tls D. Benjamin
Internet-Draft Google, LLC.
Intended status: Standards Track C. Wood
Expires: April 4, 2020 Apple, Inc.
October 02, 2019
Importing External PSKs for TLS
draft-ietf-tls-external-psk-importer-01
Abstract
This document describes an interface for importing external PSK (Pre-
Shared Key) into TLS 1.3.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
4. Key Import . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Deprecating Hash Functions . . . . . . . . . . . . . . . . . 5
6. Incremental Deployment . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
10.1. Normative References . . . . . . . . . . . . . . . . . . 6
10.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8
Appendix B. Addressing Selfie . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
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, external PSKs can be bound to a specific
KDF and hash function when used in TLS 1.3, even if they are
associated with a different hash function when provisioned. This
document specifies an interface by which external PSKs may be
imported for use in a TLS 1.3 connection to achieve this goal. In
particular, it describes how KDF-bound PSKs can be differentiated by
the target (D)TLS protocol version and KDF for which the PSK will be
used. This produces a set of candidate PSKs, each of which are bound
to a specific target protocol and KDF. This expands what would
normally have been a single PSK identity into a set of PSK
identities. However, importantly, it requires no change to the TLS
1.3 key schedule.
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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
Key importers mirror the concept of key exporters in TLS in that they
diversify a key based on some contextual information before use in a
connection. In contrast to key exporters, wherein differentiation is
done via an explicit label and context string, the key importer
defined herein uses an optional context string along with a target
protocol and KDF identifier to differentiate an external PSK into one
or more PSKs for use.
Imported keys do not require negotiation for use, as a client and
server will not agree upon identities if not imported correctly.
Thus, importers induce no protocol changes with the exception of
expanding the set of PSK identities sent on the wire. 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 TLS MUST NOT use these keys for
any other purpose. Moreover, each external PSK MUST be associated
with at most one hash function.
3.1. Terminology
o 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).
o Base Key: The secret value of an EPSK.
o External Identity: The identity of an EPSK.
o Target protocol: The protocol for which a PSK is imported for use.
o Target KDF: The KDF for which a PSK is imported for use.
o Imported PSK (IPSK): A PSK derived from an EPSK, external
identity, optional context string, and target protocol and KDF.
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o Imported Identity: The identity of an Imported PSK as sent on the
wire.
4. Key Import
A key importer 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 supported
(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 "target_kdf" values is maintained by IANA as described in
Section 9. External PSKs MUST NOT be imported for versions of (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. 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].
ImportedIdentity.target_protocol MUST be the (D)TLS protocol version
for which the PSK is being imported. For example, TLS 1.3 [RFC8446]
and QUICv1 [QUIC] use 0x0304. Note that this means future versions
of TLS will increase the number of PSKs derived from an external PSK.
An Imported PSK derived from an EPSK with base key 'epsk' bound to
this identity is then computed as follows:
epskx = HKDF-Extract(0, epsk)
ipskx = HKDF-Expand-Label(epskx, "derived psk",
Hash(ImportedIdentity), L)
L is corresponds to the KDF output length of
ImportedIdentity.target_kdf as defined in Section 9. For hash-based
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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.
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 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].
Endpoints 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.
The resulting IPSK base key 'ipskx' is then used as the binder key in
TLS 1.3 with identity ImportedIdentity. With knowledge of the
supported KDFs, one may import PSKs before the start of a connection.
EPSKs may 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 ALPN, QUIC transport settings, etc., must be provisioned
alongside these EPSKs.
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, target protocol, and target
KDF. 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
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does not preclude endpoints from using non-imported PSKs for TLS 1.2.
Indeed, this is necessary for incremental deployment.
7. Security Considerations
DISCLAIMER: This is a WIP draft and has not yet seen significant
security analysis.
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:
+-------------+-----+ | Description | Value | +-------------+-----+ |
Reserved | 0x0000 | | | | | HKDF_SHA256 | 0x0001 | | | | |
HKDF_SHA384 | 0x0002 | +-------------+-----+
New target KDF values are allocated according to the following
process:
o Values in the range 0x0000-0xfeff are assigned via Specification
Required [RFC8126].
o Values in the range 0xff00-0xffff are reserved for Private Use
[RFC8126].
10. References
10.1. Normative References
[QUIC] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-23 (work
in progress), September 2019.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
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[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>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.
[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>.
10.2. Informative References
[CCB] Bhargavan, K., Delignat-Lavaud, A., and A. Pironti,
"Verified Contributive Channel Bindings for Compound
Authentication", Proceedings 2015 Network and Distributed
System Security Symposium, DOI 10.14722/ndss.2015.23277,
2015.
[Selfie] Drucker, N. and S. Gueron, "Selfie: reflections on TLS 1.3
with PSK", 2019, <https://eprint.iacr.org/2019/347.pdf>.
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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.
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
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Christopher A. Wood
Apple, Inc.
Email: cawood@apple.com
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