Importing External PSKs for TLS
draft-ietf-tls-external-psk-importer-07

tls                                                          D. Benjamin
Internet-Draft                                              Google, LLC.
Intended status: Standards Track                              C. A. Wood
Expires: 8 September 2022                                     Cloudflare
                                                            7 March 2022

                    Importing External PSKs for TLS
                draft-ietf-tls-external-psk-importer-07

Abstract

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

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
   https://github.com/tlswg/draft-ietf-tls-external-psk-importer.

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
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   This Internet-Draft will expire on 8 September 2022.

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   Copyright (c) 2022 IETF Trust and the persons identified as the
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   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  PSK Import  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  External PSK Diversification  . . . . . . . . . . . . . .   4
     4.2.  Binder Key Derivation . . . . . . . . . . . . . . . . . .   6
   5.  Deprecating Hash Functions  . . . . . . . . . . . . . . . . .   7
   6.  Incremental Deployment  . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  12
   Appendix B.  Addressing Selfie  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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.  In cases where this is not possible,
   e.g., there are already deployed external PSKs or provisioning is
   otherwise limited, re-using external PSKs across different versions
   of TLS may produce related outputs, which may in turn lead to
   security problems; see [RFC8446], Section E.7.

   To mitigate against such problems, 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

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   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
   identities.

2.  Conventions and Definitions

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

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 the keys that are
   input to the import process for any purpose other than the importer,
   and MUST NOT use the derived keys for any purpose other than TLS
   PSKs.  Moreover, each external PSK fed to the importer process MUST
   be associated with at most one hash function.  This is analogous to
   the rules in Section 4.2.11 of [RFC8446].  See Section 7 for more
   discussion.

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3.1.  Terminology

   The following terms are used throughout this document:

   *  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.

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

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

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

   This document uses presentation language from [RFC8446], Section 3.

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;

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   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 determine
   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
   document.

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

      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,
   e.g., 32 octets for SHA256.  This is required for the IPSK to be of
   length suitable for supported ciphersuites.  Internally, HKDF-Expand-
   Label uses a label corresponding to ImportedIdentity.target_protocol,
   e.g., "tls13" for TLS 1.3, as per [RFC8446], Section 7.1, or "dtls13"
   for DTLS 1.3, as per [I-D.ietf-tls-dtls13], Section 5.10.

   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 PSK
   input for the TLS 1.3 key schedule is 'ipskx'.

   As the maximum size of the PSK extension is 2^16 - 1 octets, an
   Imported Identity that exceeds this size is likely to cause a
   decoding error.  Therefore, the PSK Importer interface SHOULD reject
   any ImportedIdentity that exceeds this size.

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   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] SHOULD 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.  Each ciphersuite
   uniquely identifies the target KDF.  Future specifications that
   change the way the KDF is negotiated will need to update this
   specification to make clear how target KDFs are determined for the
   import process.

   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 the protocol
   settings and configuration that are required for sending early data.
   Minimally, that means Application-Layer Protocol Negotiation value
   [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.

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:

              0
              |
              v
    PSK ->  HKDF-Extract = Early Secret
              |
              +-----> Derive-Secret(., "ext binder" | "res binder", "")
              |                     = binder_key
              V

   Imported PSKs use the string "imp binder" rather than "ext binder" or
   "res binder" when deriving binder_key.  This means the binder key is
   computed as follows:

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              0
              |
              v
    PSK ->  HKDF-Extract = Early Secret
              |
              +-----> Derive-Secret(., "ext binder"
              |                      | "res binder"
              |                      | "imp binder", "")
              |                     = binder_key
              V

   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.

6.  Incremental Deployment

   The mechanism defined in this document requires that an EPSK is only
   ever used as an EPSK and not for any other purpose.  In particular,
   this requirement disallows direct use of the EPSK as a PSK in TLS
   1.2.  The importer process produces distinct IPSKs derived from the
   target protocol and KDF, which in turn protects against cross-
   protocol collisions for protocol versions using this process by
   ensuring that each IPSK can only be used with one protocol and KDF.
   This is a distinct contrast to TLS 1.2, where a given PSK might be
   used with multiple KDFs in different handshakes, and importers are
   not available.  Furthermore, the KDF used in TLS 1.2 might be the
   same KDF used by the importer mechanism itself.

   In deployments that already have PSKs provisioned and in use with TLS
   1.2, attempting to incrementally deploy the importer mechanism would
   then result in concurrent use of the already provisioned PSK both
   directly as a TLS 1.2 PSK and as an EPSK, which in turn could mean
   that the same KDF and key would be used in two different protocol
   contexts.  There are no known related outputs or security issues that
   would arise from this arrangement.  However, only limited analysis
   has been done, and as such is not a recommended configuration.

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   However, the benefits of using TLS 1.3 and of using PSK importers may
   prove sufficiently compelling that existing deployments choose to
   enable this noncompliant configuration for a brief transition period
   while new software (using TLS 1.3 and importers) is deployed.
   Operators are advised to make any such transition period as short as
   possible.

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
       connection.

   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
       KDFs.

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   There are no known related outputs or security issues caused from 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, as
   noted in Section 6.  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
   identifier.

   Note also that ImportedIdentity.context is visible in cleartext on
   the wire as part of the PSK identity.  Unless otherwise protected by
   a mechanism such as TLS Encrypted ClientHello [ECH], applications
   SHOULD not put sensitive information in this field.

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
   process:

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

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   *  Values in the range 0xff00-0xffff are reserved for Private Use
      [RFC8126].

   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-
              dtls13-43, 30 April 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              dtls13-43>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
              <https://www.rfc-editor.org/rfc/rfc5056>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/rfc/rfc5869>.

   [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,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

   [RFC8447]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8447>.

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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,
              2015, <https://doi.org/10.14722/ndss.2015.23277>.

   [ECH]      Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-14, 13 February 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              esni-14>.

   [I-D.ietf-tls-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-
              dtls13-43, 30 April 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              dtls13-43>.

   [Kraw10]   Krawczyk, H., "Cryptographic Extraction and Key
              Derivation: The HKDF Scheme", Proceedings of CRYPTO 2010 ,
              2010, <https://eprint.iacr.org/2010/264>.

   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-34, 14 January 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              transport-34>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/rfc/rfc5246>.

   [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, <https://www.rfc-editor.org/rfc/rfc7301>.

   [Selfie]   Drucker, N. and S. Gueron, "Selfie: reflections on TLS 1.3
              with PSK", 2019, <https://eprint.iacr.org/2019/347.pdf>.

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

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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
   considerations.  Martin Thomson, Jonathan Hoyland, Scott Hollenbeck,
   Benjamin Kaduk, and others all provided reviews, feedback, and
   suggestions for improving the document.

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
   itself.  Note that this case can also occur when there are two nodes
   sharing a PSK without predetermined roles.

   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..2^8-1>;
       opaque server_mac<0..2^8-1>;
     } Context;

   If an attacker then redirects a ClientHello intended for one node to
   a different node, including the node that generated the ClientHello,
   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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   Email: davidben@google.com

   Christopher A. Wood
   Cloudflare
   Email: caw@heapingbits.net

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