Internet Engineering Task Force (IETF) D. Benjamin

Internet Engineering Task Force (IETF) D. Benjamin
Request for Comments: 9258 Google, LLC.
Category: Standards Track C. A. Wood
ISSN: 2070-1721 Cloudflare
July 2022

Importing External Pre-Shared Keys (PSKs) for TLS 1.3

Abstract

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

Status of This Memo

This is an Internet Standards Track document.

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). Further information on
Internet Standards is available in 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
https://www.rfc-editor.org/info/rfc9258.

Copyright Notice

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Table of Contents

1. Introduction
2. Conventions and Definitions
3. Terminology
4. Overview
5. PSK Importer
5.1. External PSK Diversification
5.2. Binder Key Derivation
6. Deprecating Hash Functions
7. Incremental Deployment
8. Security Considerations
9. Privacy Considerations
10. IANA Considerations
11. References
11.1. Normative References
11.2. Informative References
Appendix A. Addressing Selfie
Acknowledgements
Authors' Addresses

1. Introduction

TLS 1.3 [RFC8446] supports Pre-Shared Key (PSK) authentication,
wherein PSKs can be established via session tickets from prior
connections or via some external, 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), reusing external PSKs across
different versions of TLS may produce related outputs, which may, in
turn, lead to security problems; see Appendix E.7 of [RFC8446].

To mitigate 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 [RFC9147]. 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) 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 from Section 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 exporter 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 not equivalent since their identities are
different. See Section 7 for more details.

Endpoints that import external keys MUST NOT use the keys that are
input to the import process for any purpose other than the importer,
and they 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 one hash function at most. This is analogous to
the rules in Section 4.2.11 of [RFC8446]. See Section 8 for more
discussion.

5. PSK Importer

This section describes the PSK importer interface and its underlying
diversification mechanism and binder key computation modification.

5.1. External PSK Diversification

As input, the PSK importer interface takes an EPSK with External
Identity external_identity and base key epsk (as defined in
Section 3) along with an optional context. It then transforms the
input into a set of PSKs and imported identities for use in a
connection based on target protocols and KDFs. In particular, for
each supported target protocol target_protocol and KDF target_kdf,
the importer constructs an ImportedIdentity structure as follows:

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

The list of ImportedIdentity.target_kdf values is maintained by IANA
as described in Section 10. External PSKs MUST NOT be imported for
(D)TLS 1.2 or prior versions. See Section 7 for discussion on how
imported PSKs for TLS 1.3 and non-imported PSKs for earlier versions
coexist for incremental deployment.

ImportedIdentity.context MUST include the context used to determine
the EPSK, if any exists. For example, ImportedIdentity.context may
include information about peer roles or identities to mitigate
Selfie-style reflection attacks [Selfie]. See Appendix A for more
details. Since the EPSK is a key derived from an external protocol
or sequence of protocols, ImportedIdentity.context MUST include a
channel binding for the deriving protocols [RFC5056]. The details of
this binding are protocol specific and out of scope for this
document.

ImportedIdentity.target_protocol MUST be the (D)TLS protocol version
for which the PSK is being imported. For example, TLS 1.3 [RFC8446]
uses 0x0304, which will therefore also be used by QUICv1 [QUIC].
Note that this means the number of PSKs derived from an EPSK is a
function of the number of target protocols.

Given an ImportedIdentity and corresponding EPSK with base key epsk,
an imported PSK IPSK with base key ipskx is computed as follows:

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

L corresponds to the KDF output length of ImportedIdentity.target_kdf
as defined in Section 10. For hash-based KDFs, such as HKDF_SHA256
(0x0001), this is the length of the hash function output, e.g., 32
octets for SHA256. This is required for the IPSK to be of length
suitable for supported ciphersuites. Internally, HKDF-Expand-Label
uses a label corresponding to ImportedIdentity.target_protocol (e.g.,
"tls13" for TLS 1.3, as per Section 7.1 of [RFC8446] or "dtls13" for
DTLS 1.3, as per Section 5.10 of [RFC9147]).

The identity of ipskx as sent on the wire is ImportedIdentity, i.e.,
the serialized content of ImportedIdentity is used as the content of
PskIdentity.identity in the PSK extension. The corresponding PSK
input for the TLS 1.3 key schedule is "ipskx".

As the maximum size of the PSK extension is 2^16 - 1 octets, an
Imported Identity that exceeds this size is likely to cause a
decoding error. Therefore, the PSK importer interface SHOULD reject
any ImportedIdentity that exceeds this size.

