Internet DRAFT - draft-jhoyla-tls-extended-key-schedule
draft-jhoyla-tls-extended-key-schedule
jhoyla J. Hoyland
Internet-Draft Cloudflare Ltd.
Intended status: Standards Track C.A. Wood
Expires: 7 June 2021 Cloudflare
4 December 2020
TLS 1.3 Extended Key Schedule
draft-jhoyla-tls-extended-key-schedule-03
Abstract
TLS 1.3 is sometimes used in situations where it is necessary to
inject extra key material into the handshake. This draft aims to
describe methods for doing so securely. This key material must be
injected in such a way that both parties agree on what is being
injected and why, and further, in what order.
Note to Readers
Discussion of this document takes place on the TLS Working Group
mailing list (tls@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/tls/
(https://mailarchive.ietf.org/arch/browse/tls/).
Source for this draft and an issue tracker can be found at
https://github.com/jhoyla/draft-jhoyla-tls-key-injection
(https://github.com/jhoyla/draft-jhoyla-tls-key-injection).
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
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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 7 June 2021.
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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
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provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Key Schedule Extension . . . . . . . . . . . . . . . . . . . 3
3.1. Handshake Secret Injection . . . . . . . . . . . . . . . 3
3.2. Main Secret Injection . . . . . . . . . . . . . . . . . . 3
4. Key Schedule Injection Negotiation . . . . . . . . . . . . . 4
5. Key Schedule Extension Structure . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Potential Use Cases . . . . . . . . . . . . . . . . 6
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Introducing additional key material into the TLS handshake is a non-
trivial process because both parties need to agree on the injection
content and context. If the two parties do not agree then an
attacker may exploit the mismatch in so-called channel
synchronization attacks, such as those described by [SLOTH].
Injecting key material into the TLS handshake allows other protocols
to be bound to the handshake. For example, it may provide additional
protections to the ClientHello message, which in the standard TLS
handshake only receives protections after the server's Finished
message has been received. It may also permit the use of combined
shared secrets, possibly from multiple key exchange algorithms, to be
included in the key schedule. This pattern is common for Post
Quantum key exchange algorithms, as discussed in
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[I-D.ietf-tls-hybrid-design]. In particular,
[I-D.ietf-tls-hybrid-design] uses the concatenation pattern described
in this draft, but does not add the requisite framing.
The goal of this document is to provide a standardised way for
binding extra context into TLS 1.3 handshakes in a way that is easy
to analyse from a security perspective, reducing the need for
security analysis of extensions that affect the key schedule. It
separates the concerns of whether an extension achieves its goals
from the concerns of whether an extension reduces the security of a
TLS handshake, either directly or through some unforseen interaction
with another extension.
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. Key Schedule Extension
This section describes two places in which additional secrets can be
injected into the TLS 1.3 key schedule.
3.1. Handshake Secret Injection
To inject extra key material into the Handshake Secret it is
recommended to prefix it, inside an appropriate frame, to the
"(EC)DHE" input, where "||" represents concatenation.
|
v
Derive-Secret(., "derived", "")
|
v
KeyScheduleInput || (EC)DHE -> HKDF-Extract = Handshake Secret
|
v
3.2. Main Secret Injection
To inject key material into the Main Secret it is recommended to
prefix it, inside an appropriate frame, to the "0" input.
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|
v
Derive-Secret(., "derived", "")
|
v
KeyScheduleInput || 0 -> HKDF-Extract = Main Secret
|
v
This structure mirrors the Handshake Injection point.
4. Key Schedule Injection Negotiation
Applications which make use of additional key schedule inputs MUST
define a mechanism for negotiating the content and type of that
input. This input MUST be framed in a KeyScheduleSecret struct, as
defined in Section 5. Applications must take care that any
negotiation that takes place unambiguously agrees a secret. It must
be impossible, even under adversarial conditions, that a client and
server agree on the transcript of the negotiation, but disagree on
the secret that was negotiated.
5. Key Schedule Extension Structure
In some cases, protocols may require more than one secret to be
injected at a particular stage in the key schedule. Thus, we require
a generic and extensible way of doing so. To accomplish this, we use
a structure - KeyScheduleInput - that encodes well-ordered sequences
of secret material to inject into the key schedule. KeyScheduleInput
is defined as follows:
struct {
KeyScheduleSecretType type;
opaque secret_data<0..2^16-1>;
} KeyScheduleSecret;
enum {
(65535)
} KeyScheduleSecretType;
struct {
KeyScheduleSecret secrets<0..2^16-1>;
} KeyScheduleInput;
Each secret included in a KeyScheduleInput structure has a type and
corresponding secret data. Each secret MUST have a unique
KeyScheduleSecretType. When encoding KeyScheduleInput as the key
schedule Input value, the KeyScheduleSecret values MUST be in
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ascending sorted order. This ensures that endpoints always encode
the same KeyScheduleInput value when using the same secret keying
material.
