MM code: Difference between revisions

Revision as of 12:25, 10 January 2008

Revision as of 12:42, 10 January 2008

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Automated Teller Machines which can read the MM code contain a special MM box which contains the circuitry to read the MM code. This box sits more than an inch away from the card. It is unclear whether the MM code is read optically from a distance during acquisition of the card from the slot, or if there is some kind of read head positioned closer to the card and connected to the MM Box. Cash machine manufacturers do not access or service the box. The MM code consists of two components, one stored on the magnetic stripe, and one hidden. During MM code verification, a cryptographic operation is performed to check that the MM code on the magnetic stripe corresponds to the hidden one. The presence of the keyed cryptographic operation means that the correct MM code for a counterfeit cannot be calculated from the magnetic stripe information alone without knowledge of the key -- it must be read from the original card itself.

In order to remain effective, the MM code reliesrelied on the continued obscurity of the reading mechanism, and possibly also on the expense and difficulty of embedding a code once known. Since the arrival of the [[EMV]] chip-base payment protocols, the MM code has reduced significance in combatting card counterfeiting.

== Operating principle ==

== Misunderstandings about MM Code encoding ==

The MM feature is encoded in the middle layer of an [[ISO 7810]] as a [[bar code]] formed by two materials with different electrical properties.<ref name="ct1996"/> A capacitive sensor head near the magstripe reader observes the alternating capacity as the card is moved past the sensor and decodes the represented number. This sensor works in a similar fashion to the magnetic read head found in a magstripe card reader, except that it senses not a change in [[magnetic flux]], but a change in the [[dielectric constant]] of the card's material. It reads a second data stripe that, unlike the magstripe, cannot easily be rewritten with off-the-shelf equipment.

Public information on the MM code is somewhat limited. A variety of theories have been proposed, some of which are the result of confusion between the MM code and other proprietary anti-counterfeiting schemes.

== Related technologies ==

The first class of explanation proposes that the MM code is encoded into the magnetic stripe using read and write heads operating diagonally to the direction of swipe in the reader. With appropriate signal processing, these can read and encode a small amount of additional data which is polarised in a different axis to the ISO standard tracks.

In addition to capacitive MM code, which has been widely used in Germany since the early 1980s, a range of similar technologies have been proposed or patented, but have never been widely deployed in ATM cards:

The second class concerns encoding the code onto the plastic base of the card using special inks (probably a bar code), or reading a code which is inherently embedded as part of the plastic manufacturing process for each batch. Such a code may only be visible under infrared illumination (or other invisible wavelength).

The*'''Angle first classmodulation of explanation proposes thatferromagnetic theparticles:''' MMA code is encodedembedded into the magnetic stripe using read and write heads operating diagonally to the direction of swipe in the reader. With appropriate signal processing, these can read and encode a small amount of additional data which is polarised in a different axis to the ISO standard tracks.

A third class considers encoding the code using the electrical properties of the plastic card, such as capacitance. Such an explanation requires a read head within close proximity to the card.

*'''Infrared barcodes:''' The second class concerns encoding the code onto the plastic base of the card using special inks (probably a bar code), or reading a code which is inherently embedded as part of the plastic manufacturing process for each batch. Such a code may only be visible under infrared illumination (or other invisible wavelength).

==References==