RFC 8773

Internet Engineering Task Force (IETF)                        R. Housley
Request for Comments: 8773                                Vigil Security
Category: Experimental                                        March 2020
ISSN: 2070-1721

TLS 1.3 Extension for Certificate-Based Authentication with an External
                             Pre-Shared Key


   This document specifies a TLS 1.3 extension that allows a server to
   authenticate with a combination of a certificate and an external pre-
   shared key (PSK).

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

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

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Motivation and Design Rationale
   4.  Extension Overview
   5.  Certificate with External PSK Extension
     5.1.  Companion Extensions
     5.2.  Authentication
     5.3.  Keying Material
   6.  IANA Considerations
   7.  Security Considerations
   8.  Privacy Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References

   Author's Address

1.  Introduction

   The TLS 1.3 [RFC8446] handshake protocol provides two mutually
   exclusive forms of server authentication.  First, the server can be
   authenticated by providing a signature certificate and creating a
   valid digital signature to demonstrate that it possesses the
   corresponding private key.  Second, the server can be authenticated
   by demonstrating that it possesses a pre-shared key (PSK) that was
   established by a previous handshake.  A PSK that is established in
   this fashion is called a resumption PSK.  A PSK that is established
   by any other means is called an external PSK.  This document
   specifies a TLS 1.3 extension permitting certificate-based server
   authentication to be combined with an external PSK as an input to the
   TLS 1.3 key schedule.

   Several implementors wanted to gain more experience with this
   specification before producing a Standards Track RFC.  As a result,
   this specification is being published as an Experimental RFC to
   enable interoperable implementations and gain deployment and
   operational experience.

2.  Terminology

   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.  Motivation and Design Rationale

   The development of a large-scale quantum computer would pose a
   serious challenge for the cryptographic algorithms that are widely
   deployed today, including the digital signature algorithms that are
   used to authenticate the server in the TLS 1.3 handshake protocol.
   It is an open question whether or not it is feasible to build a
   large-scale quantum computer, and if so, when that might happen.
   However, if such a quantum computer is invented, many of the
   cryptographic algorithms and the security protocols that use them
   would become vulnerable.

   The TLS 1.3 handshake protocol employs key agreement algorithms and
   digital signature algorithms that could be broken by the development
   of a large-scale quantum computer [TRANSITION].  The key agreement
   algorithms include Diffie-Hellman (DH) [DH1976] and Elliptic Curve
   Diffie-Hellman (ECDH) [IEEE1363]; the digital signature algorithms
   include RSA [RFC8017] and the Elliptic Curve Digital Signature
   Algorithm (ECDSA) [FIPS186].  As a result, an adversary that stores a
   TLS 1.3 handshake protocol exchange today could decrypt the
   associated encrypted communications in the future when a large-scale
   quantum computer becomes available.

   In the near term, this document describes a TLS 1.3 extension to
   protect today's communications from the future invention of a large-
   scale quantum computer by providing a strong external PSK as an input
   to the TLS 1.3 key schedule while preserving the authentication
   provided by the existing certificate and digital signature

4.  Extension Overview

   This section provides a brief overview of the
   "tls_cert_with_extern_psk" extension.

   The client includes the "tls_cert_with_extern_psk" extension in the
   ClientHello message.  The "tls_cert_with_extern_psk" extension MUST
   be accompanied by the "key_share", "psk_key_exchange_modes", and
   "pre_shared_key" extensions.  The client MAY also find it useful to
   include the "supported_groups" extension.  Since the
   "tls_cert_with_extern_psk" extension is intended to be used only with
   initial handshakes, it MUST NOT be sent alongside the "early_data"
   extension.  These extensions are all described in Section 4.2 of
   [RFC8446], which also requires the "pre_shared_key" extension to be
   the last extension in the ClientHello message.

   If the client includes both the "tls_cert_with_extern_psk" extension
   and the "early_data" extension, then the server MUST terminate the
   connection with an "illegal_parameter" alert.

