Spack Package Signing
The goal of package signing in Spack is to provide data integrity assurances around official packages produced by the automated Spack CI pipelines. These assurances directly address the security of Spack’s software supply chain by explaining why a security-conscious user can be reasonably justified in the belief that packages installed via Spack have an uninterrupted auditable trail back to change management decisions judged to be appropriate by the Spack maintainers. This is achieved through cryptographic signing of packages built by Spack CI pipelines based on code that has been transparently reviewed and approved on GitHub. This document describes the signing process for interested users.
Risks, Impact and Threat Model
This document addresses the approach taken to safeguard Spack’s reputation with regard to the integrity of the package data produced by Spack’s CI pipelines. It does not address issues of data confidentiality (Spack is intended to be largely open source) or availability (efforts are described elsewhere). With that said the main reputational risk can be broadly categorized as a loss of faith in the data integrity due to a breach of the private key used to sign packages. Remediation of a private key breach would require republishing the public key with a revocation certificate, generating a new signing key, an assessment and potential rebuild/resigning of all packages since the key was breached, and finally direct intervention by every spack user to update their copy of Spack’s public keys used for local verification.
The primary threat model used in mitigating the risks of these stated impacts is one of individual error not malicious intent or insider threat. The primary objective is to avoid the above impacts by making a private key breach nearly impossible due to oversight or configuration error. Obvious and straightforward measures are taken to mitigate issues of malicious interference in data integrity and insider threats but these attack vectors are not systematically addressed. It should be hard to exfiltrate the private key intentionally, and almost impossible to leak the key by accident.
Pipeline Overview
Spack pipelines build software through progressive stages where packages in later stages nominally depend on packages built in earlier stages. For both technical and design reasons these dependencies are not implemented through the default GitLab artifacts mechanism; instead built packages are uploaded to AWS S3 mirrors (buckets) where they are retrieved by subsequent stages in the pipeline. Two broad categories of pipelines exist: Pull Request (PR) pipelines and Develop/Release pipelines.
PR pipelines are launched in response to pull requests made by trusted and untrusted users. Packages built on these pipelines upload code to quarantined AWS S3 locations which cache the built packages for the purposes of review and iteration on the changes proposed in the pull request. Packages built on PR pipelines can come from untrusted users so signing of these pipelines is not implemented. Jobs in these pipelines are executed via normal GitLab runners both within the AWS GitLab infrastructure and at affiliated institutions.
Develop and Release pipelines sign the packages they produce and carry strong integrity assurances that trace back to auditable change management decisions. These pipelines only run after members from a trusted group of reviewers verify that the proposed changes in a pull request are appropriate. Once the PR is merged, or a release is cut, a pipeline is run on protected GitLab runners which provide access to the required signing keys within the job. Intermediary keys are used to sign packages in each stage of the pipeline as they are built and a final job officially signs each package external to any specific packages’ build environment. An intermediate key exists in the AWS infrastructure and for each affiliated instritution that maintains protected runners. The runners that execute these pipelines exclusively accept jobs from protected branches meaning the intermediate keys are never exposed to unreviewed code and the official keys are never exposed to any specific build environment.
Key Architecture
Spack’s CI process uses public-key infrastructure (PKI) based on GNU Privacy Guard (gpg) keypairs to sign public releases of spack package metadata, also called specs. Two classes of GPG keys are involved in the process to reduce the impact of an individual private key compromise, these key classes are the Intermediate CI Key and Reputational Key. Each of these keys has signing sub-keys that are used exclusively for signing packages. This can be confusing so for the purpose of this explanation we’ll refer to Root and Signing keys. Each key has a private and a public component as well as one or more identities and zero or more signatures.
Intermediate CI Key
The Intermediate key class is used to sign and verify packages between stages within a develop or release pipeline. An intermediate key exists for the AWS infrastructure as well as each affiliated institution that maintains protected runners. These intermediate keys are made available to the GitLab execution environment building the package so that the package’s dependencies may be verified by the Signing Intermediate CI Public Key and the final package may be signed by the Signing Intermediate CI Private Key.
Intermediate CI Key (GPG) |
|
---|---|
Root Intermediate CI Private Key (RSA 4096)# |
Root Intermediate CI Public Key (RSA 4096) |
Signing Intermediate CI Private Key (RSA 4096) |
Signing Intermediate CI Public Key (RSA 4096) |
Identity: “Intermediate CI Key <maintainers@spack.io>” |
|
Signatures: None |
The Root intermediate CI Private KeyIs stripped out of the GPG key and stored offline completely separate from Spack’s infrastructure. This allows the core development team to append revocation certificates to the GPG key and issue new sub-keys for use in the pipeline. It is our expectation that this will happen on a semi regular basis. A corollary of this is that this key should not be used to verify package integrity outside the internal CI process.
