All kernel releases are cryptographically signed using OpenPGP-compliant signatures. Everyone is strongly encouraged to verify the integrity of downloaded kernel releases by verifying the corresponding signatures.
Every kernel release comes with a cryptographic signature from the person making the release. This cryptographic signature allows anyone to verify whether the files have been modified or otherwise tampered with after the developer created and signed them. The signing and verification process uses public-key cryptography and it is next to impossible to forge a PGP signature without first gaining access to the developer's private key. If this does happen, the developers will revoke the compromised key and will re-sign all their previously signed releases with the new key.
To learn more about the way PGP works, please consult Wikipedia.
Kernel.org web of trust
PGP keys used by members of kernel.org are cross-signed by other members of the Linux kernel development community (and, frequently, by many other people). If you wanted to verify the validity of any key belonging to a member of kernel.org, you could review the list of signatures on their public key and then make a decision whether you trust that key or not. See the Wikipedia article on the subject of the Web of Trust.
Using the Web Key Directory
If the task of maintaining your own web of trust is too daunting to you, you can opt to shortcut this process by using the "Trust on First Use" (TOFU) approach and rely on the kernel.org Web Key Directory (WKD).
To import keys belonging to many kernel developers, you can use the following command:
$ gpg2 --locate-keys [username]@kernel.org
For example, to import keys belonging to Linus Torvalds and Greg Kroah-Hartman, you would use:
$ gpg2 --locate-keys email@example.com firstname.lastname@example.org
This command will verify the TLS certificate presented by kernel.org before importing these keys into your keyring.
Using GnuPG to verify kernel signatures
All software released via kernel.org has detached PGP signatures you can use to verify the integrity of your downloads.
To illustrate the verification process, let's use Linux 4.6.6 release as a walk-through example. First, use "curl" to download the release and the corresponding signature:
$ curl -OL https://www.kernel.org/pub/linux/kernel/v4.x/linux-4.6.6.tar.xz $ curl -OL https://www.kernel.org/pub/linux/kernel/v4.x/linux-4.6.6.tar.sign
You will notice that the signature is made against the uncompressed version of the archive. This is done so there is only one signature required for .gz and .xz compressed versions of the release. Start by uncompressing the archive, using unxz in our case:
$ unxz linux-4.6.6.tar.xz
Now verify the .tar archive against the signature:
$ gpg2 --verify linux-4.6.6.tar.sign
You can combine these steps into a one-liner:
$ xz -cd linux-4.6.6.tar.xz | gpg2 --verify linux-4.6.6.tar.sign -
It's possible that you get a "No public key error":
gpg: Signature made Wed 10 Aug 2016 06:55:15 AM EDT using RSA key ID 38DBBDC86092693E gpg: Can't check signature: No public key
Please use the "gpg2 --locate-keys" command listed above to download the key for Greg Kroah-Hartman and Linus Torvalds and then try again:
$ gpg2 --locate-keys email@example.com firstname.lastname@example.org $ gpg2 --verify linux-4.6.6.tar.sign gpg: Signature made Wed 10 Aug 2016 06:55:15 AM EDT gpg: using RSA key 38DBBDC86092693E gpg: Good signature from "Greg Kroah-Hartman <email@example.com>" [unknown] gpg: WARNING: This key is not certified with a trusted signature! gpg: There is no indication that the signature belongs to the owner. Primary key fingerprint: 647F 2865 4894 E3BD 4571 99BE 38DB BDC8 6092 693E
To make the "WARNING" message go away you can indicate that you choose to trust that key using TOFU:
$ gpg2 --tofu-policy good 38DBBDC86092693E $ gpg2 --trust-model tofu --verify linux-4.6.6.tar.sign gpg: Signature made Wed 10 Aug 2016 06:55:15 AM EDT gpg: using RSA key 38DBBDC86092693E gpg: Good signature from "Greg Kroah-Hartman <firstname.lastname@example.org>" [full] gpg: email@example.com: Verified 1 signature in the past 53 seconds. Encrypted 0 messages.
Note that you may have to pass "--trust-model tofu" the first time you run the verify command, but it should not be necessary after that.
The scripted version
If you need to perform this task in an automated environment or simply prefer a more convenient tool, you can use the following helper script to properly download and verify Linux kernel tarballs:
Please review the script before adopting it for your needs.
Here are key fingerprints for Linus Torvalds and Greg Kroah-Hartman, who are most likely to be releasing kernels:
|Linus Torvalds||ABAF 11C6 5A29 70B1 30AB E3C4 79BE 3E43 0041 1886|
|Greg Kroah-Hartman||647F 2865 4894 E3BD 4571 99BE 38DB BDC8 6092 693E|
Please verify the TLS certificate for this site in your browser before trusting the above information.
If you get "BAD signature"
If at any time you see "BAD signature" output from "gpg2 --verify", please first check the following first:
- Make sure that you are verifying the signature against the .tar version of the archive, not the compressed (.tar.xz) version.
- Make sure the the downloaded file is correct and not truncated or otherwise corrupted.
If you repeatedly get the same "BAD signature" output, please email firstname.lastname@example.org, so we can investigate the problem.
Kernel.org checksum autosigner and sha256sums.asc
We have a dedicated off-the-network system that connects directly to our central attached storage and calculates checksums for all uploaded software releases. The generated sha256sums.asc file is then signed with a PGP key generated for this purpose and that doesn't exist outside of that system.
These checksums are NOT intended to replace developer signatures. It is merely a way for someone to quickly verify whether contents on one of the many kernel.org mirrors match the contents on the master mirror. While you may use them to quickly verify whether what you have downloaded matches what we have on our central storage system, you should continue to use developer signatures for best assurance.
Kernel releases prior to September, 2011
Prior to September, 2011 all kernel releases were signed automatically by the same PGP key:
pub 1024D/517D0F0E 2000-10-10 [revoked: 2011-12-11] Key fingerprint = C75D C40A 11D7 AF88 9981 ED5B C86B A06A 517D 0F0E uid Linux Kernel Archives Verification Key <email@example.com>
Due to the kernel.org systems compromise, this key has been retired and revoked. It will no longer be used to sign future releases and you should NOT use this key to verify the integrity of any archives. It is almost certain that this key has fallen into malicious hands.
All kernel releases that were previously signed with this key were cross-checked and signed with another key, created specifically for this purpose:
pub 3072R/C4790F9D 2013-08-08 Key fingerprint = BFA7 DD3E 0D42 1C9D B6AB 6527 0D3B 3537 C479 0F9D uid Linux Kernel Archives Verification Key (One-off resigning of old releases) <firstname.lastname@example.org>
The private key used for this purpose has been destroyed and cannot be used to sign any releases produced after 2011.