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Importing External PSKs for TLS

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9258.
Authors David Benjamin , Christopher A. Wood
Last updated 2021-02-24 (Latest revision 2020-12-03)
Replaces draft-wood-tls-external-psk-importer
RFC stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication
Associated WG milestone
Jan 2021
Submit "Importing External PSKs for TLS" to the IESG
Document shepherd Joseph A. Salowey
Shepherd write-up Show Last changed 2020-05-23
IESG IESG state Became RFC 9258 (Proposed Standard)
Consensus boilerplate Yes
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Responsible AD Roman Danyliw
Send notices to Joseph Salowey <>
IANA IANA review state IANA OK - Actions Needed
tls                                                          D. Benjamin
Internet-Draft                                              Google, LLC.
Intended status: Standards Track                               C.A. Wood
Expires: 6 June 2021                                          Cloudflare
                                                         3 December 2020

                    Importing External PSKs for TLS


   This document describes an interface for importing external Pre-
   Shared Keys (PSKs) into TLS 1.3.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 6 June 2021.

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   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  PSK Import  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  External PSK Diversification  . . . . . . . . . . . . . .   4
     4.2.  Binder Key Derivation . . . . . . . . . . . . . . . . . .   6
   5.  Deprecating Hash Functions  . . . . . . . . . . . . . . . . .   6
   6.  Incremental Deployment  . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  10
   Appendix B.  Addressing Selfie  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   TLS 1.3 [RFC8446] supports Pre-Shared Key (PSK) authentication,
   wherein PSKs can be established via session tickets from prior
   connections or externally via some out-of-band mechanism.  The
   protocol mandates that each PSK only be used with a single hash
   function.  This was done to simplify protocol analysis.  TLS 1.2
   [RFC5246], in contrast, has no such requirement, as a PSK may be used
   with any hash algorithm and the TLS 1.2 pseudorandom function (PRF).
   While there is no known way in which the same external PSK might
   produce related output in TLS 1.3 and prior versions, only limited
   analysis has been done.  Applications SHOULD provision separate PSKs
   for TLS 1.3 and prior versions.

   To mitigate against any interference, this document specifies a PSK
   Importer interface by which external PSKs may be imported and
   subsequently bound to a specific key derivation function (KDF) and
   hash function for use in TLS 1.3 [RFC8446] and DTLS 1.3 [DTLS13].  In
   particular, it describes a mechanism for differentiating external
   PSKs by the target KDF, (D)TLS protocol version, and an optional
   context string.  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

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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.  Overview

   The PSK Importer interface mirrors that of the TLS Exporters
   interface in that it diversifies a key based on some contextual
   information.  In contrast to the Exporters interface, wherein
   differentiation is done via an explicit label and context string, the
   PSK Importer interface defined herein takes an external PSK and
   identity and "imports" it into TLS, creating a set of "derived" PSKs
   and identities.  Each of these derived PSKs are bound a target
   protocol, KDF identifier, and optional context string.  Additionally,
   the resulting PSK binder keys are modified with a new derivation
   label to prevent confusion with non-imported PSKs.  Through this
   interface, importing external PSKs with different identities yields
   distinct PSK binder keys.

   Imported keys do not require negotiation for use since a client and
   server will not agree upon identities if imported incorrectly.
   Endpoints may incrementally deploy PSK Importer support by offering
   non-imported keys for TLS versions prior to TLS 1.3.  Non-imported
   and imported PSKs are distinct since their identities are different
   on the wire.  See Section 6 for more details.

   Endpoints which import external keys MUST NOT use either the external
   keys or the derived keys for any other purpose.  Moreover, each
   external PSK MUST be associated with at most one hash function, as
   per the rules in Section 4.2.11 from [RFC8446].  See Section 7 for
   more discussion.

3.1.  Terminology

   *  External PSK (EPSK): A PSK established or provisioned out-of-band,
      i.e., not from a TLS connection, which is a tuple of (Base Key,
      External Identity, Hash).

   *  Base Key: The secret value of an EPSK.

