The SSLKEYLOGFILE Format for TLS

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Table of Contents

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.¶

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 other protocols that also use the TLS key schedule. Use of this format could complement other protocol-specific logging such as QLOG [QLOG].¶

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.¶

Implementations that record secrets to a file do so continuously as those secrets are generated.¶

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.¶

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.¶

Logged secret values are not annotated with the cipher suite or other connection parameters. A record of the TLS handshake might therefore be needed to use the logged secrets.¶

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.¶

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 rely on TLS for generating 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 require separate authorization to enable logging of exporter secrets.¶

Using an environment variable, such as SSLKEYLOGFILE, to enable logging implies that access to the launch context for the application is needed to authorize logging. On systems that support specially-named files, logs might be directed to these names so that logging does not result in storage, but enable consumption by other programs. In both cases, applications might require special authorization or they might rely on system-level access control to limit access to these capabilities.¶

Forward secrecy guarantees provided in TLS 1.3 (see Section 1.2 and Appendix E.1 of [RFC8446]) and some modes of TLS 1.2 (such as those in Sections 2.2 and 2.4 of [RFC4492]) do not hold if key material is recorded. Access to key material allows an attacker to decrypt data exchanged in any previously logged TLS connections.¶

Logging the TLS 1.2 "master" secret provides the recipient of that secret far greater access to an active connection than TLS 1.3 secrets. In addition to reading and altering protected messages, the TLS 1.2 "master" secret confers the ability to resume the connection and impersonate either endpoint, insert records that result in renegotiation, and forge Finished messages. Implementations can avoid the risks associated with these capabilities 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:¶

Type name:

application¶

Subtype name:

sslkeylogfile¶

Required parameters:

N/A¶

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¶

Appendix A. Example

The following is a sample of a file in this format, including secrets from a TLS 1.3 connection.¶

# NOTE: '\' line wrapping per RFC 8792

CLIENT_HANDSHAKE_TRAFFIC_SECRET \
  cf34899b3dcb8c9fe7160ceaf95d354a294793b67a2e49cb9cca4d69b43593a0 \
  be4a28d81ce41242ff31c6d8a6615852178f2cd75eaca2ee8768f9ed51282b38
SERVER_HANDSHAKE_TRAFFIC_SECRET \
  cf34899b3dcb8c9fe7160ceaf95d354a294793b67a2e49cb9cca4d69b43593a0 \
  258179721fa704e2f1ee16688b4b0419967ddea5624cd5ad0863288dc5ead35f
CLIENT_TRAFFIC_SECRET_0 \
  cf34899b3dcb8c9fe7160ceaf95d354a294793b67a2e49cb9cca4d69b43593a0 \
  e9ca165bcb762fab8086068929d26c532e90ef2e2daa762d8b52346951a34c02
SERVER_TRAFFIC_SECRET_0 \
  cf34899b3dcb8c9fe7160ceaf95d354a294793b67a2e49cb9cca4d69b43593a0 \
  4f93c61ac1393008d4c820f3723db3c67494f06574b65fcc21c9eef22f90071a
EXPORTER_SECRET \
  cf34899b3dcb8c9fe7160ceaf95d354a294793b67a2e49cb9cca4d69b43593a0 \
  011c900833468f837f7c55d836b2719beebd39b1648fdeda58772f48d94a1ffa

The following shows a log entry for a TLS 1.2 connection.¶

# NOTE: '\' line wrapping per RFC 8792

CLIENT_RANDOM \
  ad52329fcadd34ee3aa07092680287f09954823e26d7b5ae25c0d47714152a6a \
  97af4c8618cfdc0b2326e590114c2ec04b43b08b7e2c3f8124cc61a3b068ba966\
  9517e744e3117c3ce6c538a2d88dfdf

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