[libmicrohttpd] [digest-auth]: bug in hash algorithm

From: 
Andreas Wehrmann <a.wehrmann@centersystems.com>
  To: 
libmicrohttpd@gnu.org
  Date: 
Today 08:58:43 am
   
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Full report:
Probability=No, score=-3.2 required=7.0 tests=AWL,BAYES_00 autolearn=ham version=3.2.5-tuminfo_1  
Hello!

I wrote a little testpage that I deliver using libmicrohttpd using 
digest authentication.
The testpage consists of four files (framed page + image file).
When I initially connected to the webserver via the browser it correctly 
challenged me
for my credentials. However, after entering the username and password 
the index file
got loaded but it happened that the browser then challenged me again for 
each
additional file to be loaded.
Since this is very annoying I tried increasing the nonce table size to 3000
(was default) but it was no good.
I then dug a little deeper and found out, that the hash algorithm to 
determine the index
for a given nonce always returned zero thus overwriting other nonces.
The offending line is at check_nonce_nc() in digestauth.c:313:

off = (off << 8) | (*np & (off >> 24));

whereas is should be:

off = (off << 8) | (*np ^ (off >> 24));

Since "off" is initialized with zero and an unsigned integer
a logical AND returns zero which is not right obviously.
After this fix, the server challenged me only once and I got "random" 
indices.
I found the problem in libmicrohttpd 0.9.5.

Best regards,
Andreas Wehrmann

-- 
Dipl.-Ing. (FH) Andreas Wehrmann
Software Development
--------------------------------------------------------------
Center Communication Systems GmbH
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AUTHORS

@@ -28,6 +28,7 @@ Geoffrey McRae <geoff@spacevs.com>
 Piotr Grzybowski <narsil.pl@gmail.com>
 Gerrit Telkamp <g.telkamp@domologic.de>
 Erik Slagter <erik@slagter.name>
+Andreas Wehrmann <a.wehrmann@centersystems.com>
 
 Documentation contributions also came from:
 Marco Maggi <marco.maggi-ipsu@poste.it>

ChangeLog

@@ -1,3 +1,8 @@
+Tue Jan 18 23:58:09 CET 2011
+	Fixing hash calculation in digest auth; old function had
+	collisions causing the browser to challenge users for
+	authentication too often. -CG/AW
+
 Fri Jan 14 19:19:45 CET 2011
 	Removing dead code, adding missing new symbols to export list.
 	Fixed two missing NULL checks after malloc operations. -CG

src/daemon/digestauth.c

@@ -310,7 +310,7 @@ check_nonce_nc (struct MHD_Connection *connection,
   np = nonce;
   while (*np != '\0')
     {
-      off = (off << 8) | (*np & (off >> 24));
+      off = (off << 8) | (*np ^ (off >> 24));
       np++;
     }
   off = off % mod;