Key Generation

The guide will explain how to generate a new key pair, how to create a signing sub key and how to strip the secret master key to avoid leaking your primary secret key identity in case your signing key (or the entire ~/.gnupg/) ever gets lost.

1) Generate new, fresh key pair on a secure machine

$ mkdir /tmp/signing 
$ chmod 0700 /tmp/signing 
$ gpg --homedir /tmp/signing --gen-key 
gpg (GnuPG) 1.4.18; Copyright © 2014 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 ‘/tmp/signing/secring.gpg’ created 
gpg: keyring ‘/tmp/signing/pubring.gpg’ created
TipPick 4 to generate an RSA-only key and choose a key size of 4096 bits. For this how-to I choose to set no expiry at all.
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? 4 
RSA keys may be between 1024 and 4096 bits long. 
What keysize do you want? (2048) 4096 
Requested keysize is 4096 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
TipGPG will ask about your user identity now, provide your real name and the mail address you intend to use for your project communication. I also suggest to provide a meaningful comment, eg. “LEDE Signing Key”
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) <<heinrichh@duesseldorf.de>>"

Real name: Jo-Philipp Wich 
Email address: <jo@mein.io> 
Comment: LEDE Signing Key 
You selected this USER-ID: 
    "Jo-Philipp Wich (LEDE Signing Key) <<jo@mein.io>>"

Change (N)ame, ©omment, (E)mail or (O)kay/(Q)uit? O 
You need a Passphrase to protect your secret key.
TipAt this point enter a good pass phrase twice to protect your secret key, the command will take a while to gather entropy and complete key until it’ll eventually print the key summary:
gpg: /tmp/signing/trustdb.gpg: trustdb created 
gpg: key 612A0E98 marked as ultimately trusted public and secret key created and signed.

gpg: checking the trustdb 
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model 
gpg: depth: 0 valid: 1 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 1u 
pub   4096R/612A0E98 2016-04-05 
      Key fingerprint = 69B2 6A27 62D0 65E6 6F59 6755 C76F DE50 612A 0E98 
uid   Jo-Philipp Wich (LEDE Signing Key) <<jo@mein.io>>

Note that this key cannot be used for encryption. You may want to use 
the command "--edit-key" to generate a subkey for this purpose.

2) Generate a sub key

$ gpg --homedir /tmp/signing --edit-key <jo@mein.io> 
gpg (GnuPG) 1.4.18; Copyright © 2014 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.

Secret key is available.

pub 4096R/612A0E98 created: 2016-04-05 expires: never usage: SC 
                   trust: ultimate validity: ultimate 
[ultimate] (1). Jo-Philipp Wich (LEDE Signing Key) <<jo@mein.io>>
TipIn the now appearing interactive gpg prompt enter “addkey” to create a new signing subkey. GnuPG will ask your to unlock the master key using the passphrase you’ve given in the previous step.
gpg> addkey 
Key is protected.

You need a passphrase to unlock the secret key for 
user: "Jo-Philipp Wich (LEDE Signing Key) <<jo@mein.io>>" 
4096-bit RSA key, ID 612A0E98, created 2016-04-05

Please select what kind of key you want: 
    (3) DSA (sign only) 
    (4) RSA (sign only) 
    (5) Elgamal (encrypt only) 
    (6) RSA (encrypt only)
TipWe’ll use a signing-only 4096bit RSA key with an validity of two years
Your selection? 4 
RSA keys may be between 1024 and 4096 bits long. 
What keysize do you want? (2048) 4096 
Requested keysize is 4096 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) 730 
Key expires at Thu Apr 5 18:19:42 2018 CEST 
Is this correct? (y/N) y 
Really create? (y/N) y
TipAt this point, GnuPG will start gathering entropy again, running an “find /” in the background is a good way to speed it up. When done it will print the sub key summary and return to the prompt. Note the ID “1584F206” of the subkey, we’ll need that in step 4.
pub 4096R/612A0E98 created: 2016-04-05 expires: never usage: SC 
                   trust: ultimate validity: ultimate 
sub 4096R/1584F206 created: 2016-04-05 expires: 2018-04-05 usage: S 
[ultimate] (1). Jo-Philipp Wich (LEDE Signing Key) <<jo@mein.io>>
TipEnter “save” to commit the new key pair and its sub key to disk, the GnuPG will automatically exit to the shell.
gpg> save

3) Put the key into a vault

At this point it is a good idea to make a reliable and secure backup of the /tmp/signing/ directory, I suggest burning it onto a CDROM or copying it onto a thumb drive which you can safely lock away or hide in your apartment :)

4) Export the private sub key only

We’ll now export just the secret sub key since that is all we’ll ever need to sign files. Use the sub key ID from step 2 followed by an exclamation mark to select the sub key to export:

$ gpg --homedir /tmp/signing --export-secret-subkeys 1584F206! \ 
    > /tmp/secret-signing-key.pgp 
$ file /tmp/secret-signing-key.pgp 
secret-signing-key.pgp: PGP\011Secret Key - 1024b created on Tue Apr 5 16:08:15 2016 - RSA (Encrypt or Sign)

5) Import the secret signing sub key into your actual key store

You can now import the secret signing sub key on any machine you’ll use for signing files in the future. To import the sub key file, pass it to gpg --import and leave out the alternative homedir argument:

$ gpg --import /tmp/secret-signing-key.pgp 
gpg: key 612A0E98: secret key imported 
gpg: key 612A0E98: public key "Jo-Philipp Wich (LEDE Signing Key) <<jo@mein.io>>" imported 
gpg: Total number processed: 1 
gpg:               imported: 1 (RSA: 1) 
gpg:       secret keys read: 1 
gpg:   secret keys imported: 1
TipYou can now issue a “gpg -K” to list all secret keys in your key store, you should see the key you’ve imported with a leading “sec#”. The hash mark here indicates that the secret master key is missing, which is what we want.
$ gpg -K 
/home/jow/.gnupg/secring.gpg
----------------------------
[...]
sec# 4096R/612A0E98 2016-04-05 
uid                 Jo-Philipp Wich (LEDE Signing Key) <<jo@mein.io>> 
ssb  4096R/1584F206 2016-04-05
TipIt is time to upload your public key part to a key server now so that others can easily fetch it by its fingerprint or your chosen mail address later on. For uploading use the primary key ID printed after the “sec#” word in the previous command.
$ gpg --keyserver hkp:%%//%%pool.sks-keyservers.net --send-keys 612A0E98 
gpg: sending key 612A0E98 to hkp server pool.sks-keyservers.net

6) Delete original

Make sure once again that your backup of the /tmp/signing directory is complete and readable, then remove the secret sub key file and the entire temporary signing directory:

$ rm -r /tmp/signing/ 
$ rm /tmp/secret-signing-key.pgp
TipYou’re now done setting up a suitable signing key pair.

7) Finish

To export your public key in ASCII format use the following command, again with the primary ID you’ve already used for uploading the pubkey.

Make sure to provide a meaningful comment so that people looking at the key file know who it belongs to without having to inspect it using GPG utilities:

$ gpg --armor --export --no-version \ 
        --comment="Public key of Jo-Philipp Wich" 612A0E98

In order to sign a file with your signing sub key, use the command below:

$ gpg --no-version -a -b -u 612A0E98 \ 
        --comment="My signature for something" -o output.sig input.file
TipUse your key ID as filename when adding your public signing key to the repository:
$ cd keyring/gpg/ 
$ gpg --armor --export --no-version \ 
  --comment="Public key of Me Myself" 612A0E98 > 612A0E98.asc 
$ git add 612A0E98.asc 
$ git commit -sm "Add my public key" 
$ git push origin master

In order to generate an usign key pair for use in LEDE release and package repositories, follow the steps below.

1) Obtain usign

Clone the usign repository and compile it. Note that the compilation requires an installed cmake to succeed.

$ git clone https://git.openwrt.org/project/usign.git 
$ cd usign/ 
$ cmake . 
$ make
TipRun ./usign to check that the binary works.
$ ./usign Usage: ./usign <command> <options> 
Commands: 
  -V:             verify (needs at least -m and -p|-P) 
  -S:             sign (needs at least -m and -s) 
  -F:             print key fingerprint of public/secret key or signature 
  -G:             generate a new keypair 
Options: 
  -c <comment>:   add comment to keys 
  -m <file>:      message file 
  -p <file>:      public key file (verify/fingerprint only) 
  -P <path>:      public key directory (verify only) 
  -q:             quiet (do not print verification result, use return code only) 
  -s <file>:      secret key file (sign/fingerprint only) 
  -x <file>:      signature file (defaults to <message file>.sig)

2) Generate key pair

Instruct the usign executable to generate a new key pair and provide a suitable comment to be able to identify the key file later on.

./usign -G -c "LEDE usign key of Jo-Philipp Wich" \
  -s secret.key -p public.key
TipStore the secret.key file in a secure and reliable location, you’ll need it to sign package repositories in the future.

3) Add public key to the repository

Obtain the fingerprint of your public key with the usign -F command and use it as filename for storing the pubkey in the keyring.git repository:

$ ./usign -F -p public.key 
72a57f2191b211e0
TipAdd the key to Git, using the fingerprint as filename:
$ cd keyring/usign/ 
$ cp /some/where/public.key 72a57f2191b211e0 
$ git add 72a57f2191b211e0 
$ git commit -sm "Add my public usign key" 
$ git push origin master