Importing External PSKs for 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶

This Internet-Draft will expire on 10 October 2020.¶

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 (https://trustee.ietf.org/license-info) 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

TLS 1.3 [RFC8446] supports Pre-Shared Key (PSK) authentication, 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 [RFC5246], 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, this document specifies a PSK Importer interface by which external PSKs may be imported and subsequently bound to a specific KDF and hash function for use in TLS 1.3. In particular, it describes a mechanism for differentiating external PSKs by the target KDF, (D)TLS protocol version, and an optional context string. This process yields a set of candidate PSKs, each of which are bound to a target KDF and protocol. This expands what would normally have been a single PSK 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. Overview

The PSK Importer interface mirrors that of the TLS Exporters interface in that it diversifies a key based on some contextual information. In contrast to the Exporters interface, wherein differentiation is done via an explicit label and context string, the PSK Importer interface defined herein takes an external PSK and identity and "imports" it into TLS, creating a set of "derived" PSKs and identities. Each of these derived PSKs are bound a target protocol, KDF identifier, and optional context string. Additionally, the resulting PSK binder keys are modified with a new derivation label to prevent confusion with non-imported PSKs.¶

Imported keys do not require negotiation for use since a client and server will not agree upon identities if imported incorrectly. Endpoints may incrementally deploy PSK Importer support by offering non-imported keys for TLS versions prior to TLS 1.3. Non-imported and imported PSKs are distinct since their identities are different on the wire. See Section 6 for more details.¶

Clients which import external keys MUST NOT use either the external keys or the derived keys for any other purpose. Moreover, each external PSK MUST be associated with at most one hash function, as per the rules in Section 4.2.11 from [RFC8446]. See Section 7 for more discussion.¶

4. PSK Import

This section describes the PSK Importer interface and its underlying diversification mechanism 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

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 remove 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.¶

6. Incremental Deployment

Recall that TLS 1.2 permits computing the TLS PRF with any hash algorithm and PSK. Thus, an EPSK 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 target KDF and protocol. Thus, PSKs imported for TLS 1.3 are distinct from those used in TLS 1.2, and thereby avoid cross-protocol collisions. Note that this does not preclude endpoints from using non-imported PSKs for TLS 1.2. Indeed, this is necessary for incremental deployment.¶

7. Security Considerations

The PSK Importer security goals can be roughly stated as follows: avoid PSK re-use across KDFs while properly authenticating endpoints. When modeled as computational extractors, KDFs assume that input keying material (IKM) is sampled from some "source" probability distribution and 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 different KDFs.¶

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

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

1. Externally provisioned PSKs imported into a TLS connection achieve compound authentication of 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 existing PSKs used for TLS 1.2 and below.¶
5. Imported PSKs do not collide with future protocol versions and KDFs.¶

8. Privacy Considerations

External PSK identities are typically static by design so that endpoints may use them to lookup keying material. However, for some systems and use cases, this identity may become a persistent tracking identifier.¶

9. IANA Considerations

This specification introduces a new registry for TLS KDF identifiers and defines the following target KDF values:¶

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].¶

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. John Mattsson provided input regarding PSK importer deployment considerations. Hugo Krawczyk provided guidance for the security considerations.¶

Appendix B. Addressing Selfie

The Selfie attack [Selfie] relies on a misuse of the PSK interface. The PSK interface makes the implicit assumption that each PSK is known only to one client and one server. If multiple clients or multiple servers with distinct roles share a PSK, TLS only authenticates the entire group. A node successfully authenticates its peer as being in the group whether the peer is another node or itself.¶

Applications which require authenticating finer-grained roles while still configuring a single shared PSK across all nodes can resolve this mismatch either by exchanging roles over the TLS connection after the handshake or by incorporating the roles of both the client and server into the IPSK context string. For instance, if an application identifies each node by MAC address, it could use the following context string.¶

  struct {
    opaque client_mac<0..2^16-1>;
    opaque server_mac<0..2^16-1>;
  } Context;

If an attacker then redirects a ClientHello intended for one node to a different node, the receiver will compute 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.¶

Authors' Addresses