6. Security Considerations
[BINDEL] provides a proof that the concatenation approach in
Section 3 is secure as long as either the concatenated secret is
secure or the existing KDF input is secure.
[[OPEN ISSUE: Is this guarantee sufficient? Do we also need to
guarantee that a malicious prefix can't weaken the resulting PRF
output?]]
7. IANA Considerations
This document requests the creation of a new IANA registry: TLS
KeyScheduleInput Types. This registry should be under the existing
Transport Layer Security (TLS) Parameters heading. It should be
administered under a Specification Required policy [RFC8126].
[[OPEN ISSUE: specify initial registry values]]
+=======+=============+=========+===========+
| Value | Description | DTLS-OK | Reference |
+=======+=============+=========+===========+
| TBD | TBD | TBD | TBD |
+-------+-------------+---------+-----------+
Table 1
8. References
8.1. 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>.
[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>.
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8.2. Informative References
[BINDEL] Bindel, N., Brendel, J., Fischlin, M., Goncalves, B., and
D. Stebila, "Hybrid Key Encapsulation Mechanisms and
Authenticated Key Exchange", Post-Quantum Cryptography pp.
206-226, DOI 10.1007/978-3-030-25510-7_12, 2019,
<https://doi.org/10.1007/978-3-030-25510-7_12>.
[I-D.friel-tls-eap-dpp]
Friel, O. and D. Harkins, "Bootstrapped TLS
Authentication", Work in Progress, Internet-Draft, draft-
friel-tls-eap-dpp-01, 13 July 2020, <http://www.ietf.org/
internet-drafts/draft-friel-tls-eap-dpp-01.txt>.
[I-D.ietf-tls-hybrid-design]
Steblia, D., Fluhrer, S., and S. Gueron, "Hybrid key
exchange in TLS 1.3", Work in Progress, Internet-Draft,
draft-ietf-tls-hybrid-design-01, 15 October 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-tls-
hybrid-design-01.txt>.
[I-D.ietf-tls-semistatic-dh]
Rescorla, E., Sullivan, N., and C. Wood, "Semi-Static
Diffie-Hellman Key Establishment for TLS 1.3", Work in
Progress, Internet-Draft, draft-ietf-tls-semistatic-dh-01,
7 March 2020, <http://www.ietf.org/internet-drafts/draft-
ietf-tls-semistatic-dh-01.txt>.
[SLOTH] Bhargavan, K. and G. Leurent, "Transcript Collision
Attacks: Breaking Authentication in TLS, IKE, and SSH",
Proceedings 2016 Network and Distributed System
Security Symposium, DOI 10.14722/ndss.2016.23418, 2016,
<https://doi.org/10.14722/ndss.2016.23418>.
Appendix A. Potential Use Cases
The draft provides a mechanism for importing additional information
into the TLS key schedule. Future applications and specifications
can use this mechanism to layer TLS on to other protocols, as opposed
to layering other protocols over TLS. For example, as discussed in
Section 1, this can be used for hybrid key exchange, which, in
effect, is layering TLS over a secondary AKE. Although the key
exchanges are interleaved, the post-quantum AKE completes first, as
demonstrated by its output key being used as an input for computing
TLS's master secret.
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This can also be used in more direct ways, such as bootstrapping EAP-
TLS as in [I-D.friel-tls-eap-dpp]. This draft also allows for more
direct implementations of things such as semi-static DH
[I-D.ietf-tls-semistatic-dh]. The aim of this draft is to be
sufficiently flexible that it can be used as the basis for layering
TLS on top of any protocol that outputs a secure channel binding,
where secure is defined by the goals of the overall layered protocol.
This draft does not provide security itself, it simply provides a
standard format for layering.
Acknowledgments
We thank Karthik Bhargavan for his comments.
Authors' Addresses
Jonathan Hoyland
Cloudflare Ltd.
Email: jonathan.hoyland@gmail.com
Christopher A. Wood
Cloudflare
Email: caw@heapingbits.net
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