Skip to main content

NSA Snoop and Error Proof Your Archives

An Englishman's Home is His Castle:

In 1760, William Pitt (the Elder) made a famous declaration of this right. "The poorest man may in his cottage bid defiance to all the force of the Crown.  It may be frail, its roof may shake, the wind may blow through it. The rain may enter. The storms may enter.  But the king of England may not enter.  All his forces dare not cross the threshold of the ruined tenement."

There is many a ruined castle in Europe.  You need to defend your castle if you want it to keep its value!

You should also consider the problem of data corruption, since any single bit error in an encrypted archive, will render the whole thing unreadable.

Also remember this: Always tar a directory, never tar a bunch of files. 

It is extremely annoying when you get an archive from someone, untar it and end up with a million files scattered throughout your home directory...

Encrypting an Archive with GPG

An easy way to secure your data against unwarranted NSA and GCHQ snoops is with gpgAlways use gpg before uploading archives to a public file server such as DropBox, Copy or Evernote.

Using pipes, you can string the tape archiver tar, bzip2 and gpg together like this:
$ tar -cj test | gpg -c >test.tar.bz2.gpg

(or use gpg -er username for key based encryption)

and to reverse it:

$ gpg -d < test.tar.bz2.gpg | tar -xj

The key configuration of GPG is shown in another post. 


Forward Error Correction

A good way to protect your data against corruption is with a Reed Solomon forward error correcting code - it is used on CDROMs.  There is a little known utility available that does it, called rsbep by Guido Fiala and you can get the source using this link:

Then build it the usual way with ./configure; make; sudo make install.  It is part of the BSD and Debian distributions.

Using pipes, you can string the tape archiver tar, bzip2, rsbep and gpg together like this:

$ tar -cj test | gpg -c | rsbep >

(or use gpg -er username for key based encryption)

and to reverse it:

$ rsbep -d < | gpg -d | tar -xj

The Reed Solomon code will protect your archive against error bursts and will help to ensure that you can read the data back from a failing archive system, many years later.


Parity Bits

There is also a utility called par2 (and the handy GUI PyPar2) which adds Reed Solomon parity bits in a series of separate files.  These are in the par2cmdline and pypar2 packages.  This can be used to protect any files, but when you copy things around, you got to remember to pass the parity data along too.

Preventive Measures Using Par2

In order to ensure that I get into and stay in the habit of using gpg, I created a couple of scripts, so I can simply do:

$ targpg directory
$ untargpg directory

and the scripts will handle the messy details.

I tested the error recovery by corrupting the archive with hexedit - it works like magic!

Make an archive script like this called /usr/local/bin/targpg:
#! /bin/bash
echo Make an encrypted archive of a directory
tar -cj "$1" | gpg -c >"$1.tar.bz2.gpg"
md5sum "$1.tar.bz2.gpg" > " $1.tar.bz2.gpg.md5"
parcreate -n1 "$1.tar.bz2.gpg"
ls -al "$1"*

as well as untargpg:
#! /bin/bash
echo Untar a GPG encrypted archive $1
RESULT=$(md5sum -c "$1.tar.bz2.gpg.md5")

if [ "$RESULT" != "$1.tar.bz2.gpg: OK" ]
  echo MD5 error - Attempt a repair
  par2repair $1.tar.bz2.gpg.par2

# Try to decrypt and untar regardless   
# because the md5 and par2 files may be missing
gpg -d < "$1.tar.bz2.gpg" | tar -xj
ls -al "$1"*

and then one day when your disk goes south, par2repair may save the day.

La voila!


Popular posts from this blog

Parasitic Quadrifilar Helical Antenna

This article was reprinted in OSCAR News, March 2018: If you want to receive Satellite Weather Pictures , then you need a decent antenna, otherwise you will receive more noise than picture. For polar orbit satellites, one needs an antenna with a mushroom shaped radiation pattern .  It needs to have strong gain towards the horizon where the satellites are distant, less gain upwards where they are close and as little as possible downwards, which would be wasted and a source of noise.  Most satellites are spin stabilized and therefore the antenna also needs circular polarization, otherwise the received signal will flutter as the antennas rotate through nulls. The helical antenna, first proposed by Kraus in 1948, is the natural solution to circular polarized satellite communications.  It is a simple twisted wire - there seems to be nothing to it.  Various papers have been published on helix antennas, so the operation is pretty well understood. Therefore,

Weather Satellite Turnstile Antennas for the 2 meter Band

NEC2, 2 m band, 146 MHz, Yagi Turnstile Simulation and Build This article describes a Turnstile Antenna for the 2 meter band, 146 MHz amateur satcom, 137 MHz NOAA and Russian Meteor weather satellites.  Weather satellite reception is described here .  A quadrifilar helical antenna is described here .   Engineering, is the art of making what you need,  from what you can get. Radiation Pattern of the Three Element Yagi-Uda Antenna Once one combine and cross two Yagis, the pattern becomes distinctly twisted. The right hand polarization actually becomes visible in the radiation pattern plot, which I found really cool. Radiation Pattern of Six Element Turnstile Antenna Only a true RF Geek can appreciate the twisted invisible inner beauty of a herring bone antenna... Six Element Turnstile Antenna Essentially, it is three crosses on a stick.  The driven elements are broken in the middle at the drive points.  The other elements can go straight throug

Patch Antenna Design with NEC2

The older free Numerical Electromagnetic Code version 2 (NEC2) from Lawrence Livermore Lab assumes an air dielectric.  This makes it hard (but not impossible) for a radio amateur to experiment with Printed Circuit Board Patch antennas and micro strip lines. Air Spaced Patch Antenna Radiation Pattern You could use the free ASAP simulation program , which handles thin dielectrics, you could shell out a few hundred Dollars for a copy of NEC4 , You could buy GEMACS if you live in the USA, or you could add distributed capacitors to a NEC2 model with LD cards (hook up one capacitor in the middle of each element.), but that is far too much money/trouble for most. More information on driving an array antenna can be found here: l Air Dielectric Patch   The obvious lazy solution is to accept the limitation and make an air dielectric patch antenna. An advantage of using air dielectric, is that the antenn