From: Sasha Levin Subject: Re: [PATCH] random: add blocking facility to urandom Date: Wed, 07 Sep 2011 21:12:59 +0300 Message-ID: <1315419179.3576.6.camel@lappy> References: <1314974248-1511-1-git-send-email-jarod@redhat.com> <1315417137-12093-1-git-send-email-jarod@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: linux-crypto@vger.kernel.org, Matt Mackall , Neil Horman , Herbert Xu , Steve Grubb , Stephan Mueller , lkml To: Jarod Wilson Return-path: Received: from mail-ww0-f44.google.com ([74.125.82.44]:51823 "EHLO mail-ww0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754347Ab1IGSNX (ORCPT ); Wed, 7 Sep 2011 14:13:23 -0400 In-Reply-To: <1315417137-12093-1-git-send-email-jarod@redhat.com> Sender: linux-crypto-owner@vger.kernel.org List-ID: On Wed, 2011-09-07 at 13:38 -0400, Jarod Wilson wrote: > Certain security-related certifications and their respective review > bodies have said that they find use of /dev/urandom for certain > functions, such as setting up ssh connections, is acceptable, but if and > only if /dev/urandom can block after a certain threshold of bytes have > been read from it with the entropy pool exhausted. Initially, we were > investigating increasing entropy pool contributions, so that we could > simply use /dev/random, but since that hasn't (yet) panned out, and > upwards of five minutes to establsh an ssh connection using an > entropy-starved /dev/random is unacceptable, we started looking at the > blocking urandom approach. Can't you accomplish this in userspace by trying to read as much as you can out of /dev/random without blocking, then reading out of /dev/urandom the minimum between allowed threshold and remaining bytes, and then blocking on /dev/random? For example, lets say you need 100 bytes of randomness, and your threshold is 30 bytes. You try reading out of /dev/random and get 50 bytes, at that point you'll read another 30 (=threshold) bytes out /dev/urandom and then you'll need to block on /dev/random until you get the remaining 20 bytes. -- Sasha.