tls                                                           R. Housley
Internet-Draft                                            Vigil Security
Intended status: Informational                                J. Hoyland
Expires: 6 May 2021                                      Cloudflare Ltd.
                                                                M. Sethi
                                                               C.A. Wood
                                                         2 November 2020

                 Guidance for External PSK Usage in TLS


   This document provides usage guidance for external Pre-Shared Keys
   (PSKs) in TLS.  It lists TLS security properties provided by PSKs
   under certain assumptions and demonstrates how violations of these
   assumptions lead to attacks.  This document also discusses PSK use
   cases, provisioning processes, and TLS stack implementation support
   in the context of these assumptions.  It provides advice for
   applications in various use cases to help meet these assumptions.
   Privacy and security properties not provided by PSKs are also

Discussion Venues

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

   Source for this draft and an issue tracker can be found at

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on 6 May 2021.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Notation  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  PSK Security Properties . . . . . . . . . . . . . . . . . . .   3
   5.  Privacy Properties  . . . . . . . . . . . . . . . . . . . . .   5
   6.  External PSK Use Cases and Provisioning Processes . . . . . .   5
     6.1.  Provisioning Examples . . . . . . . . . . . . . . . . . .   7
     6.2.  Provisioning Constraints  . . . . . . . . . . . . . . . .   7
   7.  Recommendations for External PSK Usage  . . . . . . . . . . .   7
     7.1.  Stack Interfaces  . . . . . . . . . . . . . . . . . . . .   8
       7.1.1.  PSK Identity Encoding and Comparison  . . . . . . . .   9
       7.1.2.  PSK Identity Collisions . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   There are many resources that provide guidance for password
   generation and verification aimed towards improving security.
   However, there is no such equivalent for external Pre-Shared Keys
   (PSKs) in TLS.  This document aims to reduce that gap.  It lists TLS
   security properties provided by PSKs under certain assumptions and
   demonstrates how violations of these assumptions lead to attacks.
   This document also discusses PSK use cases, provisioning processes,
   and TLS stack implementation support in the context of these

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   assumptions.  It provides advice for applications in various use
   cases to help meet these assumptions.

   The guidance provided in this document is applicable across TLS
   [RFC8446], DTLS [I-D.ietf-tls-dtls13], and Constrained TLS

2.  Conventions and Definitions

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

3.  Notation

   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

   External PSK authentication in TLS allows endpoints to authenticate
   connections using previously established keys.  These keys do not
   provide protection of endpoint identities (see Section 5), nor do
   they provide non-repudiation (one endpoint in a connection can deny
   the conversation).  PSK authentication security implicitly assumes
   one fundamental property: each PSK is known to exactly one client and
   one server, and that these never switch roles.  If this assumption is
   violated, then the security properties of TLS are severely weakened.

   As discussed in Section 6, there are use cases where it is desirable
   for multiple clients or multiple servers to share a PSK.  If this is
   done naively by having all members share a common key, then TLS only
   authenticates the entire group, 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 PSK with DH is used, then compromise of a group member that
       actively completes connections with other group members can read
       (and modify) traffic.

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   3.  If PSK without DH is used, then compromise of any group member
       allows the attacker to passively read (and modify) all traffic.

   4.  If a group member is compromised, then the attacker can perform
       all of the above attacks.

   Additionally, a malicious non-member can reroute handshakes between
   honest group members to connect them in unintended ways, as described
   below.  (Note that this class of attack is not possible if each
   member uses the SNI extension [RFC6066] and terminates the connection
   on mismatch.  See [Selfie] for details.)  Let the group of peers who
   know the key be "A", "B", and "C".  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 "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".

   This attack violates the peer authentication property, and if "C"
   supports a weaker set of cipher suites than "B", this attack also
   violates the downgrade protection property.  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 both as TLS server and 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 affects deployments.  For example, revocation of
   individual group members is not possible without changing the
   authentication key for all members.

   Entropy properties of external PSKs may also affect TLS security
   properties.  In particular, if a high entropy PSK is used, then PSK-
   only key establishment modes are secure against both active and
   passive attack.  However, they lack forward security.  Forward
   security may be achieved by using a PSK-DH mode.

   In contrast, if a low entropy PSK is used, then PSK-only key
   establishment modes are subject to passive exhaustive search passive
   attacks which will reveal the traffic keys.  PSK-DH modes are subject

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   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 PAKE is used with TLS.  The Crypto Forum Research Group (CFRG) is
   currently working on specifying a standard PAKE (see
   [I-D.irtf-cfrg-cpace] and [I-D.irtf-cfrg-opaque]).

