from MD4 to MD5 (http://rsync.samba.org/ftp/rsync/src/rsync-3.0.0-NEWS).
My understanding is that MD5 is a more secure, slower version of MD4 but I
am not convinced that the added security of MD5 would alone have merited
the change from MD4 (particularly since MD4 is ~30% faster than MD5). I
wonder if I am missing other reasons which made the change
necessary/desirable?
I am looking at ways to optimise Rsync (for speed) hence my interest in
this,
Thanks,
Nick
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<br><font size=2 face="sans-serif">I don't know why
rsync made this move.
My guess is that it does not look good for rsync to use a discredited algorithm.
See http://tools.ietf.org/html/draft-turner-md4-to-historic-00.</font>
<br>
<br><font size=2 face="sans-serif">Creating secure hashing
functions is
notoriously difficult. Several times algorithms previously thought secure
have been shown to be vunerable to certain attacks. MD5 has also been discovered
to be vunerable. See the article "MD5 considered harmful
today"
at http://www.win.tue.nl/hashclash/rogue-ca.</font>
<br>
<br><font size=2 face="sans-serif">So the question is,
does rsync need
a hashing algorithm that is cryptographically secure? I suppose it's due
in part to the likelyhood of different chunks hashing to the same value.
With the MD5 vunerability one has to specially engineer it. IMO it is extremely
unlikely that it would happen by chance when used by rsync. If anyone worries
about this then maybe rsync would move to SHA-1 at some point. And then
what if someone finds a problem with SHA-1? Indeed, Bruce Schneier has
an article on this at
http://www.schneier.com/blog/archives/2005/02/sha1_broken.html.
Again, I reckon that the SHA-1 vunerability would have no practical effect
if SHA-1 was used in rsync. Just my $0.02.</font>
<br>
<br><font size=2 face="sans-serif">rsync uses the hashing
function to fingerprint
the chunks. I do not see why this needs to have all the strengths and safeguards
of a cryptographic algorithm. Unless rsync is supposed to be defending
against protocol attack? Is it? I didn't think so but I could be wrong,
I don't know enough about this bit of the rsync code. If it is trying to
defend against this then IMO it should be using an HMAC rather than just
a hash code. Assuming it doesn't need these strengths/safeguards then maybe
it should use a cheaper (i.e. quicker) hashing algorithm.</font>
<br>
<br><font size=2 face="sans-serif">Regards,<br>
<br>
Andrew Marlow<br>
</font>
<br>
<br>
<br>
<table width=100%>
<tr valign=top>
<td width=33%><font size=4 face="helv"><b>Internet
</b></font>
<br><font size=1 face="sans-serif"><b>Nick.McCarthy
at replify.com</b></font>
<p><font size=1 face="sans-serif"><b>Sent by:
rsync-bounces at lists.samba.org</b></font>
<p><font size=1 face="sans-serif">04/08/2010
09:46</font>
<td width=66%>
<table width=100%>
<tr valign=top>
<td>
<div align=right><font size=1
face="sans-serif">To</font></div>
<td><font size=1 face="sans-serif">rsync at
lists.samba.org</font>
<tr valign=top>
<td>
<div align=right><font size=1
face="sans-serif">cc</font></div>
<td>
<tr valign=top>
<td>
<div align=right><font size=1
face="sans-serif">Subject</font></div>
<td><font size=1 face="sans-serif">Optimising the Rsync
algorithm for speed
by reverting to MD4 hashing</font></table>
<br>
<table>
<tr valign=top>
<td>
<td></table>
<br></table>
<br>
<br>
<br><font size=2 face="Calibri">Hi,</font>
<br><font size=2 face="Calibri"> </font>
<br><font size=2 face="Calibri">From v3.0.0 onwards the
hash function implemented
by Rsync was changed from MD4 to MD5 (</font><a
href="http://rsync.samba.org/ftp/rsync/src/rsync-3.0.0-NEWS"><font
size=2 color=blue
face="Arial"><u>http://rsync.samba.org/ftp/rsync/src/rsync-3.0.0-NEWS</u></font></a><font
size=2 face="Arial">).
My understanding is that MD5 is a more secure, slower version of MD4 but
I am not convinced that the added security of MD5 would alone have merited
the change from MD4 (particularly since MD4 is ~30% faster than MD5). I
wonder if I am missing other reasons which made the change
necessary/desirable?</font>
<br><font size=2 face="Arial"> </font>
<br><font size=2 face="Arial">I am looking at ways to
optimise Rsync (for
speed) hence my interest in this,</font>
<br><font size=2 face="Arial"> </font>
<br><font size=2 face="Arial">Thanks,</font>
<br><font size=2 face="Arial"> </font>
<br><font size=2 face="Arial">Nick</font>
<br><font size=2
face="Calibri"> </font><font
size=2><tt>-- <br>
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<br><pre>
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</pre>
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