PGP Best Practices - How to Create Strong and Secure Keys

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I wrote about PGP a while ago and showed how to generate a key. Since then I’ve managed to do what you should never, ever do: forget your key’s passphrase. And I didn’t generate a revocation cert to boot. So I was screwed and had to generate a new one. Forunately not many, and by “not many” I mean zero, people were using that public key which I had uploaded to public keyservers.

I generated a new one and spent a lot of time reading about PGP best practices. Here’s what I learned.

How to Generate Strong Keys and Secure Them

  • Use GnuPG version >= 1.4 to make it easier to generate and manage keys
    • You can brew install gpg2 on a Mac
  • Append the following to ~/.gnupg/gpg.conf (or whatever the latest strongest encryption algorithms are):
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personal-digest-preferences SHA256
cert-digest-algo SHA256
default-preference-list SHA512 SHA384 SHA256 SHA224 AES256 AES192 AES CAST5 ZLIB BZIP2 ZIP Uncompressed
  • gpg --gen-key
  • Choose 1 at this prompt
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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)
  • Set an expiration date of at most two years (you can always extend it)
  • Set a strong passphrase that you’ll remember!
  • Use the maximum number of bits for your key
  • Generate revocation cert(s)
    • gpg --gen-revoke <key id> key id can be the portion of the fingerprint you see in gpg --list-keys or the uid if it’s unique
  • Use subkeys for encryption (this is automatically done if you choose 1 above) and another for signing. The following was copied from the Debian wiki.
    • Here’s the best paragraph from that wiki page:

You should keep your private master key very, very safe. However, keeping all your keys extremely safe is inconvenient: every time you need to sign a new package upload, you need to copy the packages onto suitable portable media, go into your sub-basement, prove to the armed guards that you’re you by using several methods of biometric and other identification, go through a deadly maze, feed the guard dogs the right kind of meat, and then finally open the safe, get out the signing laptop, and sign the packages. Then do the reverse to get back up to your Internet connection for uploading the packages.

  • gpg --edit-key <your master key id>
  • At the prompt, enter addkey
  • Enter passphrase
  • Choose the “RSA (sign only)” key type and pick the maximum bits
  • Choose an expiry date
  • save
  • Backup the ~/.gnupg directory and store it somewhere secure and offline. Do this before deleting the private master key in the next step.
  • Check gpg --export-secret-key <your master key id> | gpg --list-packets does not show gnu-dummy 52K, algo: 0, .... This means your private master key is there.
  • gpg --export-secret-subkeys <your master key id> > secret-subkeys
  • gpg --delete-secret-key <your master key id>
  • gpg --import secret-subkeys
  • Verify that gpg -K shows a “sec#” instead of just “sec” for your private key. That means the secret key is not really there. You can see the presence of a dummy OpenPGP packet if you run gpg --export-secret-key <your master key id> | gpg --list-packets.
  • gpg --keyserver some.keyserver.com --send-keys <your master key id> to publish it to the world

How to Revoke

If you ever need to revoke your keys,

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gpg --import path/to/revocation-cert
gpg --keyserver some.keyserver.com --send-key <key id>

How to Sign Other People’s keys

Sign their key with a certain level of trust.

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gpg -u david@davidxia.com --sign-key --ask-cert-level harvey@harveyxia.com

Export the signed key and email it back to the signee.

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gpg --armor --output 00AA11BB-signedBy-1A2B3C4D.asc --export 00AA11BB

The signee needs to import the signed key

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gpg --import 1A2B3C4D-signedBy-00AA11BB.asc

and send your certification to the keyserver.

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gpg --send-key 1A2B3C4D

Encrypting messages

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gpg -u david@davidxia.com --recipient harvey@harveyxia.com --output msg.asc --encrypt msg

Recipients decrypt with gpg -u harvey@harveyxia.com --output msg --decrypt msg.asc.

Signing messages

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gpg -u david@davidxia.com --output msg.sig --detach-sign msg

Recipients verify with gpg --verify msg.sig msg. They should see “Good signature from…”

References

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