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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

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The former. Your PGP private key is stored on your disk in encrypted form. In particular, It is encrypted under 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 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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