I have an offline environment for managing changes to my GPG keys. It's an old laptop with no WIFI card and no permanent internal storage. This is a quick TL;DR summary of what you could do.
I use a stock Debian LiveCD (here). For this CD the username/password is user/live. You can use sudo without a password. It has GPG already installed.
You will need a USB flash drive for copying files between the live environment and a machine with an internet connection. You will need a 2nd USB flash drive for long term offline storage of your master key.
Initial environment set up
Ideally you should generate your key in this environment and never expose it to an internet connected system. Attilla Lendvai has a great script and a great explanation of some of the concepts. It's worth a read, but the TL;DR is as follows.
We need a good ~/.gnupg/gpg.conf:
fixed-list-mode
keyid-format 0xlong
personal-digest-preferences SHA512
default-preference-list SHA512 SHA384 SHA256 SHA224 AES256 AES192 AES CAST5 BZIP2 ZLIB ZIP Uncompressed
verify-options show-uid-validity
list-options show-uid-validity
cert-digest-algo SHA512
Now we can generate the main key. This key will only be used to sign subkeys. It is important to keep this keep protected - if a subkey is lost it can be revoked and replaced without falling out of the web of trust. If your main key is compromised you will need to start over. Go for a 4096 RSA only key:
$ gpg --gen-key
gpg (GnuPG) 1.4.20; Copyright (C) 2015 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: directory '/home/john/.gnupg' created
gpg: new configuration file '/home/john/.gnupg/gpg.conf' created
gpg: WARNING: options in '/home/john/.gnupg/gpg.conf' are not yet active during this run
gpg: keyring '/home/john/.gnupg/secring.gpg' created
gpg: keyring '/home/john/.gnupg/pubring.gpg' created
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) 0
Key does not expire at all
Is this correct? (y/N) y
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) <heinrichh@duesseldorf.de>"
Real name: John Smith
Email address: john.smith@example.com
Comment:
You selected this USER-ID:
"John Smith <john.smith@example.com>"
Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O
You need a Passphrase to protect your secret key.
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
..+++++
..................................................+++++
gpg: /home/john/.gnupg/trustdb.gpg: trustdb created
gpg: key B05E3F37 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/B05E3F37 2016-10-26
Key fingerprint = 4C0A CCA7 8170 E95A 22FE C7B2 EA57 D591 B05E 3F37
uid John Smith <john.smith@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.
Generate the subkeys:
$ gpg --edit-key EA57D591B05E3F37
gpg (GnuPG) 1.4.20; Copyright (C) 2015 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/B05E3F37 created: 2016-10-26 expires: never usage: SC
trust: ultimate validity: ultimate
[ultimate] (1). John Smith <john.smith@example.com>
gpg> addkey
Key is protected.
You need a passphrase to unlock the secret key for
user: "John Smith <john.smith@example.com>"
4096-bit RSA key, ID B05E3F37, created 2016-10-26
gpg: gpg-agent is not available in this session
Please select what kind of key you want:
(3) DSA (sign only)
(4) RSA (sign only)
(5) Elgamal (encrypt only)
(6) RSA (encrypt only)
Your selection? 6
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) 3y
Key expires at Sat 26 Oct 2019 08:50:37 AM PDT
Is this correct? (y/N) y
Really create? (y/N) y
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
..................+++++
..............................+++++
pub 4096R/B05E3F37 created: 2016-10-26 expires: never usage: SC
trust: ultimate validity: ultimate
sub 4096R/BF4CEBBA created: 2016-10-26 expires: 2019-10-26 usage: E
[ultimate] (1). John Smith <john.smith@example.com>
gpg> addkey
Key is protected.
You need a passphrase to unlock the secret key for
user: "John Smith <john.smith@example.com>"
4096-bit RSA key, ID B05E3F37, created 2016-10-26
Please select what kind of key you want:
(3) DSA (sign only)
(4) RSA (sign only)
(5) Elgamal (encrypt only)
(6) RSA (encrypt only)
Your selection? 4
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (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) 3y
Key expires at Sat 26 Oct 2019 08:52:00 AM PDT
Is this correct? (y/N) y
Really create? (y/N) y
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
.+++++
+++++
pub 4096R/B05E3F37 created: 2016-10-26 expires: never usage: SC
trust: ultimate validity: ultimate
sub 4096R/BF4CEBBA created: 2016-10-26 expires: 2019-10-26 usage: E
sub 2048R/45F1A70C created: 2016-10-26 expires: 2019-10-26 usage: S
[ultimate] (1). John Smith <john.smith@example.com>
gpg> save
You can also generate authentication keys in --expert mode. I use a GPG authentication key for SSH.
