A straddling checkerboard is a device for converting an alphanumericplaintext into digits whilst simultaneously achieving fractionation and data compression relative to other schemes using digits. It also is known as a monôme-binôme cipher.
History
In 1555, Pope Paul IV created the office of Cipher Secretary to the Pontiff. In the late 1580s, this position was held by members of the Argenti family, most notably Giovanni Batista and his nephew, Matteo. Matteo is credited for designing what is now called the straddling checkerboard cipher. In more modern times it was used by communist forces during the Spanish Civil War in order to protect their radio and written transmissions. It was later used as the basis for the message-to-digits step in the VIC cipher.
Mechanics
Setup
A straddling checkerboard is set up something like this:
0
1
2
3
4
5
6
7
8
9
E
T
A
O
N
R
I
S
2
B
C
D
F
G
H
J
K
L
M
6
P
Q
/
U
V
W
X
Y
Z
.
The header row is populated with the ten digits, 0-9. They can be presented in order, as in the above table, or scrambled for additional security. The second row is typically set up with eight high-frequency letters, leaving two blank spots; this row has no row coordinate in the first column. The remaining two rows are labeled with one of the two digits that were not assigned a letter in the second row, and then filled out with the rest of the alphabet, plus the two symbols '.' and '/'.
Similar to the ordering of the digits in the header row, the alphabet characters can be presented in order, or scrambled based on a secret keyword/phrase.
Enciphering
Letter-Encipherment: To encipher a letter in the second row is simply replaced by the number labeling its column. Characters in the third and fourth rows are replaced by a two-digit number representing their row and column numbers Digit-Encipherment: To encipher a digit, there are a few possible methods :
Single Digit Escape: Encode the numerical escape character as per any letter, then write the required digit 'in-clear'. This means a digit is encrypted by 3 ciphertext characters; 2 for the escape character, 1 for the digit itself. In this scheme, each digit requires an escape character encoded before it.
Double-Digit Scheme: If the escape character is encoded by two different digits, then multiple digits can be encoded by writing each out twice. To 'escape' back to text the escape character is used. In this way a stream of digits can be encoded with only one escape character. This method cannot be used if the escape character is itself encoded by a double digit combination.
Triple-Digit Scheme: As per the double-digit scheme above, but triple digits are used. This was the mechanism used in the VIC cipher.'
Example
Here is an example using 'ATTACK AT DAWN'
A
T
T
A
C
K
A
T
D
A
W
N
3
1
1
3
21
27
3
1
22
3
65
5
The resulting message, 3113212731223655, may be sent directly, but needs to be processed through additional cipher stages to make it secure, such as transposition or substitution. As a simple example, we will add a secret key number using modular arithmetic: Optionally, we could then use the same straddling checkerboard to convert the ciphertext back into letters:
3
5
65
25
7
9
3
5
7
4
3
0
0
7
A
N
W
H
R
S
A
N
R
O
A
E
E
R
Deciphering is simply the reverse of these processes. Although the size of groups can vary, deciphering is unambiguous because whenever the next element to be deciphered starts with a 2 or a 6, it is a pair; otherwise, it is a singleton. If the letter decoded is the numerical escape character '/' then the next 1+ digits are to be interpreted as a digit.
Cryptographic properties
Compression: The more common characters are encoded by only one character, instead of two, this reduces the ciphertext size and potentially the cipher's proneness to a frequency attack. Fractionation: Unlike in the Polybius Square, a straddling checkerboard will not encrypt each character with the same number of ciphertext digits. This makes it harder for a cryptanalysts to determine the boundaries between plaintext characters. This may be combined with a transposition in order to locate the ciphertext letters of the same plaintext character at unknown locations in the ciphertext.
