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Explained in detail: How magstripes are encoded, i


NOTICE: TO ALL CONCERNED Certain text files and messages contained on this site deal with activities and devices which would be in violation of various Federal, State, and local laws if actually carried out or constructed. The webmasters of this site do not advocate the breaking of any law. Our text files and message bases are for informational purposes only. We recommend that you contact your local law enforcement officials before undertaking any project based upon any information obtained from this or any other web site. We do not guarantee that any of the information contained on this system is correct, workable, or factual. We are not responsible for, nor do we assume any liability for, damages resulting from the use of any information on this site.
Card-O-Rama: Magnetic Stripe Technology and Beyond
or
"A Day in the Life of a Flux Reversal"

Written by

oooOO Count Zero OOooo
Restricted Data Transmissions

November 99, 1999

Look in your wallet. Chances are you own at least 3 cards that have magnetic
stripes on the back. ATM cards, credit cards, calling cards, frequent flyer
cards, ID cards, passcards,...cards, cards, cards! And chances are you have
NO idea what information is on those stripes or how they are encoded. This
detailed document will enlighten you and hopefully spark your interest in
this fascinating field. None of this info is "illegal"...but MANY
organizations (the government, credit card companies, security firms, etc.)
would rather keep you in the dark. Also, many people will IMMEDIATELY
assume that you are a CRIMINAL if you merely "mention" that you are
"interested in how magnetic stripe cards work." Watch yourself, ok? Just
remember that there is nothing wrong with wanting to know how things work,
although in our present society, you may be labelled a "deviant"
(or worse, <gasp> a "hacker")!

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Anyway, I will explain in detail how magstripes are encoded and give several
examples of the data found on some common cards. I will also cover the
technical theory behind magnetic encoding, and discuss magnetic encoding
alternatives to magstripes (Wiegand, barium ferrite). Non-magnetic card
technology (bar code, infrared, etc.) will be described. Finally, there will
be an end discussion on security systems and the ramifications of emergent
"smartcard" and biometric technologies.

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*DISCLAIMER*

Use this info to EXPLORE, not to EXPLOIT. This text is presented for
informational purposes only, and I cannot be held responsible for anything
you do or any consequences thereof. I do not condone fraud, larceny,
or any other criminal activities.

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_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_

*A WARNING*

Lately, I've noticed a few "books" and "magazines" for sale that were filled
with files on a variety of computer topics. These file were originally
released into the Net with the intention of distributing them for free.
However, these files are now being packaged and sold for profit. This really
pisses me off. I am writing this to be shared for free, and I ask no
payment. Feel free to reprint this in hardcopy format and sell it if you must
but no profits must be made. Not a f***ing dime ,Deutschmark, Punt, Lira,
Pound, or Centime! If anyone reprints this file and tries to sell it for a
profit, I will hunt you down and make your life miserable.
?????????????????????????????????????????????????????????????????????????????
How?
?????????????????????????????????????????????????????????????????????????????
Use your imagination. The reality will be worse.

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_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_

** MAGSTRIPE FIELDS, HEADS, ENCODING/READING **

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Now, I'll get down to business!

First, I am going to explain the basics behind fields, heads, encoding and
reading. Try and absorb the theory behind encoding/reading. This will help
you greatly if you ever decide to build your own encoder/reader from scratch
(more on that later). Ferromagnetic materials are substances that retain
magnetism after an external magnetizing field is removed. This principle is
the basis of all magnetic recording and playback. Magnetic poles always occur
in pairs within magnetized material, and magnetic flux lines emerge from the
north pole and terminate at the south. The elemental parts of megstripes are
ferromagnetic particles about 20 millionths of an inch long, each of which acts
like a tiny bar magnet. These particles are rigidly held together by a resin
binder. The magnetic particles are made by companies which make coloring
pigments for the paint industry, and are usually called pigments. When making
the magstripe media, the elemental magnetic particles are aligned with their
North-South axes parallel to the magnetic stripe by means of an external
magnetic fields while the binder hardens.

These particles are actually permanent bar magnets with two stable polarities.
If a magnetic particle is placed in a strong external magnetic field of the
opposite polarity, it will reverse its own polarity (North becomes South,
South becomes North). The external magnetic field strength required to
produce this flip is called the coercive force, and is a measure of the
coercivity of the particle. Magnetic pigments are available in a variety of
coercivities (more on that later on).

An unencoded magstripe is actually a series of North-South magnetic domains
(see Figure 1). The adjacent N-S fluxes merge, and the entire stripe acts as a
single bar magnet with North and South poles at its ends.

Figure 1: N-S.N-S.N-S.N-S.N-S.N-S.N-S.N-S <-particles in stripe
---------
represented as-> N-----------------------------S

However, if a S-S interface is created somewhere on the stripe, the fluxes will
repel, and we get a concentration of flux lines around the S-S interface (same
with N-N interface). Encoding consists of creating S-S and N-N interfaces, and
reading consists of (you guessed it) detecting 'em. The S-S and N-N interfaces
are called flux transitions, or flux reversals.

||| ||| <-flux lines
Figure 2: N------------N-N-S-S-----------------S
--------- flux lines -> ||| |||

The external magnetic field used to flip the polarities is produced by a
solenoid, which can reverse its polarity by reversing the direction of current.
An encoding head solenoid looks like a bar magnet bent into the shape of a ring
so that the North/South poles are very close and face each other across a tiny
gap. The field of the solenoid is concentrated across this gap, and when
elemental magnetic particles of the magstripe are exposed to this field, they
polarize to the opposite (unlike poles attract). Movement of the stripe past
the solenoid gap during which the polarity of the solenoid is reversed will
produce a single flux reversal (see Figure 3). To erase a magstripe, the
encoding head is held at a constant polarity and the entire stripe is moved
past it. No flux reversals, no data.

| | <----wires leading to solenoid
| | (wrapped around ring)
/-|-|-\??, ?-?-?-? ? ?? / \
Figure 3: | | <----solenoid (has JUST changed polarity)

--------- \ /
\ N S / <---gap in ring.. NS polarity across gap
N----------------------SS-N-------------------------S
U???F ?h ??x ; t
?? ?????? ?x +??? U??h??FX-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-X
 
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