(c) 2000,2008 Peter McCollum
Encryption via a One-Time
Pad
This section presents an example of how message encryption was
typically done using a "one-time pad" (OTP). An old-timer kindly
provided the information and pictures for this section.
A one-time pad is essentially a pad of paper on which each page
has a unique set of random letters. The sender and receiver have identical
pads. Each letter on the pad is used to determine a single letter of the
enciphered message. Since the letters on the pad are random, there is no
formula that can be determined by studying the letters. Assuming that the pad
is not compromised, and each page is used only once, the OTP system is
unbreakable.
The key letters on the pad, and the messages themselves, are
typically written in 5-letter groups. This helped the communicators to collate
and verify the length of the message, and if something was misunderstood, the
receiving person could ask for a certain group to be repeated, etc.
"OTP is a very simple
yet completely unbreakable symmetric cipher. To use a one time pad you need 2
copies of the "pad" which will vary in size from something around 8 x
10 inches, or approximately half that size. There are two pads issued to each
user. One for encipher and one for decipher, and the key text is printed in red
for encipher and black for decipher. Each page of the pad is sealed and must
not be opened until actually enciphering or deciphering. Typically the pads are
set up in blocks of five letter random groups. The key text may not be reused
and pages should be destroyed after each use."
To use the OTP, a method is needed for correlating a letter of
plain text with the next letter of the key text (from the pad), to produce a
letter of enciphered text. The method used is called a "Vigenere's
Tableau", or Vigenere's square (see the first picture, below). The table
has the alphabet in the left-most column, and also across the top (printed in
black in the picture). For each row, there is a shifted-reverse alphabet
(printed in red). So, the "A" row lists the alphabet backwards,
beginning with Z. The "B" row begins with Y and ends with
"…CBAZ", etc.
To encipher the first letter in a message, go to the row
corresponding to the plain-text letter, then go to the column indicated by the
first letter on your OTP. The letter at the row-column intersection is the
enciphered letter. Note that the Vigenere's table itself does not contain any
'secret' information - it simply provides the mechanism for combining plain and
key text into enciphered text.
For example, suppose that the message is "Dead drop Alpha
three AM tonight" :
DEADD ROPAL PHATH REEAM TONIG HTXXX ----- this is the plain text
BNJEX KQPBC LZCXV PKTUY QFHNG QWERT ----- this is the key text
(from OTP)
VIQSZ YVVYM ZTXJX TLCFP QGFEN CKYLJ ----- this is the enciphered
text
One of the two disks that comes with the GRA-71 burst coder
device has an extra reversed alphabet enscribed on it, thus allowing it to be
used in place of a printed table. The red mark on the coder is aligned with the
red letter on the wheel indicating the desired row of the table, then the 2nd
and 3rd members of the triad are read from the white letters
directly below the red mark.
The two pictures below are the front and back of an actual
Vigenere's table (this table can also be called a "Beaufort square",
since the alphabets are reversed). The front side is the complete table, while
the back is a set of certain 3-letter combinations ("triads") taken
from the main table. The triads on the back of the card are chosen because they
are relatively easy to remember: AIR, FOG, BAY, HAS, SUN, etc. The more triads
that can be memorized, the faster the radio operator can finish his job.
"A memory aide appears
on the back of the card. Very helpful until you get good at it. A good radio
operator could take a look at the plain text, and key text, and encipher as he
sent."
The front side of a
Vigenere's table card.
The reverse side of the
Vigenere's table card.
Use of the OTP in the Asia
Theater
The following info was provided by Russell Shepherd:
I am only knowledgeable of the systems used by the
Republic of China (Taiwan) and the US army Special Forces (Asia). The Chinese
used a code book in which the kanji characters were listed by their page
number, column number & line number. For example, the character for ‘man’
might be represented by 7234; or page 72, column 3, line 4. I assume this book
was not so highly classified, since memorizing a large number of characters
would be very difficult.
The crypto pads were physically similar to the
literal pads with the exception of using four number groups in place of the
five letter groups. [The term ‘literal pads’ refers to the type shown above,
which used alphabetic characters. Russ says this was called the ‘Diana
system’.] The pads were sealed like the literal (Diana) pads were. The
number-based system was called ‘Calypso’.
Upon receiving a group of numbers you would enter it
under the group in the decode pad and then do a ‘false subtraction’ on it. For
example, pad group 9321 minus 7234 equals 2197. [‘False subtraction’ means to
subtract without borrowing.]
The Chinese also sent their numbers in a ‘cut
numbers’ fashion to speed up transmission time.
One .---- was .-
Two ..--- was ..-
Three …-- was …--
Four ….- was ….-
Five ….. was …..
Six -…. was -….
Seven --… was --…
Eight ---.. was -..
Nine ----. was -.
Zero ----- was -
The American Special Forces, and I am sure other
units in Asia used the same type of numbered pads. The letters of the alphabet
were just given a numerical value.