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Dav Harzin

One Time Pads – Numbers or Letters?

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One Time Pads – Numbers or Letters?


One Time Pads (OTP) are a cryptographic system that uses a table of random digits as a key to encrypt a plain text message. Used as intended, with truly random keys used only one time, it is an unbreakable encryption method. Encrypting and decrypting is done by using a translation table or codex against the plain text first before adding or subtracting the key digits to complete the encryption (subtraction) or decryption (addition). This codex maps letters and special characters to numeric values that will work in the above operations and serves to obfuscate text, changing the length of the message vs. the plain text.





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This is an unbreakable cipher method, one traditionally used by the Soviets for high-value communications. It requires both transmitter and receiver to have the same 'one-time pad'. For transmitting encrypted documents that must be rendered letter-perfect, and where the receiver has the means to decode it,  it cannot be beat. I assume that it would be very easy to put each 'pad' into digital form and have a computer do the encoding and decoding.


However, in a battlefield situation, this may not be possible.  You want to be able to broadcast over-the-air messages which need to be understood and acted on in real time. Here, it seems to me that codes, not ciphers, are required.


A cipher is a method of translating each and every symbol of  plain text into a code symbol, and then back again.  So,  a message like "Send casevac to MGRF 3PKB 324 862"  which consistes of 33 characters, would get turned into 33 other characters,  like hdone lhald hh9hb 39ndl ahmnx wqrpp ft.  


In the improvised, ad hoc, hasty situation in which a local militia may find itself,  sophisticated communications equipment may not be available.


In that case, a code -- not a cipher -- system may be preferable.  In a code system, each general concept (which can include simple nouns,  verbs and adjectives) is attached to a word or phrase.

So,  the concept of "send casevac" might be rendered as "The meal is ready"  and that particular Grid Reference might be rendered as "Lollipops are tasty". 


A code, like all but Public Key ciphers, requires that both transmitter and receiver have a copy of the 'key'.


Codes are vulnerable to being figured out if they are used too often. So if the enemy hears "The meal is ready, lollipops are tasty",  several times, and notes that a casuality is always then evacuated

from location 3PKB 324 862, he can make an educated guess as to the meaning of those phrases.  So a code system should be changed regularly.


The Japanese had several elaborate codes before and during WWII with tens of thousands of entries. But the US was able to break many of them.  A really good, and very easy to read, history of codes and ciphers has been written by the popular science writer Simon Singh: https://en.wikipedia.org/wiki/The_Code_Book  

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