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Add more GnuPG bits

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Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
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Konstantin Ryabitsev 5 years ago
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developer-security-hygiene.md
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@ -226,9 +226,9 @@ from other PGP users you come across.
#### Installing GnuPG
If you are using Linux, you should already have GnuPG installed. On a Mac,
you should install [GPG-Suite](https://gpgtools.org). For all other platforms,
you'll need to do your own research to find the correct places to download and
install GnuPG.
you should install [GPG-Suite](https://gpgtools.org) or you can use `brew
install gnupg2`. For all other platforms, you'll need to do your own research
to find the correct places to download and install GnuPG.
##### GnuPG 1 vs. 2
@ -248,3 +248,267 @@ command:
If you see `gpg (GnuPG) 2.x.x`, then you are good to go. This guide will
assume you have the version 2.2 of GnuPG (or later).
##### Making sure you always use GnuPG2
If you have both `gpg` and `gpg2` commands, you should make sure you are
always using GnuPG v2, not the legacy version. You can make sure of it by
setting the alias:
alias gpg='/usr/bin/gpg2'
You can put that in your `.bashrc` to make sure it's always loaded whenever
you use the gpg commands.
## Generating and protecting your master PGP key
### Checklist
- [ ] Generate the 4096-bit RSA master key _(ESSENTIAL)_
- [ ] Back up the master key using paperkey _(ESSENTIAL)_
- [ ] Add all relevant identities _(ESSENTIAL)_
### Considerations
#### Understanding the "Master" (Certify) key
In this and next section we'll talk about the "master key" and "subkeys". It
is important to understand the following:
1. There are no technical differences between the "master key" and "subkeys."
2. At creation time, we assign functional limitations to each key by
giving it specific capabilities.
3. A PGP key can have 4 capabilities.
- **[S]** Key can be used for signing
- **[E]** Key can be used for encryption
- **[A]** Key can be used for authentication
- **[C]** Key can be used for certifying other keys
4. A single key may have multiple capabilities.
The key carrying the **[C]** (certify) capability is considered the "master"
key because it is the only key that can be used to indicate relationship with
other keys. Only the **[C]** key can be used to:
- add or revoke other keys (subkeys) with S/E/A capabilities
- add, change or revoke identities (uids) associated with the key
- add or change the expiration date on itself or any subkey
- sign other people's keys for the web of trust purposes
In the Free Software world, the **[C]** key is your digital identity. Once you
create the key, you should take extra care to protect it and prevent it from
falling into malicious hands.
#### Before you create the master key
Before you create your master key you need to pick your primary identity and
your master passphrase.
##### Primary identity
An identity is basically in the same format as the From field in emails:
Alice Engineer <alice.engineer@example.org>
You can create new identities and revoke old ones, and you can also change
which identity is your "primary" one at a later time. Since the primary
identity is shown in all GnuPG operations, you should pick an
address/description that is both professional and the most likely one to be
used for PGP-enforced communication, such as your work address or the address
you use for signing off on project commits.
##### Passphrase
The passphrase is used exclusively for encrypting the private key with a
symmetric algorithm while it is stored on disk. If the contents of your
`.gnupg` directory ever get leaked, a good passphrase is the last line of
defense between the thief and them being able to impersonate you online, which
is why it is important to set up a good passphrase.
A good guideline for a strong passphrase is 3-4 words from a rich or mixed
dictionary that are not quotes from popular sources (songs, books, slogans).
You won't need to type the **[C]** key passphrase very frequently, so it does
not need to be easy to type, just easy to remember.
##### Algorithm and key strength
Even though GnuPG has supported Elliptic Curve crypto for a while now, we'll be
sticking to RSA keys, at least for a little while longer. While it is possible
to start using ED25519 keys right now, it is possible that you will come
across tools and hardware devices that will not be able to handle them
correctly.
For this reason, we will be generating RSA keys. For our master key, we'll use
4096 bits, and for our subkeys we'll stick to 2048 bits -- it is easy enough
to replace subkeys with stronger ones, but the master key must live on for a
long time.
#### Generate the master key
To generate your key, issue the following command, putting in the right values
instead of Alice Engineer:
gpg --quick-generate-key 'Alice Engineer <alice@example.org>' rsa4096 cert
A dialog will pop up asking to enter the passphrase. Then, you may need to
move your mouse around or type on some keys to generate enough entropy until
the command completes.
Review the output of the command, it will be something like this:
pub rsa4096 2017-12-06 [C] [expires: 2019-12-06]
111122223333444455556666AAAABBBBCCCCDDDD
uid Alice Engineer <alice@example.org>
Note the long string on the 2nd line -- that is the full fingerprint of your
newly generated key. Key ID can be represented in three different forms:
- **fingerprint**, a full 40-character key identifier
- **long**, last 16-characters of the fingerprint (`AAAABBBBCCCCDDDD`)
- **short**, last 8 characters of the fingerprint (`CCCCDDDD`)
You should avoid using 8-character "short key IDs" as they are not
sufficiently unique.
At this point, I suggest you open a text editor, copy the fingerprint of your
new key and paste it there. You'll need to use it for the next few steps.
