tls D. Benjamin
Internet-Draft Google, LLC.
Intended status: Standards Track C.A. Wood
Expires: 6 June 2021 Cloudflare
3 December 2020
Importing External PSKs for TLS
draft-ietf-tls-external-psk-importer-06
Abstract
This document describes an interface for importing external Pre-
Shared Keys (PSKs) into TLS 1.3.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 6 June 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
4. PSK Import . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. External PSK Diversification . . . . . . . . . . . . . . 4
4.2. Binder Key Derivation . . . . . . . . . . . . . . . . . . 6
5. Deprecating Hash Functions . . . . . . . . . . . . . . . . . 6
6. Incremental Deployment . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Appendix B. Addressing Selfie . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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 TLS 1.3 and prior versions.
To mitigate against any interference, 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 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, 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.
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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. 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 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.
Endpoints 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.
3.1. Terminology
* 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.
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* Target KDF: The KDF for which a PSK is imported for use.
* Imported PSK (IPSK): A PSK derived from an EPSK, External
Identity, optional context string, target protocol, and target
KDF.
* Imported Identity: A sequence of bytes used to identify an IPSK.
4. PSK Import
This section describes the PSK Importer interface and its underlying
diversification mechanism and binder key computation modification.
4.1. External PSK Diversification
The PSK Importer interface takes as input an EPSK with External
Identity "external_identity" and base key "epsk", as defined in
Section 3.1, along with an optional context, and transforms it 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 9. External PSKs MUST NOT be imported for
(D)TLS 1.2 or prior versions. See Section 6 for discussion on how
imported PSKs for TLS 1.3 and non-imported PSKs for earlier versions
co-exist for incremental deployment.
ImportedIdentity.context MUST include the context used to derive 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 B for more
details. If the EPSK is a key derived from some other 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]
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uses 0x0304, which will therefore also be used by QUICv1 [QUIC].
Note that this means future versions of TLS will increase the number
of PSKs derived from an external PSK.
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 9. For hash-based KDFs, such as
HKDF_SHA256(0x0001), this is the length of the hash function output,
i.e., 32 octets. This is required for the IPSK to be of length
suitable for supported ciphersuites.
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 TLS 1.3 binder key is "ipskx".
The hash function used for HKDF [RFC5869] is that which is associated
with the EPSK. It is not the hash function associated with
ImportedIdentity.target_kdf. If no hash function is specified,
SHA-256 [SHA2] MUST be used. Diversifying EPSK by
ImportedIdentity.target_kdf ensures that an IPSK is only used as
input keying material to at most one KDF, thus satisfying the
requirements in [RFC8446]. See Section 7 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.
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 protocol
settings and configurations that would otherwise be required for
early data with normal (ticket-based PSK) resumption. Minimally,
that means Application-Layer Protocol Negotiation [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.
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4.2. 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 replace the string "ext binder" with "imp 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.
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.
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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. Specifically,
existing applications using TLS 1.2 with non-imported PSKs can safely
enable TLS 1.3 with imported PSKs in clients and servers without
interoperability risk.
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.
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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 is no known interference between 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. 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.
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,
titled "TLS KDF Identifiers", under the existing "Transport Layer
Security (TLS) Parameters" heading.
The entries in the registry are:
+-----------------+--------+-----------+
| KDF Description | Value | Reference |
+=================+========+===========+
| Reserved | 0x0000 | N/A |
+-----------------+--------+-----------+
| HKDF_SHA256 | 0x0001 | [RFC5869] |
+-----------------+--------+-----------+
| HKDF_SHA384 | 0x0002 | [RFC5869] |
+-----------------+--------+-----------+
Table 1: Target KDF Registry
New target KDF values are allocated according to the following
process:
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* 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].
10. References
10.1. Normative References
[DTLS13] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
dtls13-39, 2 November 2020, <http://www.ietf.org/internet-
drafts/draft-ietf-tls-dtls13-39.txt>.
[QUIC] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", Work in Progress, Internet-Draft,
draft-ietf-quic-transport-32, 20 October 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-quic-
transport-32.txt>.
[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>.
[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>.
[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>.
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[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>.
10.2. Informative References
[BAA15] Bhargavan, K., Delignat-Lavaud, A., and A. Pironti,
"Verified Contributive Channel Bindings for Compound
Authentication", DOI 10.14722/ndss.2015.23277, Proceedings
2015 Network and Distributed System Security Symposium,
2015, <https://doi.org/10.14722/ndss.2015.23277>.
[Kraw10] Krawczyk, H., "Cryptographic Extraction and Key
Derivation: The HKDF Scheme", Proceedings of CRYPTO 2010 ,
2010, <https://eprint.iacr.org/2010/264>.
[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", 2019, <https://eprint.iacr.org/2019/347.pdf>.
[SHA2] National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-3 , October 2008.
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
and others all provided reviews, feedback, and suggestions for
improving the document.
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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
David Benjamin
Google, LLC.
Email: davidben@google.com
Christopher A. Wood
Cloudflare
Email: caw@heapingbits.net
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