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[OT] openPGP: keeping primary key offline?

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  [OT] openPGP: keeping primary key offline? Marco Möller <talby@debianlists.mobilxpress.net> - 2021-06-22 10:50 +0200
    Re: [OT] openPGP: keeping primary key offline? Andrei POPESCU <andreimpopescu@gmail.com> - 2021-06-22 12:20 +0200
    Re: [OT] openPGP: keeping primary key offline? Teemu Likonen <tlikonen@iki.fi> - 2021-06-22 13:10 +0200

#236264 — [OT] openPGP: keeping primary key offline?

Regarding openPGP usage, it is recommended in several user guides to 
keep the primary key offline and keeping on the main computer only 
subkeys. The argumentation is that if keeping it on the main computer 
then it could be used in an unauthorized way.
Of course, what is offline, like a good backup, is safer than what is 
kept online, simply because it cannot be accessed on the computer.
But, would it really be so easy for someone to to use a primary key? 
Isn't the primary key stored passphrase protected, thus stored encrypted?
Except the case that an attacker would be in my system and actively 
spying on me with a keylogger and other bad tools, I would assume that 
if someone would simply steel (copy and carry away) my complete ~/.gnupg 
folder with the primary key and revocation certificates somewhere being 
stored in there, the usage of these keys and certificates would only be 
possible after decryption upon the correct answer at the passphrase 
request? Isn't it like this?

Marco.

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#236265

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On Ma, 22 iun 21, 10:46:58, Marco Möller wrote:
> Regarding openPGP usage, it is recommended in several user guides to keep
> the primary key offline and keeping on the main computer only subkeys. The
> argumentation is that if keeping it on the main computer then it could be
> used in an unauthorized way.
> Of course, what is offline, like a good backup, is safer than what is kept
> online, simply because it cannot be accessed on the computer.
> But, would it really be so easy for someone to to use a primary key? Isn't
> the primary key stored passphrase protected, thus stored encrypted?
> Except the case that an attacker would be in my system and actively spying
> on me with a keylogger and other bad tools, I would assume that if someone
> would simply steel (copy and carry away) my complete ~/.gnupg folder with
> the primary key and revocation certificates somewhere being stored in there,
> the usage of these keys and certificates would only be possible after
> decryption upon the correct answer at the passphrase request? Isn't it like
> this?

Yes, for using the key. They could also just use your revocation 
certificate for a denial-of-service attack (as far as I recall these are 
not protected by a passphrase).

However, with a copy of your private key an attacker can try to 
brute-force your passphrase. Because this is done locally they can throw 
whatever processing power they have access to at the problem[1].

For most other passwords this is not an issue, because an attacker 
trying to brute-force a password will quickly find themselves rate 
limited and/or banned and/or the account will be locked after a certain 
number of tries.

So it's basically a defence-in-depth approach, which may be relatively 
easy for you to implement (the master key is needed only to issue or 
revoke sub-keys and sign other keys), but increases the difficulty 
significantly for an attacker.

[1] obligatory XKCD: https://xkcd.com/538/

Hope this helps,
Andrei
-- 
http://wiki.debian.org/FAQsFromDebianUser

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#236270

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* 2021-06-22 10:46:58+0200, Marco Möller wrote:

> Regarding openPGP usage, it is recommended in several user guides to 
> keep the primary key offline and keeping on the main computer only 
> subkeys.

I have read some of such guides. They imply considerations about
relevant threats but unfortunately they often fail to define their
threat model. They also don't acknowledge the fact that their threats
are not relevant to all people. Crypto nerds like to create perfect (?)
security systems.

A user certainly can keep his primary key outside his usual key ring. It
may build an extra barrier (together with passphrase) for others to
steal and use the key -- or for the owner to lose it. But if you don't
consider this a relevant threat then there is no need to keep your
primary key far and offline. Key management guides don't know what is
relevant to you.

-- 
/// Teemu Likonen - .-.. https://www.iki.fi/tlikonen/
// OpenPGP: 4E1055DC84E9DFF613D78557719D69D324539450

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