#### Back up your master key
For disaster recovery purposes -- and especially if you intend to use the Web
of Trust and collect key signatures from other project developers -- you
should create a hardcopy backup of your private key. This is supposed to be a
"last resort" measure in case all other backup mechanisms have failed.
The best way to create a printable hardcopy of your private key is using
`paperkey` software written for this very purpose. Paperkey is available on
all Linux distros, as well installable via `brew install paperkey` on Macs.
Run the following command, replacing `[fpr]` with the full fingerprint of your
key:
gpg --export-secret-key [fpr] | paperkey > /tmp/key-backup.txt
The output will be in a format that is easy to OCR or input by hand, should
you ever need to recover it. Print out that file, then take a pen and write
the key passphrase on the margin of the paper. This is a required step because
the key printout is still encrypted with the passphrase, and if you ever
change the passphrase on your key, you will not remember what it used to be
when you had first created it -- guaranteed.
Put the resulting printout and the hand-written passphrase into an envelope
and store in a secure and well-protected place that is away from your home,
such as your bank vault.
**NOTE ON PRINTERS**: Long gone are days when printers were dumb devices
connected to the computer's parallel port. These days they have full operating
systems, hard drives, and cloud integration. Since the key content we send to
the printer will be encrypted with the passphrase, this is a fairly safe
operation, but use your best paranoid judgement.
#### Add relevant identities
If you have multiple relevant email addresses (personal, work, open-source
project, etc), you should add them to your master key. You don't need to do
this for any addresses that you don't expect to use with PGP (e.g. probably
not your school alumni address).
The command is (put the full key fingerprint instead of `[fpr]`):
gpg --quick-add-uid [fpr] 'Alice Engineer <allie@example.net>'
You can review the IDs you've already added using:
gpg --list-key [fpr] | grep ^uid
##### Pick the primary UID
GnuPG will make the latest UID you add as your primary UID, so if that is
different from what you want, you should fix it back:
gpg --quick-set-primary-uid [fpr] 'Alice Engineer <alice@example.org>'
## Generating PGP subkeys
### Checklist
- [ ] Generate a 2048-bit Encryption key _(ESSENTIAL)_
- [ ] Generate a 2048-bit Signing key _(ESSENTIAL)_
- [ ] Generate a 2048-bit Authentication key _(NICE)_
### Considerations
Now that we've created the master key, let's create the keys you'll actually
be using for day-to-day work. We create 2048-bit keys because a lot of
specialized hardware (we'll discuss this further) does not handle larger keys,
but also for pragmatic reasons. If we ever find ourselves in a world where
2048-bit RSA keys are not considered good enough, it will be because of
fundamental problems with the RSA protocol and longer 4096-bit keys will not
make much difference.
#### Create the Sign and Encrypt subkeys
To create the subkeys, run:
gpg --quick-add-key [fpr] rsa2048 encr
gpg --quick-add-key [fpr] rsa2048 sign
You can also create the Authentication key, which will allow you to use your
PGP key for ssh purposes (covered in other guides):
gpg --quick-add-key [fpr] rsa2048 auth
You can review your key information using `gpg --list-key [fpr]`:
pub rsa4096 2017-12-06 [C] [expires: 2019-12-06]
111122223333444455556666AAAABBBBCCCCDDDD
uid [ultimate] Alice Engineer <alice@example.org>
uid [ultimate] Alice Engineer <allie@example.net>
sub rsa2048 2017-12-06 [E]
sub rsa2048 2017-12-06 [S]
## Moving your master key to offline storage
### Checklist
- [ ] Prepare encrypted detachable storage _(ESSENTIAL)_
- [ ] Back up your GnuPG directory _(ESSENTIAL)_
- [ ] Remove the master key from your home directory _(NICE)_
#### Back up your GnuPG directory
It is important to have a readily available backup of your PGP keys should you
need to recover them (this is different from the disaster-level preparedness
we did with `paperkey`).
**This step is especially important if you are going to remove your master key
or use smartcard hardware. Do not skip this step!**
#### Prepare detachable encrypted storage
Start by getting a detachable USB drive (preferably two) that you will use for
backup purposes. They do not need to be large. You will first need to encrypt
them:
- [Apple instructions](https://support.apple.com/kb/PH25745)
- [Linux instructions](https://help.ubuntu.com/community/EncryptedFilesystemsOnRemovableStorage)
For the encryption passphrase, you can use the same one as on your master key.
#### Back up your GnuPG directory
Once the encryption process is over, re-insert the USB drive and make sure it
gets properly mounted. Find out the full mount point of the device, for
example by running the `mount` command (under Linux, external media usually
gets mounted under `/media/disk`, under Mac it's `/Volumes`).
Once you know the full mount path, copy your entire GnuPG directory there:
cp -rp $HOME/.gnupg [/media/disk/some/path]/gnupg-backup
You should now test to make sure it still works:
gpg --homedir=[/media/disk/some/path]/gnupg-backup --list-key [fpr]
If you don't get any errors, then you should be good to go. Unmount the USB
drive, label it accordingly so you don't blow it away next time you need to
use a quick USB drive, and put in a safe place.

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