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<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE article PUBLIC "-//FreeBSD//DTD DocBook XML V5.0-Based Extension//EN"
"../../../share/xml/freebsd50.dtd">
<!--
The FreeBSD Documentation Project
$FreeBSD$
-->
<article xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" version="5.0" xml:lang="en">
<info><title>Independent Verification of IPsec Functionality in FreeBSD</title>
<author><personname><firstname>David</firstname><surname>Honig</surname></personname><affiliation>
<address><email>honig@sprynet.com</email></address>
</affiliation></author>
<pubdate>1999-05-03</pubdate>
<legalnotice xml:id="trademarks" role="trademarks">
&tm-attrib.freebsd;
&tm-attrib.opengroup;
&tm-attrib.general;
</legalnotice>
<releaseinfo>$FreeBSD$</releaseinfo>
<abstract>
<para>You installed IPsec and it seems to be working. How do you
know? I describe a method for experimentally verifying that IPsec is
working.</para>
</abstract>
</info>
<sect1 xml:id="problem">
<title>The Problem</title>
<para>First, lets assume you have <link linkend="ipsec-install">
installed <emphasis>IPsec</emphasis></link>. How do you know
it is <link linkend="caveat">working</link>? Sure, your
connection will not work if it is misconfigured, and it will work
when you finally get it right. &man.netstat.1; will list it.
But can you independently confirm it?</para>
</sect1>
<sect1 xml:id="solution">
<title>The Solution</title>
<para>First, some crypto-relevant info theory:</para>
<orderedlist>
<listitem>
<para>Encrypted data is uniformly distributed, i.e., has maximal
entropy per symbol;</para>
</listitem>
<listitem>
<para>Raw, uncompressed data is typically redundant, i.e., has
sub-maximal entropy.</para>
</listitem>
</orderedlist>
<para>Suppose you could measure the entropy of the data to- and
from- your network interface. Then you could see the difference
between unencrypted data and encrypted data. This would be true
even if some of the data in <quote>encrypted mode</quote> was
not encrypted---as the outermost IP header must be if the
packet is to be routable.</para>
<sect2 xml:id="MUST">
<title>MUST</title>
<para>Ueli Maurer's <quote>Universal Statistical Test for Random
Bit Generators</quote>(<link xlink:href="http://www.geocities.com/SiliconValley/Code/4704/universal.pdf">
<acronym>MUST</acronym></link>) quickly measures the entropy
of a sample. It uses a compression-like algorithm. <link linkend="code">The code is given below</link> for a variant
which measures successive (~quarter megabyte) chunks of a
file.</para>
</sect2>
<sect2 xml:id="tcpdump">
<title>Tcpdump</title>
<para>We also need a way to capture the raw network data. A
program called &man.tcpdump.1; lets you do this, if you have
enabled the <emphasis>Berkeley Packet Filter</emphasis>
interface in your <link linkend="kernel">kernel's config
file</link>.</para>
<para>The command:</para>
<screen><userinput>tcpdump -c 4000 -s 10000 -w dumpfile.bin</userinput></screen>
<para>will capture 4000 raw packets to
<replaceable>dumpfile.bin</replaceable>. Up to 10,000 bytes per
packet will be captured in this example.</para>
</sect2>
</sect1>
<sect1 xml:id="experiment">
<title>The Experiment</title>
<para>Here is the experiment:</para>
<procedure>
<step>
<para>Open a window to an IPsec host and another window to an
insecure host.</para>
</step>
<step>
<para>Now start <link linkend="tcpdump">capturing
packets</link>.</para>
</step>
<step>
<para>In the <quote>secure</quote> window, run the &unix;
command &man.yes.1;, which will stream the <literal>y</literal>
character. After a while, stop this. Switch to the
insecure window, and repeat. After a while, stop.</para>
</step>
<step>
<para>Now run <link linkend="code">MUST</link> on the
captured packets. You should see something like the
following. The important thing to note is that the secure
connection has 93% (6.7) of the expected value (7.18), and
the <quote>normal</quote> connection has 29% (2.1) of the
expected value.</para>
<screen>&prompt.user; <userinput>tcpdump -c 4000 -s 10000 -w ipsecdemo.bin</userinput>
&prompt.user; <userinput>uliscan ipsecdemo.bin</userinput>
Uliscan 21 Dec 98
L=8 256 258560
Measuring file ipsecdemo.bin
Init done
Expected value for L=8 is 7.1836656
