Status of This Memo

This is an Internet Standards Track document.¶

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Table of Contents

1.Introduction

TLS 1.3[RFC8446] supports Pre-Shared Key (PSK) authentication, wherein PSKscan be established via session tickets from prior connections or via some external, out-of-bandmechanism. 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 theTLS 1.2 pseudorandom function (PRF). While there is no known way in which the sameexternal PSK might produce related output in TLS 1.3 and prior versions, only limitedanalysis has been done. ApplicationsSHOULD provision separate PSKs for (D)TLS 1.3 andprior versions. In cases where this is not possible (e.g., there are already-deployedexternal PSKs or provisioning is otherwise limited), reusing external PSKs across differentversions of TLS may produce related outputs, which may, in turn, lead to security problems;seeAppendix E.7 of [RFC8446].¶

To mitigate such problems, this document specifies a PSK importerinterface by which external PSKs may be imported and subsequently bound to a specifickey derivation function (KDF) and hash function for use in TLS 1.3[RFC8446]and DTLS 1.3[RFC9147]. In particular, it describes amechanism for importing PSKs derived from external PSKs by including the target KDF,(D)TLS protocol version, and an optional context string to ensure uniqueness. This process yields a set of candidatePSKs, each of which are bound to a target KDF and protocol, that are separate from thoseused in (D)TLS 1.2 and prior versions. This expands what would normally have been a singlePSK and identity into a set of PSKs and identities.¶

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.¶

3.Terminology

The following terms are used throughout this document:¶

External PSK (EPSK):

A PSK established or provisioned out of band (i.e., notfrom a TLS connection) that is a tuple of (Base Key, External Identity, Hash).¶

Base Key:

The secret value of an EPSK.¶

External Identity:

A sequence of bytes used to identify an EPSK.¶

Target protocol:

The protocol for which a PSK is imported for use.¶

Target KDF:

The KDF for which a PSK is imported for use.¶

Imported PSK (IPSK):

A TLS PSK derived from an EPSK, optional context string, target protocol, and target KDF.¶

Non-imported PSK:

An EPSK that is used directly as a TLS PSK without being imported.¶

Imported Identity:

A sequence of bytes used to identify an IPSK.¶

This document uses presentation language fromSection 3 of [RFC8446].¶

4.Overview

The PSK importer interface mirrors that of the TLS exporter interface (see[RFC8446])in that it diversifies a key based on some contextual information. In contrast to the exporterinterface, wherein output uniqueness is achieved via an explicit label and context string,the PSK importer interface defined herein takes an external PSK and identity and "imports" it intoTLS, creating a set of "derived" PSKs and identities that are each unique. Each of thesederived PSKs are bound to a target protocol, KDF identifier, and optional context string.Additionally, the resulting PSK binder keys are modified with a new derivation labelto prevent confusion with non-imported PSKs. Through this interface, importing externalPSKs with different identities yields distinct PSK binder keys.¶

Imported keys do not require negotiation for use since a client and server will not agree uponidentities if imported incorrectly. Endpoints may incrementally deploy PSK importer supportby offering non-imported PSKs for TLS versions prior to TLS 1.3.Non-imported and imported PSKsare not equivalent since their identities are different. SeeSection 7 for more details.¶

Endpoints that import external keysMUST NOT use the keys that are input to theimport process for any purpose other than the importer, and theyMUST NOT use the derivedkeys for any purpose other than TLS PSKs. Moreover, each external PSK fed to theimporter processMUST be associated with one hash function at most. Thisis analogous to the rules inSection 4.2.11 of [RFC8446]. SeeSection 8 formore discussion.¶

5.PSK Importer

This section describes the PSK importer interface and its underlying diversificationmechanism and binder key computation modification.¶

struct {   opaque external_identity<1...2^16-1>;   opaque context<0..2^16-1>;   uint16 target_protocol;   uint16 target_kdf;} ImportedIdentity;

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

           0           |           v PSK ->  HKDF-Extract = Early Secret           |           +-----> Derive-Secret(., "ext binder" | "res binder", "")           |                     = binder_key           V

           0           |           v PSK ->  HKDF-Extract = Early Secret           |           +-----> Derive-Secret(., "ext binder"           |                      | "res binder"           |                      | "imp binder", "")           |                     = binder_key           V

6.Deprecating Hash Functions

If a client or server wishes to deprecate a hash function and no longer use it for TLS 1.3,it removes the corresponding KDF from the set of target KDFs used for importing keys.This does not affect the KDF operation used to derive imported PSKs.¶

