Verifying GPG signatures for Temurin downloads

July 27, 2022 – posted by   Stewart X Addison

With the latest releases from July 2022 (8u342, 11.0.16, 17.0.4 and 18.0.2) we provide GPG signatures along with our releases which you can use to verify that the downloads you have are genuine and have not been tampered with since we built them. This gives a quick crib sheet of what you need to do to verify the downloads.

What is GPG signing?

In the use case we are talking about here, GPG signing is a cryptographic process whereby a private/public keypair is used to confirm that a file has not been tampered with between when it was published and it being delivered to the end user. The private key is used by Adoptium to produce the signature file which can be downloaded along with the OpenJDK binaries, and the public key can be obtained by you and used to verify the the signature is valid, proving the integrity of the file and also that it was signed by Adoptium and not modified by a third party.

The download site already provides SHA256 checksums. Why would I need this?

While going into the details of public key encryption is beyond the scope of this article, if there was a man in the middle attack that resulted in the downloads you receive being compromised, then the corresponding SHA checksums could also be compromised. GPG signing avoids this issue by using a separately certified signature which you can initially trust and then verify subsequent downloads.

What do I need to do to obtain the signatures?

The link to the signatures is provided in the metadata for our releases. In the following examples I will use JDK17 but you can change the calls appropriately for other versions:

The metadata that contains the URL of the signature file can be obtained as follows:

curl 'https://api.adoptium.net/v3/assets/feature_releases/17/ga' > adopt.json

Once you’ve downloaded that you need to extract the link and signature_link entries using your preferred JSON parsing tool. The information you want is in the binaries[0].package.link and binaries[0].package.signature_link section of the file for the most recent version.

The following example uses the jq command-line JSON parsing tool to pull out the information for Linux/x64 and download the product and the GPG signature using the curl command (change accordingly for other platforms)

jq '.[0].binaries[] | select(.architecture=="x64") | select (.os=="linux") | select (.image_type=="jdk") .package.link' adopt.json | xargs curl -LO
jq '.[0].binaries[] | select(.architecture=="x64") | select (.os=="linux") | select (.image_type=="jdk") .package.signature_link' adopt.json | xargs curl -LO

Note that as mentioned in the previous section, we also provide the sha256sums which can be obtained from the JSON file with

jq '.[0].binaries[] | select(.architecture=="x64") | select (.os=="linux") | select (.image_type=="jdk") .package.checksum' adopt.json

The SHA checksums can be verified against the output from running one of the following commands depending on your operating system:

• Windows: certUtil -hashfile file SHA256
• MacOS: shasum -a 256 file
• UNIX/Linux: sha256sum file

The SHA checksum allows you to verift that the download has occurred without errors, and the GPG checksum additionally verifies the binaries are those released by the Adoptium project.

How do I verify the signatures once I have them?

You will need to have the gpg tool installed in order to verify the signatures. You can then run the following command to check the signature by supplying the signature file and the corresponding file which the signature is for e.g.:

gpg --verify OpenJDK17U-jdk_x64_linux_hotspot_17.0.4_8.tar.gz.sig OpenJDK17U-jdk_x64_linux_hotspot_17.0.4_8.tar.gz

If you do not currently have the Adoptium project’s public signing key you will get a message such as this:

gpg: directory '/home/sxa/.gnupg' created
gpg: keybox '/home/sxa/.gnupg/pubring.kbx' created
gpg: Signature made Mon Jul  4 18:20:31 2022 UTC
gpg:                using RSA key 3B04D753C9050D9A5D343F39843C48A565F8F04B
gpg: Can't check signature: No public key

To resolve this message you need to acquire the public key that was used to sign the binaries. You can download it from a trusted GPG server, for example to use the Ubuntu key servers run this command:

gpg --keyserver keyserver.ubuntu.com --recv-keys 3B04D753C9050D9A5D343F39843C48A565F8F04B

If you then run the verify command you will get a message indicating that the newly imported key has not been trusted:

gpg: Good signature from "Adoptium GPG Key (DEB/RPM Signing Key)
<[email protected]>" [unknown] gpg: WARNING: This key is not certified
with a trusted signature!  gpg: There is no indication that the signature
belongs to the owner.

While the “Good signature” message gives you some confidence that the download is valid, to fully trust the certificate and remove the final warning you can run the following then follow the prompts to grant ultimate trust to it:

gpg --edit-key 3B04D753C9050D9A5D343F39843C48A565F8F04B trust

The verification should then succeed as follows:

gpg: Signature made Mon Jul  4 18:20:31 2022 UTC
gpg:                using RSA key 3B04D753C9050D9A5D343F39843C48A565F8F04B
gpg: checking the trustdb
gpg: marginals needed: 3  completes needed: 1  trust model: pgp
gpg: depth: 0  valid:   1  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 1u
gpg: Good signature from "Adoptium GPG Key (DEB/RPM Signing Key) <[email protected]>" [ultimate]

OK I know what commands to run, but what are the implications of those steps?

For more information on GPG signing and the impliations of the different steps in the process above, see the integrity checking article from Eclipse

Do you have questions or want to discuss this post? Hit us up on the Adoptium Slack workspace!