Reputational Key
The Reputational Key is the public facing key used to sign complete groups of development and release packages. Only one key pair exists in this class of keys. In contrast to the Intermediate CI Key the Reputational Key should be used to verify package integrity. At the end of develop and release pipeline a final pipeline job pulls down all signed package metadata built by the pipeline, verifies they were signed with an Intermediate CI Key, then strips the Intermediate CI Key signature from the package and re-signs them with the Signing Reputational Private Key. The officially signed packages are then uploaded back to the AWS S3 mirror. Please note that separating use of the reputational key into this final job is done to prevent leakage of the key in a spack package. Because the Signing Reputational Private Key is never exposed to a build job it cannot accidentally end up in any built package.
Reputational Key (GPG) |
|
---|---|
Root Reputational Private Key (RSA 4096)# |
Root Reputational Public Key (RSA 4096) |
Signing Reputational Private Key (RSA 4096) |
Signing Reputational Public Key (RSA 4096) |
Identity: “Spack Project <maintainers@spack.io>” |
|
Signatures: Signed by core development team [1] |
The Root Reputational Private Key is stripped out of the GPG key and stored offline completely separate from Spack’s infrastructure. This allows the core development team to append revocation certificates to the GPG key in the unlikely event that the Signing Reputation Private Key is compromised. In general it is the expectation that rotating this key will happen infrequently if at all. This should allow relatively transparent verification for the end-user community without needing deep familiarity with GnuPG or Public Key Infrastructure.
Build Cache Signing
For an in-depth description of the layout of a binary mirror, see the documentation covering binary caches. The key takeaway from that discussion that applies here is that the entry point to a binary package is it’s manifest. The manifest refers unambiguously to the spec metadata and compressed archive, which are stored as content-addressed blobs.
The manifest files can either be signed or unsigned, but are always given
a name ending with .spec.manifest.json
regardless. The difference between
signed and unsigned manifests is simply that the signed version is wrapped in
a gpg cleartext signature, as illustrated below:
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512
{
"version": 3,
"data": [
{
"contentLength": 10731083,
"mediaType": "application/vnd.spack.install.v2.tar+gzip",
"compression": "gzip",
"checksumAlgorithm": "sha256",
"checksum": "0f24aa6b5dd7150067349865217acd3f6a383083f9eca111d2d2fed726c88210"
},
{
"contentLength": 1000,
"mediaType": "application/vnd.spack.spec.v5+json",
"compression": "gzip",
"checksumAlgorithm": "sha256",
"checksum": "fba751c4796536737c9acbb718dad7429be1fa485f5585d450ab8b25d12ae041"
}
]
}
-----BEGIN PGP SIGNATURE-----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=RrFX
-----END PGP SIGNATURE-----
If a user has trusted the public key associated with the private key used to sign the above manifest file, the signature can be verified with gpg, as follows:
$ gpg --verify gcc-runtime-12.3.0-s2nqujezsce4x6uhtvxscu7jhewqzztx.spec.manifest.json
When attempting to install a binary package that has been signed, spack will
attempt to verify the signature with one of the trusted keys in its keyring,
and will fail if unable to do so. While not recommended, it is possible to
force installation of a signed package without verification by providing the
--no-check-signature
argument to spack install ...
.
Internal Implementation
The technical implementation of the pipeline signing process includes components defined in Amazon Web Services, the Kubernetes cluster, at affilicated institutions, and the GitLab/GitLab Runner deployment. We present the technical implementation in two interdependent sections. The first addresses how secrets are managed through the lifecycle of a develop or release pipeline. The second section describes how Gitlab Runner and pipelines are configured and managed to support secure automated signing.
Secrets Management
As stated above the Root Private Keys (intermediate and reputational) are stripped from the GPG keys and stored outside Spack’s infrastructure.
Warning
- TODO
Explanation here about where and how access is handled for these keys.
Both Root private keys are protected with strong passwords
Who has access to these and how?