$ gpg --expert --edit-key EA57D591B05E3F37
gpg (GnuPG) 1.4.20; Copyright (C) 2015 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/B05E3F37 created: 2016-10-26 expires: never usage: SC
trust: ultimate validity: ultimate
sub 4096R/BF4CEBBA created: 2016-10-26 expires: 2019-10-26 usage: E
sub 2048R/45F1A70C created: 2016-10-26 expires: 2019-10-26 usage: S
[ultimate] (1). John Smith <john.smith@example.com>
gpg> addkey
Key is protected.
You need a passphrase to unlock the secret key for
user: "John Smith <john.smith@example.com>"
4096-bit RSA key, ID B05E3F37, created 2016-10-26
gpg: gpg-agent is not available in this session
Please select what kind of key you want:
(3) DSA (sign only)
(4) RSA (sign only)
(5) Elgamal (encrypt only)
(6) RSA (encrypt only)
(7) DSA (set your own capabilities)
(8) RSA (set your own capabilities)
Your selection? 8
Possible actions for a RSA key: Sign Encrypt Authenticate
Current allowed actions: Sign Encrypt
(S) Toggle the sign capability
(E) Toggle the encrypt capability
(A) Toggle the authenticate capability
(Q) Finished
Your selection? A
Possible actions for a RSA key: Sign Encrypt Authenticate
Current allowed actions: Sign Encrypt Authenticate
(S) Toggle the sign capability
(E) Toggle the encrypt capability
(A) Toggle the authenticate capability
(Q) Finished
Your selection? S
Possible actions for a RSA key: Sign Encrypt Authenticate
Current allowed actions: Encrypt Authenticate
(S) Toggle the sign capability
(E) Toggle the encrypt capability
(A) Toggle the authenticate capability
(Q) Finished
Your selection? E
Possible actions for a RSA key: Sign Encrypt Authenticate
Current allowed actions: Authenticate
(S) Toggle the sign capability
(E) Toggle the encrypt capability
(A) Toggle the authenticate capability
(Q) Finished
Your selection? Q
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) 3y
Key expires at Sat 26 Oct 2019 08:56:53 AM PDT
Is this correct? (y/N) y
Really create? (y/N) y
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
....+++++
.......+++++
pub 4096R/B05E3F37 created: 2016-10-26 expires: never usage: SC
trust: ultimate validity: ultimate
sub 4096R/BF4CEBBA created: 2016-10-26 expires: 2019-10-26 usage: E
sub 2048R/45F1A70C created: 2016-10-26 expires: 2019-10-26 usage: S
sub 4096R/AF3A5B1C created: 2016-10-26 expires: 2019-10-26 usage: A
[ultimate] (1). John Smith <john.smith@example.com>
gpg> save
It's a good idea to generate a revocation cert:
$ gpg -a --output /media/revocation.asc --gen-revoke EA57D591B05E3F37
sec 4096R/B05E3F37 2016-10-26 John Smith <john.smith@example.com>
Create a revocation certificate for this key? (y/N) y
Please select the reason for the revocation:
0 = No reason specified
1 = Key has been compromised
2 = Key is superseded
3 = Key is no longer used
Q = Cancel
(Probably you want to select 1 here)
Your decision? 1
Enter an optional description; end it with an empty line:
> Revocation certificate generated when key itself was; if this cert is used then i no longer have access to the key and it should be treated as compromised
>
Reason for revocation: Key has been compromised
Revocation certificate generated when key itself was; if this cert is used then i no longer have access to the key and it should be treated as compromised
Is this okay? (y/N) y
You need a passphrase to unlock the secret key for
user: "John Smith <john.smith@example.com>"
4096-bit RSA key, ID B05E3F37, created 2016-10-26
gpg: gpg-agent is not available in this session
Revocation certificate created.
Please move it to a medium which you can hide away; if Mallory gets
access to this certificate he can use it to make your key unusable.
It is smart to print this certificate and store it away, just in case
your media become unreadable. But have some caution: The print system of
your machine might store the data and make it available to others!
Finally you need to export an offline copy of your keys:
gpg -a --export EA57D591B05E3F37 > /media/public-key.asc
gpg -a --export-secret-key EA57D591B05E3F37 > /media/master-key.asc
gpg -a --export-secret-subkeys EA57D591B05E3F37 > /media/secret-sub-keys.asc
Subsequent usage of live environment
When I boot the machine I copy over my gpg.conf from an SD card and import my keys:
mount /dev/mmcblk0 /media
mkdir ~/.gnupg
cp /media/gpg.conf ~/.gnupg/gpg.conf
gpg --import /media/public-key.asc
gpg --import /media/master-key.asc
(I actually have that scripted, so i mount /media and run /media/init).
At that point you can perform whatever changes needed, for example gpg --edit-key.
When i've done making changes i export the keys back to the SD card. Again, there is a bash script at /media/save:
cp ~/.gnupg/gpg.conf /media/gpg.conf
gpg -a --export > /media/public-key.asc
gpg -a --export-secret-key > /media/master-key.asc
gpg -a --export-secret-subkeys > /media/secret-sub-keys.asc
(You'll want to give those commands your key id).