5.  Privacy Properties

   PSK privacy properties are orthogonal to security properties
   described in Section 4.  Traditionally, 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 it
   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 oppress handshakes or application
   data from a specific device by blocking, delaying, or rate-limiting
   traffic.  Techniques for mitigating these risks require 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.

6.  External PSK Use Cases and Provisioning Processes

   PSK ciphersuites were first specified for TLS in 2005.  Now, PSKs are
   an integral part of the TLS version 1.3 specification [RFC8446].  TLS
   1.3 also uses PSKs for session resumption.  It distinguishes these
   resumption PSKs from external PSKs which have been provisioned out-
   of-band (OOB).  Below, we list some example use-cases where pair-wise
   external PSKs (i.e., external PSKs that are shared between only one
   server and one client) have been used for authentication in TLS.

   *  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.

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   *  Intra-data-center communication.  Machine-to-machine communication
      within a single data center or PoP may use externally provisioned
      PSKs, primarily for the purposes of supporting TLS connections
      with early data.

   *  Certificateless server-to-server communication.  Machine-to-
      machine communication may use externally provisioned PSKs,
      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
      capabilities.  [RFC7925] 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 Technical Specification [LwM2M]
      states that LwM2M servers MUST support the PSK mode of DTLS.

   *  Use of PSK ciphersuites are optional when securing RADIUS
      [RFC2865] with TLS as specified in [RFC6614].

   *  The Generic Authentication Architecture (GAA) defined by 3GGP
      mentions that TLS-PSK can be used between a server and user
      equipment for authentication [GAA].

   *  Smart Cards.  The electronic German ID (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 [RFC8773])
      because of the protection they provide against quantum computers.

   There are also use cases where PSKs are shared between more than two
   entities.  Some examples below (as noted by Akhmetzyanova et

   *  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.

   *  Internet of Things (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).

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   The exact provisioning process depends on the system requirements and
   threat model.  Generally, use of a single PSK shared between more
   than one node is not recommended, even if other accommodations are
   made, such as client certificate authentication after PSK-based
   connection establishment.  See Section 7.

6.1.  Provisioning Examples

   *  Many industrial protocols assume that PSKs are distributed and
      assigned manually via one of the following approaches: typing the
      PSK into the devices, or via web server masks (using a Trust On
      First Use (TOFU) approach with a device completely unprotected
      before the first login did take place).  Many devices have very
      limited UI.  For example, they may only have a numeric keypad or
      even less number of 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
      syncing protocol.

   *  Some secrets may be baked into or 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.

6.2.  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 pair-wise 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

7.  Recommendations for External PSK Usage

   If an application uses external PSKs, the external PSKs MUST adhere
   to the following requirements:

   1.  Each PSK SHOULD be derived from at least 128 bits of entropy,
       MUST be at least 128 bits long, and SHOULD be combined with a DH
       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

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       entropy, are subject to attack and SHOULD be avoided.  If only
       low-entropy keys are available, then key establishment mechanisms
       such as Password Authenticated Key Exchange (PAKE) that mitigate
       the risk of offline dictionary attacks SHOULD be employed.  Note
       that no such mechanisms have yet been standardised, and further
       that these mechanisms will not necessarily follow the same
       architecture as the process for incorporating EPSKs described in

   2.  Unless other accommodations are made, 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 nodes MAY be the same, in different roles.)  Two
       acceptable accommodations are described in
       [I-D.ietf-tls-external-psk-importer]: (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 EPSK importer.

   3.  Nodes using TLS 1.3 SHOULD use external PSK importers
       [I-D.ietf-tls-external-psk-importer] when configuring PSKs for a
       client-server pair.  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 in
       [I-D.ietf-tls-external-psk-importer] for more information.

   4.  Where possible the main PSK (that which is fed into the importer)
       SHOULD be deleted after the imported keys have been generated.
       This protects 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.

7.1.  Stack Interfaces

   Most major TLS implementations support external PSKs.  Stacks
   supporting external PSKs provide interfaces that applications may use
   when supplying them for individual connections.  Details about
   existing stacks at the time of writing are below.

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   *  OpenSSL and BoringSSL: Applications can specify support for
      external PSKs via distinct ciphersuites in TLS 1.2 and below.
      They also 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.  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, either as raw byte
      strings or hexadecimal strings.  The PSK identity and key size are
      not validated.

   *  wolfSSL: Applications configure PSKs with callbacks similar to

7.1.1.  PSK Identity Encoding and Comparison

   Section 5.1 of [RFC4279] 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 identites are configured
   manually it is important to be aware that due to encoding issues
   visually identical strings may, in fact, differ.

   TLS version 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).  [RFC8446] also requires
   that the PSK identities are at least 1 byte and at the most 65535
   bytes in length.  Although [RFC8446] does not place strict
   requirements on the format of PSK identities, we do however 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].
      gnuTLS for example treats PSK identities as usernames.

