USPS Hacking (Autumn, 1991)
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By The Devil's Advocate

The U.S. Postal Service (USPS) is just like any other system. It is huge and
complicated, with lots of acronyms and technical jargon. It is riddled with
inconsistencies, and is prone to human error. Most importantly, it beckons to
be explored by that very same bunch who are so fond of creative exploration:
Hackers!

POSTNET

The Postal Numeric Encoding Technique (POSTNET) is a bar code system initiated
in 1983 to help accelerate the sorting of letter mail by automated equipment.
The term "POSTNET" refers to a bar code, which represents either a five digit
ZIP code, or a nine digit ZIP+4 code. POSTNET is most often preprinted on
business or courtesy reply mail by businesses. POSTNET can also be jet sprayed
on envelopes that are processed by an Optical Character Reader (OCR) machine.

POSTNET consists of a combination of 22 long bars and 30 short bars. The 52
bars encode a nine digit ZIP+4 code plus a checksum number. Learning to read
POSTNET is easy for anyone familiar with binary. The first and last bars
(always long) are guide bars, and play no part in determining the encoded
ZIP+4. Each group of five bars after the first guide bar represents one ZIP+4
number. The group consists of a combination of two long bars and three short
bars. The position in the group has a corresponding value. The values from left
to right are 7-4-2-1-0. A ZIP+4 number is obtained by adding the values of the
positions containing the two long bars. The only special case is when the added
values equal eleven. In this case, the number represented is zero. POSTNET also
includes a checksum number at the end for the purpose of error detection. You
can determine what the checksum number should be by adding the numbers of your
ZIP+4. The last digit of the resulting sum, when subtracted from 10, will yield
the checksum number. For instance, if your ZIP+4 is 11953-0752, then the sum is
1+1+9+5+3+0+7+5+2=33, the last digit of the sum is 3, and the checksum is
10-3=7.

The USPS encourages companies to preprint the ZIP+4 POSTNET on business reply
mail by offering reduced postage rates. The advantage of using POSTNET is not
only in savings but in speed. Letter mail that uses POSTNET is processed faster
and more accurately than mail that does not use POSTNET.

MARK

The MARK facer-canceller serves three purposes:

 1. It cancels and postmarks letter mail.
 2. It arranges letters so that they all face in the same direction.
 3. It separates POSTNET letter mail from mail that does not use POSTNET.

The MARK utilizes fluorescent and phosphorescent detectors that enable it to
detect the presence of minute traces of phosphor on stamps, prestamped
postcards or envelopes, and meter marks. The MARK is also capable of detecting
preprinted Facing Identification Marks (FIM).

FIM

Open any magazine and you will find business reply mail cards inside. Nearly
every card will contain a FIM. These six-line bar codes are much taller than
POSTNET, but not nearly as wide. They are located at the top of the card, just
left of the postage area. The MARK recognizes four types of FIMS:

FIM A five vertical lines (2,1,2)  Letter uses POSTNET and needs postage. Used
                                   for courtesy reply mail.

FIM B six vertical lines (1,2,2,1) Letter does not use POSTNET and does not
                                   need postage. Used for business reply mail.

FIM C six vertical lines (2,1,1,2) Letter uses POSTNET and does not need
                                   postage. Used for business reply mail.

FIM D seven vertical lines (3,1,3) Letter does not use POSTNET, needs postage,
                                   and is OCR readable. Used for courtesy reply
				   window envelopes.

Business reply mail that uses FIM B or FIM C (indicating that no postage is
necessary) must also use horizontal bars to indicate that USPS must collect
postage from the business to which the mail is addressed. The horizontal bars
are located on the right hand side of the cards and allow clerks to easily spot
these cards in a tray full of other letters.

The MARK first checks to see that a letter has postage (stamp, meter mark, or
FIM). After passing this test, the letter is then canceled, postmarked, and
directed to one of eight bins based upon the orientation of the letter and the
presence of POSTNET. Four of the eight bins are for POSTNET letter mail, while
the other four bins are for mail that does not use POSTNET. Each group of four
bins accepts letters according to their orientation. Because letters can enter
the machine right side up, upside down, backwards, or forwards, the MARK must
have a bin for every possible orientation.