7.Incremental Deployment

In deployments that already have PSKs provisioned and in use with TLS 1.2, attemptingto incrementally deploy the importer mechanism would result in concurrent use ofthe already-provisioned PSK directly as both a TLS 1.2 PSK and an EPSK, which, in turn, could mean that the same KDF and key would be used in two different protocol contexts.This is not a recommended configuration; seeSection 8 for more details.However, the benefits of using TLS 1.3 and PSK importers may prove sufficientlycompelling that existing deployments choose to enable this noncompliant configuration fora brief transition period while new software (using TLS 1.3 and importers) is deployed.Operators are advised to make any such transition period as short as possible.¶

8.Security Considerations

The PSK importer security goals can be roughly stated as follows: avoid PSK reuse across KDFs while properly authenticating endpoints. When modeled as computational extractors, KDFsassume that input keying material (IKM) is sampled from some "source" probability distributionand that any two IKM values are chosen independently of each other[Kraw10]. This source-independence requirement implies that the same IKM value cannot be used for two differentKDFs.¶

PSK-based authentication is functionally equivalent to session resumption in that a connectionuses existing key material to authenticate both endpoints. Following the work of[BAA15], this is a form of compound authentication. Looselyspeaking, compound authentication is the property that an execution of multiple authenticationprotocols, wherein at least one is uncompromised, jointly authenticates all protocols. Therefore, authenticating with an externally provisioned PSK should ideally authenticate boththe TLS connection and the external provisioning process. Typically, the external provisioning processproduces a PSK and corresponding context from which the PSK was derived and in which it shouldbe used. If available, this is used as the ImportedIdentity.context value. We refer to anexternal PSK without such context as "context-free".¶

Thus, in considering the source-independence and compound authentication requirements, the PSKimporter interface described in this document aims to achieve the following goals:¶

1. Externally provisioned PSKs imported into a TLS connection achieve compound authenticationof the provisioning process and connection.¶
2. Context-free PSKs only achieve authentication within the context of a single connection.¶
3. Imported PSKs are not used as IKM for two different KDFs.¶
4. Imported PSKs do not collide with future protocol versions and KDFs.¶

There are no known related outputs or security issues caused from the processfor computing imported PSKs from an external PSK and the processing of existingexternal PSKs used in (D)TLS 1.2 and below, as noted inSection 7. However,only limited analysis has been done, which is an additional reason why applicationsSHOULD provision separate PSKs for (D)TLS 1.3 and prior versions, even when theimporter interface is used in (D)TLS 1.3.¶

The PSK importer does not prevent applications from constructing non-importer PSK identitiesthat collide with imported PSK identities.¶

9.Privacy Considerations

External PSK identities are commonly static by design so that endpoints may use them tolook up keying material. As a result, for some systems and use cases, this identitymay become a persistent tracking identifier.¶

Note also that ImportedIdentity.context is visible in cleartext on the wire as part ofthe PSK identity. Unless otherwise protected by a mechanism such as TLS EncryptedClientHello[ECH], applicationsSHOULD NOT put sensitive informationin this field.¶

10.IANA Considerations

IANA has created the "TLS KDF Identifiers" registry under the existing "Transport Layer Security (TLS) Parameters" registry.¶

The entries in the registry are as follows:¶

New target KDF values are allocated according to the following process:¶

• Values in the range 0x0000-0xfeff are assigned via Specification Required[RFC8126].¶
• Values in the range 0xff00-0xffff are reserved for Private Use[RFC8126].¶

The procedures for requesting values in the Specification Required space are specified inSection 17 of [RFC8447].¶

Appendix A.Addressing Selfie

The Selfie attack[Selfie] relies on a misuse of the PSK interface.The PSK interface makes the implicit assumption that each PSKis known only to one client and one server. If multiple clients ormultiple servers with distinct roles share a PSK, TLS onlyauthenticates the entire group. A node successfully authenticatesits peer as being in the group whether the peer is another nodeor itself. Note that this case can also occur when there are two nodes sharing a PSK without predetermined roles.¶

Applications that require authenticating finer-grained roles while stillconfiguring a single shared PSK across all nodes can resolve thismismatch either by exchanging roles over the TLS connection afterthe handshake or by incorporating the roles of both the client and the serverinto the IPSK context string. For instance, if an applicationidentifies each node by the Media Access Control (MAC) address, it could use the followingcontext string.¶

  struct {    opaque client_mac<0..2^8-1>;    opaque server_mac<0..2^8-1>;  } Context;

If an attacker then redirects a ClientHello intended for one node to a differentnode, including the node that generated the ClientHello, the receiver willcompute 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.¶

Acknowledgements

The authors thankEric Rescorla andMartin Thomson for discussions that led to theproduction of this document, as well asChristian Huitema for input regarding privacyconsiderations of external PSKs.John Preuß Mattsson provided input regarding PSK importerdeployment considerations.Hugo Krawczyk provided guidance for the security considerations.Martin Thomson,Jonathan Hoyland,Scott Hollenbeck,Benjamin Kaduk, and others allprovided reviews, feedback, and suggestions for improving the document.¶

Authors' Addresses