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Information on Automatic Teller Machines
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*************** Track Layouts ************************
This is off the top of my head, but is 99% there. Also I'll ignore
some obsolete stuff.
The physical layout of the cards are standard. The LOGICAL makeup
varies from institution to institution. There are some generally
followed layouts, but not mandatory.
There are actually up to three tracks on a card.
Track 1 was designed for airline use. It contains your name and
usually your account number. This is the track that is used when
the ATM greets you by name. There are some glitches in how things
are ordered so occasionally you do get "Greetings Bill Smith Dr."
but such is life. This track is also used with the new airline
auto check in (PSA, American, etc)
Track 3 is the "OFF-LINE" ATM track. It contains such nifty
information as your daily limit, limit left, last access, account
number, and expiration date. (And usually anything I describe in track
2). The ATM itself could have the ability to rewrite this track to
update information.
Track 2 is the main operational track for online use. The first thing
on track to is the PRIMARY ACCOUNT NUMBER (PAN). This is pretty
standard for all cards, though no guarantee. Some additional info
might be on the card such as expiration date. One interesting item
is the PIN offset. When an ATM verifies a PIN locally, it usually
uses an encryption scheme involving the PAN and a secret KEY.
This gives you a "NATURAL PIN" (i.e. when they mail you your pin, this
is how it got generated.) If you want to select your own PIN, they
would put the PIN OFFSET in the clear on the card. Just do modulo 10
arithmetic on the Natural PIN plus the offset, and you have the
selected PIN. YOUR PIN IS NEVER IN THE CLEAR ON YOUR CARD. Knowing
the PIN OFFSET will not give you the PIN. This will required the
SECRET KEY.
Hope that answers your question
************ Deposits at ATMs ************************
Deposits on ATM:
Various banks have various systems. As an example, at CITIbank
a deposit was made to a specific account. Your account was updated
with a MEMO update, i.e. it would show up on your balance. However
it did not become AVAILABLE funds until it was verified by a teller.
On the envelope was Customer ID number, the envelope number and
the Entered dollar amount, the branch # and the Machine #.
There was also a selection for OTHER PAYMENTS. This allowed you to
dump any deposit into the ATM.
What are you assured then when you deposit to an ATM ?
1) You have a banking RECORD (not a reciept at Citibank). If you
have this record, there is a VERY high percentage that you
deposited something at that ATM.
2) Some banks have ways of crediting your deposit RIGHT NOW.
This could be done by a balance in another account (i.e. a long
term C.D. or a line of credit.) That way they can get you if
you lied.
************** ATM Splitting a Card in half ***************
I've worked with about 75% of the types of machines on the market
and NONE of them split a card in half upon swallow. However, some
NETWORKS have a policy of slicing a card to avoid security
problems.
Trusting an ATM.
Intresting you should bring this up, I'm just brusing up a paper
describing a REAL situation where your card and PIN are in the clear.
This involves a customer using a bank that is part of a network.
All the information was available to folks in DP, if they put in some
efforts to get it.
Mis-Implementation of an ATM PIN security system
1. Synopsis
In an EFT (Electronic Funds Transfer) network, a single node which does
not implement the proper security can have effects throughout the
network. In this paper, the author describes an example of how security
features were ignored, never-implemented, and/or incorrectly designed.
The human factors involved in the final implementation are explored by
showing several major vulnerabilites caused by a Savings and Loan and a
regional EFT network's lack of vigilance in installing an EFT network
node. While using an EFT system as an example, the concepts can be
extrapolated into the implementation of other secured systems.
2. Background
A small Savings and Loan was setting up a small (10 to 16 ATMs)
proprietary Automatic Teller Machine (ATM) network. This network was
then intended to link up to a regional network. The manufacturer of the
institution's online banking processor sent an on-site programmer to
develop the required interfaces.
An ATM network consists of three main parts. The first is the ATM
itself. An ATM can have a range of intelligence. In this case the ATM
was able to decode a PIN (Personal Identification Number) using an
institution supplied DES (Data Encryption Standard) key. It was then
required to send a request for funds to the host where it would receive
authorization.
The second portion of the network is the ATM controller. The controller
monitors the transaction, and routes the message to the authorization
processor. The controller would also generally monitor the physical
devices and statuses of the ATM.
The third portion of the network is the authorization system. In this
case customers of the local institution would have the transaction
authorized on the same processor. Customers from foreign (i.e. one
that does not belong to the institution that runs the ATM) institutions
would be authorized by the regional network. Authorization could be
from a run-up file which maintains establishes a limit on withdrawals
for a given account during a given period. A better method is
authorization direct from the institution which issued the card.
3. Security
The system has a two component key system to allow access to the network
by the customer. The first is the physical ATM card which has a
magnetic stripe. The magnetic stripe contains account information. The
second component is the Personal Identification Number (PIN). The PIN
is hand entered by the customer into the ATM at transaction time. Given
these two parts, the network will assume that the user is the
appropriate customer and allow the transaction to proceed.
