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The title pretty much explains it all: I have a very (read: very) strong passphrase for my private key. How reckless can I be with my .gpg directory? Put another way, how many bits of entropy would I need in my passphrase to safely post my .gpg directory on the web? Put yet another way, how strong is the encryption algorithm protecting my secret key?

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You don't need a strong passphrase, you just need to reduce the validity time of your keys to something like 1 month. – BatchyX Jan 12 '13 at 9:28
up vote 4 down vote accepted

This is not a statement about the security of public key pairs or gpg encryption, but WRT entropy and passwords, given a password taken from the range of ASCII alphanumeric characters (A-Z, a-z, 0-9), the possible number of combinations in 16 characters is:

n = 62^16 = 47672401706823533450263330816

If I have your key and try to "brute force" the password by peeling through all those possibilities, 1 million times per second, then:

n / 1000000 / 3600 / 8760 = 1511681941489838

3600 being the number of seconds in an hour and 8760 being the number of hours in a year, it could, as a worst case scenario, take more than 1.5e13 centuries (1.5 million aeons).

Which is why it is very naive to believe that anyone tries to break a password that way. Passwords are important and strong passwords are better than weak passwords, but having "a very (read: very) strong passphrase" is not going to protect you any better from attempts to steal the password, spoof exchanges, or other methods which are not simply about brute forcing the passphrase. An analogy might be cancer -- there are people who've never smoked, eat well, exercise, always wear sunscreen, etc., and still get cancer. That doesn't mean you might as well run out and eat plutonium.

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"Spoof exchanges"? Also, I may have given the wrong impression in my question. I'm not trying to be reckless, I am just a bit paranoid. I don't want anyone to get my keys, but I also want redundancy in case my hard disk burns out. So, a slightly better phrasing might go along the lines of, "If I want to put my keys on potentially insecure media such as USB drives, web storage services such as Dropbox, etc., can I just use the default encryption on my keys or do I need to do more?" – InkBlend Oct 30 '14 at 4:49
By "spoof exchanges" I presume I meant something like man-in-the-middle style attacks -- having someone exploit a weakness in a mechanism in order to trick you/your software into giving things away inappropriately. I'm not a crypto expert; my thing about probability reflects ideally how a password should work, and how a key ideally should work, but WRT to the latter even, e.g. AES might have attacks that reduce this ideal relationship to probability (i.e., cracks that are not brute force) but are still too absurd to be generally viable. If that stuff was truly broken, presumably... – goldilocks Oct 30 '14 at 8:25
...they'd let us know. So whatever the default password encryption on your key is, it can't be no good what-so-ever. WRT stunts such as "safely post my .gpg directory on the web", probably you do want to learn and do some more research. I kinda presumed someone who knew more would post a more interesting answer. Not that I think this one is wrong, it is just a common sensical thing highlighting that central concept of what exponential growth in possibilities means to brute force. Oh and not to eat plutonium anyway ;) – goldilocks Oct 30 '14 at 8:29

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