Gronsfeld Cipher

[iamtheky]

The CTF we were playing this evening had a gronsfeld cipher, so here is my first stab at a function that would solve it. Due to the way I am rolling through the alphabet all characters must be uppercase (and specials removed or stripped in the func, i opted for removing them manually for example purposes), any improvements are welcome.

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local $aTestKey[8] = [0,7,2,7,1,9,7,8]

$aTestStrCreate = stringsplit(stringupper("ICECTFIDONTTHINKGRONSFELDLIKESMONDAYS") , "" , 2)

$aTestStrSolve = stringsplit(stringupper("IjgJUOPLOUVAIRUSGYQUTOLTDSKRFBTWNKCFT") , "", 2)


ConsoleWrite(_GronsfeldCipher($aTestKey , $aTestStrCreate , 0) & @LF)
ConsoleWrite(_GronsfeldCipher($aTestKey , $aTestStrSolve , 1) & @LF)


Func _GronsfeldCipher($aKey , $aStr , $action)  ; 0 = Create , 1 = Solve

$k = -1
$aOut = ""

If $action = 1 Then

For $i = 0 to ubound($astr) - 1
    $k+=1
     If asc($astr[$i]) - $aKey[$k] < 65 Then
         $aOut &= chrw(asc($astr[$i]) + 26 - $aKey[$k])
        Else
         $aOut &= chrw(asc($astr[$i]) - $aKey[$k])
     EndIf
    If $k = ubound($aKey) - 1 Then $k = -1
Next

Else

For $i = 0 to ubound($astr) - 1
    $k+=1
     If asc($astr[$i]) + $aKey[$k] > 90 Then
         $aOut &= chrw(asc($astr[$i]) - 26 + $aKey[$k])
        Else
         $aOut &= chrw(asc($astr[$i]) + $aKey[$k])
     EndIf
    If $k = ubound($aKey) - 1 Then $k = -1
Next

EndIf

return $aOut

EndFunc

  

Edited September 10, 2018 by iamtheky
improvements

[iamtheky]

added to the Func so you can both create and solve them, suppose I'll get around to fixing it to add lowercase support as well.