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Re[2]: Crypto Question
From: Vishal <dhrakol () myrealbox com>
Date: Mon, 17 Nov 2003 17:14:27 -0500

Hi N407ER

Saturday, November 15, 2003, 8:36:58 PM, you wrote:

N> If I'm not mistaken, though, the passphrase on the PGP private key is
N> simply a bit of symmetric-key encryption to help protect your private 
N> key in the event that the key itself is compromized.

The passphrase is hashed to produce a symmetric key that is then used to
encrypt your private key.

N> But if you've got your key secured on, say, a CD in a locked drawer, and
N> you send an e-mail encrypted with that key, the passphrase (or lack) is
N> irrelevent;

Not quite. Say someone wanted to impersonate you by digitally signing a
message. Or read a confidential message you had been sent. He would need your
private key in both these scenarios. And if he managed to break into that
drawer, the only thing standing between your private key and him is the
passphrase. A strong passphrase can make it orders of magnitude more difficult
to access your private key either using brute force or dictionary attacks
against the passphrase.

Drawer locks aren't usually too difficult to pick. If you had that CD in a
locked safe in a locked room guarded by utterly incorruptible guards, you
might be justified in picking a weak passphrase. Since it's a drawer, backing
up that protection with a strong passphrase is a good idea if maintaining your
privacy is important to you.

N> an attacker would still have to break the RSA encryption, of
N> which the only current known mean is bruteforce. The passphrase really only
N> comes into play if your private key is compromized; e.g. the attacker
N> breaks into your system and steals your key. Am I incorrect in this
N> assumption?

Since brute force attacks are impractical for the average attacker, the way he
will get around the system is by accessing your private key. This makes the
"if your private key is compromised" scenario very much the issue in question,
since that is where his efforts will be directed.

N> I've never really looked at the internal workings of PGP (but I
N> was under the impression its fairly stock RSA).

Not always RSA. A number of different symmetric(IDEA, 3DES etc.) and asymmetric
(RSA, DH/DSS etc.) ciphers are used. DH/DSS gained popularity due to licensing
issues with RSA in recent years.

Cheers,

-Vishal 


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