One-Time Pad encryption is a multi-layered process using random numbers to encrypt a message. The Soviets (as well as other countries) routinely used the One-Time Pad method
from WWII into the beginning of the Cold War. Some entities may still be using use this system. The elegance of this encryption technique is obvious. If necessary, encryption could be done entirely by hand, and if done properly the encryption is believed to be uncrackable even with today's supercomputers. However, if human error had occurred, then one's enemies might be able to decypher an intercepted message.
HOW THE ENCRYPTION IS DONE
There are many variations in how the One-Time Pad encryption can be done. We will be using the method that was used by the Soviets.
Let's assume that the message to be encrypted is "Retreat at dawn."
STEP 1.
To begin our hypothetical encryption, we will be using a Code Book (sort of a
"code dictionary"). A Code Book has words, listed alphabetically, along with corresponding numbers that represent each word (the period at the end of the sentence is also considered to be a "word"). The Soviets used 4-digit numbers in their Code Book. The Code Book can be re-used for future messages.
So, let's say that, after consulting our Code Book, "Retreat at dawn." translates to the following four numbers:
0441 0412 2123 9000
[the number "9000" represents a period at the end of the sentence]
Note that, even though we have converted the characters into numbers, the message is not encrypted enough to be uncrackable by an enemy. To achieve a high level of encryption, we must proceed to the next step.
STEP 2.
Next, we rearrange the 4-digit numbers into 5-digit numbers by moving the left-most digit from the number group to the right, over to the adjacent number group on the left. Since there is a shift of some digits one digit to the left, you will find that you run out of digits for the last (right-most) numbers, so simply add zeros until the last number has 5 digits:
It is easier to show it than to explain it. Shifting one digit from each number to the left, and adding some zeros to the right-most number, the code now becomes:
04410 41221 23900 00000
STEP 3.
To finish the encryption of our message, we consult yet another book (called the One-Time Pad Book, a reference book that has rows of randomly generated 5-digit numbers). There are only two copies of the One-Time Pad Book: the sender has one copy, and the recipient has the other copy. We must arithmetically add these random numbers to the coded message. So, the 1st random number in the row is added to the 1st coded word in the message; the 2nd random number is added to the 2nd coded word in the message; and so on. The numbers on the One-Time Pad page are read from left to right across a row, and then the next row of random numbers is used. NOTE: To do the addition correctly, we must use a method called "non-carrying addition", meaning that any digit carried over is not used in the addition.
So, let's assume that the first four random numbers on the top row of the One-Time Pad page are 23402 89524 94742 and 00425. The non-carrying math results in the following:
04410 41221 23900 00000 <----The message (rearranged 5-digit numbers from Code Book).
+
23402 89524 94742 00425 <-----random numbers copied from the One-Time Pad book.
----- ----- ----- ------ ------
27812 20745 17642 00425 <-----resultant encrypted message.
STEP 4 (OPTIONAL).
Now, if you wish, you can convert the resulting sum back into characters, using
a chart. Note that this step does not add another layer of security to the encrypted message. So, if it doesn't add any extra security to the message, why go through the trouble of converting the numbers to characters? The reason was economics; transmitting characters via wire was cheaper than transmitting numbers (the Western Union company charged its customers a premium for sending numbers), and one could save a lot of money by using the conversion chart provided by the Western Union Company (shown below). Therefore, the Soviets converted the numbers back into alphabetic characters soley to save money.
WESTERN UNION TELEGRAPH COMPANY'S NUMBER-TO-CHARACTER CONVERSION TABLE
0=O
1=I
2=U
3=Z
4=T
5=R
6=E
7=W
8=A
9=P
If the option of encoding the message back into characters is chosen, then the encrypted message "Retreat at dawn." now becomes:
UWAIU UOWTR IWETU OOTUR
Lastly, the sender destroys the one-time pad page(s) that were used in the encryption. The page(s) are ripped out of the book and are burned or shredded. The remaining pages in the One-Time Pad Book will be used for encrypting future messages.
The message is now ready to be transmitted via telegraph or by other means.
Obviously, the person at the receiving end of the transmission must decrypt the message in reverse order. In order to do this, both he and the sender must have duplicate one-time pad books. The recipient also destroys the one-time pad page(s) that he had used to decode the message.
Okay. The sender has successfully encrypted the message. Now see if you can decrypt it. First, try it without help, then confirm that you followed the steps.
HOW TO CRACK THE CODE? (Answer: wait for your enemy to make a mistake)
Two huge unknowns confront the code-breaker: 1) The randomly-generated numbers that were on a particular One-Time Pad Book's page(s); and 2) the numeric code from the Code Book and its corresponding Russian words. If some random numbers on a One-Time Pad sheet could be identified, then it might be possible to uncover some of the numerical "words" in the Code Book. In theory, this should be impossible. But what if the encrypter/sender made a mistake?
