Why does gpg use so much entropy from /dev/random?

[Philip Potter]

Thanks very much, I didn't know about the --gen-random command and the
quality level option. I'll have a look at the source code and see if I can
understand further.

Can you set the quality level for other generation commands, or just for
--gen-random?


On 31 March 2013 18:33, Hauke Laging <mailinglisten at hauke-laging.de> wrote:

> Am So 31.03.2013, 10:45:54 schrieb Philip Potter:
>
> > GPG uses /dev/random as its entropy source. It pulls a lot of entropy
> from
> > this source. More entropy, in fact, than the linux /dev/random manpage
> > suggests it should. Quoting from the manpage:
>
> I don't know the gpg source, the following (3) is just a guess.
>
>
> > Recently when generating a 2048-bit key, I got a message that GPG needed
> > 280 *bytes* more entropy. This is far more than 256 bits.
>
> 1) If you don't do anything special then two keys are generated (mainkey
> and
> subkey).
>
> 2) A 2048 bit RSA key is supposed to be as secure as a 112 bit symmetric
> key.
> I don't know whether you can map a 112 bit symmetric key directly to RSA
> key
> values. You need find primes after all. Maybe the algorithm to do that
> consumes additional entropy.
>
> 3) Who knows how random the /dev/random output really is? I guess that the
> entropy quality can be increased by consuming more ("make one good bit
> from 16
> bad bits").
>
> strace -e trace=open,read gpg --armor --gen-random 0 16
> [...]
> open("/dev/urandom", O_RDONLY)          = 3
> read(3, "\332\376J\314\1[\357\n7ee\303\372\3555h", 16) = 16
>
>
> strace -e trace=open,read gpg --armor --gen-random 1 16
> [...]
> open("/dev/urandom", O_RDONLY)          = 3
> read(3, "\3471=\307+n\3656\204\31!\232\270\303\324[", 16) = 16
>
> (Strange. Werner, have I found a bug? :-)  )
>
>
> strace -e trace=open,read gpg --armor --gen-random 2 16
> [...]
> open("/dev/random", O_RDONLY)           = 4
> read(4, "\1\362P\231..."..., 300) = 128
> read(4, "+7m\2314|\353..."..., 172) = 128
> read(4, "\233\272~\237\..."..., 44) = 44
>
> So we see: If high quality entropy is required then gpg reads
> (128+128+44)/16=18.75 times as much entropy from /dev/random as demanded.
>
>
> Hauke
> --
> ☺
> PGP: 7D82 FB9F D25A 2CE4 5241 6C37 BF4B 8EEF 1A57 1DF5 (seit 2012-11-04)
> http://www.openpgp-schulungen.de/
>
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