The SSLKEYLOGFILE Format for TLS
draft-thomson-tls-keylogfile-01
| Document | Type | Active Internet-Draft (tls WG) | |
|---|---|---|---|
| Author | Martin Thomson | ||
| Last updated | 2023-11-06 (Latest revision 2023-07-28) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | Adopted by a WG | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
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| Send notices to | (None) |
draft-thomson-tls-keylogfile-01
Transport Layer Security M. Thomson
Internet-Draft Mozilla
Intended status: Informational 28 July 2023
Expires: 29 January 2024
The SSLKEYLOGFILE Format for TLS
draft-thomson-tls-keylogfile-01
Abstract
A format that supports the logging information about the secrets used
in a TLS connection is described. Recording secrets to a file in
SSLKEYLOGFILE format allows diagnostic and logging tools that use
this file to decrypt messages exchanged by TLS endpoints.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://martinthomson.github.io/sslkeylogfile/draft-thomson-tls-
keylogfile.html. Status information for this document may be found
at https://datatracker.ietf.org/doc/draft-thomson-tls-keylogfile/.
Discussion of this document takes place on the Transport Layer
Security Working Group mailing list (mailto:tls@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/tls/. Subscribe
at https://www.ietf.org/mailman/listinfo/tls/.
Source for this draft and an issue tracker can be found at
https://github.com/martinthomson/sslkeylogfile.
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."
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Copyright Notice
Copyright (c) 2023 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
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Applicability Statement . . . . . . . . . . . . . . . . . 3
1.2. Conventions and Definitions . . . . . . . . . . . . . . . 3
2. The SSLKEYLOGFILE Format . . . . . . . . . . . . . . . . . . 3
2.1. Secret Labels for TLS 1.3 . . . . . . . . . . . . . . . . 4
2.2. Secret Labels for TLS 1.2 . . . . . . . . . . . . . . . . 5
3. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Normative References . . . . . . . . . . . . . . . . . . 7
5.2. Informative References . . . . . . . . . . . . . . . . . 7
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Debugging or analyzing protocols can be challenging when TLS [TLS13]
is used to protect the content of communications. Inspecting the
content of encrypted messages in diagnostic tools can enable more
thorough analysis.
Over time, multiple TLS implementations have informally adopted a
file format that logging the secret values generated by the TLS key
schedule. In many implementations, the file that the secrets are
logged to is specified in an environment variable named
"SSLKEYLOGFILE", hence the name of SSLKEYLOGFILE format. Note the
use of "SSL" as this convention originally predates the adoption of
TLS as the name of the protocol.
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This document describes the SSLKEYLOGFILE format. This format can be
used for TLS 1.2 [TLS12] and TLS 1.3 [TLS13]. The format also
supports earlier TLS versions, though use of earlier versions is
forbidden [RFC8996]. This format can also be used with DTLS
[DTLS13], QUIC [RFC9000][RFC9001], and protocols that also use the
TLS key schedule. Use of this format could complement other
protocol-specific logging such as QLOG [QLOG].
1.1. Applicability Statement
The artifact that this document describes - if made available to
entities other than endpoints - completely undermines the core
guarantees that TLS provides. This format is intended for use in
systems where TLS only protects test data. While the access that
this information provides to TLS connections can be useful for
diagnosing problems while developing systems, this mechanism MUST NOT
be used in a production system.
1.2. 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.
2. The SSLKEYLOGFILE Format
A file in SSLKEYLOGFILE format is a text file. This document
specifies the character encoding as UTF-8 [RFC3629]. Though the
format itself only includes ASCII characters [RFC0020], comments MAY
contain other characters. Though Unicode is permitted in comments,
the file MUST NOT contain a unicode byte order mark (U+FEFF).
Lines are terminated using the line ending convention of the platform
on which the file is generated. Tools that process these files MUST
accept CRLF (U+13 followed by U+10), CR (U+13), or LF (U+10) as a
line terminator. Lines are ignored if they are empty or if the first
character is an octothorp character ('#', U+23). Other lines of the
file each contain a single secret.