   *  PSK identities MAY have a domain name suffix for roaming and

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   *  Deployments should take care that the length of the PSK identity
      is sufficient to avoid collisions.

7.1.2.  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
   behaviour 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 will be given precedence
   over how to handle the PSK.

8.  Security Considerations

   It is NOT RECOMMENDED to share the same PSK between more than one
   client and server.  However, as discussed in Section 6, there are
   application scenarios that may rely on sharing the same PSK among
   multiple nodes.  [I-D.ietf-tls-external-psk-importer] 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 [I-D.ietf-tls-external-psk-importer].  It is RECOMMENDED
   that each endpoint selects one globally unique identifier and uses it
   in all PSK handshakes.  The unique identifier can, for example, be
   one of its MAC addresses, a 32-byte random number, or its Universally
   Unique IDentifier (UUID) [RFC4122].  Each endpoint SHOULD know the
   identifier of the other endpoint with which its wants to connect and
   SHOULD compare it with the other endpoint's identifier used in
   ImportedIdentity.context.  It is however important to remember that
   endpoints sharing the same group PSK can always impersonate each

9.  IANA Considerations

   This document makes no IANA requests.

10.  References

10.1.  Normative References

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              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-38, 29 May 2020, <

              Benjamin, D. and C. Wood, "Importing External PSKs for
              TLS", Work in Progress, Internet-Draft, draft-ietf-tls-
              external-psk-importer-05, 19 May 2020,

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

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

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

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

10.2.  Informative References

              Akhmetzyanova, L., Alekseev, E., Smyshlyaeva, E., and A.
              Sokolov, "Continuing to reflect on TLS 1.3 with external
              PSK", 2019, <>.

   [GAA]      "TR33.919 version 12.0.0 Release 12", n.d.,

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              Rescorla, E., Barnes, R., and H. Tschofenig, "Compact TLS
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              ctls-01, 2 November 2020, <

              Abdalla, M., Haase, B., and J. Hesse, "CPace, a balanced
              composable PAKE", Work in Progress, Internet-Draft, draft-
              irtf-cfrg-cpace-00, 28 July 2020, <

              Krawczyk, H., Lewi, K., and C. Wood, "The OPAQUE
              Asymmetric PAKE Protocol", Work in Progress, Internet-
              Draft, draft-irtf-cfrg-opaque-00, 28 September 2020,

              Mattsson, J., 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, <

   [Krawczyk] Krawczyk, H., "SIGMA: The ‘SIGn-and-MAc’ Approach to
              Authenticated Diffie-Hellman and Its Use in the IKE
              Protocols", Annual International Cryptology Conference.
              Springer, Berlin, Heidelberg , 2003,

   [LwM2M]    "Lightweight Machine to Machine Technical Specification",

   [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,

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, Ed., "Extensible Authentication Protocol
              (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,

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   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,

   [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,

   [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,

   [RFC7925]  Tschofenig, H., Ed. and T. Fossati, "Transport Layer
              Security (TLS) / Datagram Transport Layer Security (DTLS)
              Profiles for the Internet of Things", RFC 7925,
              DOI 10.17487/RFC7925, July 2016,

   [RFC8773]  Housley, R., "TLS 1.3 Extension for Certificate-Based
              Authentication with an External Pre-Shared Key", RFC 8773,
              DOI 10.17487/RFC8773, March 2020,

   [Selfie]   Drucker, N. and S. Gueron, "Selfie: reflections on TLS 1.3
              with PSK", 2019, <>.

   [Sethi]    Sethi, M., Peltonen, A., and T. Aura, "Misbinding Attacks
              on Secure Device Pairing and Bootstrapping", Proceedings
              of the 2019 ACM Asia Conference on Computer and
              Communications Security , 2019,

              "Technical Guideline TR-03112-7 eCard-API-Framework –
              Protocols", 2015, <

Appendix A.  Acknowledgements

   This document is the output of the TLS External PSK Design Team,
   comprised of the following members: Benjamin Beurdouche, Bjoern
   Haase, Christopher Wood, Colm MacCarthaigh, Eric Rescorla, Jonathan
   Hoyland, Martin Thomson, Mohamad Badra, Mohit Sethi, Oleg Pekar, Owen
   Friel, and Russ Housley.

Housley, et al.            Expires 6 May 2021                  [Page 13]

Internet-Draft   Guidance for External PSK Usage in TLS    November 2020

Authors' Addresses

   Russ Housley
   Vigil Security


   Jonathan Hoyland
   Cloudflare Ltd.


   Mohit Sethi


   Christopher A. Wood


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