Pacman/Package signing

Related articles

• GnuPG
• DeveloperWiki:Package signing

To determine if packages are authentic, pacman uses GnuPG keys in a web of trust model. The current Master Signing Keys are found here. At least three of these Master Signing Keys are used to sign each of the Developer's and Trusted User's own keys which then in turn are used to sign their packages. The user also has a unique PGP key which is generated when you set up pacman-key. So the web of trust links the user's key to the Master Keys.

Examples of webs of trust:

• Custom packages: You made the package yourself and signed it with your own key.
• Unofficial packages: A developer made the package and signed it. You used your key to sign that developer's key.
• Official packages: A developer made the package and signed it. The developer's key was signed by the Arch Linux master keys. You used your key to sign the master keys, and you trust them to vouch for developers.

Note: The HKP protocol uses 11371/tcp for communication. In order to get the signed keys from the servers (using pacman-key), this port is required for communication.

Setup

Configuring pacman

The SigLevel option in /etc/pacman.conf determines how much trust is required to install a package. For a detailed explanation of SigLevel see the pacman.conf man page and the comments in the file itself. Signature checking may be set globally or per repository. If SigLevel is set globally in the [options] section to require all packages to be signed, then packages you build will also need to be signed using makepkg.

Note: Although all official packages are now signed, as of June 2012 signing of the databases is a work in progress. If Required is set then DatabaseOptional should also be set.

A default configuration can be used to only install packages that are signed by trusted keys:

/etc/pacman.conf

SigLevel = Required DatabaseOptional

This is because TrustedOnly is a default compiled-in pacman parameter. So above leads to the same result as a global option of:

SigLevel = Required DatabaseOptional TrustedOnly

The above can be achieved too on a repository level further below in the configuration, e.g.:

[core]
SigLevel = PackageRequired
Include = /etc/pacman.d/mirrorlist

explicitly adds signature checking for the packages of the repository, but does not require the database to be signed. Optional here would turn off a global Required for this repository

Warning: The SigLevel TrustAll option exists for debugging purposes and makes it very easy to trust keys that have not been verified. You should use TrustedOnly for all official repositories.

Initializing the keyring

To set up the pacman keyring use:

# pacman-key --init

For this initialization, entropy is required. Moving your mouse around, pressing random characters at the keyboard or running some disk-based activity (for example in another console running ls -R / or find / -name foo or dd if=/dev/sda8 of=/dev/tty7) should generate entropy. If your system does not already have sufficient entropy, this step may take hours; if you actively generate entropy, it will complete much more quickly.

The randomness created is used to set up a keyring (/etc/pacman.d/gnupg) and the GPG signing key of your system.

Note: If you need to run pacman-key --init on computer that does not generate much entropy (e.g. a headless server), key generation may take a very long time. To generate pseudo-entropy, install either haveged or rng-tools on the target machine and start the corresponding service before running pacman-key --init.

Managing the keyring

Verifying the master keys

The initial setup of keys is achieved using:

# pacman-key --populate archlinux

Take time to verify the Master Signing Keys when prompted as these are used to co-sign (and therefore trust) all other packager's keys.

PGP keys are too large (2048 bits or more) for humans to work with, so they are usually hashed to create a 40-hex-digit fingerprint which can be used to check by hand that two keys are the same. The last eight digits of the fingerprint serve as a name for the key known as the '(short) key ID' (the last sixteen digits of the fingerprint would be the 'long key ID').

Adding developer keys

The official developer and Trusted Users (TU) keys are signed by the master keys, so you do not need to use pacman-key to sign them yourself. Whenever pacman encounters a key it does not recognize, it will prompt to download it from a keyserver configured in /etc/pacman.d/gnupg/gpg.conf (or by using the --keyserver option on the command line). Wikipedia maintains a list of keyservers.

Once you have downloaded a developer key, you will not have to download it again, and it can be used to verify any other packages signed by that developer.

Note: The archlinux-keyring package, which is a dependency of pacman, contains the latest keys. However keys can also be updated manually using pacman-key --refresh-keys (as root). While doing --refresh-keys, your local key will also be looked up on the remote keyserver, and you will receive a message about it being not found. This is nothing to be concerned about.

Adding unofficial keys

This method can be used, for example, to add your own key to the pacman keyring, or to enable signed unofficial user repositories.