   *  External Identity: A sequence of bytes used to identify an EPSK.

   *  Target protocol: The protocol for which a PSK is imported for use.

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   *  Target KDF: The KDF for which a PSK is imported for use.

   *  Imported PSK (IPSK): A PSK derived from an EPSK, External
      Identity, optional context string, target protocol, and target

   *  Imported Identity: A sequence of bytes used to identify an IPSK.

4.  PSK Import

   This section describes the PSK Importer interface and its underlying
   diversification mechanism and binder key computation modification.

4.1.  External PSK Diversification

   The PSK Importer interface takes as input an EPSK with External
   Identity "external_identity" and base key "epsk", as defined in
   Section 3.1, along with an optional context, and transforms it into a
   set of PSKs and imported identities for use in a connection based on
   target protocols and KDFs.  In particular, for each supported target
   protocol "target_protocol" and KDF "target_kdf", the importer
   constructs an ImportedIdentity structure as follows:

   struct {
      opaque external_identity<1...2^16-1>;
      opaque context<0..2^16-1>;
      uint16 target_protocol;
      uint16 target_kdf;
   } ImportedIdentity;

   The list of ImportedIdentity.target_kdf values is maintained by IANA
   as described in Section 9.  External PSKs MUST NOT be imported for
   (D)TLS 1.2 or prior versions.  See Section 6 for discussion on how
   imported PSKs for TLS 1.3 and non-imported PSKs for earlier versions
   co-exist for incremental deployment.

   ImportedIdentity.context MUST include the context used to derive the
   EPSK, if any exists.  For example, ImportedIdentity.context may
   include information about peer roles or identities to mitigate
   Selfie-style reflection attacks [Selfie].  See Appendix B for more
   details.  If the EPSK is a key derived from some other protocol or
   sequence of protocols, ImportedIdentity.context MUST include a
   channel binding for the deriving protocols [RFC5056].  The details of
   this binding are protocol specific and out of scope for this

   ImportedIdentity.target_protocol MUST be the (D)TLS protocol version
   for which the PSK is being imported.  For example, TLS 1.3 [RFC8446]

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   uses 0x0304, which will therefore also be used by QUICv1 [QUIC].
   Note that this means future versions of TLS will increase the number
   of PSKs derived from an external PSK.

   Given an ImportedIdentity and corresponding EPSK with base key
   "epsk", an Imported PSK IPSK with base key "ipskx" is computed as

      epskx = HKDF-Extract(0, epsk)
      ipskx = HKDF-Expand-Label(epskx, "derived psk",
                                Hash(ImportedIdentity), L)

   L corresponds to the KDF output length of ImportedIdentity.target_kdf
   as defined in Section 9.  For hash-based KDFs, such as
   HKDF_SHA256(0x0001), this is the length of the hash function output,
   i.e., 32 octets.  This is required for the IPSK to be of length
   suitable for supported ciphersuites.

   The identity of "ipskx" as sent on the wire is ImportedIdentity,
   i.e., the serialized content of ImportedIdentity is used as the
   content of PskIdentity.identity in the PSK extension.  The
   corresponding TLS 1.3 binder key is "ipskx".

   The hash function used for HKDF [RFC5869] is that which is associated
   with the EPSK.  It is not the hash function associated with
   ImportedIdentity.target_kdf.  If no hash function is specified,
   SHA-256 [SHA2] MUST be used.  Diversifying EPSK by
   ImportedIdentity.target_kdf ensures that an IPSK is only used as
   input keying material to at most one KDF, thus satisfying the
   requirements in [RFC8446].  See Section 7 for more details.

   Endpoints SHOULD generate a compatible "ipskx" for each target
   ciphersuite they offer.  For example, importing a key for
   TLS_AES_128_GCM_SHA256 and TLS_AES_256_GCM_SHA384 would yield two
   PSKs, one for HKDF-SHA256 and another for HKDF-SHA384.  In contrast,
   if TLS_AES_128_GCM_SHA256 and TLS_CHACHA20_POLY1305_SHA256 are
   supported, only one derived key is necessary.

