TLS 1.3 Extended Key Schedule
draft-jhoyla-tls-extended-key-schedule-01
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Jonathan Hoyland , Christopher A. Wood | ||
| Last updated | 2020-03-09 (Latest revision 2019-11-04) | ||
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draft-jhoyla-tls-extended-key-schedule-01
jhoyla J. Hoyland
Internet-Draft Cloudflare Ltd.
Intended status: Standards Track C.A. Wood
Expires: 10 September 2020 Apple, Inc.
9 March 2020
TLS 1.3 Extended Key Schedule
draft-jhoyla-tls-extended-key-schedule-01
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 10 September 2020.
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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/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Key Schedule Extension . . . . . . . . . . . . . . . . . . . 3
3.1. Handshake Secret Injection . . . . . . . . . . . . . . . 3
3.2. Master Secret Injection . . . . . . . . . . . . . . . . . 3
4. Key Schedule Extension Structure . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
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.
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
[I-D.stebila-tls-hybrid-design].
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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. 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 key material into the Handshake Secret it is recommended to
use an extra derive secret.
|
v
Derive-Secret(., "derived early", "")
|
v
Input -> HKDF-Extract
|
v
Derive-Secret(., "derived", "")
|
v
(EC)DHE -> HKDF-Extract = Handshake Secret
|
v
As shown in the figure above, the key schedule has an extra derive
secret and HKDF-Extract step. This extra step isolates the Input
material from the rest of the handshake secret, such that even
maliciously chosen values cannot weaken the security of the key
schedule overall.
The additional Derive-Secret with the "derived early" label enforces
the separation of the key schedule from vanilla TLS handshakes,
because HKDFs can be assumed to ensure that keys derived with
different labels are independent.
3.2. Master Secret Injection
To inject key material into the Master Secret it is recommended to
use an extra derive secret.
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|
v
Derive-Secret(., "derived early", "")
|
v
Input -> HKDF-Extract
|
v
Derive-Secret(., "derived", "")
|
v
0 -> HKDF-Extract = Master Secret
|
v
This structrue mirrors the Handshake Injection point, the key
schedule has an extra Extract, Derive-Secret pattern. This, again,
should isolate the Input material from the rest of the Master Secret.
4. 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
ascending sorted order. This ensures that endpoints always encode
the same KeyScheduleInput value when using the same secret keying
material.
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5. Security Considerations
[[OPEN ISSUE: This draft has not seen any security analysis.]]
6. IANA Considerations
[[TODO: define secret registry structure]]
7. References
7.1. 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>.
[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>.
7.2. Informative References
[I-D.stebila-tls-hybrid-design]
Steblia, D., Fluhrer, S., and S. Gueron, "Hybrid key
exchange in TLS 1.3", Work in Progress, Internet-Draft,
draft-stebila-tls-hybrid-design-03, 12 February 2020,
<http://www.ietf.org/internet-drafts/draft-stebila-tls-
hybrid-design-03.txt>.
Acknowledgments
We thank Karthik Bhargavan for his comments.
Authors' Addresses
Jonathan Hoyland
Cloudflare Ltd.
Email: jonathan.hoyland@gmail.com
Christopher A. Wood
Apple, Inc.
Email: cawood@apple.com
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