GNU bug report logs - #6683
mktemp foo.XXXXXXXXXXX is not sufficiently random

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#6683: closed (mktemp foo.XXXXXXXXXXX is not sufficiently random)
Date: Mon, 08 Aug 2011 07:41:02 +0000

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Your message dated Mon, 08 Aug 2011 00:39:30 -0700
with message-id <4E3F92B2.4010906 <at> cs.ucla.edu>
and subject line Re: bug#6683: mktemp foo.XXXXXXXXXXX is not sufficiently random
has caused the GNU bug report #6683,
regarding mktemp foo.XXXXXXXXXXX is not sufficiently random
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


-- 
6683: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=6683
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From: Paul Eggert <eggert <at> CS.UCLA.EDU>
To: Bug Coreutils <bug-coreutils <at> gnu.org>
Subject: mktemp foo.XXXXXXXXXXX is not sufficiently random
Date: Tue, 20 Jul 2010 10:21:11 -0700

While looking at the random-number stuff I found a theoretical
randomness bug in mktemp.  The mktemp command currently uses 8 bytes
of randomness to generate a file name, so with an invocation like
this:

$ mktemp foo.XXXXXXXXXXX

the file name is not sufficiently random.  There are 62 possibilities
for each X, so one needs log2(62**11) random bits to generate a random
11-character value for the Xs, which is about 65.5 bits, but we are
generating only 64 bits.  The more Xs, the more randomness is needed,
so the bug gets more "serious" as the number of Xs grows.

Here's a simple patch to fix this.  Should I install this by
generating a new gl/lib/tempname.c.diff by hand, and pushing that?

--- old/tempname.c	2010-07-20 09:41:36.774229000 -0700
+++ new/tempname.c	2010-07-20 10:14:33.391452000 -0700
@@ -245,7 +245,7 @@ gen_tempname_len (char *tmpl, int suffix
   XXXXXX = &tmpl[len - x_suffix_len - suffixlen];
 
   /* Get some more or less random data.  */
-  rand_src = randint_all_new (NULL, 8);
+  rand_src = randint_all_new (NULL, x_suffix_len);
   if (! rand_src)
     return -1;
 

Here's a fancier patch that uses fewer random bits, but on
futher thought I don't think it's worth the extra machine
instructions for a purely-theoretical bug:

--- old/tempname.c	2010-07-20 09:41:36.774229000 -0700
+++ new/tempname.c	2010-07-20 09:45:00.685972000 -0700
@@ -19,6 +19,7 @@
 
 #if !_LIBC
 # include <config.h>
+# include <limits.h>
 # include "tempname.h"
 # include "randint.h"
 #endif
@@ -189,6 +190,17 @@ check_x_suffix (char const *s, size_t le
 static const char letters[] =
 "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
 
+/* Upper bound on the number bytes of random information needed to
+   generate N random letters.  There are 62 letters, and 2**6 is 64,
+   so 6N bits = 6N/CHAR_BIT bytes is an upper bound.  Return ceil (6.0
+   * N / CHAR_BIT) without rounding error or overflow.  */
+static size_t
+randomness_bound (size_t n)
+{
+  return ((n / CHAR_BIT) * 6
+	  + ((n % CHAR_BIT) * 6 + CHAR_BIT - 1) / CHAR_BIT);
+}
+
 /* Generate a temporary file name based on TMPL.  TMPL must match the
    rules for mk[s]temp (i.e. end in at least X_SUFFIX_LEN "X"s,
    possibly with a suffix).
@@ -245,7 +257,7 @@ gen_tempname_len (char *tmpl, int suffix
   XXXXXX = &tmpl[len - x_suffix_len - suffixlen];
 
   /* Get some more or less random data.  */
-  rand_src = randint_all_new (NULL, 8);
+  rand_src = randint_all_new (NULL, randomness_bound (x_suffix_len));
   if (! rand_src)
     return -1;
 

[Message part 3 (message/rfc822, inline)]

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Jim Meyering <jim <at> meyering.net>
Cc: 6683-done <at> debbugs.gnu.org
Subject: Re: bug#6683: mktemp foo.XXXXXXXXXXX is not sufficiently random
Date: Mon, 08 Aug 2011 00:39:30 -0700

On 08/07/2011 10:04 AM, Jim Meyering wrote:

> Yes, please do.

OK, thanks, I installed the one-line change as change to the diff.
This is the first time I've updated a diff file in a while (ever?),
so I hope I did it right.  I'm marking the bug done.

From 8e2767a3f0c279d355f067e53be2c63173959eb1 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert <at> cs.ucla.edu>
Date: Mon, 8 Aug 2011 00:29:46 -0700
Subject: [PATCH] mktemp: stir in enough entropy (Bug#6683)

* gl/lib/tempname.c.diff (gen_tempname_len):
Use x_suffix_len bytes' worth of entropy, not 8 bytes.
---
 gl/lib/tempname.c.diff |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/gl/lib/tempname.c.diff b/gl/lib/tempname.c.diff
index fcacf53..3e30c97 100644
--- a/gl/lib/tempname.c.diff
+++ b/gl/lib/tempname.c.diff
@@ -100,7 +100,7 @@ index 2da5afe..562955a 100644
 -  }
 -#endif
 -  value += random_time_bits ^ __getpid ();
-+  rand_src = randint_all_new (NULL, 8);
++  rand_src = randint_all_new (NULL, x_suffix_len);
 +  if (! rand_src)
 +    return -1;
 
-- 
1.7.4.4

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