Each secret is described using a single line composed of three values
that are separated by a single space character (U+20). These values
are:
label: The label identifies the type of secret that is being
conveyed; see Section 2.1 for a description of the labels that are
defined in this document.
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client_random: The 32 byte value of the Random field from the
ClientHello message that established the TLS connection. This
value is encoded as 64 hexadecimal characters. Including this
field allows a single file to include secrets from multiple
connections and for the secrets to be applied to the correct
connection, though this depends on being able to match records to
the correct ClientHello message.
secret: The value of the identified secret for the identified
connection. This value is encoded in hexadecimal, with a length
that depends on the size of the secret.
For the hexadecimal values of the client_random or secret, no
convention exists for the case of characters 'a' through 'f' (or 'A'
through 'F'). Files can be generated with either, so either form
MUST be accepted when processing a file.
Diagnostic tools that accept files in this format might choose to
ignore lines that do not conform to this format in the interest of
ensuring that secrets can be obtained from corrupted files.
2.1. Secret Labels for TLS 1.3
An implementation of TLS 1.3 produces a number of values as part of
the key schedule (see Section 7.1 of [TLS13]). Each of the following
labels correspond to the equivalent secret produced by the key
schedule:
CLIENT_EARLY_TRAFFIC_SECRET:
This secret is used to protect records sent by the client as early
data, if early data is attempted by the client. Note that a
server that rejects early data will not log this secret, though a
client that attempts early data can do so unconditionally.
EARLY_EXPORTER_MASTER_SECRET:
This secret is used for early exporters. Like the
CLIENT_EARLY_TRAFFIC_SECRET, this is only generated when early
data is attempted and might not be logged by a server if early
data is rejected.
CLIENT_HANDSHAKE_TRAFFIC_SECRET:
This secret is used to protect handshake records sent by the
client.
SERVER_HANDSHAKE_TRAFFIC_SECRET:
This secret is used to protect handshake records sent by the
server.
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CLIENT_TRAFFIC_SECRET_0:
This secret is used to protect application_data records sent by
the client immediately after the handshake completes. This secret
is identified as client_application_traffic_secret_0 in the TLS
1.3 key schedule.
SERVER_TRAFFIC_SECRET_0:
This secret is used to protect application_data records sent by
the server immediately after the handshake completes. This secret
is identified as server_application_traffic_secret_0 in the TLS
1.3 key schedule.
EXPORTER_SECRET:
This secret is used in generating exporters Section 7.5 of
[TLS13].
Each of the preceding labels are identified using the lowercase form
of the label in [TLS13], except as noted. Note that the order that
labels appear here corresponds to the order in which they are
presented in [TLS13], but there is no guarantee that implementations
will log secrets strictly in this order.
Key updates (Section 7.2 of [TLS13]) result in new secrets being
generated for protecting application_data records. The label used
for these secrets comprises a base label of "CLIENT_TRAFFIC_SECRET_"
for a client or "SERVER_TRAFFIC_SECRET_" for a server, plus the
decimal value of a counter. This counter identifies the number of
key updates that occurred to produce this secret. This counter
starts at 0, which produces the first application data traffic
secret, as above.
2.2. Secret Labels for TLS 1.2
An implementation of TLS 1.2 [TLS12] (and also earlier versions) use
the label "CLIENT_RANDOM" to identify the "master" secret for the
connection.
3. Security Considerations
Access to the content of a file in SSLKEYLOGFILE format allows an
attacker to break the confidentiality protection on any TLS
connections that are included in the file. This includes both active
connections and connections for which encrypted records were
previously stored. Ensuring adequate access control on these files
therefore becomes very important.
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Implementations that support logging this data need to ensure that
logging can only be enabled by those who are authorized. Allowing
logging to be initiated by any entity that is not otherwise
authorized to observe or modify the content of connections for which
secrets are logged could represent a privilege escalation attack.
