Importing External PSKs for TLS 1.3
draft-wood-tls-external-psk-importer-00

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tls                                                              C. Wood
Internet-Draft                                               Apple, Inc.
Intended status: Experimental                           October 22, 2018
Expires: April 25, 2019

                  Importing External PSKs for TLS 1.3
                draft-wood-tls-external-psk-importer-00

Abstract

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

Status of This Memo

   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 April 25, 2019.

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   Copyright (c) 2018 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  . . . . . . . . . . . . . . . . .   4
   6.  TLS 1.2 Compatibility . . . . . . . . . . . . . . . . . . . .   4
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

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",
   "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.

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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