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><A
NAME="KEYPAD"
>11.2. Integrity of the samhain executable</A
></H1
><P
> Each samhain executable contains a compiled-in key, that is used
when the signatures of emails and/or logfile entries are verified.
By default, a cryptographically strong random key
is generated by the <B
CLASS="COMMAND"
>configure</B
> script at compile time.
Thus, each build is unique, and signature verification will fail
if a different build is used, except if the compiled-in key was set
to a common value for both builds.
</P
><P
> To set a
user-defined key, there is an option
</P
><P
> <B
CLASS="COMMAND"
>./configure --enable-base=<TT
CLASS="REPLACEABLE"
><I
>B1,B2</I
></TT
></B
>
</P
><P
> where B1,B2 should be two integers in the range 0...2147483647.
</P
><P
> The key generated by <B
CLASS="COMMAND"
>configure</B
> is printed in
the configure script's output. It is recommended that you save this key
and use it for further builds.
</P
><P
> Whenever you try to verify the integrity of e-mails or log file entries,
this compiled-in key is used (to be more specific: the signature key
is encrypted with a one-time pad generated from the message itself and
the compiled-in key). As a result, if executable B is used to
verify the integrity of e-mails sent by executable A, <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>integrity
verification
will fail</I
></SPAN
> if the compiled-in keys of A and B do not match.
This can be used to
check the integrity of A in a straightforward way (check e-mails on another
host, using a different executable compiled with the same key).
</P
><P
> Obviously, this scheme can be broken, but it requires an intruder to
disassemble/decompile and analyze the existing
<SPAN
CLASS="APPLICATION"
>samhain</SPAN
> executable, rather
than simply replace it with a precompiled trojan.
</P
><P
> However, if you use a <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>precompiled</I
></SPAN
>
<SPAN
CLASS="APPLICATION"
>samhain</SPAN
> executable (e.g. from a
binary distribution), in principle a prospective intruder could easily
obtain a copy of the executable and analyze it in advance. This will
enable her/him to generate fake audit trails and/or generate
a trojan for this particular binary distribution.
</P
><P
> For this reason, it is possible for the user to add more key material into
the binary executable. This is done with the command:
</P
><P
> <B
CLASS="COMMAND"
>samhain --add-key=<TT
CLASS="REPLACEABLE"
><I
>key@/path/to/samhain_executable</I
></TT
></B
>
</P
><P
> This will read the file <TT
CLASS="FILENAME"
>/path/to/samhain_executable</TT
>,
add the key <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>key</I
></SPAN
>,
which can be a string of arbitrary length, except that it should not
contain a '@' (because it has a special meaning, separating
key from path), and write the new binary to the location
<TT
CLASS="FILENAME"
>/path/to/executable.out</TT
> (i.e.
with .out appended).
</P
><DIV
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>For Clarification</B
></TH
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> </TD
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><P
>Please note that --add-key does not replace a compiled-in key but only
adds to it. Integrity verification depends on both the compiled-in and
any added key material, and integrity verification using different binaries
will therefore only work if all were compiled with the same key and
had the same extra key material (if any) added in.
</P
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>WARNING</B
></TH
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><TD
> </TD
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><P
> Using a precompiled samhain executable from a binary
package distribution is not recommended unless you add in key material as
described above.
</P
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