   If the server is willing to use one of the external PSKs listed in
   the "pre_shared_key" extension and perform certificate-based
   authentication, then the server includes the
   "tls_cert_with_extern_psk" extension in the ServerHello message.  The
   "tls_cert_with_extern_psk" extension MUST be accompanied by the
   "key_share" and "pre_shared_key" extensions.  If none of the external
   PSKs in the list provided by the client is acceptable to the server,
   then the "tls_cert_with_extern_psk" extension is omitted from the
   ServerHello message.

   When the "tls_cert_with_extern_psk" extension is successfully
   negotiated, the TLS 1.3 key schedule processing includes both the
   selected external PSK and the (EC)DHE shared secret value.  (EC)DHE
   refers to Diffie-Hellman over either finite fields or elliptic
   curves.  As a result, the Early Secret, Handshake Secret, and Master
   Secret values all depend upon the value of the selected external PSK.
   Of course, the Early Secret does not depend upon the (EC)DHE shared

   The authentication of the server and optional authentication of the
   client depend upon the ability to generate a signature that can be
   validated with the public key in their certificates.  The
   authentication processing is not changed in any way by the selected
   external PSK.

   Each external PSK is associated with a single hash algorithm, which
   is required by Section 4.2.11 of [RFC8446].  The hash algorithm MUST
   be set when the PSK is established, with a default of SHA-256.

5.  Certificate with External PSK Extension

   This section specifies the "tls_cert_with_extern_psk" extension,
   which MAY appear in the ClientHello message and ServerHello message.
   It MUST NOT appear in any other messages.  The
   "tls_cert_with_extern_psk" extension MUST NOT appear in the
   ServerHello message unless the "tls_cert_with_extern_psk" extension
   appeared in the preceding ClientHello message.  If an implementation
   recognizes the "tls_cert_with_extern_psk" extension and receives it
   in any other message, then the implementation MUST abort the
   handshake with an "illegal_parameter" alert.

   The general extension mechanisms enable clients and servers to
   negotiate the use of specific extensions.  Clients request extended
   functionality from servers with the extensions field in the
   ClientHello message.  If the server responds with a HelloRetryRequest
   message, then the client sends another ClientHello message as
   described in Section 4.1.2 of [RFC8446], including the same
   "tls_cert_with_extern_psk" extension as the original ClientHello
   message, or aborts the handshake.

   Many server extensions are carried in the EncryptedExtensions
   message; however, the "tls_cert_with_extern_psk" extension is carried
   in the ServerHello message.  Successful negotiation of the
   "tls_cert_with_extern_psk" extension affects the key used for
   encryption, so it cannot be carried in the EncryptedExtensions
   message.  Therefore, the "tls_cert_with_extern_psk" extension is only
   present in the ServerHello message if the server recognizes the
   "tls_cert_with_extern_psk" extension and the server possesses one of
   the external PSKs offered by the client in the "pre_shared_key"
   extension in the ClientHello message.

   The Extension structure is defined in [RFC8446]; it is repeated here
   for convenience.

     struct {
         ExtensionType extension_type;
         opaque extension_data<0..2^16-1>;
     } Extension;

   The "extension_type" identifies the particular extension type, and
   the "extension_data" contains information specific to the particular
   extension type.

   This document specifies the "tls_cert_with_extern_psk" extension,
   adding one new type to ExtensionType:

     enum {
         tls_cert_with_extern_psk(33), (65535)
     } ExtensionType;

   The "tls_cert_with_extern_psk" extension is relevant when the client
   and server possess an external PSK in common that can be used as an
   input to the TLS 1.3 key schedule.  The "tls_cert_with_extern_psk"
   extension is essentially a flag to use the external PSK in the key
   schedule, and it has the following syntax:

     struct {
         select (Handshake.msg_type) {
             case client_hello: Empty;
             case server_hello: Empty;
     } CertWithExternPSK;

5.1.  Companion Extensions

   Section 4 lists the extensions that are required to accompany the
   "tls_cert_with_extern_psk" extension.  Most of those extensions are
   not impacted in any way by this specification.  However, this section
   discusses the extensions that require additional consideration.

