Received-SPF: pass (google.com: best guess record for domain of linux-ext4-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: Linux 5.3-rc8
To:     "Theodore Y. Ts'o" <tytso@mit.edu>,
        Martin Steigerwald <martin@lichtvoll.de>
Cc:     Willy Tarreau <w@1wt.eu>, Matthew Garrett <mjg59@srcf.ucam.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Ahmed S. Darwish" <darwish.07@gmail.com>,
        Vito Caputo <vcaputo@pengaru.com>,
        Lennart Poettering <mzxreary@0pointer.de>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Jan Kara <jack@suse.cz>, Ray Strode <rstrode@redhat.com>,
        William Jon McCann <mccann@jhu.edu>,
        zhangjs <zachary@baishancloud.com>, linux-ext4@vger.kernel.org,
        lkml <linux-kernel@vger.kernel.org>
References: <CAHk-=wgs65hez6ctK7J2k46BdQzvKU5avExPOTTJsZu6iqA-ow@mail.gmail.com>
 <C4F7DC65-50B9-4D70-8E9B-0A6FF5C1070A@srcf.ucam.org>
 <20190917052438.GA26923@1wt.eu> <2508489.jOnZlRuxVn@merkaba>
 <20190917121156.GC6762@mit.edu>
From:   "Alexander E. Patrakov" <patrakov@gmail.com>
Message-ID: <6601569a-f339-bc2c-0459-5548a21d0595@gmail.com>
Date:   Tue, 17 Sep 2019 18:11:04 +0500
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17.09.2019 17:11, Theodore Y. Ts'o =D0=BF=D0=B8=D1=88=D0=B5=D1=82:
> There are only two ways out of this mess.  The first option is we take
> functionality away from a userspace author who Really Wants A Secure
> Random Number Generator.  And there are an awful lot of programs who
> really want secure crypto, becuase this is not a hypothetical.  The
> result in "Mining your P's and Q's" did happen before.  If we forget
> the history, we are doomed to repeat it.

You cannot take away functionality that does not really exist. Every=20
time somebody tries to use it, there is a huge news, "the boot process=20
is blocked on application FOO", followed by an insecure fallback to=20
/dev/urandom in the said application or library.

Regarding the "Mining your P's and Q's" paper: I would say it is a=20
combination of TWO faults, only one of which (poor, or, as explained=20
below, "marginally poor" entropy) is discussed and the other one (not=20
really sound crypto when deriving the RSA key from the=20
presumedly-available entropy) is ignored.

The authors of the paper factored the weak keys by applying the=20
generalized GCD algorithm, thus looking for common factors in the RSA=20
public keys. For two RSA public keys to be detected as faulty, they must =

share exactly one of their prime factors. In other words: repeated keys=20
were specifically excluded from the study by the paper authors.

Sharing only one of the two primes means that that the systems in=20
question behaved identically when they generated the first prime, but=20
diverged (possibly due to the extra entropy becoming available) when=20
they generated the second one. And asking the randomness for p and for q =

separately is what I would call the application bug here that nobody=20
wants to talk about: both p and q should have been derived from a CSPRNG =

seeded by a single read from a random source. If that practice were=20
followed, then it would either result in a duplicate key (which is not=20
as bad as a factorable one), or in completely unrelated keys.

--=20
Alexander E. Patrakov


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