Editing Straddling checkerboard (section)

==Mechanics==
===Setup===
A straddling checkerboard is set up something like this:
{| class="wikitable"
|-
! &nbsp; || 0 || 1 || 2 || 3 || 4
! 5 || 6 || 7 || 8 || 9
|-
! &nbsp; 
| E || T || &nbsp; || A || O
| N || &nbsp; || 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 (based on a secret key value) for additional security.  The second row is typically set up with eight high-frequency letters (mnemonics for the English language include; 'ESTONIA-R', 'A SIN TO ER(R)', 'AT ONE SIR', 'AERO TINS'), 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 '/'.
* The period '.' is used as a full stop or [[decimal separator]],
* The slash '/' is used as a [[Escape character|numeric escape character]] (indicating that a numeral follows).

Similar to the ordering of the digits in the header row, the alphabet characters can be presented in order (as it is here), 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 (with the row coordinate written first, i.e. B=20)

'''Digit-Encipherment:''' To encipher a digit, there are a few possible methods (which must be known/agreed beforehand):
* ''Single Digit Escape'': Encode the numerical escape character (i.e. the slash '/') 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 (e.g. '26' in the example above), 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]].'<ref>
David Kahn.
[https://web.archive.org/web/20080110104120/https://www.cia.gov/library/center-for-the-study-of-intelligence/kent-csi/vol5no4/html/v05i4a09p_0001.htm "Number One From Moscow"].
1993.
</ref>

====Example====
The table below shows the conversion of string "ATTACK AT DAWN"<ref>[https://asecuritysite.com/coding/straddling?word=attackatdawn "Straddling Cipher"]</ref> into numbers using the straddling checkerboard setup above.
{| class="wikitable"
|-
|| Plaintext || A || T || T || A || C || K || A || T || D || A || W || N
|-
|| Numbers || 3&nbsp; || 1&nbsp; || 1&nbsp; || 3&nbsp; || 21 || 27 || 3&nbsp; || 1&nbsp; || 22 || 3&nbsp; || 65 || 5&nbsp;
|}
The resulting message, 3113212731223655 has to be secured by other means if the straddling checkerboard table is not scrambled. By passing digits through an additional [[transposition cipher|transposition]] or [[substitution cipher|substitution]] cipher stage can be used to secure message -- to whatever extent transposition or substitution can be considered secure.

For example, securing the message above using as a secret key digits "0452" -- digits of the secret key are repeated to get at least as many digits as there are digits in the message -- and [[Modular arithmetic|adding modulo]] 10 for transposition produces:
{| class="wikitable"
|-
| Message digits
| 3 || 1 || 1 || 3
| 2 || 1 || 2 || 7
| 3 || 1 || 2 || 2
| 3 || 6 || 5 || 5
|-
| Key digits
| 0 || 4 || 5 || 2
| 0 || 4 || 5 || 2
| 0 || 4 || 5 || 2
| 0 || 4 || 5 || 2
|-
| Cipher digits
| 3 || 5 || 6 || 5
| 2 || 5 || 7 || 9
| 3 || 5 || 7 || 4
| 3 || 0 || 0 || 7
|}
The message can also be ciphered with a substitution cipher using the same straddling checkerboard to convert the digits back to letters again:
{| class="wikitable"
|-
| Message numbers
| 3 || 5 || 65 || 25 || 7
| 9 || 3 || 5 || 7 || 4
| 3 || 0 || 0 || 7
|-
| Ciphertext
| 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 (depending on scheme) are to be interpreted as a digit.