tls                                                          D. Benjamin
Internet-Draft                                              Google, LLC.
Intended status: Experimental Standards Track                                 C. Wood
Expires: November 15, 2019 April 4, 2020                                       Apple, Inc.
                                                            May 14,
                                                        October 02, 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 is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 15, 2019. April 4, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   2   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Key Import  . . . . . . . . . . . . . . . . . . . . . . . . .   3   4
   5.  Label Values  . . . . . . .  Deprecating Hash Functions  . . . . . . . . . . . . . . . . .   5
   6.  Deprecating Hash Functions  . . .  Incremental Deployment  . . . . . . . . . . . . . .   5
   7.  Backwards Compatibility and Incremental Deployment . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   9.   6
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   6
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     11.2.   6
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8   7
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   8
   Appendix B.  Addressing Selfie  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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 KDF (and hash function) 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 different hash
   algorithms to produce
   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 hash function. 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.

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

   Intuitively, key

   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 label target
   protocol and set of hash algorithms 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.  (Negotiation  Non-imported and use of imported
   PSKs requires both endpoints support are distinct since their identities are different on the importer API
   described herein.) 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, KDF).  The associated KDF (and hash function)
      may be undefined. 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.

   o  Imported Identity: The identity of a 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', "epsk", as defined in Section 3.1,
   along with an optional label, 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 HashAlgorithm
   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 label<0..2^8-1>;
          HashAlgorithm hash; context<0..2^16-1>;
      uint16 target_protocol;
      uint16 target_kdf;
   } ImportedIdentity;

   [[TODO: An alternative design might combine label and hash into the
   same field so that future protocols which don't have a notion

   The list of
   HashAlgorithm don't need this field.]]

   ImportedIdentity.label "target_kdf" values is maintained by IANA as described in
   Section 9.  External PSKs MUST NOT be bound to the protocol imported for versions of (D)TLS
   1.2 or prior versions.  See Section 6 for discussion on how imported
   PSKs for which the
   key is imported.  Thus, TLS 1.3 and QUICv1 [I-D.ietf-quic-transport] non-imported PSKs for earlier versions co-exist
   for incremental deployment.

   ImportedIdentity.context MUST use "tls13" as 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 label.  Similarly, 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 1.2 1.3 [RFC8446]
   and all prior TLS
   versions should QUICv1 [QUIC] use "tls12" as ImportedIdentity.label, as well as
   SHA256 as ImportedIdentity.hash. 0x0304.  Note that this means future versions
   of TLS will increase the number of PSKs derived from an external PSK.

   A unique and imported

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

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

   [[TODO: The L)

   L is corresponds to the KDF output length of ipskx MUST match that
   ImportedIdentity.target_kdf as defined in Section 9.  For hash-based
   KDFs, such as HKDF_SHA256(0x0001), this is the length of the corresponding and hash
   function output, i.e., 32 octets.  This is required for the IPSK to
   be of length suitable for supported ciphersuites.]] 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 external PSK. EPSK.  It is not bound to ImportedIdentity.hash. the hash function associated with
   ImportedIdentity.target_kdf.  If no hash function is specified,
   SHA-256 MUST be used.  Differentiating
   epsk  Diversifying EPSK by ImportedIdentity.hash
   ImportedIdentity.target_kdf ensures that each imported PSK an IPSK is only used with as
   input keying material to at most one hash function, KDF, thus satisfying the
   requirements in [RFC8446].

