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First, some crypto-relevant info theory:
Encrypted data is uniformly distributed, i.e., has maximal entropy per symbol;
Raw, uncompressed data is typically redundant, i.e., has sub-maximal entropy.
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.
Ueli Maurer's <quote>Universal Statistical Test for Random Bit Generators</quote>(<link xlink:href=""> <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.
We also need a way to capture the raw network data. A program called <citerefentry><refentrytitle>tcpdump</refentrytitle><manvolnum>1</manvolnum></citerefentry> 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>.
The command:
<userinput>tcpdump -c 4000 -s 10000 -w <replaceable>dumpfile.bin</replaceable></userinput>
will capture 4000 raw packets to <replaceable>dumpfile.bin</replaceable>. Up to 10,000 bytes per packet will be captured in this example.
The Experiment
Here is the experiment:
Open a window to an IPsec host and another window to an insecure host.
Now start <link linkend="tcpdump">capturing packets</link>.
In the <quote>secure</quote> window, run the <trademark class="registered">UNIX</trademark> command <citerefentry><refentrytitle>yes</refentrytitle><manvolnum>1</manvolnum></citerefentry>, which will stream the <literal>y</literal> character. After a while, stop this. Switch to the insecure window, and repeat. After a while, stop.
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.
<prompt>%</prompt> <userinput>tcpdump -c 4000 -s 10000 -w <replaceable>ipsecdemo.bin</replaceable></userinput>
<prompt>%</prompt> <userinput>uliscan <replaceable>ipsecdemo.bin</replaceable></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 -----------------
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.
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.
Installing IPsec
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 <citerefentry><refentrytitle>setkey</refentrytitle><manvolnum>8</manvolnum></citerefentry> command.
A comprehensive guide on running IPsec on FreeBSD is provided in <link xlink:href="@@URL_RELPREFIX@@/doc/en_US.ISO8859-1/books/handbook/ipsec.html">FreeBSD Handbook</link>.
This needs to be present in the kernel config file in order to capture network data with <citerefentry><refentrytitle>tcpdump</refentrytitle><manvolnum>1</manvolnum></citerefentry>. Be sure to run <citerefentry><refentrytitle>config</refentrytitle><manvolnum>8</manvolnum></citerefentry> after adding this, and rebuild and reinstall.
device bpf
Maurer's Universal Statistical Test (for block size=8 bits)
You can find the same code at <link xlink:href=""> this link</link>.
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

ULISCAN filename
outputs to stdout

#define L 8
#define V (1&lt;&lt;L)
#define Q (10*V)
#define K (100 *Q)
#define MAXSAMP (Q + K)

#include &lt;stdio.h&gt;
#include &lt;math.h&gt;

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 &lt; 2) {
printf("Usage: Uliscan filename\n");
} else {
printf("Measuring file %s\n", argv[1]);

fptr = fopen(argv[1],"rb");

if (fptr == NULL) {
printf("Can't find %s\n", argv[1]);

for (i = 0; i &lt; V; i++) {
table[i] = 0;

for (i = 0; i &lt; 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 &amp;&amp; i &lt; Q + K; i++) {
j = i;
b = fgetc(fptr);

if (b &lt; 0)
run = 0;

if (run) {
if (table[b] &gt; 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 &lt; (int)(sum*8.0 + 0.50); i++)


/* refill initial table */
if (0) {
for (i = 0; i &lt; Q; i++) {
b = fgetc(fptr);
if (b &lt; 0) {
run = 0;
} else {
table[b] = i;


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(itstool) path: step/para
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articles/ipsec-must.pot, string 29