On Mon, 2016-08-01 at 13:48 +0200, Nikos Mavrogiannopoulos wrote: > This is an updated patch reflecting the recent discussion in linux- > crypto: > http://www.mail-archive.com/linux-crypto@xxxxxxxxxxxxxxx/msg20400.html Hi, I'm resending the patch with few typo fixes, and adding Ted in CC for review. Ted would you like to review this patch for the random(4) manpage? regards, Nikos
From 7952cf6bbabf36a4be50a3ba953278077f7b5157 Mon Sep 17 00:00:00 2001 From: Nikos Mavrogiannopoulos <nmav@xxxxxxxxxx> Date: Thu, 7 Apr 2016 09:08:14 +0200 Subject: [PATCH] Update the random(4) documentation towards a more accurate view on /dev/urandom This documents the "property" of /dev/urandom of being able to serve numbers prior to pool being initialized, and removes any suggested usages of /dev/random which are disputable (i.e., one-time pad). Document the fact /dev/random is a legacy interface and only suitable for applications which can afford indeterminate delays since very few applications can do so. Signed-off-by: Nikos Mavrogiannopoulos <nmav@xxxxxxxxxx> --- man4/random.4 | 51 ++++++++++++++++++++++++++------------------------- 1 file changed, 26 insertions(+), 25 deletions(-) diff --git a/man4/random.4 b/man4/random.4 index b67c46f..c7afaf4 100644 --- a/man4/random.4 +++ b/man4/random.4 @@ -13,8 +13,9 @@ .\" 2004-04-08, AEB, Improved description of read from /dev/urandom .\" 2008-06-20, George Spelvin <linux@xxxxxxxxxxx>, .\" Matt Mackall <mpm@xxxxxxxxxxx> -.\" Add a Usage subsection that recommends most users to use -.\" /dev/urandom, and emphasizes parsimonious usage of /dev/random. +.\" 2016-10-20, Nikos Mavrogiannopoulos <nmav@xxxxxxxxxx> +.\" Mention that /dev/random is a legacy interface and removed suggested +.\" uses of /dev/random. .\" .TH RANDOM 4 2016-10-08 "Linux" "Linux Programmer's Manual" .SH NAME @@ -37,11 +38,22 @@ The generator also keeps an estimate of the number of bits of noise in the entropy pool. From this entropy pool random numbers are created. .LP -When read, the \fI/dev/random\fP device will return random bytes -only within the estimated number of bits of noise in the entropy -pool. -\fI/dev/random\fP should be suitable for uses that need very -high quality randomness such as one-time pad or key generation. +When read, the \fI/dev/urandom\fP device return random bytes using a pseudorandom +number generator seeded from the entropy pool. That operation is +non-blocking. When used during early boot time, this device may return +data prior to the entropy pool being initialized. +If this is of concern in your application, use +.BR getrandom(2) +or \fI/dev/random\fP instead. + +.LP +The \fI/dev/random\fP device is a legacy interface which dates back to +a time where the cryptographic primitives used in the implementation +were not widely trusted. It will return random bytes +only within the estimated number of bits of fresh noise in the entropy +pool, blocking if necessary. +\fI/dev/random\fP is suitable for applications that need very +high quality randomness, and can afford indeterminate delays. When the entropy pool is empty, reads from \fI/dev/random\fP will block until additional environmental noise is gathered. If @@ -60,18 +72,8 @@ will return -1 and .I errno will be set to .BR EAGAIN . -.LP -A read from the \fI/dev/urandom\fP device will not block -waiting for more entropy. -If there is not sufficient entropy, a pseudorandom number generator is used -to create the requested bytes. -As a result, in this case the returned values are theoretically vulnerable to a -cryptographic attack on the algorithms used by the driver. -Knowledge of how to do this is not available in the current unclassified -literature, but it is theoretically possible that such an attack may -exist. -If this is a concern in your application, use \fI/dev/random\fP -instead. + +The flag .B O_NONBLOCK has no effect when opening .IR /dev/urandom . @@ -82,6 +84,8 @@ for the device signals will not be handled until after the requested random bytes have been generated. + + Since Linux 3.16, .\" commit 79a8468747c5f95ed3d5ce8376a3e82e0c5857fc a @@ -104,14 +108,11 @@ This means that it will impact the contents read from both files, but it will not make reads from \fI/dev/random\fP faster. .SS Usage -If you are unsure about whether you should use +The .IR /dev/random -or +interface is considered a legacy interface, and .IR /dev/urandom , -then probably you want to use the latter. -As a general rule, -.IR /dev/urandom -should be used for everything except long-lived GPG/SSL/SSH keys. +is recommended for general use. If a seed file is saved across reboots as recommended below (all major Linux distributions have done this since 2000 at least), the output is -- 2.7.4