   EPSKs MAY be imported before the start of a connection if the target
   KDFs, protocols, and context string(s) are known a priori.  EPSKs MAY
   also be imported for early data use if they are bound to protocol
   settings and configurations that would otherwise be required for
   early data with normal (ticket-based PSK) resumption.  Minimally,
   that means Application-Layer Protocol Negotiation [RFC7301], QUIC
   transport parameters (if used for QUIC), and any other relevant
   parameters that are negotiated for early data MUST be provisioned
   alongside these EPSKs.

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4.2.  Binder Key Derivation

   To prevent confusion between imported and non-imported PSKs, imported
   PSKs change the PSK binder key derivation label.  In particular, the
   standard TLS 1.3 PSK binder key computation is defined as follows:

    PSK ->  HKDF-Extract = Early Secret
              +-----> Derive-Secret(., "ext binder" | "res binder", "")
              |                     = binder_key

   Imported PSKs replace the string "ext binder" with "imp binder" when
   deriving "binder_key".  This means the binder key is computed as

    PSK ->  HKDF-Extract = Early Secret
              +-----> Derive-Secret(., "ext binder"
              |                      | "res binder"
              |                      | "imp binder", "")
              |                     = binder_key

   This new label ensures a client and server will negotiate use of an
   external PSK if and only if (a) both endpoints import the PSK or (b)
   neither endpoint imports the PSK.  As "binder_key" is a leaf key,
   changing its computation does not affect any other key.

5.  Deprecating Hash Functions

   If a client or server wish to deprecate a hash function and no longer
   use it for TLS 1.3, they remove the corresponding KDF from the set of
   target KDFs used for importing keys.  This does not affect the KDF
   operation used to derive Imported PSKs.

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6.  Incremental Deployment

   Recall that TLS 1.2 permits computing the TLS PRF with any hash
   algorithm and PSK.  Thus, an EPSK may be used with the same KDF (and
   underlying HMAC hash algorithm) as TLS 1.3 with importers.  However,
   critically, the derived PSK will not be the same since the importer
   differentiates the PSK via the identity and target KDF and protocol.
   Thus, PSKs imported for TLS 1.3 are distinct from those used in TLS
   1.2, and thereby avoid cross-protocol collisions.  Note that this
   does not preclude endpoints from using non-imported PSKs for TLS 1.2.
   Indeed, this is necessary for incremental deployment.  Specifically,
   existing applications using TLS 1.2 with non-imported PSKs can safely
   enable TLS 1.3 with imported PSKs in clients and servers without
   interoperability risk.

7.  Security Considerations

   The PSK Importer security goals can be roughly stated as follows:
   avoid PSK re-use across KDFs while properly authenticating endpoints.
   When modeled as computational extractors, KDFs assume that input
   keying material (IKM) is sampled from some "source" probability
   distribution and that any two IKM values are chosen independently of
   each other [Kraw10].  This source-independence requirement implies
   that the same IKM value cannot be used for two different KDFs.

   PSK-based authentication is functionally equivalent to session
   resumption in that a connection uses existing key material to
   authenticate both endpoints.  Following the work of [BAA15], this is
   a form of compound authentication.  Loosely speaking, compound
   authentication is the property that an execution of multiple
   authentication protocols, wherein at least one is uncompromised,
   jointly authenticates all protocols.  Authenticating with an
   externally provisioned PSK, therefore, should ideally authenticate
   both the TLS connection and the external provisioning process.
   Typically, the external provision process produces a PSK and
   corresponding context from which the PSK was derived and in which it
   should be used.  If available, this is used as the
   ImportedIdentity.context value.  We refer to an external PSK without
   such context as "context-free".

   Thus, in considering the source-independence and compound
   authentication requirements, the PSK Import interface described in
   this document aims to achieve the following goals:

   1.  Externally provisioned PSKs imported into a TLS connection
       achieve compound authentication of the provisioning process and

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   2.  Context-free PSKs only achieve authentication within the context
       of a single connection.

