GPG keys for multiple email accounts

[Hauke Laging]

Am Sa 06.07.2013, 19:00:47 schrieb atair:
> (1) I create one key pair for each email account. In case one key gets
> compromised the possible damage is limited to one email account.

That's a strange argument for several reasons. The most important being: Why 
should just one key be compromised if they are used on the same system?

Wouldn't it make more sense to put the saved effort for creating 19 additional 
keys into securing the system, making it less probable that the key gets 
compromised?


> However, as drawback I'd have to (1) remember 20 passphrases (with for
> example 20-40 characters each)

Even with the default settings a 19-digits passphrase (upper and lower case 
ASCII letters and digits) is as hard as AES (without flaws). If the passphrase 
is completely random then it is ridiculous to make it longer than 19 chars 
(unless you store it in two halves (with about 18 chars each) in different 
places).

You need only one good passphrase for all your offline-mainkeys. It makes 
little sense to protect a key with a huge passphrase in the system on which 
the key is used (unless the system does not have disk encryption). The real 
threat to keys in this situation are online attacks. And if someone manages to 
break into your system then he probably manages to read your keyboard input, 
too. gpg-agent (as a running process) is not really protected by default, too.


> and (2) type them every time I want to
> read the emails. This does not seem to be very convenient... (*)

You can write a script which reads the passphrase once and loads it into gpg-
agent for several keys.


> (2) I create one key with several sub keys for each email account.

That doesn't make sense. User IDs belong to email addresses. Subkeys belong to 
their mainkey only (on the same level as user IDs belong to a mainkey).


> (3) I create independent keys (with several sub keys) for groups of
> email accounts (private/official/work/...).

My recommendation:
Separate keys by email address type:

a) private (one group)
b) each business separate
c) each organization separate

Also separate the private addresses by

a) security level (some may not need OpenPGP at all; some may be read via 
webmail others only on systems you control)
b) seriousness (hauke.laging at example.org maybe should not be grouped with 
superman123 at rpgchat.example.net)


> (4) I create independent keys (without sub keys) and use one key for
> multiple email accounts.

You should NEVER use mainkeys outside a safe environment (boot from CD/DVD). 
Only subkeys should be used on normal systems.


> (*) additionally, all senders of emails to me would have to choose the
> right keys for the account to send the email to. (related to 3.)

That's not the problem. The problem is that you have others to verify all your 
keys. If all your contacts use just one of your addresses each then that is 
not a big difference though. But you should always have some slips of paper 
with your fingerprint with you. That may be a bit annoying for 20 separate 
keys.


> 2. Maintenance:
> Usually, I keep all (important) old emails locally on my hard disk.
> But how should this be done with encrypted emails since the private
> key might get lost or compromised one day? So far, I think it would be
> necessary to decrypt all emails before archiving and store them
> (unencrypted) on the encrypted (LUKS etc.) hard disk.

Why should the risk of losing the private key be higher than the risk of 
losing access to the LUKS volume? If your key is protected by an 18-chars 
passphrase then you can put a backup on your web site.


> Does it create problems to attach a fake email
> address to the key (e.g. @example.com)?

Problems like not being taken seriously?


> Since I read my emails on laptop and PC, I need to copy the private
> key to both computers. This is against the normal intention of a
> "private key".

Says who?


> How is/should this be usually done?

If you do not trust the channel (SSH, USB stick) then you make sure that the 
passphrase is hard enough before you copy it.


Hauke
-- 
Crypto für alle: http://www.openpgp-schulungen.de/fuer/bekannte/
OpenPGP: 7D82 FB9F D25A 2CE4 5241 6C37 BF4B 8EEF 1A57 1DF5
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