 1.3 [RFC8446
] follows the same practice of specifying the PSK
identity as a sequence of opaque bytes (shown as opaque
identity<1..2^16-1> in the specification) that thus is compared on a
byte-by-byte basis. [RFC8446
] also requires that the PSK identities
are at least 1 byte and at the most 65535 bytes in length. Although
] does not place strict requirements on the format of PSK
identities, note that the format of PSK identities can vary depending
on the deployment:
* The PSK identity MAY
be a user-configured string when used in
protocols like Extensible Authentication Protocol (EAP) [RFC3748
For example, gnuTLS treats PSK identities as usernames.
* PSK identities MAY
have a domain name suffix for roaming and
federation. In applications and settings where the domain name
suffix is privacy sensitive, this practice is NOT RECOMMENDED
* Deployments should take care that the length of the PSK identity
is sufficient to avoid collisions.
6.1.2. PSK Identity Collisions
It is possible, though unlikely, that an external PSK identity may
clash with a resumption PSK identity. The TLS stack implementation
and sequencing of PSK callbacks influences the application's behavior
when identity collisions occur. When a server receives a PSK
identity in a TLS 1.3 ClientHello, some TLS stacks execute the
application's registered callback function before checking the
stack's internal session resumption cache. This means that if a PSK
identity collision occurs, the application's external PSK usage will
typically take precedence over the internal session resumption path.
Because resumption PSK identities are assigned by the TLS stack
implementation, it is RECOMMENDED
that these identifiers be assigned
in a manner that lets resumption PSKs be distinguished from external
PSKs to avoid concerns with collisions altogether.
7. Privacy Considerations
PSK privacy properties are orthogonal to security properties
described in Section 4
. TLS does little to keep PSK identity
information private. For example, an adversary learns information
about the external PSK or its identifier by virtue of the identifier
appearing in cleartext in a ClientHello. As a result, a passive
adversary can link two or more connections together that use the same
external PSK on the wire. Depending on the PSK identity, a passive
attacker may also be able to identify the device, person, or
enterprise running the TLS client or TLS server. An active attacker
can also use the PSK identity to suppress handshakes or application
data from a specific device by blocking, delaying, or rate-limiting
traffic. Techniques for mitigating these risks require further
analysis and are out of scope for this document.
In addition to linkability in the network, external PSKs are
intrinsically linkable by PSK receivers. Specifically, servers can
link successive connections that use the same external PSK together.
Preventing this type of linkability is out of scope.
8. Security Considerations
Security considerations are provided throughout this document. It
bears repeating that there are concerns related to the use of
external PSKs regarding proper identification of TLS 1.3 endpoints
and additional risks when external PSKs are known to a group.
It is NOT RECOMMENDED
to share the same PSK between more than one
client and server. However, as discussed in Section 5.1
, there are
application scenarios that may rely on sharing the same PSK among
multiple nodes. [RFC9258
] helps in mitigating rerouting and Selfie-
style reflection attacks when the PSK is shared among multiple nodes.
This is achieved by correctly using the node identifiers in the
ImportedIdentity.context construct specified in [RFC9258
solution would be for each endpoint to select one globally unique
identifier to use in all PSK handshakes. The unique identifier can,
for example, be one of its Media Access Control (MAC) addresses, a
32-byte random number, or its Universally Unique IDentifier (UUID)
]. Note that such persistent, global identifiers have
privacy implications; see Section 7
Each endpoint SHOULD
know the identifier of the other endpoint with
which it wants to connect and SHOULD
compare it with the other
endpoint's identifier used in ImportedIdentity.context. However, it
is important to remember that endpoints sharing the same group PSK
can always impersonate each other.
Considerations for external PSK usage extend beyond proper
identification. When early data is used with an external PSK, the
random value in the ClientHello is the only source of entropy that
contributes to key diversity between sessions. As a result, when an
external PSK is used more than one time, the random number source on
the client has a significant role in the protection of the early
9. IANA Considerations
This document has no IANA actions.