The MARK also utilizes a ninth bin for letters that are rejected by the machine
for lack of postage. For example, if a letter does not have postage, and the
letter does not have FIM B or FIM C (indicating that no postage is necessary),
then the letter will end up in the reject bin. Sometimes letters that do have
legitimate postage may end up in the reject bin. If a stamp is not placed in
the upper right hand corner of an envelope, then the MARK's sensors may not
detect the phosphor, and the letter will be rejected. A clerk manually goes
over all of the rejected letters individually to determine why they were not
processed.

LSM

The Letter Sorting Machine (LSM) was first used by the USPS in the late 1950s.
The huge semiautomatic beast requires a group of operators to sit in front of
twelve consoles while letters are zipping by at a rate of one per second. The
machine automatically positions a letter in front of an operator, who then has
one second to key in the first three digits of the ZIP code. The letter is then
whisked away to one of several hundred bins according to the keys that were
depressed. If an operator fails to key in anything then the letter will go to a
reject bin and will eventually be fed back into the LSM. If an operator happens
to key in the wrong code, then a slight possibility exists that the misguided
letter will be caught by a clerk before it is shipped. Otherwise, the letter
will be delivered to that location, wherever it may be, and will eventually be
delivered back again.

LSM places a marker on the back of every letter that is processed. The marker
consists of two alphanumeric symbols. The first symbol is always a letter
ranging from A to Z. The second symbol is either a letter ranging from A to C,
or a number ranging from 1 to 9. The marker can therefore be one of 319
possibilities. The marker may also be one of several different colors, although
the color does not indicate any useful information. According to USPS LSM
operators, the marker indicates which console processed the letter. However,
this information is fairly useless because we still do not know which specific
LSM processed the letter. The USPS uses hundreds of LSMs nationwide, and each
of those LSMs has twelve consoles. I am uncertain how to translate a specific
marker into a specific console, nor do I understand why the marker can be one
of 319 possibilities if there are only twelve consoles.

BCS

The Bar Code Sorter (BCS) processes POSTNET letter mail. The BCS is therefore
limited to sorting only business reply mail and other high volume mail that
incorporates the POSTNET. At a sorting rate of ten letters per second, the BCS
is considered slightly faster than your average clerk. The letters must be
properly positioned and fed into the machine manually by an operator. This is
accomplished by stacking trays of letters received from the MARK onto a feeder
unit. The operator does not have to properly position each letter because the
letters received from the MARK are already facing the same way.

MLOCR

The Multiline Optical Character Reader (MLOCR) is the latest and most advanced
machine in the USPS letter sorting arsenal. This million-dollar monster is
capable of reading all of the lines that comprise a letter s address. It then
takes this information and compares it against its own internally stored
address directory. Finally, an appropriate POSTNET is jet sprayed on the letter
so that it can be further processed by a BCS. The purpose of the MLOCR is
therefore to spray POSTNET on letters that do not use POSTNET, so that they can
be processed by a BCS.

The advantage of the MLOCR is that it can determine an address even if parts of
the address are illegible, incorrect, or missing. For instance, if someone
forgets to include a ZIP code, or uses the wrong ZIP code by mistake, then the
MLOCR can still determine the correct ZIP code by comparing the street, city,
and state with its own address directory. It will then spray the letter with
the correct ZIP+4 code (the MLOCR will always try to spray the letter with a
ZIP+4, even if the letter uses a five digit ZIP code).

Early OCRs could only read type or clearly printed handwriting. In the near
future, however, the MLOCR will recognize script as well. The MLOCR is capable
of reading the address even if it is skewed (i.e., printed at an angle). The
MLOCR does not have the capability of knowing whether or not a letter already
has POSTNET, nor can it sort mail according to POSTNET. Therefore, it is
possible to receive a letter that has two overlapping POSTNET bar codes.

Like the BCS, the MLOCR only accepts trays of properly positioned machinable
letters that must be fed into the machine manually by an operator.

Mail Hacks

There are at least three things that everyone familiar with the USPS would like
to do: 1) Mail letters for free; 2) Get their letters delivered quicker; 3)
Find out why it takes so long for their letters to arrive.