The Magnetic stripe is in the clear and may be assume to be reproducible
using various methods, thus the PIN is crucial security.
Security PIN security
3.1. PIN security
3.1.1. PIN key validation method
PINs can be linked up to a particular card in a number of ways. One
method puts the PIN into a central data base in a one-way encrypted
format. When a PIN is presented, it would be encrypted against the
format in the data base. This method requires a method of encrypting
the PIN given at the ATM, until it can be verified at the central site.
Problems can also occur if the institution wants to move the PIN data
base to another processor, especially from a different computer vendor.
Another method is to take information on the card, combine it with an
institution PIN encryption key (PIN key) and use that to generate the
PIN. The institution in question used the PIN key method. This allows
the customer to be verified at the ATM itself and no transmission of the
PIN is required. The risk of the system is the PIN key must be
maintained under the tightest of security.
The PIN key is used to generate the natural PIN. This is derived by
taking the account number and using DES upon it with the PIN key. The
resulting number then is decimialized by doing a lookup on a 16 digit
decimalization table to convert the resulting hexadecimal digits to
decimal digits. An ATM loaded with the appropriate PIN key can then
validate a customer locally with no need to send PIN information to the
network, thereby reducing the risk of compromise.
The PIN key requires the utmost security. Once the PIN key is known,
any customer's ATM card, with corresponding PIN can be created given a
customer account number. The ATM allows for the PIN to be entered at
the ATM in two parts, thus allowing each of two bank officers to know
only one half of the key. If desired, a terminal master key can be
loaded and then the encrypted PIN key loaded from the network.
The decimalization table usually consists of 0 to 9 and 0 to 5, ("0" to
"F" in hexadecimal where "F" = 15). The decimalization table can be put
into any order, scrambling the digits and slowing down an attacker. (As
a side note, it could be noted that using the "standard" table, the PIN
digits are weighted to 0 through 5, each having a 1/8 chance of being
the digit, while 6 through 9 has only a 1/16 chance.)
When handling a foreign card, (i.e. one that does not belong to the
institution that runs the ATM), the PIN must be passed on to the network
in encrypted form. First, however, it must be passed from the ATM to
the ATM controller. This is accomplished by encrypting the PIN entered
at the ATM using a communication key (communication key), The
communication key is entered at the ATM much like the PIN key. In
addition, it can be downloaded from the network. The PIN is decrypted
at the controller and then reencrypted with the network's communication
key.
- 2 -
Security
PIN security
PIN key validation method
Maintaining the the security of the foreign PIN is of critical
importance. Given the foreign PIN along with the ATM card's magnetic
image, the perpetrator has access to an account from any ATM on the
network. This would make tracking of potential attackers quite
difficult, since the ATM and the institution they extract funds from can
be completely different from the institution where the information was
gleaned.
Given that the encrypted PIN goes through normal communication
processes, it could be logged on the normal I/O logs. Since it is
subject to such logging, the PIN in any form should be denied from the
logging function.
3.2. Security Violations
While the EFT network has potential to run in a secured mode given some
of the precautions outlined above, the potential for abuse of security
is quite easy. In the case of this system, security was compromised in
a number of ways, each leading to the potential loss of funds, and to a
loss of confidence in the EFT system itself.
3.2.1. Violations of the PIN key method
The two custodian system simply wasn't practical when ATMs were being
installed all over the state. Two examples show this: When asked by
the developer for the PIN key to be entered into a test ATM, there was
first a massive search for the key, and then it was read to him over the
phone. The PIN key was written on a scrap of paper which was not
secured. This is the PIN key that all the customer PINs are based on,
and which compromise should require the reissue of all PINs.)
The importance of a system to enter the PIN key by appropriate officers
of the bank should not be overlooked. In practice the ATM installer
might be the one asked to enter the keys into the machine. This indeed
was demonstrated in this case where the ATM installer not only had the
keys for the Savings and Loan, but also for other institutions in the
area. This was kept in the high security area of the notebook in the
installer's front pocket.
Having a Master key entered into the ATM by officers of the bank might
add an additional layer of security to the system. The actual PIN key
would then be loaded in encrypted form from the network. In the example
above, if the installer was aware of the terminal master key, he would
have to monitor the line to derive the actual PIN key.
The use of a downline encrypted key was never implemented, due to the
potential complications and added cost of such a system. Even if it
was, once violated, security can only be regained by a complete reissue
of customer PINs with the resulting confusion ensuing.
- 3 -
Security
Security Violations
Network validated PIN Security violations
3.2.2. Network validated PIN Security violations
Given the potential for untraced transactions, the maintenance of the
foreign PINs security was extremely important. In the PIN key example
above, any violation would directly affect the institution of the
violators. This would limit the scope of an investigation, and enhance
the chance of detection and apprehension. The violation of foreign PIN
information has a much wider sphere of attack, with the corresponding
lower chance of apprehension.