10.1. Normative References
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119
Key Words", BCP 14, RFC 8174
, DOI 10.17487/RFC8174
May 2017, <https://www.rfc-editor.org/info/rfc8174
] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446
, DOI 10.17487/RFC8446
, August 2018,
] Benjamin, D. and C. A. Wood, "Importing External Pre-
Shared Keys (PSKs) for TLS 1.3", RFC 9258
, July 2022,
10.2. Informative References
[AASS19] Akhmetzyanova, L., Alekseev, E., Smyshlyaeva, E., and A.
Sokolov, "Continuing to reflect on TLS 1.3 with external
PSK", April 2019, <https://eprint.iacr.org/2019/421.pdf
[CPACE] Abdalla, M., Haase, B., and J. Hesse, "CPace, a balanced
composable PAKE", Work in Progress, Internet-Draft, draft-
irtf-cfrg-cpace-06, 24 July 2022,
[CTLS] Rescorla, E., Barnes, R., Tschofenig, H., and B. M.
Schwartz, "Compact TLS 1.3", Work in Progress, Internet-
Draft, draft-ietf-tls-ctls-06, 9 July 2022,
Mattsson, J. P., Sethi, M., Aura, T., and O. Friel, "EAP-
TLS with PSK Authentication (EAP-TLS-PSK)", Work in
Progress, Internet-Draft, draft-mattsson-emu-eap-tls-psk-
00, 9 March 2020, <https://datatracker.ietf.org/doc/html/
[GAA] ETSI, "Digital cellular telecommunications system (Phase
2+); Universal Mobile Telecommunications System (UMTS);
LTE; 3G Security; Generic Authentication Architecture
(GAA); System description", version 12.0.0, ETSI TR 133
919, October 2014, <https://www.etsi.org/deliver/
[Krawczyk] Krawczyk, H., "SIGMA: The 'SIGn-and-MAc' Approach to
Authenticated Diffie-Hellman and Its Use in the IKE
Protocols", DOI 10.1007/978-3-540-45146-4_24, 2003,
[LwM2M] Open Mobile Alliance, "Lightweight Machine to Machine
Technical Specification", version 1.0, February 2017,
[OPAQUE] Bourdrez, D., Krawczyk, H., Lewi, K., and C. A. Wood, "The
OPAQUE Asymmetric PAKE Protocol", Work in Progress,
Internet-Draft, draft-irtf-cfrg-opaque-09, 6 July 2022,
] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", RFC 2865
, DOI 10.17487/RFC2865
, June 2000,
] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748
, DOI 10.17487/RFC3748
, June 2004,
] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122
, July 2005,
] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)", RFC 4279
, DOI 10.17487/RFC4279
, December 2005,
] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066
, January 2011,
] Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
"Transport Layer Security (TLS) Encryption for RADIUS", RFC 6614
, DOI 10.17487/RFC6614
, May 2012,
] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925
, July 2016,
] Housley, R., "TLS 1.3 Extension for Certificate-Based
Authentication with an External Pre-Shared Key", RFC 8773
, March 2020,
] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147
, DOI 10.17487/RFC9147
, April 2022,
[Selfie] Drucker, N. and S. Gueron, "Selfie: reflections on TLS 1.3
with PSK", DOI 10.1007/s00145-021-09387-y, May 2021,
[Sethi] Sethi, M., Peltonen, A., and T. Aura, "Misbinding Attacks
on Secure Device Pairing and Bootstrapping",
DOI 10.1145/3321705.3329813, May 2019,
Bundesamt für Sicherheit in der Informationstechnik,
"Technical Guideline TR-03112-7 eCard-API-Framework -
Protocols", version 1.1.5, April 2015, <https://www.bsi.bu
This document is the output of the TLS External PSK Design Team,
comprised of the following members: Benjamin Beurdouche, Björn Haase,
Christopher Wood, Colm MacCarthaigh, Eric Rescorla, Jonathan Hoyland,
Martin Thomson, Mohamad Badra, Mohit Sethi, Oleg Pekar, Owen Friel,
and Russ Housley.
This document was improved by high-quality reviews by Ben Kaduk and
John Preuß Mattsson.
Vigil Security, LLC
Christopher A. Wood