RPM Signing with Koji

What is a GPG keypair?

A GPG keypair has a public key that you can share with the world and a private key that you keep secret.

Here are some example commands for working with RPM and GPG.

Example of generating a GPG keypair for testing:

gpg --quick-generate-key security@example.com
# For testing, simply press "Enter" when prompted for a password.

Exporting your public key:

gpg --armor --export --output /path/to/my-signing-key.asc

Signing all RPMs in the current directory with this key:

rpmsign --define "_gpg_name security@example.com" --addsign *.rpm

Inspecting an RPM signature

In order to install a signed RPM on clients, each client must trust (import) the public GPG key into their RPMDB:

rpm --import /path/to/my-signing-key.asc

Example: No GPG signature at all (an unsigned RPM):

rpm -Kv python3-cherrypy-18.6.0-1.fc33.noarch.rpm
python3-cherrypy-18.6.0-1.fc33.noarch.rpm:
  Header SHA256 digest: OK
  Header SHA1 digest: OK
  Payload SHA256 digest: OK
  MD5 digest: OK

Note there is no “RSA/SHA256 Signature” header field on the RPM here.

Example: A GPG signature that rpmdb DOES trust:

rpm -Kv python3-cherrypy-18.4.0-4.fc32.noarch.rpm
python3-cherrypy-18.4.0-4.fc32.noarch.rpm:
  Header V3 RSA/SHA256 Signature, key ID 12c944d0: OK
  Header SHA256 digest: OK
  Header SHA1 digest: OK
  Payload SHA256 digest: OK
  V3 RSA/SHA256 Signature, key ID 12c944d0: OK
  MD5 digest: OK

Example: A GPG signature that rpmdb does NOT trust:

rpm -Kv python-cherrypy-18.6.0-1.el8.src.rpm
python-cherrypy-18.6.0-1.el8.src.rpm:
  Header V4 RSA/SHA256 Signature, key ID 782096ac: NOKEY
  Header SHA256 digest: OK
  Header SHA1 digest: OK
  Payload SHA256 digest: OK
  V4 RSA/SHA256 Signature, key ID 782096ac: NOKEY
  MD5 digest: OK

Note the signature is syntatically valid here, but “NOKEY” here means RPMDB does not trust the GPG key that signed this RPM.

A lower-level command that shows the signature on an RPM file (the RSAHEADER field piped through RPM’s pgpsig formatter):

rpm -q --qf '%{NAME} %{RSAHEADER:pgpsig}\n' -p python-routes-2.5.1-1.el8.src.rpm

Uploding signed RPMs to Koji

Koji does not sign RPMs. Instead, Koji imports RPMs that are signed with a separate key.

To sign an RPM from Koji, you should make a copy of the file, sign it with the appropriate rpm command, and import the signature. Note that you should not simply sign the file directly under /mnt/koji, as this causes an inconsistency between the filesystem and the database (hence the copy step).

In this example, we download an unsigned build from Koji, then sign it, and then upload the signed copy with kojidev import-sig:

koji download-build --debuginfo bash-5.0.17-2.fc32
rpmsign --define "_gpg_name security@example.com" --addsign *.rpm
koji import-sig *.rpm

The koji import-sig command uploads the signed RPM headers to the Koji Hub, which stores the headers on disk alongside the main unsigned RPM. It also writes out a full signed RPM.

Downloading a signed RPM from Koji

Specify the --key option to koji download-build:

koji download-build --key=3AF362BAB bash-5.0.17-2.fc32

Signing a build with multiple keys

Currently RPM’s file format only allows one single GPG signature per file.

Koji allows users to upload multiple GPG signatures for a single RPM. it stores each signature alongside the RPM build and splices the signature headers in to generate full signed RPMs. Here are some use-cases of this feature:

Creating repos of signed RPMs

You can put signed RPMs into Yum repos three different ways.

  1. Create dist-repos manually with the koji dist-repo command, that takes a GPG key argument.

  2. Install and configure the tag2distrepo hub plugin to automatically export dist-repos for certain tags.

  3. Pungi can create signed repos (“composes”).

See Exporting repositories for more information.

How to automate signing?

For a small testing environment, you can simply sign RPMs with a GPG key on a workstation and run koji import-sig. This is not secure and it does not scale.

See the Sigil and Robosignatory projects for more advanced workflows.

Koji cryptography best-practices

  • Use HTTPS everywhere (kojihub + kojiweb)

  • Understand checksums (md5)

  • Understand signatures (GPG)

How do RPM signatures relate to HTTPS?

HTTPS is transport-layer security. When you install a package over HTTPS you verify that:

  • The web server is who they say they are

  • The information the web server sends is private

As soon as you download that build or copy it to another location, those security guarantees are lost.

In a release pipeline, you end up copying builds to many locations, and while it’s important to use HTTPS for copying, it’s even more important to have a strong cryptographic signature follow each build.

This means that even if someone or some thing mirrors your build elsewhere, that signature will go along with the build. In the case of RPMs, the GPG signatures are actually embedded in the RPMs themselves that we deliver to users.

Another reason this is important is for image-based artifacts that might use many RPMs. If you think of cloud images or container images where you’re delivering an image with “preinstalled” RPMs, if you use signed RPMs in the images you distribute, you’re providing an extra layer of security.