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.
$ 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
|Tip||Pick 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
|Tip||GPG 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) <<firstname.lastname@example.org>>" Real name: Jo-Philipp Wich Email address: <email@example.com> Comment: LEDE Signing Key You selected this USER-ID: "Jo-Philipp Wich (LEDE Signing Key) <<firstname.lastname@example.org>>" Change (N)ame, ©omment, (E)mail or (O)kay/(Q)uit? O You need a Passphrase to protect your secret key.
|Tip||At 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) <<email@example.com>> 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.
$ gpg --homedir /tmp/signing --edit-key <firstname.lastname@example.org> 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) <<email@example.com>>
|Tip||In 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) <<firstname.lastname@example.org>>" 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)
|Tip||We’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
|Tip||At 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) <<email@example.com>>
|Tip||Enter “save” to commit the new key pair and its sub key to disk, the GnuPG will automatically exit to the shell.|
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 :)
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)
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) <<firstname.lastname@example.org>>" imported gpg: Total number processed: 1 gpg: imported: 1 (RSA: 1) gpg: secret keys read: 1 gpg: secret keys imported: 1
|Tip||You 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) <<email@example.com>> ssb 4096R/1584F206 2016-04-05
|Tip||It 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
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
|Tip||You’re now done setting up a suitable signing key pair.|
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
|Tip||Use 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.
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
$ ./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)
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
Obtain the fingerprint of your public key with the
usign -F command and use it as filename for storing the pubkey in the
$ ./usign -F -p public.key 72a57f2191b211e0
|Tip||Add 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