Implementations that enable logging also need to ensure that access
to logged secrets is limited, using appropriate file permissions or
equivalent access control mechanisms.
In order to support decryption, the secrets necessary to remove
record protection are logged. However, as the keys that can be
derived from these secrets are symmetric, an adversary with access to
these secrets is also able to encrypt data for an active connection.
This might allow for injection or modification of application data on
a connection that would otherwise be protected by TLS.
As some protocols that depend on TLS generate encryption keys, the
SSLKEYLOGFILE format includes keys that identify the secret used in
TLS exporters or early exporters (Section 7.5 of [TLS13]. Knowledge
of these secrets can enable more than inspection or modification of
encrypted data, depending on how an application protocol uses
exporters. For instance, exporters might be used for session
bindings (e.g., [RFC8471]), authentication (e.g., [RFC9261]), or
other derived secrets that are used in application context. An
adversary that obtains these secrets might be able to use this
information to attack these applications. A TLS implementation might
either choose to omit these secrets in contexts where the information
might be abused or to require separate authorization to enable
logging of exporter secrets.
Logging the TLS 1.2 "master" secret provides the recipient of a file
in SSLKEYLOGFILE far greater access to an active connection. This
can include the ability to resume the connection and impersonate
either endpoint, insert records that result in renegotiation, or even
forge Finished messages. Implementations might avoid these risks by
not logging this secret value.
4. IANA Considerations
The "application/sslkeylogfile" media type can be used to describe
content in the SSLKEYLOGFILE format. IANA [has added/is requested to
add] the following information to the "Media Types" registry at
https://www.iana.org/assignments/media-types
(https://www.iana.org/assignments/media-types):
Type name: application
Subtype name: sslkeylogfile
Required parameters: N/A
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Optional parameters: N/A
Encoding considerations: 8bit (Unicode without BOM or ASCII only)
Security considerations: See Section 3.
Interoperability considerations: Line endings might differ from
platform convention
Published specification: This document
Applications that use this media type: Diagnostic and analysis tools
that need to decrypt data that is otherwise protected by TLS.
Fragment identifier considerations: N/A
Additional information: Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): N/A
Macintosh file type code(s): N/A
Person & email address to contact for further information: See the
Authors' Addresses section.
Intended usage: COMMON
Restrictions on usage: N/A
Author: See the Authors' Addresses section.
Change controller: IESG
5. References
5.1. 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/rfc/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/rfc/rfc3629>.
[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/rfc/rfc8174>.
[TLS12] 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/rfc/rfc5246>.
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
5.2. Informative References
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[DTLS13] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/rfc/rfc9147>.
[QLOG] Marx, R., Niccolini, L., Seemann, M., and L. Pardue, "Main
logging schema for qlog", Work in Progress, Internet-
Draft, draft-ietf-quic-qlog-main-schema-06, 10 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
qlog-main-schema-06>.
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/rfc/rfc20>.
[RFC8471] Popov, A., Ed., Nystroem, M., Balfanz, D., and J. Hodges,
"The Token Binding Protocol Version 1.0", RFC 8471,
DOI 10.17487/RFC8471, October 2018,
<https://www.rfc-editor.org/rfc/rfc8471>.
[RFC8996] Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
<https://www.rfc-editor.org/rfc/rfc8996>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
[RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/rfc/rfc9001>.
[RFC9261] Sullivan, N., "Exported Authenticators in TLS", RFC 9261,
DOI 10.17487/RFC9261, July 2022,
<https://www.rfc-editor.org/rfc/rfc9261>.
Acknowledgments
The SSLKEYLOGFILE format originated in the NSS project, but it has
evolved over time as TLS has changed. Many people contributed to
this evolution. The author is only documenting the format as it is
used.
Author's Address
Martin Thomson
Mozilla
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Email: mt@lowentropy.net
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