jhoyla J. Hoyland
Internet-Draft Cloudflare Ltd.
Intended status: Standards Track C. Wood
Expires: May 7, 2020 Apple, Inc.
November 04, 2019
TLS 1.3 Extended Key Schedule
draft-jhoyla-tls-extended-key-schedule-00
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/ [1].
Source for this draft and an issue tracker can be found at
https://github.com/jhoyla/draft-jhoyla-tls-key-injection [2].
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 7, 2020.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Key Schedule Extension . . . . . . . . . . . . . . . . . . . 3
3.1. Early Secret Injection . . . . . . . . . . . . . . . . . 3
3.2. Handshake Secret Injection . . . . . . . . . . . . . . . 4
4. Key Schedule Extension Structure . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 6
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
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
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Quantum key exchange algorithms, as discussed in
[I-D.stebila-tls-hybrid-design].
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 ways in which additional secrets can be
injected into the TLS 1.3 key schedule.
3.1. Early Secret Injection
TLS provides exporter keys that allow for other protocols to provide
data authenticated by the TLS channel. This can be used to bind a
protocol to a specific TLS handshake, giving joint authentication
guarantees. In a similar way, one may wish to introduce externally
authenticated and pre-shared data to the early secret derivation.
This can be used to bind TLS to an external protocol.
To achieve this, pre-shared keys modify the binder key computation.
This is needed since it ensures that both parties agree on both the
authenticated data and the context in which it was used.
The binder key computation change is as follows:
0
|
v
PSK -> HKDF-Extract = Early Secret
|
+-----> Derive-Secret(., "ext binder"
| | "res binder"
| | "imp ext binder"
| | "imp res binder", "")
| = binder_key
v
Use of the "imp ext binder" label implies that both parties agree
that there is some context that has been agreed, and that they are
using an external PSK. Use of the "imp res binder" label implies
that both parties agree that there is some context that has been
agreed, and that they are using an resumption PSK. This assumes the
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PSK has some mechanism by which additional context is included.
[I-D.ietf-tls-external-psk-importer] describes one way by which such
context may be included.
struct {
opaque external_identity<1...2^16-1>;
opaque context<0...2^16>;
} PSKIDWithAdditionalData;
external_identity is the "PSK_ID" that would have been used if the
additional data were not agreed upon.
context is an opaque value that is bound to the agreed upon
additional data.
Those using the "imp ext binder" or "imp res binder" label MUST
include a "context" field, to allow the additional data.
Note that this structure is recursive. If this mechanism is used
multiple times then the "external_identity" field will contain
previous contexts in sequential order. If the client does not know
in advance which pieces of additional data the server will be willing
to agree on, it can provide multiple binders with different subsets
of the additional data. The server can then select a binder with
which it is willing to proceed. The binders MUST be verified in an
all-or-nothing manner, and only one binder SHOULD be checked. A
server MUST NOT accept a binder for which it only agrees upon some of
the data.
3.2. 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
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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.
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.
5. Security Considerations
[[OPEN ISSUE: This draft has not seen any security analysis.]]
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6. IANA Considerations
[[TODO: define secret registry structure]]
7. References
7.1. Normative References
[I-D.ietf-tls-external-psk-importer]
Benjamin, D. and C. Wood, "Importing External PSKs for
TLS", draft-ietf-tls-external-psk-importer-01 (work in
progress), October 2019.
[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, "Design issues
for hybrid key exchange in TLS 1.3", draft-stebila-tls-
hybrid-design-01 (work in progress), July 2019.
7.3. URIs
[1] https://mailarchive.ietf.org/arch/browse/tls/
[2] https://github.com/jhoyla/draft-jhoyla-tls-key-injection
Acknowledgments
We thank Karthik Bhargavan for his comments.
Authors' Addresses
Jonathan Hoyland
Cloudflare Ltd.
Email: jonathan.hoyland@gmail.com
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Christopher A. Wood
Apple, Inc.
Email: cawood@apple.com
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