   The "psk_key_exchange_modes" extension is defined in of Section 4.2.9
   of [RFC8446].  The "psk_key_exchange_modes" extension restricts the
   use of both the PSKs offered in this ClientHello and those that the
   server might supply via a subsequent NewSessionTicket.  As a result,
   when the "psk_key_exchange_modes" extension is included in the
   ClientHello message, clients MUST include psk_dhe_ke mode.  In
   addition, clients MAY also include psk_ke mode to support a
   subsequent NewSessionTicket.  When the "psk_key_exchange_modes"
   extension is included in the ServerHello message, servers MUST select
   the psk_dhe_ke mode for the initial handshake.  Servers MUST select a
   key exchange mode that is listed by the client for subsequent
   handshakes that include the resumption PSK from the initial

   The "pre_shared_key" extension is defined in Section 4.2.11 of
   [RFC8446].  The syntax is repeated below for convenience.  All of the
   listed PSKs MUST be external PSKs.  If a resumption PSK is listed
   along with the "tls_cert_with_extern_psk" extension, the server MUST
   abort the handshake with an "illegal_parameter" alert.

     struct {
         opaque identity<1..2^16-1>;
         uint32 obfuscated_ticket_age;
     } PskIdentity;

     opaque PskBinderEntry<32..255>;

     struct {
         PskIdentity identities<7..2^16-1>;
         PskBinderEntry binders<33..2^16-1>;
     } OfferedPsks;

     struct {
         select (Handshake.msg_type) {
             case client_hello: OfferedPsks;
             case server_hello: uint16 selected_identity;
     } PreSharedKeyExtension;

   "OfferedPsks" contains the list of PSK identities and associated
   binders for the external PSKs that the client is willing to use with
   the server.

   The identities are a list of external PSK identities that the client
   is willing to negotiate with the server.  Each external PSK has an
   associated identity that is known to the client and the server; the
   associated identities may be known to other parties as well.  In
   addition, the binder validation (see below) confirms that the client
   and server have the same key associated with the identity.

   The "obfuscated_ticket_age" is not used for external PSKs.  As stated
   in Section 4.2.11 of [RFC8446], clients SHOULD set this value to 0,
   and servers MUST ignore the value.

   The binders are a series of HMAC [RFC2104] values, one for each
   external PSK offered by the client, in the same order as the
   identities list.  The HMAC value is computed using the binder_key,
   which is derived from the external PSK, and a partial transcript of
   the current handshake.  Generation of the binder_key from the
   external PSK is described in Section 7.1 of [RFC8446].  The partial
   transcript of the current handshake includes a partial ClientHello up
   to and including the PreSharedKeyExtension.identities field, as
   described in Section of [RFC8446].

   The "selected_identity" contains the index of the external PSK
   identity that the server selected from the list offered by the
   client.  As described in Section 4.2.11 of [RFC8446], the server MUST
   validate the binder value that corresponds to the selected external
   PSK, and if the binder does not validate, the server MUST abort the
   handshake with an "illegal_parameter" alert.

5.2.  Authentication

   When the "tls_cert_with_extern_psk" extension is successfully
   negotiated, authentication of the server depends upon the ability to
   generate a signature that can be validated with the public key in the
   server's certificate.  This is accomplished by the server sending the
   Certificate and CertificateVerify messages, as described in Sections
   4.4.2 and 4.4.3 of [RFC8446].

   TLS 1.3 does not permit the server to send a CertificateRequest
   message when a PSK is being used.  This restriction is removed when
   the "tls_cert_with_extern_psk" extension is negotiated, allowing
   certificate-based authentication for both the client and the server.
   If certificate-based client authentication is desired, this is
   accomplished by the client sending the Certificate and
   CertificateVerify messages as described in Sections 4.4.2 and 4.4.3
   of [RFC8446].

5.3.  Keying Material

   Section 7.1 of [RFC8446] specifies the TLS 1.3 key schedule.  The
   successful negotiation of the "tls_cert_with_extern_psk" extension
   requires the key schedule processing to include both the external PSK
   and the (EC)DHE shared secret value.

   If the client and the server have different values associated with
   the selected external PSK identifier, then the client and the server
   will compute different values for every entry in the key schedule,
   which will lead to the client aborting the handshake with a
   "decrypt_error" alert.

