Straddling checkerboard: Difference between revisions

=={{header|REXX}}==

Extra coding was added to:

*   allow the use of any table   (specifiable on the invocation of the   '''genCipher'''   subroutine, the 5^th line in the REXX program),

*   no hard-coding of the location of blanks,

*   no hard-coding or the numbered lines in the straddled checkerboard table,

*   no hard-coding of the location of the escape character,

*   support the usage of a blank in the 1^st character (the top line of the table).

<lang rexx>/*REXX program uses the straddling checkerboard cipher to encrypt/decrypt a message. */

arg msg /*obtain optional message from the C.L.*/

if msg='' then msg= 'One night-it was the twentieth of March, 1888-I was returning'

say 'plain text=' msg

call genCipher 'et aon ris', 'bcdfghjklm', 'pq/uvwxyz.' /*build the cipher (board)*/

enc=encrypt(msg); say ' encrypted=' enc /*encrypt message and show encryption. */

dec=decrypt(enc); say ' decrypted=' dec /*decrypt " " " decryption. */

exit /*stick a fork in it, we're all done. */

/*──────────────────────────────────────────────────────────────────────────────────────*/

genCipher: @.=; arg @..,two,three; z=-1; @.z= @.. /*build top row of cipher.*/

_=pos(' ', @.. ) - 1; @._= two /* " 2nd " " " */

_=pos(' ', @.., _+2) - 1; @._= three /* " 3rd " " " */

do j=0 for 9; @..=@.. || @.j /*construct a table for fast searching.*/

if @.j\=='' then @.r=@.r || j

_=pos('/', @.j) /*is the escape character in this row? */

if _\==0 then @.dig=j || (_-1) /*define " " for numerals.*/

end /*j*/

@..=space(@.., 0) /*purify the table of encryptable chars*/

return

/*──────────────────────────────────────────────────────────────────────────────────────*/

encrypt: procedure expose @.; arg !,,$ /*$: output (encrypted text) so far.*/

do j=1 for length(!) /*process each of the plain─text chars.*/

x=substr(!, j, 1) /*obtain a message char to be encrypted*/

if datatype(x, 'W') then do; $=$ || @.dig || x; iterate; end /*numeral? */

if pos(x, @..)==0 then iterate /*Not one of the allowable chars? Skip*/

do k=-1 for 10; y=pos(x, @.k) /*traipse thru rows, looking for match.*/

if y==0 then iterate /*Not in this row? Then keep looking.*/

z=k; if z==-1 then z= /*construct the index of the cypher row*/

$=$ || z || (y-1) /*add an encrypted character to output.*/

iterate j /*go and process the next msg character*/

end /*k*/

end /*j*/

return $

/*──────────────────────────────────────────────────────────────────────────────────────*/

decrypt: procedure expose @.; parse arg !,,$ /*$: output (decrypted text) so far.*/

do j=1 to length(!); rw=-1 /*process each of the encypted numbers.*/

x=substr(!,j,1) /*obtain a message char to be decrypted*/

if substr(!,j,2)==@.dig then do; j=j+2; $=$ || substr(!, j, 1); iterate; end

if pos(x, @.r)\==0 then do; j=j+1; rw=x; x=substr(!, j, 1); end

$=$ || substr(@.rw, x+1, 1) /*add a character to decrypted message.*/

end /*j*/

return $</lang>

{{out|output|text=&nbsp; when using the default input:}}

<pre>

plain text= One night-it was the twentieth of March, 1888-I was returning

encrypted= 4505824251816539125016505180125423293721256216286286288653970163758524

decrypted= ONENIGHTITWASTHETWENTIETHOFMARCH1888IWASRETURNING

</pre>