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Date: Thu, 7 Dec 2017 23:53:32 +0100
From: Solar Designer <solar@...nwall.com>
To: oss-security@...ts.openwall.com
Subject: Re: Recommendations GnuPG-2 replacement
On Thu, Dec 07, 2017 at 10:01:34PM +0100, Solar Designer wrote:
> On Thu, Dec 07, 2017 at 06:32:11AM +0000, halfdog wrote:
> > Thus the Debian switch from gpg1 to gpg2 just introduced efforts
> > fiddling with functionality I do not need and cannot disable,
> > provides a keymanagement that cannot be configured easily to
> > protect against the threats it should mitigate (theft of key material)
> > and creating additional attack surface without any recognizable
> > benefit.
>
> I think the benefit is being on a version upstream intends to maintain
> to a greater extent and for a longer time. For example, when yet
> another side-channel leak was reported against GnuPG 1 & 2 recently,
> upstream officially patched it for GnuPG 2 only and said that GnuPG 1
> probably contains many other side-channel leaks anyway:
>
> http://openwall.com/lists/oss-security/2017/07/06/8
Turns out upstream shortly released GnuPG 1.4.22 fixing this. It's good
news, which I had missed.
Noteworthy changes in version 1.4.22 (2017-07-19)
-------------------------------------------------
* Mitigate a flush+reload side-channel attack on RSA secret keys
dubbed "Sliding right into disaster". For details see
<https://eprint.iacr.org/2017/627>. [CVE-2017-7526]
* Fix some minor bugs.
> Are you saying "--s2k-count" option to "gpg2" is ignored, and moreover
> that this is documented? gnupg-2.1.23/doc/gpg.texi says (formatted):
>
> `--s2k-count `n''
> Specify how many times the passphrase mangling is repeated. This
> value may range between 1024 and 65011712 inclusive. The default
> is inquired from gpg-agent. Note that not all values in the
> 1024-65011712 range are legal and if an illegal value is selected,
> GnuPG will round up to the nearest legal value. This option is
> only meaningful if `--s2k-mode' is 3.
I should have looked at GnuPG 2.2.3, but its description of the above
option is the same, and it describes the corresponding option to
gpg-agent as follows:
'--s2k-count N'
Specify the iteration count used to protect the passphrase. This
option can be used to override the auto-calibration done by
default. The auto-calibration computes a count which requires
100ms to mangle a given passphrase.
To view the actually used iteration count and the milliseconds
required for an S2K operation use:
gpg-connect-agent 'GETINFO s2k_count' /bye
gpg-connect-agent 'GETINFO s2k_time' /bye
To view the auto-calibrated count use:
gpg-connect-agent 'GETINFO s2k_count_cal' /bye
Looks sane to me, and this might also answer your question:
> > PS: I do not know, how much the gpg-agent calibration under
> > increased system load reduced the KDF complexity, as I failed
> > to extract the KDF rounds value from the gpg data structures,
> > but the value seems to be at least below 70ms due to total time
> > measurements for gpg-agent (math, interprocess communication,
> > filesystem) to unlock a key on an idle system.
Alexander
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