---
title: Independent Verification of IPsec Functionality in FreeBSD
authors:
  - author: David Honig
    email: honig@sprynet.com
description: Independent Verification of IPsec Functionality in FreeBSD
trademarks: ["freebsd", "opengroup", "general"]
tags: ["IPsec", "verification", "FreeBSD"]
---

= Independent Verification of IPsec Functionality in FreeBSD
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:images-path: articles/ipsec-must/

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[.abstract-title]
Abstract

You installed IPsec and it seems to be working.
How do you know? I describe a method for experimentally verifying that IPsec is working.

'''

toc::[]

[[problem]]
== The Problem

First, lets assume you have crossref::ipsec-must[ipsec-install, Installing IPsec].
How do you know it is crossref::ipsec-must[caveat, Caveat]? 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?

[[solution]]
== The Solution

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 "encrypted mode" was not encrypted---as the outermost IP header must be if the packet is to be routable.

[[MUST]]
=== MUST

Ueli Maurer's "Universal Statistical Test for Random Bit Generators"(https://web.archive.org/web/20011115002319/http://www.geocities.com/SiliconValley/Code/4704/universal.pdf[MUST]) quickly measures the entropy of a sample.
It uses a compression-like algorithm.
crossref::ipsec-must[code, Maurer's Universal Statistical Test (for block size8 bits)] for a variant which measures successive (~quarter megabyte) chunks of a file.

[[tcpdump]]
=== Tcpdump

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
_Berkeley Packet Filter_ interface in your crossref::ipsec-must[kernel,src/sys/i386/conf/KERNELNAME].

The command:

[source,shell]
....
 tcpdump -c 4000 -s 10000 -w dumpfile.bin
....

will capture 4000 raw packets to _dumpfile.bin_.
Up to 10,000 bytes per packet will be captured in this example.

[[experiment]]
== The Experiment

Here is the experiment:

[.procedure]
====
. Open a window to an IPsec host and another window to an insecure host.
. Now start crossref::ipsec-must[tcpdump, Tcpdump].
. In the "secure" window, run the UNIX(R) command man:yes[1], which will stream the `y` character. After a while, stop this. Switch to the insecure window, and repeat. After a while, stop.
. Now run crossref::ipsec-must[code, Maurer's Universal Statistical Test (for block size8 bits)] 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 "normal" connection has 29% (2.1) of the expected value.
+
[source,shell]
....
% tcpdump -c 4000 -s 10000 -w ipsecdemo.bin
% uliscan ipsecdemo.bin
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 -----------------
....
====

[[caveat]]
== Caveat

This experiment shows that IPsec _does_ seem to be distributing the payload data __uniformly__, as encryption should.
However, the experiment described here _cannot_ 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.

[[IPsec]]
== IPsec---Definition

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; SSH secures only a login; PGP secures only a specified file or message.
IPsec encrypts everything between two hosts.

[[ipsec-install]]
== Installing IPsec

Most of the modern versions of FreeBSD have IPsec support in their base source.
So you will need to include the `IPSEC` option in your kernel config and, after kernel rebuild and reinstall, configure IPsec connections using man:setkey[8] command.

A comprehensive guide on running IPsec on FreeBSD is provided in extref:{handbook}[FreeBSD Handbook, ipsec].

[[kernel]]
== src/sys/i386/conf/KERNELNAME

This needs to be present in the kernel config file to capture network data with man:tcpdump[1].
Be sure to run man:config[8] after adding this, and rebuild and reinstall.

[.programlisting]
....
device	bpf
....

[[code]]
== Maurer's Universal Statistical Test (for block size=8 bits)

You can find the same code at https://web.archive.org/web/20031204230654/http://www.geocities.com:80/SiliconValley/Code/4704/uliscanc.txt[this link].

[.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;
        }
      }
    }
  }
}
....