Signing Your Work

01 Mar 2017

Git is cryptographically secure, but it’s not foolproof. If you’re taking work from others on the internet and want to verify that commits are actually from a trusted source, Git has a few ways to sign and verify work using GPG.

GPG Introduction

First, if you want to sign anything you need to get GPG configured, and your personal key installed.

gpg --list-keys

Output:

/home/nasermirzaei89/.gnupg/pubring.gpg
---------------------------------------
pub   2048R/0857423A 2017-03-01
uid                  Naser Mirzaei <[email protected]>
sub   2048R/8369A0DC 2017-03-01

If you don’t have a key installed, you can generate one with gpg --gen-key:

gpg --gen-key

Steps:

gpg (GnuPG) 1.4.20; Copyright (C) 2015 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

gpg: keyring `/home/nasermirzaei89/.gnupg/secring.gpg' created
Please select what kind of key you want:
   (1) RSA and RSA (default)
   (2) DSA and Elgamal
   (3) DSA (sign only)
   (4) RSA (sign only)
Your selection?
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048)
Requested keysize is 2048 bits
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0)
Key does not expire at all
Is this correct? (y/N) y

You need a user ID to identify your key; the software constructs the user ID
from the Real Name, Comment and Email Address in this form:
    "Heinrich Heine (Der Dichter) <[email protected]>"

Real name: Naser Mirzaei
Email address: [email protected]
Comment:
You selected this USER-ID:
    "Naser Mirzaei <[email protected]>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O
You need a Passphrase to protect your secret key.

We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.

If you got an error like this:

Not enough random bytes available.  Please do some other work to give the OS a chance to collect more entropy!

Run this command in another terminal to help to complete key generation:

ls / -R

Once you have a private key to sign with, you can configure Git to use it for signing things by setting the user.signingkey config setting.

git config --global user.signingkey 0857423A

Now Git will use your key by default to sign tags and commits if you want.

Signing Tags

If you have a GPG private key setup, you can now use it to sign new tags. All you have to do is use -s instead of -a:

$ git tag -s v1.5 -m 'my signed 1.5 tag'
You need a passphrase to unlock the secret key for
user: "Ben Straub <[email protected]>"
2048-bit RSA key, ID 800430EB, created 2014–05–04

If you run git show on that tag, you can see your GPG signature attached to it:

$ git show v1.5
tag v1.5
Tagger: Ben Straub <[email protected]>
Date: Sat May 3 20:29:41 2014 -0700
my signed 1.5 tag
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1
iQEcBAABAgAGBQJTZbQlAAoJEF0+sviABDDrZbQH/09PfE51KPVPlanr6q1v4/Ut
LQxfojUWiLQdg2ESJItkcuweYg+kc3HCyFejeDIBw9dpXt00rY26p05qrpnG+85b
hM1/PswpPLuBSr+oCIDj5GMC2r2iEKsfv2fJbNW8iWAXVLoWZRF8B0MfqX/YTMbm
ecorc4iXzQu7tupRihslbNkfvfciMnSDeSvzCpWAHl7h8Wj6hhqePmLm9lAYqnKp
8S5B/1SSQuEAjRZgI4IexpZoeKGVDptPHxLLS38fozsyi0QyDyzEgJxcJQVMXxVi
RUysgqjcpT8+iQM1PblGfHR4XAhuOqN5Fx06PSaFZhqvWFezJ28/CLyX5q+oIVk=
=EFTF
-----END PGP SIGNATURE-----
commit ca82a6dff817ec66f44342007202690a93763949
Author: Naser Mirzaei <[email protected]>
Date: Mon Mar 17 21:52:11 2008 -0700
changed the version number

Verifying Tags

To verify a signed tag, you use git tag -v [tag-name]. This command uses GPG to verify the signature. You need the signer’s public key in your keyring for this to work properly:

$ git tag -v v1.4.2.1
object 883653babd8ee7ea23e6a5c392bb739348b1eb61
type commit
tag v1.4.2.1
tagger Junio C Hamano <[email protected]> 1158138501 -0700
GIT 1.4.2.1
Minor fixes since 1.4.2, including git-mv and git-http with alternates.
gpg: Signature made Wed Sep 13 02:08:25 2006 PDT using DSA key ID F3119B9A
gpg: Good signature from "Junio C Hamano <[email protected]>"
gpg: aka "[jpeg image of size 1513]"
Primary key fingerprint: 3565 2A26 2040 E066 C9A7 4A7D C0C6 D9A4 F311 9B9A

If you don’t have the signer’s public key, you get something like this instead:

gpg: Signature made Wed Sep 13 02:08:25 2006 PDT using DSA key ID F3119B9A
gpg: Can't check signature: public key not found
error: could not verify the tag 'v1.4.2.1'

Signing Commits

In more recent versions of Git (v1.7.9 and above), you can now also sign individual commits. If you’re interested in signing commits directly instead of just the tags, all you need to do is add a -s to your git commit command.

