Getting started with GPG key for signing git commit

Configuring Git and GPG

After installing git, you need to add git's binary path to the PATH environment, located in %ProgramFiles%\Git\usr\bin.

GPG Key

Create new key

Generate a key: gpg --default-new-key-algo rsa4096 --gen-key. After that, check again with this command: gpg --list-secret-keys --keyid-format LONG, result example:

$ gpg --list-secret-keys --keyid-format LONG
/c/Users/fmaktum/.gnupg/pubring.gpg
-----------------------------------
sec   rsa4096/E170165D27E434C2 2018-07-22 [SC] [expires: 2022-07-23]
      FE428E022494CC3ED85ACDD3E170165D27E434C2
uid                 [ultimate] Fakhrulhilal Maktum <fakhrulhilal@gmail.com>
uid                 [ultimate] Fakhrulhilal Maktum <fakhrulhilal@outlook.com>
uid                 [ultimate] [jpeg image of size 13093]
ssb   rsa4096/C0D8267ED759FC4B 2018-07-22 [E] [expires: 2022-07-23]

in that case, key ID is 3AA5C34371567BD2.

Next, we need to associate with the email address. To do that, we need to edit first by this command: gpg --edit-key 3AA5C34371567BD2

gpg> adduid
Real name: Fakhrulhilal Maktum
Email address: fakhrulhilal@outlook.com
Comment: 
You selected this USER-ID:
    "Fakhrulhilal Maktum <fakhrulhilal@outlook.com>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o

Optionally, we can add the picture (suggested to use 240x288)

gpg> addphoto

After all changes, we can know save it

gpg> save

Extending Expired Public Key

You need to edit the key by using this command: gpg --edit-key:

gpg> expire
Changing expiration time for a subkey.
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) 2y
Key expires at Sun Jul 24 06:36:28 2022 SEAST
Is this correct? (y/N) y

sec  rsa4096/E170165D27E434C2
     created: 2018-07-22  expires: 2023-07-23  usage: SC
     trust: ultimate      validity: ultimate
ssb* rsa4096/C0D8267ED759FC4B
     created: 2018-07-22  expires: 2022-07-23  usage: E
[ultimate] (1). Fakhrulhilal Maktum <fakhrulhilal@gmail.com>
[ultimate] (2)  Fakhrulhilal Maktum <fakhrulhilal@outlook.com>
[ultimate] (3)  [jpeg image of size 13093]

gpg> key 1

sec  rsa4096/E170165D27E434C2
     created: 2018-07-22  expires: 2023-07-23  usage: SC
     trust: ultimate      validity: ultimate
ssb  rsa4096/C0D8267ED759FC4B
     created: 2018-07-22  expires: 2022-07-23  usage: E
[ultimate] (1). Fakhrulhilal Maktum <fakhrulhilal@gmail.com>
[ultimate] (2)  Fakhrulhilal Maktum <fakhrulhilal@outlook.com>
[ultimate] (3)  [jpeg image of size 13093]

gpg> expire
Changing expiration time for the primary key.
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) 2y
Key expires at Sun Jul 24 06:36:36 2022 SEAST
Is this correct? (y/N) y

sec  rsa4096/E170165D27E434C2
     created: 2018-07-22  expires: 2022-07-23  usage: SC
     trust: ultimate      validity: ultimate
ssb  rsa4096/C0D8267ED759FC4B
     created: 2018-07-22  expires: 2022-07-23  usage: E
[ultimate] (1). Fakhrulhilal Maktum <fakhrulhilal@gmail.com>
[ultimate] (2)  Fakhrulhilal Maktum <fakhrulhilal@outlook.com>
[ultimate] (3)  [jpeg image of size 13093]

The first key is for extending primary key, the second command is for extending sub encryption key.

Backup GPG Key

The easy way to backup all keys is by copy-paste the database

public keys: %UserProfile%\.gnupg\pubring.gpg
secret keys: %UserProfile%\.gnupg\secring.gpg
trust db: %UserProfile%\.gnupg\trustdb.gpg

GPG manual suggests this command to backup trust db: gpg --export-ownertrust > gpg-owner-trust.txt.

To backup individual key:

public key: gpg --armor --export E170165D27E434C2 > public.gpg
secret key: gpg --armor --export-secret-key E170165D27E434C2> secret.asc

Or you can use the email address instead of the key ID, f.e. git --armor --export fakhrulhilal@gmail.com > public.gpg. Note that, secret key always contains public key.

We can also publish the GPG key to public server with this command: gpg --keyserver [server address] --send-keys fakhrulhilal@gmail.com. Some notable PGP public key servers:

pgp.mit.edu
pgp.key-server.io
keyserver.pgp.com

Import/Restore GPG Key

Importing secret key (along with public key): gpg --import fakhrulhilal@gmail.com.asc. After that, import all owner trust: gpg --import-ownertrust gpg-owner-trust.txt. Alternatively, we can trust by each key:

$ gpg --edit-key fakhrulhilal@gmail.com
gpg> trust
Your decision? 5 (Ultimate trust)

Sharing GPG key to public key server

Below is currently active keyservers:

pgp.mit.edu
keyserver.ubuntu.com
keys.openpgp.org
keyserver1.pgp.com

To upload the key using gpg command, use gpg --keyserver the_server --send-keys E170165D27E434C2. Another way is by uploading manually to them. So we need to go their website and upload the key, commonly, they accept ASCII version of public key (gpg --export --armor E170165D27E434C2)

Associating Git with GPG

Setting GPG key for git commit

Set the key by using this command: git config user.signingkey E170165D27E434C2. And then we can sign the commit by -S option. Alternatively, we can force all commit to be signed using this command git config commit.gpgsign true, so we don't have to specify -S parameter each time committing the change.

Uploading public key to github

First, we need to backup the public key as follows: gpg --armor --export E170165D27E434C2 > fakhrulhilal.gpg

Login to your github account
Go to menu Settings > SSH and GPG keys
Add new gpg key
Copy-paste from fakhrulhilal.gpg content then save it

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