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How to hash and sign files with GPG and a bit of Bash

When making a release of your software or sending some important documents, it's pretty common practice (especially amongst larger projects) to distribute hashes and GPG signatures along with release binaries themselves. Example projects that do this include:

This is great practice, since it allows downloaders to verify that their download has not been corrupted, and that it was you who released them and not some imposter.

In this post, I'm going to outline how you can do this too.

I've recently both verified a number of signatures and generated some of my own too, so I thought I'd post about it here to show others how to do it too.

Verification

Before we get into the generation of hashes and signatures, we should first talk about verifying them. I'm mentioned this is good practice already, so it makes sense to briefly talk about verification first. First, let's download Nomad version 0.10.5. You can grab the files here. Download the following files:

Verifying the hashes is easier, so let's do that first. We can see from the filenames that we have SHA 256 hashes, so we'll want the sha256sum command. Windows users will need to use the Windows Subsystem for Linux, or setup an msys environment:

sha256sum --ignore-missing --check nomad_0.10.5_SHA256SUMS

It should output an OK message and return an exit code zero (to check this in a script, you can do echo "$?" directly after running it to check the exit code). 99% of the time this check will succeed, but you'll be glad that you checked the 1% of the time it fails.

Checking the hashes here is the bit that ensures that the files haven't been corrupted. Next, we'll verify the GPG signature. This is the bit that ensures that the files we've downloaded were actually originally released by who we think they were. Do that like this:

gpg --verify nomad_0.10.5_SHA256SUMS.sig

Doing this, you may get an error message telling you that it can't verify the signature because you haven't got the public key of the signer imported into your keyring. To remedy this, look for the bit that tells you the key id (e.g. using RSA key 51852D87348FFC4C). Copy it, and then do this:

gpg --recv-keys 51852D87348FFC4C

This will download it and import the key id into your local GPG keyring. Then re-run the gpg --verify command above, and it should work.

Generation

Now that we know how to verify a signature, let's generate our own. First, put the files you want to hash into a directory and cd into it in your terminal. Then, let's generate the hash file:

# Hash files
find . -type f -not -name "*.SHA256" -print0 | xargs -0 -n1 -I{} -P"$(nproc)" sha256sum -b "{}" >HASHES.SHA256

We use find here to locate all the files (other than the hash file itself), and then pass them to xargs, which calls sha256sum to hash the files in question. Finally, we write the hashes to HASHES.SHA256.

Next, lets generate a GPG signature for the hash file. For this, you'll need a GPG key. That's out of scope of this post really, but this tutorial looks like it will show you how to do it. Note that in order for other people to verify the GPG signature you create, you'll probably need to upload your GPG public key to a keyserver (the article I link to shows you how to do this too).

Once done, generate the signature like this:

# Sign the hashes
gpg --sign --detach-sign --armor HASHES.SHA256

Specifically, we generate a detached signature here - meaning that it's in a separate file to the file that is being signed. The --armor there just means to wrap the signature in base64 encoding (I'm pretty sure) and some text so that it's not a raw binary file that might confuse the uninitiated.

Finally, let's verify the signature we just created - just in case:

# Verify the signature, and check we used the right key
gpg --verify HASHES.SHA256.asc

If all's good, it should tell you that the signature is ok. If you've got multiple keys, ensure that you signed it with the correct key here. GPG will sign things with the key you have marked as the default key.

Found this interesting? Having trouble? Want to say hi? Comment below!

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