The communication key itself was never secured. In this case, the
developer handed the key to the bank officers, to ensure the
communication key didn't get misplaced as the PIN key did (This way he
could recall it in case it got lost). Given the communication key, the
security violation potential is simple enough. The programmer could
simply tap the line between the ATM and the controller. This
information could then generate a set of PIN and card image pairs. He
would even have account balances.
Tapping the line would have been an effort, and worse yet he could get
caught. However, having the I/O logs could serve the same purpose.
While originally designed to obscure PIN information in the I/O logs,
the feature was disabled due to problems caused by the regional network
during testing. The I/O logs would be sent to the developer any time
there was a problem with the ATM controller or the network interface.
The generation of PIN and card image pairs has a potential for even the
most secured system on the network to be attacked by the lapse in
security of a weaker node. Neither the communication key, nor the PIN
should ever be available in the clear. This requires special hardware
at the controller to store this information. In this case, the
institution had no desire to install a secured box for storing key
information. The communication key was available in software, and the
PIN was in the clear during the process of decrypting from the ATM and
re-encrypting with the network key. Any programmer on the system with
access to the controller could put in a log file to tap off the PINs at
that point.
The largest failure of the system, though, was not a result of the items
described above. The largest failure in the system was in the method of
encrypting the PIN before going to the network. This is due to the
failure of the network to have a secured key between sites. The PIN was
to be encrypted with a network key. The network key was sent in
encrypted form from the network to the ATM controller. However, the key
to decrypt the network key was sent almost in the clear as part of the
start-of-day sequence.
Any infiltrator monitoring the line would be able to get all key
information by monitoring the start-of-day sequence, doing the trivial
decryption of the communication key, and proceeding to gather card image
and PIN pairs. The infiltrator could then generate cards and attack the
system at his leisure.
- 4 -
Security
Security Violations
Network validated PIN Security violations
The network-ATM controller security failure is the most critical feature
since it was defined by a regional network supporting many institutions.
The network was supposedly in a better position to understand the
security requirements.
4. The Human Factors in Security Violation
It is important the users of a system be appraised of the procedures for
securing the system. They should understand the risks, and know what
they are protecting. The bank officers in charge of the program had
little experience with ATM systems. They were never fully indoctrinated
in the consequences of a PIN key or communication key compromise. The
officers showed great surprise when the developer was able to generate
PINs for supplied test cards. Given the potential risk, nothing more
was done to try to change the PIN key, even though, they were quite
aware that the PIN key was in the developer's possession. They once
even called the developer for the PIN key when they weren't able to find
it.
The developer had a desire to maintain a smooth running system and cut
down on the development time of an already over-budget project. Too
much security, for example modifying I/O logs, could delay the isolation
or repair of a problem.
The regional network was actually a marketing company who subcontracted
out the data processing tasks. They failed to recognized the security
problem of sending key information with extremely weak encryption. The
keys were all but sent in the clear. There seemed to be a belief that
the use of encryption in and of itself caused a network to be secured.
The use of DES with an unsecured communication key gave the appearance
of a secured link.
The lack of audits of the system, both in design and implementation was
the final security defect which allowed the system to be compromised in
so many ways. An example of the Savings and Loan's internal auditors
failure to understand the problems or technology is when the auditors
insisted that no contract developers would be allowed physically into
the computer room. The fact was, access to the computer room was never
required to perform any of the described violations.
5. Security Corrections
As in any system where security was required, the time to implement it
is at the beginning. This requires the review of both implementation
ormed to
verify that the procedures are followed as described in the plan.
Financing, scheduling and man power for such audits must be allocated so
security issues can be addressed.
For this institution, the first step would have been to indoctrinate the
- 5 -
Security Corrections
banking officers of the risks in the ATM network, the vulnerabilites,
and the security measures required.
Custodians of all keys should be well aware of their responsibilities
for those keys. A fall back system of key recovery must be in place in
case an officer is not available for key entry.
The cost of installing hardware encryption units at the host should be
included in the cost of putting in the system. The host unit could
generate down-line keys for both the PIN key and the communication key
thus making it more difficult to derive these keys without collusion
from at least three people.
A secured communications key should be established between the Network
and the institution. This would allow for the exchange of working
communication keys. This key should be changed with a reasonable
frequency.
All these areas should be audited in both the system specification and
implementation to make sure they are not being abridged in the name of
expediency.
6. Summary
In this view of a single institution, a number of failures in the
security system were shown. There was shown a definite failure to
appreciate what was required in the way of security for PINs and keys
used to derive PIN information. An avoidance of up front costs for
security lead to potentially higher cost in the future. The key area
was the lack of audits of the EFT system by both the institution and the
network, causing potential loss to all institutions on the network.
- 6 -
For those of you who would like a deeper view of thes of ATM
PIN stuff, I'm merging some previous postings along with a paper
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