6.  IANA Considerations

   IANA has updated the "TLS ExtensionType Values" registry [IANA] to
   include "tls_cert_with_extern_psk" with a value of 33 and the list of
   messages "CH, SH" in which the "tls_cert_with_extern_psk" extension
   may appear.

7.  Security Considerations

   The Security Considerations in [RFC8446] remain relevant.

   TLS 1.3 [RFC8446] does not permit the server to send a
   CertificateRequest message when a PSK is being used.  This
   restriction is removed when the "tls_cert_with_extern_psk" extension
   is offered by the client and accepted by the server.  However, TLS
   1.3 does not permit an external PSK to be used in the same fashion as
   a resumption PSK, and this extension does not alter those
   restrictions.  Thus, a certificate MUST NOT be used with a resumption

   Implementations must protect the external pre-shared key (PSK).
   Compromise of the external PSK will make the encrypted session
   content vulnerable to the future development of a large-scale quantum
   computer.  However, the generation, distribution, and management of
   the external PSKs is out of scope for this specification.

   Implementers should not transmit the same content on a connection
   that is protected with an external PSK and a connection that is not.
   Doing so may allow an eavesdropper to correlate the connections,
   making the content vulnerable to the future invention of a large-
   scale quantum computer.

   Implementations must generate external PSKs with a secure key-
   management technique, such as pseudorandom generation of the key or
   derivation of the key from one or more other secure keys.  The use of
   inadequate pseudorandom number generators (PRNGs) to generate
   external PSKs can result in little or no security.  An attacker may
   find it much easier to reproduce the PRNG environment that produced
   the external PSKs and search the resulting small set of
   possibilities, rather than brute-force searching the whole key space.
   The generation of quality random numbers is difficult.  [RFC4086]
   offers important guidance in this area.

   If the external PSK is known to any party other than the client and
   the server, then the external PSK MUST NOT be the sole basis for
   authentication.  The reasoning is explained in Section 4.2 of
   [K2016].  When this extension is used, authentication is based on
   certificates, not the external PSK.

   In this extension, the external PSK preserves confidentiality if the
   (EC)DH key agreement is ever broken by cryptanalysis or the future
   invention of a large-scale quantum computer.  As long as the attacker
   does not know the PSK and the key derivation algorithm remains
   unbroken, the attacker cannot derive the session secrets, even if
   they are able to compute the (EC)DH shared secret.  Should the
   attacker be able compute the (EC)DH shared secret, the forward-
   secrecy advantages traditionally associated with ephemeral (EC)DH
   keys will no longer be relevant.  Although the ephemeral private keys
   used during a given TLS session are destroyed at the end of a
   session, preventing the attacker from later accessing them, these
   private keys would nevertheless be recoverable due to the break in
   the algorithm.  However, a more general notion of "secrecy after key
   material is destroyed" would still be achievable using external PSKs,
   if they are managed in a way that ensures their destruction when they
   are no longer needed, and with the assumption that the algorithms
   that use the external PSKs remain quantum-safe.

   TLS 1.3 key derivation makes use of the HMAC-based Key Derivation
   Function (HKDF) algorithm, which depends upon the HMAC [RFC2104]
   construction and a hash function.  This extension provides the
   desired protection for the session secrets, as long as HMAC with the
   selected hash function is a pseudorandom function (PRF) [GGM1986].

   This specification does not require that the external PSK is known
   only by the client and server.  The external PSK may be known to a
   group.  Since authentication depends on the public key in a
   certificate, knowledge of the external PSK by other parties does not
   enable impersonation.  Since confidentiality depends on the shared
   secret from (EC)DH, knowledge of the external PSK by other parties
   does not enable eavesdropping.  However, group members can record the
   traffic of other members and then decrypt it if they ever gain access
   to a large-scale quantum computer.  Also, when many parties know the
   external PSK, there are many opportunities for theft of the external
   PSK by an attacker.  Once an attacker has the external PSK, they can
   decrypt stored traffic if they ever gain access to a large-scale
   quantum computer, in the same manner as a legitimate group member.

