tls                                                          D. Benjamin
Internet-Draft                                              Google, LLC.
Intended status: Standards Track                                 C. Wood
Expires: May 7, 2020                                         Apple, Inc.
                                                       November 04, 2019

                    Importing External PSKs for TLS


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

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   This Internet-Draft will expire on May 7, 2020.

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Table of Contents

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

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 PRF.  This means that
   external PSKs could possibly be re-used in two different contexts
   with the same hash functions during key derivation.  Moreover, it
   requires external PSKs to be provisioned for specific hash functions.

   To mitigate these problems, external PSKs can be bound to a specific
   KDF and hash function when used in TLS 1.3, even if they are
   associated with a different hash function when provisioned.  This
   document specifies an interface by which external PSKs may be
   imported for use in a TLS 1.3 connection to achieve this goal.  In
   particular, it describes how KDF-bound PSKs can be differentiated by
   the target (D)TLS protocol version and KDF for which the PSK will be
   used.  This produces a set of candidate PSKs, each of which are bound
   to a specific target protocol and KDF.  This expands what would
   normally have been a single PSK identity into a set of PSK
   identities.  However, importantly, it requires no change to the TLS
   1.3 key schedule.

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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

   Key importers mirror the concept of key exporters in TLS in that they
   diversify a key based on some contextual information before use in a
   connection.  In contrast to key exporters, wherein differentiation is
   done via an explicit label and context string, the key importer
   defined herein uses an optional context string along with a target
   protocol and KDF identifier to differentiate an external PSK into one
   or more PSKs for use.

   Imported keys do not require negotiation for use, as a client and
   server will not agree upon identities if not imported correctly.
   Thus, importers induce no protocol changes with the exception of
   expanding the set of PSK identities sent on the wire.  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.

   Clients which import external keys TLS MUST NOT use these keys for
   any other purpose.  Moreover, each external PSK MUST be associated
   with at most one hash function.

3.1.  Terminology

   o  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).

   o  Base Key: The secret value of an EPSK.

   o  External Identity: The identity of an EPSK.

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

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

   o  Imported PSK (IPSK): A PSK derived from an EPSK, external
      identity, optional context string, and target protocol and KDF.

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   o  Imported Identity: The identity of an Imported PSK as sent on the

4.  Key Import

   A key importer 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 supported
   (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 "target_kdf" values is maintained by IANA as described in
   Section 9.  External PSKs MUST NOT be imported for versions of (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.  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].

   ImportedIdentity.target_protocol MUST be the (D)TLS protocol version
   for which the PSK is being imported.  For example, TLS 1.3 [RFC8446]
   and QUICv1 [QUIC] use 0x0304.  Note that this means future versions
   of TLS will increase the number of PSKs derived from an external PSK.

   An Imported PSK derived from an EPSK with base key 'epsk' bound to
   this identity is then computed as follows:

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

   L is corresponds to the KDF output length of
   ImportedIdentity.target_kdf as defined in Section 9.  For hash-based

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   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.

   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 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].

   Endpoints 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.

   The resulting IPSK base key 'ipskx' is then used as the binder key in
   TLS 1.3 with identity ImportedIdentity.  With knowledge of the
   supported KDFs, one may import PSKs before the start of a connection.

   EPSKs may 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 ALPN, QUIC transport settings, etc., must be provisioned
   alongside these EPSKs.

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.

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, target protocol, and target
   KDF.  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

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   does not preclude endpoints from using non-imported PSKs for TLS 1.2.
   Indeed, this is necessary for incremental deployment.

7.  Security Considerations

   The Key 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 provision process.
   Typically, the external provision process produces a PSK and
   corresponding context from which the PSK was derived and in wihch it
   should be used.  We refer to an external PSK without such context as
   "context free".

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

   1.  Externally provisioned PSKs imported into TLS achieve compound
       authentication of the provision step(s) and connection.

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

   3.  Imported PSKs are used as IKM for two different KDFs.

   4.  Imported PSKs do not collide with existing PSKs used for TLS 1.2
       and below.

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

   [[ TODO: point to stable reference which describes the analysis of
   these goals ]]

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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
   and defines the following target KDF values:

   +-------------+-----+ | Description | Value | +-------------+-----+ |
   Reserved | 0x0000 | | | | | HKDF_SHA256 | 0x0001 | | | | |
   HKDF_SHA384 | 0x0002 | +-------------+-----+

   New target KDF values are allocated according to the following

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

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

10.  References

10.1.  Normative References

   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-23 (work
              in progress), September 2019.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [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,

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   [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,

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,

   [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,

   [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

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

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

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

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

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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

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.


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   Christopher A. Wood
   Apple, Inc.


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