First, get the key ID (keyid) from its owner. Then add it to the keyring using one of the two methods:

1. If the key is found on a keyserver, import it with:

   # pacman-key --recv-keys keyid

2. If otherwise a link to a keyfile is provided, download it and then run:

   # pacman-key --add /path/to/downloaded/keyfile

It is recommended to verify the fingerprint, as with any master key or any other key you are going to sign:

$ pacman-key --finger keyid

Finally, you must locally sign the imported key:

# pacman-key --lsign-key keyid

You now trust this key to sign packages.

Debugging with gpg

For debugging purposes, you can access pacman's keyring directly with gpg, e.g.:

# gpg --homedir /etc/pacman.d/gnupg --list-keys

Troubleshooting

Warning: pacman-key depends on time. If your system clock is wrong, you'll get:

error: PackageName: signature from "User <email@archlinux.org>" is invalid
error: failed to commit transaction (invalid or corrupted package (PGP signature))
Errors occured, no packages were upgraded.

Cannot import keys

There are multiple possible sources of this problem:

• An outdated archlinux-keyring package.
• Incorrect date.
• Your ISP blocked the port used to import PGP keys.
• Your pacman cache contains copy of unsigned packages from previous attempts.
• dirmngr is not correctly configured

You might be stuck because of outdated archlinux-keyring package when doing an upgrade synchronization. Try if upgrading the system can fix it first.

If you are still having issues, make sure the following file exists /root/.gnupg/dirmngr_ldapservers.conf and that you can successfully run # dirmngr. Create an empty file if it doesn't and run # dirmngr again.

If it does not help and your date is correct, you could try to switch to the MIT keyserver, which provides an alternate port. To do this, edit /etc/pacman.d/gnupg/gpg.conf and change the keyserver line to:

keyserver hkp://pgp.mit.edu:11371

If even port 80 doesn't work (for example when company uses some kind of http-only "transparent" proxy instead of routing, following could work:

keyserver hkps://hkps.pool.sks-keyservers.net:443

If you have IPv6 disabled, gpg will fail when it found some IPv6 address. In this case try with an IPv4-only keyserver like:

keyserver hkp://ipv4.pool.sks-keyservers.net:11371

If you happen to forget to run pacman-key --populate archlinux you might get some errors while importing keys.

If none of this helps, your pacman cache, located at /var/cache/pacman/pkg/ might contain unsigned packages from previous attempts. Try cleaning the cache manually or run:

# pacman -Sc

which removes all cached packages that have not been installed.

Disabling signature checking

Warning: Use with caution. Disabling package signing will allow pacman to install untrusted packages automatically.

If you are not concerned about package signing, you can disable PGP signature checking completely. Edit /etc/pacman.conf and uncomment the following line under [options]:

SigLevel = Never

You need to comment out any repository-specific SigLevel settings too because they override the global settings. This will result in no signature checking, which was the behavior before pacman 4. If you decide to do this, you do not need to set up a keyring with pacman-key. You can change this option later if you decide to enable package verification.

Resetting all the keys

If you want to remove or reset all the keys installed in your system, you can remove /etc/pacman.d/gnupg folder as root and rerun pacman-key --init and following that add the keys as preferred.

Removing stale packages

If the same packages keep failing and you are sure you did all the pacman-key stuff right, try removing them like so rm /var/cache/pacman/pkg/badpackage* so that they are freshly downloaded.

This might actually be the solution if you get a message like error: linux: signature from "Some Person <Some.Person@example.com>" is invalid or similar when upgrading (i.e. you might not be the victim of a MITM attack after all, your downloaded file was simply corrupt).

Updating keys via proxy

In order to use a proxy when updating keys the honor-http-proxy option must be set in both /etc/gnupg/dirmngr.conf and /etc/pacman.d/gnupg/dirmngr.conf. See GnuPG#Use a keyserver for more information.

Note: If pacman-key is used without the honor-http-proxy option and fails, a reboot may solve the issue.

See also

• DeveloperWiki:Package Signing Proposal for Pacman
• Pacman Package Signing – 1: Makepkg and Repo-add
• Pacman Package Signing – 2: Pacman-key
• Pacman Package Signing – 3: Pacman
• Pacman Package Signing – 4: Arch Linux