During the Cold War, some microfilm discovered in a hollow nickel contained a lengthy message encrypted using a "manual cipher" that was never broken. (It was eventually revealed by a spy who defected.) For details see
http://www.fbi.gov/libref/historic/famcases/abel/abel.htm
http://www.quadibloc.com/crypto/pp1324.htm
Here's a simplified recreational version of the part of that cipher that converted letters to decimal digits, as adapted to English (with J=I) and using only the nine digits of Sudoku ...
- Code: Select all
9 4 8 7 2 3 6 5 1
-------------------
- | E T A O I N S - -
5 | B C D F G H K L M
1 | P Q R U V W X Y Z
The arrangement of letters and blanks ('-') in the table does not change, whereas the nine-digit heading is the secret key -- some permutation of 123456789 -- that varies from message to message. The two digits on the left always just repeat the rightmost pair of digits in the key.
To encrypt a letter, just replace it by the digit (if any) to its left in the table, followed by the digit directly above it in the heading; e.g., CENTRALPARK --> 54 9 3 4 18 8 55 19 8 18 56 --> 54934188551981856
Decryption works by reversing these steps in the obvious way, first using the "pairing digits" (here 5,1) to group the digits correctly; e.g., 54934188551981856 --> 54 9 3 4 18 8 55 19 8 18 56 --> CENTRALPARK
==
To decrypt this message ... 7916984874843635174191375748269378966 ... the key is row three of the solution to this Sudoku ...
- Code: Select all
9 4 . | . 2 . | 6 5 .
2 . 1 | . . 4 | . . .
6 . . | . . 1 | . . .
------+-------+------
. 8 . | . . 7 | . . .
. 9 . | . 4 . | . 7 .
. . . | 3 . . | . 4 .
------+-------+------
. . . | 1 . . | . . 7
. . . | 5 . . | 4 . 9
. 1 7 | . 9 . | . 2 6
EDIT: It was by the defection of the spy (Reino Hayhanen) that the encryption method was revealed. (I'd said he was captured.)
Also, 
