TLS 1.3 Extension for Certificate-Based Authentication with an External Pre-Shared Key
RFC 8773

Document Type RFC - Experimental (March 2020; No errata)
Author Russ Housley 
Last updated 2020-03-29
Replaces draft-housley-tls-tls13-cert-with-extern-psk
Stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication
Document shepherd Joseph Salowey
Shepherd write-up Show (last changed 2019-07-01)
IESG IESG state RFC 8773 (Experimental)
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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.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   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
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