   3.  Imported PSKs are not used as IKM for two different KDFs.

   4.  Imported PSKs do not collide with future protocol versions and

   There is no known interference between the process for computing
   Imported PSKs from an external PSK and the processing of existing
   external PSKs used in (D)TLS 1.2 and below.  However, only limited
   analysis has been done, which is an additional reason why
   applications SHOULD provision separate PSKs for (D)TLS 1.3 and prior
   versions, even when the importer interface is used in (D)TLS 1.3.

   The PSK Importer does not prevent applications from constructing non-
   importer PSK identities that collide with imported PSK identities.

8.  Privacy Considerations

   External PSK identities are typically static by design so that
   endpoints may use them to lookup keying material.  However, for some
   systems and use cases, this identity may become a persistent tracking

9.  IANA Considerations

   This specification introduces a new registry for TLS KDF identifiers,
   titled "TLS KDF Identifiers", under the existing "Transport Layer
   Security (TLS) Parameters" heading.

   The entries in the registry are:

                 | KDF Description | Value  | Reference |
                 | Reserved        | 0x0000 | N/A       |
                 | HKDF_SHA256     | 0x0001 | [RFC5869] |
                 | HKDF_SHA384     | 0x0002 | [RFC5869] |

                       Table 1: Target KDF Registry

   New target KDF values are allocated according to the following

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   *  Values in the range 0x0000-0xfeff are assigned via Specification
      Required [RFC8126].

   *  Values in the range 0xff00-0xffff are reserved for Private Use

   The procedures for requesting values in the Specification Required
   space are specified in Section 17 of [RFC8447].

10.  References

10.1.  Normative References

   [DTLS13]   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-
              dtls13-39, 2 November 2020, <

   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-32, 20 October 2020,

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

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

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   [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,

   [RFC8447]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,

10.2.  Informative References

   [BAA15]    Bhargavan, K., Delignat-Lavaud, A., and A. Pironti,
              "Verified Contributive Channel Bindings for Compound
              Authentication", DOI 10.14722/ndss.2015.23277, Proceedings
              2015 Network and Distributed System Security Symposium,
              2015, <>.

   [Kraw10]   Krawczyk, H., "Cryptographic Extraction and Key
              Derivation: The HKDF Scheme", Proceedings of CRYPTO 2010 ,
              2010, <>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <>.

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

   [SHA2]     National Institute of Standards and Technology, "Secure
              Hash Standard", FIPS PUB 180-3 , October 2008.

Appendix A.  Acknowledgements

   The authors thank Eric Rescorla and Martin Thomson for discussions
   that led to the production of this document, as well as Christian
   Huitema for input regarding privacy considerations of external PSKs.
   John Mattsson provided input regarding PSK importer deployment
   considerations.  Hugo Krawczyk provided guidance for the security
   considerations.  Martin Thomson, Jonathan Hoyland, Scott Hollenbeck
   and others all provided reviews, feedback, and suggestions for
   improving the document.

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Appendix B.  Addressing Selfie

   The Selfie attack [Selfie] relies on a misuse of the PSK interface.
   The PSK interface makes the implicit assumption that each PSK is
   known only to one client and one server.  If multiple clients or
   multiple servers with distinct roles share a PSK, TLS only
   authenticates the entire group.  A node successfully authenticates
   its peer as being in the group whether the peer is another node or

   Applications which require authenticating finer-grained roles while
   still configuring a single shared PSK across all nodes can resolve
   this mismatch either by exchanging roles over the TLS connection
   after the handshake or by incorporating the roles of both the client
   and server into the IPSK context string.  For instance, if an
   application identifies each node by MAC address, it could use the
   following context string.

     struct {
       opaque client_mac<0..2^16-1>;
       opaque server_mac<0..2^16-1>;
     } Context;

   If an attacker then redirects a ClientHello intended for one node to
   a different node, the receiver will compute a different context
   string and the handshake will not complete.

   Note that, in this scenario, there is still a single shared PSK
   across all nodes, so each node must be trusted not to impersonate
   another node's role.

Authors' Addresses

   David Benjamin
   Google, LLC.


   Christopher A. Wood


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