$ git commit -a -S -m 'signed commit'
You need a passphrase to unlock the secret key for
user: "Naser Mirzaei (Git signing key) <[email protected]>"
2048-bit RSA key, ID 0A46826A, created 2014–06–04
[master 5c3386c] signed commit
 4 files changed, 4 insertions(+), 24 deletions(-)
 rewrite Rakefile (100%)
 create mode 100644 lib/git.rb

To sign every commit without -s add global commit.gpgsign=true parameter in .gitconfig file:

git config --global commit.gpgsign true

To see and verify these signatures, there is also a --show-signature option to git log.

$ git log --show-signature -1
commit 5c3386cf54bba0a33a32da706aa52bc0155503c2
gpg: Signature made Wed Jun 4 19:49:17 2014 PDT using RSA key ID 0A46826A
gpg: Good signature from "Naser Mirzaei (Git signing key) <[email protected]>"
Author: Naser Mirzaei <[email protected]>
Date: Wed Jun 4 19:49:17 2014 -0700
signed commit

Additionally, you can configure git log to check any signatures it finds and list them in its output with the %G? format.

$ git log --pretty="format:%h %G? %aN %s"
5c3386c G Naser Mirzaei signed commit
ca82a6d N Naser Mirzaei changed the version number
085bb3b N Naser Mirzaei removed unnecessary test code
a11bef0 N Naser Mirzaei first commit

Here we can see that only the latest commit is signed and valid and the previous commits are not.

In Git 1.8.3 and later, git merge and git pull can be told to inspect and reject when merging a commit that does not carry a trusted GPG signature with the --verify-signatures command.

If you use this option when merging a branch, and it contains commits that are not signed and valid, the merge will not work.

$ git merge --verify-signatures non-verify
fatal: Commit ab06180 does not have a GPG signature.

If the merge contains only valid signed commits, the merge command will show you all the signatures it has checked and then move forward with the merge.

$ git merge --verify-signatures signed-branch
Commit 13ad65e has a good GPG signature by Naser Mirzaei (Git signing key) <[email protected]>
Updating 5c3386c..13ad65e
Fast-forward
 README | 2 ++
 1 file changed, 2 insertions(+)

You can also use the -S option with the git merge command itself to sign the resulting merge commit itself. The following example both verifies that every commit in the branch to be merged is signed and furthermore signs the resulting merge commit.

$ git merge --verify-signatures -S signed-branch
Commit 13ad65e has a good GPG signature by Naser Mirzaei (Git signing key) <[email protected]>
You need a passphrase to unlock the secret key for
user: "Naser Mirzaei (Git signing key) <[email protected]>"
2048-bit RSA key, ID 0A46826A, created 2014–06–04
Merge made by the 'recursive' strategy.
 README | 2 ++
 1 file changed, 2 insertions(+)

Export PGP Public Key

Use the gpg --list-secret-keys --keyid-format LONG command to list GPG keys for which you have both a public and private key. A private key is required for signing commits or tags.

gpg --list-secret-keys --keyid-format LONG

Note: Some GPG installations on Linux may require you to use gpg2 --list-keys --keyid-format LONG to view a list of your existing keys instead. In this case you will also need to configure Git to use gpg2 by running git config --global gpg.program gpg2.

From the list of GPG keys, copy the GPG key ID you’d like to use. In this example, the GPG key ID is 3AA5C34371567BD2:

Output:

/home/nasermirzaei89/.gnupg/secring.gpg
---------------------------------------
sec   2048R/4E4E444D0857423A 2017-03-01
uid                          Naser Mirzaei <[email protected]>
ssb   2048R/0E098B718369A0DC 2017-03-01

Paste the text below, substituting in the GPG key ID you’d like to use. In this example, the GPG key ID is 4E4E444D0857423A:

gpg --armor --export 4E4E444D0857423A

Prints the GPG key ID, in ASCII armor format

Your GPG key, beginning with -----BEGIN PGP PUBLIC KEY BLOCK----- and ending with -----END PGP PUBLIC KEY BLOCK-----.

Everyone Must Sign

Signing tags and commits are great, but if you decide to use this in your normal workflow, you’ll have to make sure everyone on your team understands how to do so. If you don’t, you’ll end up spending a lot of time helping people figure out how to rewrite their commits with signed versions. Make sure you understand GPG, and the benefits of signing things before adopting this as part of your standard workflow.

References

git GitHub signing GPG