Free Mail

It is not difficult for someone to mail a letter for free. It is, however,
extremely difficult to mail many letters for free. The USPS is always looking
out for mail fraud, and has an entire agency devoted to just this task. Even if
a good mail hack works once, it is not likely to work if used repeatedly.
Therefore, if you are reading this article with the intent of saving money by
tricking the USPS and mailing letters for free, then you would do better to
give up now before you are busted. Of course, anyone with even the slightest
iota of curiosity would want to know some of the methods.

Perhaps one of the oldest scams in the book is to switch the destination
address with the return address and mail the letter without postage. The USPS
will then return the letter to its "sender" for postage. Of course, the USPS is
not that stupid, and this trick rarely works for nonlocal mail.

A much better mail hack would be to use a laser printer to print a FIM B on an
envelope. The MARK will then treat this letter like a business reply mail card,
and will not reject it for lack of postage. Of course, the problem with this
technique is that a mail carrier will almost certainly notice the missing
postage before the letter even gets to a MARK. Therefore, you would have to
bundle this letter with another letter that has postage, place the letter with
postage on top of the illegitimate letter, and use a rubber band to bundle them
together. The mail carrier will not disturb this bundle. Eventually, the bundle
will reach a General Mail Facility (GMF) where clerks quickly separate bundles
on a conveyer belt. It is extremely unlikely that they will notice the
illegitimate letter at this point. From the conveyor belt, the letter will
journey to the MARK. Once the MARK processes the letter, it is unlikely that
anyone will notice the missing postage until the letter reaches its
destination. The final obstacle is the mail carrier who will physically deliver
the letter to its destination. At this point, the letter is postmarked, so one
can only hope that the mail carrier is not too nosy.

Fast Mail

Getting your letters mailed quickly is a much better hack than trying to mail
your letters for free. Not only is it legal but the results are guaranteed.

Normally, a letter reaches a MARK where it is processed and sent to an MLOCR.
If the address on the envelope is readable by the MLOCR, then it is jet sprayed
with a POSTNET and sent to a BCS. Otherwise, the letter is rejected and sent to
an LSM. The one thing you really want to avoid is having your letter processed
by an LSM. The operators who run these machines are notorious for keying in the
wrong code, causing your letter to journey out of its way to strange and exotic
parts of the country. Never write the address on your envelope in script unless
you want to delay your letter.

One way you can get your letters processed quicker is to have your letters skip
some of the steps in the sorting process. The method involves using a laser
printer to print a FIM A and a POSTNET on an envelope. The FIM A will instruct
the MARK to treat the envelope as courtesy reply mail. The MARK will look for
postage, which you have thoughtfully provided, and then send the letter into a
bin with all of the other POSTNET mail. This mail will then be placed in a tray
and sent directly to a BCS, skipping the MLOCR and completely avoiding the LSM.

By using POSTNET, you are taking advantage of the same multimillion dollar
equipment that is used by businesses. Another advantage to using this method is
that your letter will be processed entirely by machines. From the moment your
letter enters the MARK until the moment it leaves the BCS, no clerk will see
your letter. In addition, the USPS will be pleased with your creative use of
their multimillion dollar machinery.

Snail Mail

Now that you know what happens to your letter when you mail it, you can use
this information to determine why it takes so long for your own mail to arrive.
The next time a letter comes in the mail, analyze it for telltale USPS markings
that may give you insight into how the letter was processed. If the letter has
POSTNET on it, then you know that the letter was processed by an MLOCR and a
BCS. You can then read the POSTNET to make sure that it represents your ZIP
code. If the POSTNET is incorrect then that would certainly explain why your
letter was delayed. You should also flip the letter over and look for LSM
markers. You should not see any more than one or two markings. If the back of
your letter is covered with them, then you know that your letter probably had
quite a journey, whipping back and forth around the country before it reached
you. Keep in mind that it is not unusual for a letter to be processed by both a
BCS and an LSM. Not all GMFs use the same machinery, and the average clerk can
screw up any letter, even if it is processed by machines.