   Endpoints MUST import and derive an ipsk generate a compatible ipskx for each hash
   function used by each target ciphersuite
   they support. offer.  For example, importing a key for TLS_AES_128_GCM_SHA256
   and TLS_AES_256_GCM_SHA384 would yield two PSKs, one for SHA256 HKDF-SHA256
   and another for SHA384. 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 hash functions, 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.  Label Values

   For clarity, the following table specifies PSK importer labels for
   varying instances of the TLS handshake.

              |             Protocol             |  Label   |
              |        TLS 1.3 [RFC8446]         | "tls13"  |
              |                                  |          |
              | QUICv1 [I-D.ietf-quic-transport] | "tls13"  |
              |                                  |          |
              |        TLS 1.2 [RFC5246]         | "tls12"  |
              |                                  |          |
              |        DTLS 1.2 [RFC6347]        | "dtls12" |
              |                                  |          |
              |  DTLS 1.3 [I-D.ietf-tls-dtls13]  | "dtls13" |

6.  Deprecating Hash Functions

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

7.  Backwards Compatibility and

6.  Incremental Deployment

   Recall that TLS 1.2 permits computing the TLS PRF with any hash
   algorithm and PSK.  Thus, an external PSK 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 identity, target protocol, and hash function. 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
   does not preclude endpoints from using non-imported PSKs for TLS 1.2.
   Indeed, this is necessary for incremental deployment.


7.  Security Considerations

   DISCLAIMER: This is a WIP draft and has not yet seen significant
   security analysis.


8.  Privacy Considerations

   DISCLAIMER: This section contains a sketch of a design for protecting
   external PSK identities.  It is not meant to be implementable as


   External PSK identities are typically static by design so that
   endpoints may use them to lookup keying material.  For  However, for some
   systems and use cases, this identity may become a persistent tracking
   identifier.  One mitigation to this problem is encryption.  Future
   drafts may specify

9.  IANA Considerations

   This specification introduces a way new registry for encrypting PSK identities using a
   mechanism similar to that of TLS KDF identifiers
   and defines the Encrypted SNI proposal
   [I-D.ietf-tls-esni].  Another approach is following target KDF values:

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

   New target KDF values are allocated according to replace the identity
   with an unpredictable or "obfuscated" value derived from the
   corresponding PSK.  One such proposal, derived from a design outlined
   in [I-D.ietf-dnssd-privacy], is as follows.  Let ipskx be the
   imported PSK with identity ImportedIdentity, and N be a unique nonce
   of length equal to that of ImportedIdentity.hash.  With these values,
   construct the following "obfuscated" identity:

      struct {
          opaque nonce[hash.length];
          opaque obfuscated_identity<1..2^16-1>;
          HashAlgorithm hash;
      } ObfuscatedIdentity;

   ObfuscatedIdentity.nonce carries N,
   ObfuscatedIdentity.obfuscated_identity carries HMAC(ipskx, N), where
   HMAC is computed with ImportedIdentity.hash, and
   ObfuscatedIdentity.hash is ImportedIdentity.hash.

   Upon receipt of such an obfuscated identity, a peer must lookup

   o  Values in the
   corresponding PSK by exhaustively trying to compute
   ObfuscatedIdentity.obfuscated_identity using ObfuscatedIdentity.nonce
   and each of its known imported PSKs.  If N is chosen range 0x0000-0xfeff are assigned via Specification
      Required [RFC8126].

   o  Values in a predictable
   fashion, e.g., as a timestamp, it may be possible for peers to
   precompute these obfuscated identities to ease the burden of trial
   decryption. range 0xff00-0xffff are reserved for Private Use

10.  IANA Considerations

   This document makes no IANA requests.

11.  References

10.1.  Normative References


   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-20 draft-ietf-quic-transport-23 (work
              in progress), April 2019.

              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-31 (work in progress), March 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, <https://www.rfc-

   [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, <https://www.rfc-

   [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-

   [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, <https://www.rfc-

   [RFC6347]  Rescorla, E.

   [RFC8126]  Cotton, M., Leiba, B., and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 6347, 8126, DOI 10.17487/RFC6347,
              January 2012, <>. 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

              Huitema, C. and D. Kaiser, "Privacy Extensions for DNS-
              SD", draft-ietf-dnssd-privacy-05 (work in progress),
              October 2018.

              Rescorla, E., Oku,

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

   [Selfie]   Drucker, N. and S. Gueron, "Selfie: reflections on TLS 1.3", draft-
              ietf-tls-esni-03 (work in progress), March 2019. 1.3
              with PSK", 2019, <>.

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

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
   Apple, Inc.