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If the former, then if the private key can be used to digitally sign messages, why doesn't your client offer to encrypt this highly sensitive piece of data?

If the latter, then if someone had access to your private key alone, would they be able to decrypt all your messages?

Thanks in advance, I haven't been able to find a clear answer.

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If you fix the question i believe i know what you are asking and I will answer it. You want to know if you need the your pgp(or rather gpg) private key and passphrase to decrypt a message correct? – dc5553 Apr 28 '12 at 12:50
Not exactly. I want to know what the passphrase is used for, is it for encrypting the private key, or used in a function with the private key to decrypt the data. – Mike Gallagher Apr 28 '12 at 17:35
up vote 1 down vote accepted

In the end, is your quesiton: "Should I encrypt my Private Key when I email it to myself, or backup on cloud storage, or some other place?"

Then the answer is yes--absolutely encrypt your PGP Private Key before "backing it up" some place, (encrypt pre-generated Revocation Certificates as well).

An example to create an encrypted ASCII Armor .gpg file, from a plain text Private Key .asc file:

gpg --symmetric --cipher-algo=AES256 -a -o 20141022.PGP.66H049E4.prv.gpg 20141022.PGP.66H049E4.prv.asc 

Use a delete utility like, "shred," to delete the original .asc key file, otherwise it can be undeleted.

The Private Key is unusable without the password. However, that Password is a whole lot easier to crack than the Private Key itself.

There are a couple of attack trees employed to take advantage of unencrypted/unsecured Private Keys:

  1. Brute Force Attack: When trying to "crack" a PGP encrypted email, we don't try to brute force what the PGP Key is--especially if we have a copy of the Private Key on hand. a.) Instead, what we do is subject the Private Key, itself, to a Brute Force attack, to get the password. b.) With that password, we then use the Private Key, to decrypt messages--a far more efficient Attack Tree.
  2. Impersonation, Harassment: An Unencrypted Private Key can be "Imported" into a PGP store, (GPG, Kleopatra, Windows, etc. a.) This creates the appearance that a particular person used a specific device. In Forensic Information Investigation, a private key on a loaded on a machine is pretty compelling proof, but not dispositive, that a person used a device; In Addition, b.) Public keys can be exported, without the password, (but Revocation Keys cannot). In the end, you can successfully simulate a "footprint" of a user on a particular system, spoof emails from them with a public key attached, etc.

Using a strong Private Key password is the best practice, and will thwart intense Brute Force Attacks.

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The former. Your PGP private key is stored on your disk in encrypted form. In particular, it is encrypted using your passphrase.

To decrypt a file, PGP needs (1) your passphrase, and (2) the encrypted private key file; from these it can reconstitute your private key, and then decrypt the file. Neither alone is sufficient to use an encrypted private key file.

To answer your question, your client does encrypt your private key. It is stored on your disk in encrypted form.

(I am assuming you are using public-key encryption, not conventional encryption.)

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I would change your wording -- the private key alone is in fact sufficient to decrypt the message. The private key simply isn't accessible until the passphrase is used to decrypt it. However, with an unencrypted private key, one would not need the passphrase to decrypt messages (a bad idea). – mikebabcock May 31 '12 at 21:02

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