Our code signing happens in discrete tasks, for both performance reasons and to limit which machines have access to the signing servers and keys.

In general, the binary-to-be-signed is generated in one task, and the request to sign it is in a second task. We verify the request via the chain of trust, sign the binary, then upload the signed binary or original binary + detached signature as artifacts.

How the Task Works

Scriptworker verifies the task definition and the upstream tasks until it determines the graph comes from a trusted tree; this is chain of trust verification. Part of this verification is downloading and verifying the shas of the upstreamArtifacts in the task payload.

An example signing task payload:

  "payload": {
    "upstreamArtifacts": [{
      "paths": ["public/build/target.dmg"],
      "formats": ["macapp"],
      "taskId": "abcde",
      "taskType": "build"
    }, {
      "paths": ["public/build/target.tar.gz"],
      "formats": ["gpg"],
      "taskId": "12345",
      "taskType": "build"

In the above example, scriptworker would download the target.dmg from task abcde and target.tar.gz from task 12345 and verify their shas and task definitions via chain of trust verification. Then it will launch signingscript, which requests a signing token from the signing server pool.

Signingscript determines it wants to sign target.dmg with the macapp format, and target.tar.gz with the gpg format. Each of the signing formats has their own behavior. After performing any format-specific checks or optimizations, it calls signtool to submit the file to the signing servers and poll them for signed output. Once it downloads all of the signed output files, it exits and scriptworker uploads the signed binaries.

We can specify multiple paths from a single task for a given set of formats, and multiple formats for a given set of paths.

Signing kinds

We currently have 12 different signing kinds. These fall into several categories:

Build internal signing: Certain package types require the internals to be signed. For certain package types, e.g. exe or dmg, we extract the internal binaries (e.g. xul.dll) and sign them. This is true for certain zipfiles, exes, and dmgs; we need to sign the internals before we [re]create the package. For linux tarballs, we don’t need special packaging, so we can sign everything in this task. These kinds include build-signing, nightly-l10n-signing, release-eme-free-repack-signing, and release-partner-repack-signing.

Build repackage signing: Once we take the signed internals and package them (known as a repackage), certain formats require a signed external package. If we have created an update MAR file from the signed internals, the MAR file will also need to be signed. These kinds include repackage-signing, release-eme-free-repack-repackage-signing, and release-partner-repack-repackage-signing.

release-source-signing and partials-signing sign the release source tarball and partial update MARs.

We generate signed checksums at the top of the releases directories, like in 60.0. To generate these, we have the checksums signing kinds, including release-generate-checksums-signing, checksums-signing, and release-source-checksums-signing

Signing formats

The known signingscript formats are listed in the fourth column of the signing password files.

The formats are specified in the upstreamArtifacts list-of-dicts. The task must have a superset of scopes to match. For example, a Firefox signing task with an upstreamArtifacts that lists both gpg and macapp formats must have both project:releng:signing:format:gpg and project:releng:signing:format:macapp in its scopes.

gpg signing results in a detached .asc signature file. Because of its nature, we gpg-sign at the end if given multiple formats for a given set of files.

jar signing is Android apk signing. After signing, we zipalign the apk. This includes the focus-jar format, which is just a way to specify a different set of keys for the Focus app.

macapp signing accepts either a dmg or tar.gz; it converts dmg files to tar.gz before submitting to the signing server. The signed binary is a tar.gz.

signcode signing takes individual binaries or a zipfile. We sign the individual file or internals of the zipfile, skipping any already-signed files and a select few blocklisted files (using the `_should_sign_windows`_ function). It returns a signed individual binary or zipfile with signed internals, depending on the input. This format includes signcode, osslsigncode, sha2signcode, and sha2signcodestub.

mar signing signs our update files (Mozilla ARchive). mar_sha384 is the same, but with a different hashing algorithm.

widevine and widevine_blessed are also video-related; see the widevine site. We sign specific files inside the package and rebuild the precomplete file that we use for updates.

Cert levels

Cert levels are how we separate signing privileges. We have the following levels:

dep is short for depend, which is a term from the Netscape days. (This refers to builds that don’t clobber, so they keep their dependency object files cached from the previous build.) These certs and keys are designed to be used for Try or on-push builds that we don’t intend to ship. Many of these are self-signed and not of high security value; they’re intended for testing purposes.

nightly refers to the Nightly product and channel. We use these keys for signing and shipping nightly builds, as well as Devedition on the beta channel. Because these are shipping keys, they are restricted; only a subset of branches can request the use of these keys.

release refers to our releases, off the beta, release, or esr channels. These are the most restricted keys.

We request a certain cert level via scopes: project:releng:signing:cert:dep-signing, project:releng:signing:cert:nightly-signing, or project:releng:signing:cert:release-signing. Each signing task is required to have exactly one of those scopes, and only nightly- and release-enabled branches are able to use the latter two scopes. If a task is scheduled with one of those restricted scopes on a non-allowlisted branch, Chain of Trust verification will raise an exception.

Signing scriptworker workerTypes

The depsigning pool handles all of the dep signing. These are heavily in use on try, mozilla-inbound, and autoland, but also other branches. These verify the chain of trust artifact but not its signature, and they don’t have a gpg key to sign their own chain of trust artifact. This is by design; the chain of trust should and will break if a production scriptworker is downstream from a depsigning worker.

The signing-linux-v1 pool is the production signing pool; it handles the nightly- and release- signing requests. As such, it verifies the upstream chain of trust and all signatures, and signs its chain of trust artifact.

The signing-linux-dev pool is intended for signingscript and scriptworker development use. Because it isn’t used on any Firefox-developer-facing branch, Mozilla Releng is able to make breaking changes on this pool without affecting any other team.