Length for AES256 symmetric encryption passphrase?
Robert J. Hansen
rjh at sixdemonbag.org
Fri Apr 4 22:14:09 CEST 2014
> To be clear, I want to use gnupgp to do symmetric encryption using
> AES256. As I understand it, the 'gpg -symmetric ...' command
> converts a pass phrase into a key, a 32 byte key in the case of
> I /assume/ that this conversion is 'deterministic' since as far as
> I can tell, the 'gpg -symmetric ...' does not store the key it
> generates. Correct me if I'm wrong.
> I am trying to decide how long a pass phrase to use. I have not
> decided how to generate the pass phrase. Assume that it is
> pseudo-randomly chosen from the an english language character set.
Then this becomes pretty straightforward. :) Let's say you use the
upper- and lower-case letters, the digits 0 through 9, as well as the
'+' and '/' marks. This character set is commonly called 'base64',
since there are 64 symbols in the set.
Using the equation René provided and I polished a bit, you have:
3 * 256 <-- 256: size of the key in bits
L = -----------
10 * log 64 <-- 64: how many letters are in your set
... 43 characters.
A quick back-of-the-envelope calculation confirms this to be the case.
base64 is known to have six bits of entropy per character. 6 * 43 =
258 bits. At 43 characters you're providing GnuPG with 258 bits of
entropy to use in creating a 256-bit symmetric key.
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