The hash function used for HMAC-based Key Derivation Function (HKDF)
[RFC5869] is that which is associated with the EPSK. It is not the
hash function associated with ImportedIdentity.target_kdf. If the
EPSK does not have such an associated hash function, SHA-256 [SHA2]
SHOULD be used. Diversifying EPSK by ImportedIdentity.target_kdf
ensures that an IPSK is only used as input keying material to one KDF
at most, thus satisfying the requirements in [RFC8446]. See
Section 8 for more details.

Endpoints SHOULD generate a compatible ipskx for each target
ciphersuite they offer. For example, importing a key for
TLS_AES_128_GCM_SHA256 and TLS_AES_256_GCM_SHA384 would yield two
PSKs: one for HKDF-SHA256 and another for HKDF-SHA384. In contrast,
if TLS_AES_128_GCM_SHA256 and TLS_CHACHA20_POLY1305_SHA256 are
supported, only one derived key is necessary. Each ciphersuite
uniquely identifies the target KDF. Future specifications that
change the way the KDF is negotiated will need to update this
specification to make clear how target KDFs are determined for the
import process.

EPSKs MAY be imported before the start of a connection if the target
KDFs, protocols, and context string(s) are known a priori. EPSKs MAY
also be imported for early data use if they are bound to the protocol
settings and configuration that are required for sending early data.
Minimally, this means that the Application-Layer Protocol Negotiation
(ALPN) value [RFC7301], QUIC transport parameters (if used for QUIC),
and any other relevant parameters that are negotiated for early data
MUST be provisioned alongside these EPSKs.

5.2. Binder Key Derivation

To prevent confusion between imported and non-imported PSKs, imported
PSKs change the PSK binder key derivation label. In particular, the
standard TLS 1.3 PSK binder key computation is defined as follows:

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

Imported PSKs use the string "imp binder" rather than "ext binder" or
"res binder" when deriving binder_key. This means the binder key is
computed as follows:

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

This new label ensures a client and server will negotiate use of an
external PSK if and only if (a) both endpoints import the PSK or (b)
neither endpoint imports the PSK. As binder_key is a leaf key,
changing its computation does not affect any other key.

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, attempting to incrementally deploy the importer mechanism would
result in concurrent use of the 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; see Section 8 for more
details. However, the benefits of using TLS 1.3 and 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 reuse 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. Therefore, authenticating with
an externally provisioned PSK should ideally authenticate both the
TLS connection and the external provisioning process. Typically, the
external provisioning 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 importer 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 look up 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

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:

+========+=================+===========+
| Value | KDF Description | Reference |
+========+=================+===========+
| 0x0000 | Reserved | RFC 9258 |
+--------+-----------------+-----------+
| 0x0001 | HKDF_SHA256 | [RFC5869] |
+--------+-----------------+-----------+
| 0x0002 | HKDF_SHA384 | [RFC5869] |
+--------+-----------------+-----------+

Table 1: TLS KDF Identifiers Registry

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].

11. References

11.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
<https://www.rfc-editor.org/info/rfc5056>.

[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/info/rfc8447>.

[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.

11.2. Informative References

[BAA15] Bhargavan, K., Delignat-Lavaud, A., and A. Pironti,
"Verified Contributive Channel Bindings for Compound
Authentication", Proceedings 2015 Network and Distributed
System Security, DOI 10.14722/ndss.2015.23277, February
2015, <https://doi.org/10.14722/ndss.2015.23277>.

[ECH] Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", Work in Progress, Internet-Draft,
draft-ietf-tls-esni-14, 13 February 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-14>.

[Kraw10] Krawczyk, H., "Cryptographic Extraction and Key
Derivation: The HKDF Scheme", Proceedings of Crypto 2010,
May 2010, <https://eprint.iacr.org/2010/264>.

[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.

[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.

[Selfie] Drucker, N. and S. Gueron, "Selfie: reflections on TLS 1.3
with PSK", DOI 10.1007/s00145-021-09387-y, May 2021,
<https://eprint.iacr.org/2019/347.pdf>.

[SHA2] National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-4,
DOI 10.6028/NIST.FIPS.180-4, August 2015,
<https://doi.org/10.6028/NIST.FIPS.180-4>.

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 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 that 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 the server into the IPSK context string. For instance, if an
application identifies each node by the Media Access Control (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.

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 Preuß 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.

Authors' Addresses

David Benjamin
Google, LLC.
Email: [email protected]

Christopher A. Wood
Cloudflare
Email: [email protected]