Importing External PSKs for TLS

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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 pseudorandom function (PRF). While there is no known way in which the same external PSK might produce related output in TLS 1.3 and prior versions, only limited analysis has been done. Applications SHOULD provision separate PSKs for (D)TLS 1.3 and prior versions. In cases where this is not possible, e.g., there are already deployed external PSKs or provisioning is otherwise limited, re-using external PSKs across different versions of TLS may produce related outputs, which may in turn lead to security problems; see [RFC8446], Section E.7.¶

To mitigate against such problems, this document specifies a PSK Importer interface by which external PSKs may be imported and subsequently bound to a specific key derivation function (KDF) and hash function for use in TLS 1.3 [RFC8446] and DTLS 1.3 [DTLS13]. In particular, it describes a mechanism 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 candidate PSKs, each of which are bound to a target KDF and protocol, that are separate from those used in (D)TLS 1.2 and prior versions. 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. Terminology

The following terms are used throughout this document:¶

• External PSK (EPSK): A PSK established or provisioned out-of-band (i.e., not from a TLS connection) which 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 which 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 from [RFC8446], Section 3.¶

4. Overview

The PSK Importer interface mirrors that of the TLS Exporters interface (see [RFC8446]) in that it diversifies a key based on some contextual information. In contrast to the Exporters interface, 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 into TLS, creating a set of "derived" PSKs and identities that are each unique. Each of these derived 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 label to prevent confusion with non-imported PSKs. Through this interface, importing external PSKs with different identities yields distinct PSK binder keys.¶

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 PSKs for TLS versions prior to TLS 1.3. Non-imported and imported PSKs are distinct since their identities are different. See Section 7 for more details.¶

Endpoints which import external keys MUST NOT use the keys that are input to the import process for any purpose other than the importer, and MUST NOT use the derived keys for any purpose other than TLS PSKs. Moreover, each external PSK fed to the importer process MUST be associated with at most one hash function. This is analogous to the rules in Section 4.2.11 of [RFC8446]. See Section 8 for more discussion.¶

5. 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

6. Deprecating Hash Functions

If a client or server wishes 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.¶

7. Incremental Deployment

In deployments that already have PSKs provisioned and in use with TLS 1.2, attempting to incrementally deploy the importer mechanism would then result in concurrent use of the already provisioned PSK both directly as a TLS 1.2 PSK and as 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; see Section 8 for more details. However, the benefits of using TLS 1.3 and of using PSK importers may prove sufficiently compelling that existing deployments choose to enable this noncompliant configuration for a 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 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 future protocol versions and KDFs.¶

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

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

9. Privacy Considerations

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

Note also that ImportedIdentity.context is visible in cleartext on the wire as part of the PSK identity. Unless otherwise protected by a mechanism such as TLS Encrypted ClientHello [ECH], applications SHOULD NOT put sensitive information in this field.¶

10. IANA Considerations

This specification introduces a new registry for TLS KDF identifiers, titled "TLS KDF Identifiers", under the existing "Transport Layer Security (TLS) Parameters" heading.¶

The entries in the registry are:¶

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 in Section 17 of [RFC8447].¶

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. Martin Thomson, Jonathan Hoyland, Scott Hollenbeck, Benjamin Kaduk, and others all provided reviews, feedback, and suggestions for improving the document.¶

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. Note that this case can also occur when there are two nodes sharing a PSK without predetermined roles.¶

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

If an attacker then redirects a ClientHello intended for one node to a different node, including the node that generated the ClientHello, 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