(mozilla-projects-nss-key-log-format)=

# NSS Key Log Format

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Key logs can be written by NSS so that external programs can decrypt TLS connections. Wireshark
1.6.0 and above can use these log files to decrypt packets. You can tell Wireshark where to find
the key file via *Edit→Preferences→Protocols→TLS→(Pre)-Master-Secret log filename*.

Key logging is enabled by setting the environment variable `SSLKEYLOGFILE` to point to a file.
Note: starting with {ref}`mozilla_projects_nss_nss_3_24_release_notes` (used by Firefox 48 and 49
only), the `SSLKEYLOGFILE` approach is disabled by default for optimized builds using the
Makefile (those using gyp via `build.sh` are *not* affected). Distributors can re-enable it at
compile time though (using the `NSS_ALLOW_SSLKEYLOGFILE=1` make variable) which is done for the
official Firefox binaries. (See [bug
1188657](https://bugzilla.mozilla.org/show_bug.cgi?id=1188657).) Notably, Debian does not have
this option enabled, see [Debian bug
842292](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=842292).

This key log file is a series of lines. Comment lines begin with a sharp character ('#') and are
ignored. Secrets follow the format `<Label> <space> <ClientRandom> <space> <Secret>` where:

- `<Label>` describes the following secret.
- `<ClientRandom>` is 32 bytes Random value from the Client Hello message, encoded as 64
  hexadecimal characters.
- `<Secret>` depends on the Label (see below).

The following labels are defined, followed by a description of the secret:

- `RSA`: 48 bytes for the premaster secret, encoded as 96 hexadecimal characters (removed in
  NSS 3.34)
- `CLIENT_RANDOM`: 48 bytes for the master secret, encoded as 96 hexadecimal characters (for
  SSL 3.0, TLS 1.0, 1.1 and 1.2)
- `CLIENT_EARLY_TRAFFIC_SECRET`: the hex-encoded early traffic secret for the client side (for
  TLS 1.3)
- `CLIENT_HANDSHAKE_TRAFFIC_SECRET`: the hex-encoded handshake traffic secret for the client
  side (for TLS 1.3)
- `SERVER_HANDSHAKE_TRAFFIC_SECRET`: the hex-encoded handshake traffic secret for the server
  side (for TLS 1.3)
- `CLIENT_TRAFFIC_SECRET_0`: the first hex-encoded application traffic secret for the client
  side (for TLS 1.3)
- `SERVER_TRAFFIC_SECRET_0`: the first hex-encoded application traffic secret for the server
  side (for TLS 1.3)
- `EARLY_EXPORTER_SECRET`: the hex-encoded early exporter secret (for TLS 1.3).
- `EXPORTER_SECRET`: the hex-encoded exporter secret (for TLS 1.3)

The `RSA` form allows ciphersuites using RSA key-agreement to be logged and was the first form
supported by Wireshark 1.6.0. It has been superseded by `CLIENT_RANDOM` which also works with
other key-agreement algorithms (such as those based on Diffie-Hellman) and is supported since
Wireshark 1.8.0.

The TLS 1.3 lines are supported since NSS 3.34 ([bug
1287711](https://bugzilla.mozilla.org/show_bug.cgi?id=1287711)) and Wireshark 2.4
(`EARLY_EXPORTER_SECRET` exists since NSS 3.35, [bug
1417331](https://bugzilla.mozilla.org/show_bug.cgi?id=1417331)). The size of the hex-encoded
secret depends on the selected cipher suite. It is 64, 96 or 128 characters for SHA256, SHA384 or
SHA512 respectively.

For Wireshark usage, see [TLS - Wireshark Wiki](https://wiki.wireshark.org/TLS).
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