Tools Index

GnuPG

1. Install

Install gnupg;

        $ sudo prt-get depinst gnupg

Create a skeleton configuration to be copied by useradd to each user home directory;

        $ sudo mkdir /etc/skel/.gnupg
        $ sudo cp /usr/share/gnupg/gpg-conf.skel /etc/skel/.gnupg/gpg.conf

        $ chmod 700 ~/.gnupg
        $ chmod -R 600 ~/.gnupg/*

2. Generate keys

Options for creating a DSA and ElGamal key;

Key Size: the defaults are ok.
Expiring Date: Choose a non-expiring key for your own use. For public at least choose yearly expiration.
Name and Email: Real name and e-mail address used to identify your key.
Comment: Can be empty, make a small comment to help indentify.
Passphrase: Password should use numebers and special chars.

Generate keys;

        $ gpg2 --full-gen-key
        gpg (GnuPG) 2.1.11; Copyright (C) 2021 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.

        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? 2
        DSA keys may be between 1024 and 3072 bits long.
        What keysize do you want? (2048) 2048
        Requested keysize is 2048 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) 6m
        Key expires at Tue May 30 20:29:36 2017 WEST
        Is this correct? (y/N) y

        GnuPG needs to construct a user ID to identify your key.

        Real name: User Name
        Email address: user@machine.example.org
        Comment: user at external dot org
        You selected this USER-ID:
            "User Name (user at core) <user@machine.example.org>"

        Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O

This will create the follow files;

        .gnupg/pubring.gpg
        .gnupg/random_seed
        .gnupg/secring.gpg
        .gnupg/trustdb.gp

List keys;

        $ gpg --list-keys
        /home/droid/.gnupg/pubring.kbx
        ------------------------------
        pub   dsa3072/EE29B7D3 2016-05-30 [SC] [expires: 2017-05-30]
        uid         [ultimate] User Name (user at core ) <user@machine.example.org>
        sub   elg2112/9BC2DC12 2016-05-30 [E] [expires: 2017-05-30]

In this case pub key ID is EE29B7D3, add to .profile;

        export GPGKEY=0xEE29B7D3

3. Key Management

Key Management;

        $ gpg --list-keys
        $ gpg --list-secret-keys
        $ gpg --fingerprint
        $ gpg --delete-key UID
        $ gpg --delete-secret-key
        $ gpg --edit-key UID

3.1. Edit key

        $ gpg --edit-key KEYID
        adduid

3.2. Revoke key

Follow the instructions and then select the user you want to revoque, where N is the UID of the user;

        $ gpg --edit-key KEYID
        uid N
        revuid
        save

4. Export and import keys

4.1. Export Key

Public keys can be exported in binary format or ASCII-armored format. To export binary format;

        $ gpg --list-keys

        $ gpg --output user.gpg --export user@localhost

Generate an ASCII version of your public key;

        $ gpg --armor --output user.asc --export 'User Name'

4.2. Export to keyserver

The primary public key's ID is referenced in the pub line after the key size, for example the key created above, the short key ID is EE29B7D3:

        gpg --keyserver search.keyserver.net --send-key EE29B7D3

4.3. Import Key

Is very easy to import public keys;

        $ gpg --import user.gpg

To list imported keys;

        $ gpg --list-keys

4.4. Key Servers

Configure GnuPG to automatically fetch public keys, uncomment following line to ~/.gnupg/gpg.conf;

        keyserver-options auto-key-retrieve

And add a server, in this example wwwkeys.pgp.net;

        keyserver wwwkeys.pgp.net
        keyserver search.keyserver.net
        keyserver pgp.mit.edu

Test your configuration as described by Justin R. Miller Mutt Gnupg Howto;

        $ gpg --recv-keys 0xC9C40C31

Confirm;

        gpg --list-keys justin

5. Encrypt, decrypt and signing

5.1. Encrypt file

To be abble to decrypt the document we need to include public key in the recipient list;

        $ gpg --output index.html.gpg --encrypt \
        --recipient user@localhost \
        --recipient bob@localhost \
        index.html

5.2. Decrypt file

To decrypt the file;

        $ gpg --output index.html --decrypt index.html.gpg

5.3. Signing a File

A digital signature certifies and timestamps a document. If the document is subsequently modified in any way, a verification of the signature will fail. A digital signature can serve the same purpose as a hand-written signature with the additional benefit of being tamper-resistant. Example on how to sign a file;

To send document uncompressed and wrapped in an ASCII-armored signature;

        $ gpg --clearsign index.html

Verify signature;

        $ gpg --verify index.html.asc

To encrypt and sign a file;

        $ gpg --output index.html.gpg --sign --encrypt \
        --recipient bob@localhost \
        index.html

To decrypt;

        $ gpg --output index.html --decrypt index.html.gpg

You should see this message on the output;

        gpg: Good signature from "User (user at localhost) <user@localhost>"

Detached signatures create a separate file for signature;

        $ gpg --output index.html.sig --detach-sig index.html

Now you can send index.html.sig and on the other end;

        $ gpg --verify index.html.sig

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