Short GnuPG reference card

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Note on commands syntax:

  • keywords are written between <> and are supposed to be replaced by something meaningful...
  • optional items are written between []
  • some options have an alternative short notation, proposed behind a |, e.g. --decrypt|-d (choose only one syntax)

Contents

Setting a keyserver as default

Edit ~/.gnupg/gpg.conf:

keyserver  hkp://<my.best.keyserver.org>

If you do so you won't need to specify the keyserver manually anymore as option in the commands given in the next sections.

WinPT: "Keyserver" menu -> right-click somewhere in the white area -> "Add" -> HKP Keyserver, Hostname: <my.best.keyserver.org>, Port: 11371 -> "Add" -> Select your keyserver and press "Set default".

encrypting (and signing) a file

gpg [[--local-user|-u <yourself@foo.com>] --sign|-s]  # if you want to sign (opt with another key than your main key)
   [--armor|-a]                                      # if you want to armor (ASCII) the output, else it's binary
   --encrypt|-e 
   --recipient|-r <someone@foo.com> 
   [--encrypt-to <second_recipient@foo.com>]         # if you want other people to be able to decrypt as well
   [--encrypt-to <yourself@foo.com>]                 # if you want yourself to be able to decrypt as well
   [--output|-o <file.asc>]
   <filename>

decrypting (and verifying) a file

gpg [--decrypt|-d]
   [--output|-o <filename>]
   <file.asc>

So, yes, without any option gpg will decrypt/verify

searching a key on the keyserver

gpg --keyserver <my.best.keyserver.org>
   --search <someone>                              # search only on full words

From here you can directly import a key by providing its numeral (1, 2, 3...)

importing a key from the keyserver

gpg --keyserver <my.best.keyserver.org>
   --recv-key <key_id>                             # key-id is the last 8 bytes of the fingerprint

exporting a key to the keyserver

gpg --keyserver <my.best.keyserver.org>
   --send-key <key_id>                             # key-id is the last 8 bytes of the fingerprint

importing a key from a file

gpg  --import <pubkey.asc>

exporting your publickey to a file

gpg  --armor|-a --output|-o <yourpubkey.asc> --export <yourself@foo.com>

printing your fingerprint

(e.g. to put in your mails footers)

gpg --fingerprint <yourself>|grep -i fingerprint

printing the fingerprint of an exported keyfile

gpg --with-fingerprint <exported_keyfile>|grep -i fingerprint

signing a key

(see also below)

gpg --sign-key <someone>

Then you can upload it to the keyserver
WinPT: When signing, don't forget to un-check the "Sign local only" box otherwise your signature cannot be exported to the server.

trusting a key owner

How far you trust this user to correctly verify other users' keys (by looking at passports, checking fingerprints from different sources, etc.)

gpg --edit-key <someone>
Command> trust

=> Choose the proper trust level

Command> quit

=> save changes Remember that trust is not exported and purely local

revoking a subkey or a UID

(old email address,...)

gpg --edit-key <yourself>

=> select UID(s) or subkey(s) by its numeral: 1, 2, 3...

Command> uid/key <n>
Command> revuid/revkey
Command> quit

=> save changes

Then you can upload it to the keyserver

extracting a session key

If you're forced to by a decryption order...

gpg --show-session-key --output|-o /dev/null <encrypted_file>

This will give you a long string: the session key
It can be used to decrypt that file

gpg --override-session-key <9:0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF>  <encrypted_file> 

Creating fresh keys

Preferably RSA keys >= 2048 bits
E.g. create first a RSA key for signing only then add a RSA encryption key:

gpg --expert --gen-key
=> (5) RSA (sign only)
=> 2048 or bigger
=> validity: forever is ok
=> your name & email
=> choose a strong passphrase and be sure not to forget it!
gpg --edit-key <yourself>
Command> addkey
=> re-type your passphrase
=> (6) RSA (encrypt only)
=> 2048 or bigger
=> validity: forever is ok
Command> quit
=> save changes

Importing a secret key from PGP

If you've already a PGP key you can import it into GnuPG:
Under PGP, choose to export your key and ack to export also the secret key, tell where to put your ascii file, e.g. mykey.asc
With GnuPG, import the ascii file with the following command:

gpg --import --allow-secret-key-import < <mykey.asc>

Recovering a public key from a secret key

In case you destroyed your public key and cannot fetch it from the keyserver, you can reconstruct it:

gpg --export-secret-key <myname> | gpgsplit --no-split --secret-to-public |gpg import

Generating a revocation certificate

just in case...

Print it and keep it safe in case your key is compromised or you definitively forgot your passphrase:

gpg --gen-revoke <yourself>

Signing keys the right way

Signing keys involve more than a few GPG commands as it implies trust.
Get the public key of someone, either via keyserver or import file
Get the owner of the key in contact with you
Verify his identity (authenticate the owner)
Show him the fingerprint of his key from your computer (when you want to sign the soft will show you the fingerprint)
He must verify his fingerprint and make sure you are about to sign *his* key (the owner authenticates his key)
Sign the key, in an exportable way
Send the signed key to the server
Skipping the social aspect, a signature via gpg will look like:

gpg --keyserver <my.best.keyserver.org> --search <someone>
# either import interactively the key or find the key_id and get it:
gpg --keyserver <my.best.keyserver.org> --recv-key <key_id>
gpg --sign-key <key_id>
gpg --keyserver <my.best.keyserver.org> --send-key <key_id>

Enumerating encryption keys of an encrypted file

gpg --no-default-keyring --secret-keyring /dev/null -a --list-only <encrypted_file>
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