Let's set up a scenario in which the sender forgot to destroy the page out of his One Time Pad book and then, mistakenly, re-used it to encrypt another message.
Let's further assume that both of these encrypted messages were intercepted and are in the possession of enemy code-breakers. The enemy can now use a technique called "pattern matching and analysis" on both of the messages to find similarities in the code structure between the two messages. The code-breaker is looking for at least two matching pairs of 5-digit numbers in the two different encrypted messages. If the same One-Time Pad page was used to encrypt both messages, then two (or more) matching pairs of numbers might be found in both messages and, further, if they are found, they must occur in the same localities within both messages (this is because the 50th random number on the One-Time Pad page will be added to the 50th code "word" in both messages. Finding these matches then triggers a search for other matching pairs of numbers in the two messages. The chances of finding such an encryption mistake are miniscule, and the task is daunting. But if matches are found, decryption can then proceed, but it will only work for those two particular messages. Any other messages that correctly used a One-Time Pad sheet only once will still remain "dark" to the code-breaker.
After some of the original random numbers from the One-Time Pad page(s) had been uncovered by the code-breakers, then another technique, called "book-breaking", is used to recreate portions of the original Code Book (i.e., the 4-digit number groups and their respective words). Tying a particular 4-digit number to its corresponding Russian word is not as difficult as it may appear. However, book-breaking requires a thorough knowledge of the language and grammar of the country who's code is being cracked. One of the best book-breakers of coded Russian language messages during the 1940s was U.S. govt. linguist Meredith Gardner. As one example, Gardner found that the Soviet Code Book number "0669" represented the English word "into". By 1946, Gardner was slowly cloning the Soviet Code Book, piece by piece, even though he didn't have a physical copy in his hands.
Around 1950, largely due to Meredith Garner's book-breaking talent and his hard work, American nuclear physicist Theodore Hall and his friend Saville Saks were uncovered as nuclear spies for the Soviet Union. While working at the Los Alamos National Laboratory between 1943-1946, Hall copied top secret nuclear physics equations and other information in invisible ink (i.e., regular milk) onto the margins of newspapers. He then mailed the newspapers to Saville Saks, who made the milk ink visible by applying heat to the newspaper with a clothes iron. Saks then handed the information over to KGB agents. The Soviet Embassy then encrypted the message using the One-Time Pad system and sent the message by telegraph to Moscow via the Western Union Company. Because of a War-time agreement, all Soviet diplomatic messages that were sent from U.S. soil were also made available to the U.S. government. The Soviets were not concerned about this, because they thought that their code was unbreakable. The Soviets rarely made proceedural mistakes with their encryption. Only 1% of all encrypted Soviet messages sent to Moscow between 1944 and 1948 contained encryption errors that allowed them to be cracked by American intellegence agencies.
[Footnote: Even though Hall and Sacks were identified as spies for the Soviet Union, neither of them was prosecuted. This is because the U. S. government didn't want the Soviets to know that portions of their messages were being decrypted. But in 1948, the Soviets finally learned, through another KGB spy named William W. Weisband, that the U. S. had breached their coded channels. The Soviets then switched over to a different encryption system. Hall and Saks lived as free men into old age. Saville Saks died in 1980. Theodore Hall died in 1999 after a distinguished career in England as a biophysicist.
Footnote #2: Ironically, Senator Joseph McCarthy (R-Wisc.), a witch hunter, a drunk and a kook who destroyed the reputations of many innocent American citizens, was totally unaware of Hall's or Saks' spying activities, nor of the U.S.'s ability to break into some Soviet encrypted messages. The moral of the story is to leave the job of spy hunting to the intelligence community and to reject the claims of pathologically paranoid zeolots and politicians.]
GLOSSARY
Code Book: A dictionary that provides a multi-digit number of each word. For instance, the word "retreat", when looked up in the dictionary, might be represented by the number 0441. The sender has a dictionary with the words listed alphabetically, and the person at the receiving end of the transmission had a "reverse dictionary" with the numbers listed in ascending order.
Non-carrying addition: A form of math in which any digit carried over in an addition is not used in the addition. It is ignored. Example: 11119 + 11111 = 22220, not 22230.
One-Time Pad Book: A book with pages of randomly-generated 5-digit numbers which are arranged in rows (read from left to right). After each 5-digit random number is used, it is lightly lined-out with a pencil (in order to ensure that it will not be accidentally used again), and then the next random number in that row is added to the next number in the message. Pages from One-Time Pad Books are used only once, and then the page(s) are ripped out of the book and are destroyed.
[All of the information on this web page is declassified information and it was obtained from the Public Domain. Much of it was taken from the NOVA documentary television program "Secrets, Lies, and Atomic Spies" (which aired on PBS), in which many of the code breakers and other players from the 1940s were interviewed. A video tape of this amazing documentary can be purchased by calling 1-800-255-9424. Other information contained on this web page comes from author Steven Budiansky's excellent history book Battle of Wits.]