]> Mozilla

mt@lowentropy.net

Zscaler

yrosomakho@zscaler.com

University of Applied Sciences Bonn-Rhein-Sieg

Hannes.Tschofenig@gmx.net

SEC tls network transparency tls blockchain This document describes a format that supports logging information about the secrets used in a TLS connection. Recording secrets to a file in SSLKEYLOGFILE format allows diagnostic and logging tools that use this file to decrypt messages exchanged by TLS endpoints. This format is intended for use in systems where TLS only protects test data.

Introduction Debugging or analyzing protocols can be challenging when TLS 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 for 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 and TLS 1.3 . The format also supports earlier TLS versions, though use of earlier versions is strongly discouraged . This format can also be used with DTLS , QUIC , and other protocols that use the TLS key schedule. Use of this format could complement other protocol-specific logging such as qlog . This document also defines labels that can be used to log information about exchanges that use Encrypted Client Hello (ECH) .

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. For software that is compiled, use of conditional compilation is the best way to ensure that deployed binaries cannot be configured to enable key logging. addresses a number of additional concerns that arise from the use of key logging.

Conventions 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 when, and only when, they appear in all capitals, as shown here.

The SSLKEYLOGFILE Format A file in SSLKEYLOGFILE format is a text file. This document specifies the character encoding as UTF-8 . Though the format itself only includes ASCII characters , 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 octothorpe 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 Sections , , and for descriptions 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. In a log that can include secrets from multiple connections, this field can be used to identify a connection.

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 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. Therefore, a record of the TLS handshake might be needed to use the logged secrets.

Secret Labels for TLS 1.3 An implementation of TLS 1.3 produces a number of values as part of the key schedule (see ). If ECH was successfully negotiated for a given connection, these labels MUST be followed by the value of the Random field from the Inner ClientHello. Otherwise, the Random field from the Outer ClientHello MUST be used. 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_SECRET:

This secret is used for early exporters. Like 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.

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 ().

These labels all appear in uppercase in the key log, but they correspond to lowercase labels in the TLS key schedule (), except for the application data secrets as noted. For example, "EXPORTER_SECRET" in the log file corresponds to the secret named exporter_secret. Note that the order that labels appear here corresponds to the order in which they are presented in , but there is no guarantee that implementations will log secrets strictly in this order.

Secret Labels for TLS 1.2 Implementations of TLS 1.2 (and also earlier versions) use the label "CLIENT_RANDOM" to identify the "master" secret for the connection.

Secret Labels for ECH With ECH , additional secrets are derived during the handshake to encrypt the Inner ClientHello message using Hybrid Public Key Encryption (HPKE) . A client can log the ECH labels described below if it offered ECH, regardless of server acceptance. The server can log the labels only if it successfully decrypted the ECH offered by the client, though it could choose to do so only when it accepts ECH. These labels MUST always use the Random from the Outer ClientHello.

ECH_SECRET:

This label corresponds to the Key Encapsulation Mechanism (KEM) shared secret used by HPKE (shared_secret in the algorithms in ). The length of the secret is defined by the KEM negotiated for use with ECH.

ECH_CONFIG:

The ECHConfig used to construct the ECH extension. The value is logged in hexadecimal representation.

Security Considerations Access to the content of a file in SSLKEYLOGFILE format allows an attacker to break the confidentiality and integrity 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. Therefore, ensuring adequate access control on these files 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 (). 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., ), authentication (e.g., ), 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 enables consumption by other programs. In both cases, applications might require special authorization or might rely on system-level access control to limit access to these capabilities. Forward secrecy guarantees provided in TLS 1.3 (see Section and Appendix of ) and some modes of TLS 1.2 (such as those in Sections and of ) 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 provide. 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. Access to the ECH_SECRET record in SSLKEYLOGFILE allows the attacker to decrypt the ECH extension and thereby reveal the content of the Inner ClientHello message, including the payload of the Server Name Indication (SNI) extension. Access to the HPKE-established shared secret used in ECH introduces a potential attack surface against the HPKE library since access to this keying material is normally not available otherwise.

IANA Considerations This document registers a media type () and creates a registry for labels ().

SSLKEYLOGFILE Media Type The "application/sslkeylogfile" media type can be used to describe content in the SSLKEYLOGFILE format. IANA has added the following information to the "Media Types" registry at :

Type name:

application

Subtype name:

sslkeylogfile

Required parameters:

N/A

Optional parameters:

N/A

Encoding considerations:

UTF-8 without BOM, or ASCII only

Security considerations:

See .

Interoperability considerations:

Line endings might differ from platform convention.

Published specification:

RFC 9850

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:

TLS WG (tls@ietf.org)

Intended usage:

COMMON

Restrictions on usage:

N/A

Author:

IETF TLS Working Group

Change controller:

IETF

TLS SSLKEYLOGFILE Labels Registry IANA has created a new registry named "TLS SSLKEYLOGFILE Labels" within the existing "Transport Layer Security (TLS) Parameters" registry group. This new registry reserves labels used for SSLKEYLOGFILE entries. The initial contents of this registry are as follows:

Value	Description	Reference
CLIENT_RANDOM	Master secret in TLS 1.2 and earlier	RFC 9850
CLIENT_EARLY_TRAFFIC_SECRET	Secret for client early data records	RFC 9850
EARLY_EXPORTER_SECRET	Early exporter secret	RFC 9850
CLIENT_HANDSHAKE_TRAFFIC_SECRET	Secret protecting client handshake	RFC 9850
SERVER_HANDSHAKE_TRAFFIC_SECRET	Secret protecting server handshake	RFC 9850
CLIENT_TRAFFIC_SECRET_0	Secret protecting client records post handshake	RFC 9850
SERVER_TRAFFIC_SECRET_0	Secret protecting server records post handshake	RFC 9850
EXPORTER_SECRET	Exporter secret after handshake	RFC 9850
ECH_SECRET	HPKE KEM shared secret used in the ECH	RFC 9850
ECH_CONFIG	ECHConfig used for construction of the ECH	RFC 9850

New assignments in the "TLS SSLKEYLOGFILE Labels" registry will be administered by IANA through the Specification Required procedure . The role of designated experts for TLS registries is described in . Designated experts for this registry are advised to ensure that the specification is publicly available. In the Reference column, it is sufficient to cite an Internet-Draft (that is posted but not published as an RFC) or a document from another standards body, an industry consortium, or any other organization. Designated experts may provide more in-depth reviews, but their approval should not be taken as an endorsement of the SSLKEYLOGFILE label.

References Normative References Independent Independent Fastly Cryptography Consulting LLC Cloudflare Informative References Akamai Meta Cloudflare

Example The following is a sample of a file in SSLKEYLOGFILE format, including secrets from two TLS 1.3 connections. The examples below use line wrapping per . Note that secrets from the two connections might be interleaved as shown here, because secrets could be logged as they are generated. The following shows a log entry for a TLS 1.2 connection. The following shows a log entry for a TLS 1.3 connection that successfully negotiated ECH.

Acknowledgments The SSLKEYLOGFILE format originated in the Network Security Services (NSS) project, but it has evolved over time as TLS has changed. Many people contributed to this evolution. The authors are only documenting the format as it is used while extending it to cover ECH.