[Spooks] going form numbers to characters ?

[Nicholas Gessler]

Hello All,

I'm new to the list, so my apologies if the answer to this question is 
obvious to you all.
Anyway, here goes:

Irrespective of whether numbers stations use code books, one-time-keys or 
quasi-random-generated-keys, I'm wondering what the algorithm is for 
translating the numbers (0-9) into characters (a-z).  The most obvious 
solutions that come to mind are:

1)  Two digits select the entry coordinates to a 10x10 table of common 
letters and letter digraphs and trigraphs (e.g. a, b, th, ion...)
2)  Two digits (modulus 25) select the alphabetic character 0-24 in a 
shortened alphabet (a-y)
3)  Two digits select an ASCII value from a shortened set of 100 characters 
(32-127 plus 5 control characters).

If digits are grouped by two, then why send them in groups of 
five?  Custom?  Clarity?  It seems to me that numbers are clearer than 
letters and female voices more clear than male voices.  Would you 
agree?  Are there any studies on this?  That is to say, are there any 
publications investigating how much noise it takes to render a female 
versus a male voice intelligible?  Or studies on the relative 
intelligibility of large symbol sets (the alphabet) versus mall symbol sets 
(the decimal digits)?  Clearly, binary digits are the most robust, so maybe 
a study would be redundant.

I'm teaching a course in computer simulation and I and a few students are 
interested in building some small computer applications demonstrating how 
it "might" work.

(I put a simple OTP simulation on the Web.  I probably should have coded 
"a" as "0" instead of "1," but it works.  It will run on any PC.)
http://www.sscnet.ucla.edu/geog/gessler/borland/crypto-alpha.exe

Cheers,
Nick