6.9396 --------------------------------------------------------
6.6177 -----------------------------------------------------
6.4100 ---------------------------------------------------
2.1101 -----------------
2.0838 -----------------
2.0983 -----------------</screen>
</step>
</procedure>
</sect1>
<sect1 xml:id="caveat">
<title>Caveat</title>
<para>This experiment shows that IPsec <emphasis>does</emphasis>
seem to be distributing the payload data
<emphasis>uniformly</emphasis>, as encryption should. However,
the experiment described here <emphasis>cannot</emphasis>
detect many possible flaws in a system (none of which do I have
any evidence for). These include poor key generation or
exchange, data or keys being visible to others, use of weak
algorithms, kernel subversion, etc. Study the source; know the
code.</para>
</sect1>
<sect1 xml:id="IPsec">
<title>IPsec---Definition</title>
<para>Internet Protocol security extensions to IPv4; required for
IPv6. A protocol for negotiating encryption and authentication
at the IP (host-to-host) level. SSL secures only one application
socket; <application>SSH</application> secures only a login;
<application>PGP</application> secures only a specified file or
message. IPsec encrypts everything between two hosts.</para>
</sect1>
<sect1 xml:id="ipsec-install">
<title>Installing IPsec</title>
<para>Most of the modern versions of FreeBSD have IPsec support
in their base source. So you will need to include the
<option>IPSEC</option> option in your kernel config and, after
kernel rebuild and reinstall, configure IPsec connections using
&man.setkey.8; command.</para>
<para>A comprehensive guide on running IPsec on FreeBSD is
provided in <link xlink:href="&url.books.handbook;/ipsec.html">FreeBSD
Handbook</link>.</para>
</sect1>
<sect1 xml:id="kernel">
<title>src/sys/i386/conf/KERNELNAME</title>
<para>This needs to be present in the kernel config file in order
to capture network data with &man.tcpdump.1;. Be sure
to run &man.config.8; after adding this, and rebuild and
reinstall.</para>
<programlisting>device bpf</programlisting>
</sect1>
<sect1 xml:id="code">
<title>Maurer's Universal Statistical Test (for block size=8
bits)</title>
<para>You can find the same code at <link xlink:href="http://www.geocities.com/SiliconValley/Code/4704/uliscanc.txt">
this link</link>.</para>
<programlisting>/*
ULISCAN.c ---blocksize of 8
1 Oct 98
1 Dec 98
21 Dec 98 uliscan.c derived from ueli8.c
This version has // comments removed for Sun cc
This implements Ueli M Maurer's "Universal Statistical Test for Random
Bit Generators" using L=8
Accepts a filename on the command line; writes its results, with other
info, to stdout.
Handles input file exhaustion gracefully.
Ref: J. Cryptology v 5 no 2, 1992 pp 89-105
also on the web somewhere, which is where I found it.
-David Honig
honig@sprynet.com
Usage:
ULISCAN filename
outputs to stdout
*/
#define L 8
#define V (1<<L)
#define Q (10*V)
#define K (100 *Q)
#define MAXSAMP (Q + K)
#include <stdio.h>
#include <math.h>
int main(argc, argv)
int argc;
char **argv;
{
FILE *fptr;
int i,j;
int b, c;
int table[V];
double sum = 0.0;
int iproduct = 1;
int run;
extern double log(/* double x */);
printf("Uliscan 21 Dec 98 \nL=%d %d %d \n", L, V, MAXSAMP);
if (argc < 2) {
printf("Usage: Uliscan filename\n");
exit(-1);
} else {
printf("Measuring file %s\n", argv[1]);
}
fptr = fopen(argv[1],"rb");
if (fptr == NULL) {
printf("Can't find %s\n", argv[1]);
exit(-1);
}
for (i = 0; i < V; i++) {
table[i] = 0;
}
for (i = 0; i < Q; i++) {
b = fgetc(fptr);
table[b] = i;
}
printf("Init done\n");
printf("Expected value for L=8 is 7.1836656\n");
run = 1;
while (run) {
sum = 0.0;
iproduct = 1;
if (run)
for (i = Q; run && i < Q + K; i++) {
j = i;
b = fgetc(fptr);
if (b < 0)
run = 0;
if (run) {
if (table[b] > j)
j += K;
sum += log((double)(j-table[b]));
table[b] = i;
}
}
if (!run)
printf("Premature end of file; read %d blocks.\n", i - Q);
sum = (sum/((double)(i - Q))) / log(2.0);
printf("%4.4f ", sum);
for (i = 0; i < (int)(sum*8.0 + 0.50); i++)
printf("-");
printf("\n");
/* refill initial table */
if (0) {
for (i = 0; i < Q; i++) {
b = fgetc(fptr);
if (b < 0) {
run = 0;
} else {
table[b] = i;
}
}
}
}
}</programlisting>
</sect1>
</article>
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