   TLS 1.3 [RFC8446] takes a conservative approach to PSKs; they are
   bound to a specific hash function and KDF.  By contrast, TLS 1.2
   [RFC5246] allows PSKs to be used with any hash function and the TLS
   1.2 PRF.  Thus, the safest approach is to use a PSK exclusively with
   TLS 1.2 or exclusively with TLS 1.3.  Given one PSK, one can derive a
   PSK for exclusive use with TLS 1.2 and derive another PSK for
   exclusive use with TLS 1.3 using the mechanism specified in [IMPORT].

   TLS 1.3 [RFC8446] has received careful security analysis, and the
   following informal reasoning shows that the addition of this
   extension does not introduce any security defects.  This extension
   requires the use of certificates for authentication, but the
   processing of certificates is unchanged by this extension.  This
   extension places an external PSK in the key schedule as part of the
   computation of the Early Secret.  In the initial handshake without
   this extension, the Early Secret is computed as:

      Early Secret = HKDF-Extract(0, 0)

   With this extension, the Early Secret is computed as:

      Early Secret = HKDF-Extract(External PSK, 0)

   Any entropy contributed by the external PSK can only make the Early
   Secret better; the External PSK cannot make it worse.  For these two
   reasons, TLS 1.3 continues to meet its security goals when this
   extension is used.

8.  Privacy Considerations

   Appendix E.6 of [RFC8446] discusses identity-exposure attacks on
   PSKs.  The guidance in this section remains relevant.

   This extension makes use of external PSKs to improve resilience
   against attackers that gain access to a large-scale quantum computer
   in the future.  This extension is always accompanied by the
   "pre_shared_key" extension to provide the PSK identities in plaintext
   in the ClientHello message.  Passive observation of the these PSK
   identities will aid an attacker in tracking users of this extension.

9.  References

9.1.  Normative References

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

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

9.2.  Informative References

   [DH1976]   Diffie, W. and M. Hellman, "New Directions in
              Cryptography", IEEE Transactions on Information Theory,
              Vol. 22, No. 6, DOI 10.1109/TIT.1976.1055638, November
              1976, <https://ieeexplore.ieee.org/document/1055638>.

   [FIPS186]  NIST, "Digital Signature Standard (DSS)", Federal
              Information Processing Standards Publication (FIPS) 186-4,
              DOI 10.6028/NIST.FIPS.186-4, July 2013,

   [GGM1986]  Goldreich, O., Goldwasser, S., and S. Micali, "How to
              construct random functions", Journal of the ACM, Vol. 33,
              No. 4, pp. 792-807, DOI 10.1145/6490.6503, August 1986,

   [IANA]     IANA, "TLS ExtensionType Values",

   [IEEE1363] IEEE, "IEEE Standard Specifications for Public-Key
              Cryptography", IEEE Std 1363-2000,
              DOI 10.1109/IEEESTD.2000.92292, August 2000,

   [IMPORT]   Benjamin, D. and C. Wood, "Importing External PSKs for
              TLS", Work in Progress, Internet-Draft, draft-ietf-tls-
              external-psk-importer-03, 15 February 2020,

   [K2016]    Krawczyk, H., "A Unilateral-to-Mutual Authentication
              Compiler for Key Exchange (with Applications to Client
              Authentication in TLS 1.3)", CCS '16: Proceedings of the
              2016 ACM Communications Security, pp. 1438-50,
              DOI 10.1145/2976749.2978325, October 2016,

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,

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

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,

              Hoffman, P., "The Transition from Classical to Post-
              Quantum Cryptography", Work in Progress, Internet-Draft,
              draft-hoffman-c2pq-06, 25 November 2019,


   Many thanks to Liliya Akhmetzyanova, Roman Danyliw, Christian
   Huitema, Ben Kaduk, Geoffrey Keating, Hugo Krawczyk, Mirja K├╝hlewind,
   Nikos Mavrogiannopoulos, Nick Sullivan, Martin Thomson, and Peter Yee
   for their review and comments; their efforts have improved this

Author's Address

   Russ Housley
   Vigil Security, LLC
   516 Dranesville Road
   Herndon, VA 20170
   United States of America

   Email: housley@vigilsec.com