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><A
NAME="HASH-FUNCTION"
>5.2. Available checksum functions</A
></H1
><P
> A cryptographic hash function is a one-way function
<B
CLASS="COMMAND"
>H(<TT
CLASS="REPLACEABLE"
><I
>foo</I
></TT
>)</B
> such
that it is easy to compute
<B
CLASS="COMMAND"
>H(<TT
CLASS="REPLACEABLE"
><I
>foo</I
></TT
>)</B
> from
<TT
CLASS="REPLACEABLE"
><I
>foo</I
></TT
>, but
infeasible to compute <TT
CLASS="REPLACEABLE"
><I
>foo</I
></TT
>
from <B
CLASS="COMMAND"
>H(<TT
CLASS="REPLACEABLE"
><I
>foo</I
></TT
>)</B
>, or
to find <TT
CLASS="REPLACEABLE"
><I
>bar</I
></TT
> such that
<B
CLASS="COMMAND"
>H(<TT
CLASS="REPLACEABLE"
><I
>bar</I
></TT
>)</B
> =
<B
CLASS="COMMAND"
>H(<TT
CLASS="REPLACEABLE"
><I
>foo</I
></TT
>)</B
> (which would allow
to replace <TT
CLASS="REPLACEABLE"
><I
>foo</I
></TT
> with
<TT
CLASS="REPLACEABLE"
><I
>bar</I
></TT
> without changing the hash function).</P
><P
> One common usage of a such a hash function is the
computation of <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>checksums</I
></SPAN
>
of files, such that any modification of a file can be noticed,
as its checksum will change. </P
><P
> For computing checksums of files, and also for some other purposes,
<SPAN
CLASS="APPLICATION"
>samhain</SPAN
> uses the TIGER hash function
developed by Ross Anderson
and Eli Biham. The output of this function is 192 bits long, and the
function can be implemented efficiently on 32-bit and 64-bit machines.
Technical details can be found at
<A
HREF="http://www.cs.technion.ac.il/~biham/Reports/Tiger/"
TARGET="_top"
>this
page</A
>.</P
><P
> As of version 1.2.10, also the MD5 and SHA-1 hash functions are available.
(You need to set the option <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>DigestAlgo=MD5</I
></SPAN
> or
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>DigestAlgo=SHA1</I
></SPAN
> in the config file to enable this).
Note that MD5 is somewhat faster, but because of security concerns
it is not recommended anymore for new applications.</P
><P
> As of version 3.1.0, also the SHA2-256 hash function is available.
(You need to set the option <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>DigestAlgo=SHA256</I
></SPAN
>
in the config file to enable this).</P
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