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Versions: 00 01 draft-ietf-tls-external-psk-importer

tls                                                          D. Benjamin
Internet-Draft                                              Google, LLC.
Intended status: Experimental                                    C. Wood
Expires: September 12, 2019                                  Apple, Inc.
                                                          March 11, 2019

                    Importing External PSKs for TLS


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

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 September 12, 2019.

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   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

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

1.  Introduction

   TLS 1.3 [RFC8446] supports pre-shared key (PSK) resumption, 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, 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
   hash function when used in TLS 1.3, even if they are associated with
   a different KDF (and 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 different hash
   algorithms to produce a set of candidate PSKs, each of which are
   bound to a specific hash function.  This expands what would normally
   have been a single PSK identity into a set of PSK identities.
   However, 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

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   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Overview

   Intuitively, 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 a label and set of hash algorithms
   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.

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.

   o  Base Key: The secret value of an EPSK.

   o  External Identity: The identity of an EPSK.

   o  Imported Identity: The identity of a PSK as sent on the wire.

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 label, and transforms it into a set of PSKs
   and imported identities for use in a connection based on supported
   HashAlgorithms.  In particular, for each supported HashAlgorithm
   'hash', the importer constructs an ImportedIdentity structure as

      struct {
          opaque external_identity<1...2^16-1>;
          opaque label<0..2^8-1>;
          HashAlgorithm hash;
      } ImportedIdentity;

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

   ImportedIdentity.label MUST be bound to the protocol for which the
   key is imported.  Thus, TLS 1.3 and QUICv1 [I-D.ietf-quic-transport]
   MUST use "tls13" as the label.  Similarly, TLS 1.2 and all prior TLS
   versions should use "tls12" as ImportedIdentity.label, as well as
   SHA256 as ImportedIdentity.hash.  Note that this means future
   versions of TLS will increase the number of PSKs derived from an
   external PSK.

   A unique and imported PSK (IPSK) with base key 'ipskx' 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 length of ipskx MUST match that of the corresponding and
   supported ciphersuites.]]

   The hash function used for HKDF [RFC5869] is that which is associated
   with the external PSK.  It is not bound to ImportedIdentity.hash.  If
   no hash function is specified, SHA-256 MUST be used.  Differentiating
   epsk by ImportedIdentity.hash ensures that each imported PSK is only
   used with at most one hash function, thus satisfying the requirements
   in [RFC8446].  Endpoints MUST import and derive an ipsk for each hash
   function used by each ciphersuite they support.  For example,
   importing a key for TLS_AES_128_GCM_SHA256 and TLS_AES_256_GCM_SHA384
   would yield two PSKs, one for SHA256 and another for 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, one may import PSKs before the start of a

   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.

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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 may remove this hash function from the set
   of hashes used during while importing keys.  This does not affect the
   KDF operation used to derive concrete PSKs.

6.  Backwards Compatibility

   Recall that TLS 1.2 permits computing the TLS PRF with any hash
   algorithm and PSK.  Thus, an external PSK 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 hash function.
   Thus, PSKs imported for TLS 1.3 are distinct from those used in TLS
   1.2, and thereby avoid cross-protocol collisions.

7.  Security Considerations

   This is a WIP draft and has not yet seen significant security

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 some systems
   and use cases, this identity may become a persistent tracking
   identifier.  One mitigation to this problem is encryption.  Future
   drafts may specify a way for encrypting PSK identities using a
   mechanism similar to that of the Encrypted SNI proposal
   [I-D.ietf-tls-esni].  Another approach is 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;

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   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 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 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

9.  IANA Considerations

   This document makes no IANA requests.

10.  References

10.1.  Normative References

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-18 (work
              in progress), January 2019.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

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

   [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,

   [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/info/rfc8174>.

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   [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, K., Sullivan, N., and C. Wood,
              "Encrypted Server Name Indication for TLS 1.3", draft-
              ietf-tls-esni-03 (work in progress), March 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.

Authors' Addresses

   David Benjamin
   Google, LLC.

   Email: davidben@google.com

   Christopher A. Wood
   Apple, Inc.

   Email: cawood@apple.com

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