Intermediate CI Key
Multiple intermediate CI signing keys exist, one Intermediate CI Key for jobs run in AWS, and one key for each affiliated institution (e.g. University of Oregon). Here we describe how the Intermediate CI Key is managed in AWS:
The Intermediate CI Key (including the Signing Intermediate CI Private Key is
exported as an ASCII armored file and stored in a Kubernetes secret called
spack-intermediate-ci-signing-key
. For convenience sake, this same secret
contains an ASCII-armored export of just the public components of the
Reputational Key. This secret also contains the public components of each of
the affiliated institutions’ Intermediate CI Key. These are potentially needed
to verify dependent packages which may have been found in the public mirror or
built by a protected job running on an affiliated institution’s infrastructure
in an earlier stage of the pipeline.
Procedurally the spack-intermediate-ci-signing-key
secret is used in
the following way:
A
large-arm-prot
orlarge-x86-prot
protected runner picks up a job taggedprotected
from a protected GitLab branch. (See Protected Runners and Reserved Tags).Based on its configuration, the runner creates a job Pod in the pipeline namespace and mounts the spack-intermediate-ci-signing-key Kubernetes secret into the build container
The Intermediate CI Key, affiliated institutions’ public key and the Reputational Public Key are imported into a keyring by the
spack gpg …
sub-command. This is initiated by the job’s build script which is created by the generate job at the beginning of the pipeline.Assuming the package has dependencies those spec manifests are verified using the keyring.
The package is built and the spec manifest is generated
The spec manifest is signed by the keyring and uploaded to the mirror’s build cache.
Reputational Key
Because of the increased impact to end users in the case of a private
key breach, the Reputational Key is managed separately from the
Intermediate CI Keys and has additional controls. First, the Reputational
Key was generated outside of Spack’s infrastructure and has been signed
by the core development team. The Reputational Key (along with the
Signing Reputational Private Key) was then ASCII armor exported to a
file. Unlike the Intermediate CI Key this exported file is not stored as
a base64 encoded secret in Kubernetes. Insteadthe key file
itselfis encrypted and stored in Kubernetes as the
spack-signing-key-encrypted
secret in the pipeline namespace.
The encryption of the exported Reputational Key (including the Signing
Reputational Private Key) is handled by AWS Key Management Store (KMS) data
keys.
The private key material is decrypted and imported at the time of signing into a
memory mounted temporary directory holding the keychain. The signing job uses
the AWS Encryption SDK
(i.e. aws-encryption-cli
) to decrypt the Reputational Key. Permission to
decrypt the key is granted to the job Pod through a Kubernetes service account
specifically used for this, and only this, function. Finally, for convenience
sake, this same secret contains an ASCII-armored export of the public
components of the Intermediate CI Keys and the Reputational Key. This allows the
signing script to verify that packages were built by the pipeline (both on AWS
or at affiliated institutions), or signed previously as a part of a different
pipeline. This is is done before importing decrypting and importing the
Signing Reputational Private Key material and officially signing the packages.
Procedurally the spack-singing-key-encrypted
secret is used in the
following way:
The
spack-package-signing-gitlab-runner
protected runner picks up a job taggednotary
from a protected GitLab branch (See Protected Runners and Reserved Tags).Based on its configuration, the runner creates a job pod in the pipeline namespace. The job is run in a stripped down purpose-built image
ghcr.io/spack/notary:latest
Docker image. The runner is configured to only allow running jobs with this image.The runner also mounts the
spack-signing-key-encrypted
secret to a path on disk. Note that this becomes several files on disk, the public components of the Intermediate CI Keys, the public components of the Reputational CI, and an AWS KMS encrypted file containing the Singing Reputational Private Key.In addition to the secret, the runner creates a tmpfs memory mounted directory where the GnuPG keyring will be created to verify, and then resign the package specs.
The job script syncs all spec manifest files from the build cache to a working directory in the job’s execution environment.
The job script then runs the
sign.sh
script built into the notary Docker image.The
sign.sh
script imports the public components of the Reputational and Intermediate CI Keys and uses them to verify good signatures on the spec.manifest.json files. If any signed manifest does not verify, the job immediately fails.Assuming all manifests are verified, the
sign.sh
script then unpacks the manifest json data from the signed file in preparation for being re-signed with the Reputational Key.The private components of the Reputational Key are decrypted to standard out using
aws-encryption-cli
directly into agpg –import …
statement which imports the key into the keyring mounted in-memory.The private key is then used to sign each of the manifests and the keyring is removed from disk.
The re-signed manifests are resynced to the AWS S3 Mirror and the public signing of the packages for the develop or release pipeline that created them is complete.
Non service-account access to the private components of the Reputational Key that are managed through access to the symmetric secret in KMS used to encrypt the data key (which in turn is used to encrypt the GnuPG key - See:Encryption SDK Documentation). A small trusted subset of the core development team are the only individuals with access to this symmetric key.