Function EncryptCaesarCypher:String(text:String, key:String, vigenere:Int = False)
	
	Local keyAlphabet:Int[26], posKey:Int
	Local encrypted:String
	
	key = key.ToLower()
	
	posKey = (Asc(key) - 97) Mod 25

	For Local i:Int = 0 To 25
		
		keyAlphabet[i] = i + posKey
		If keyAlphabet[i] > 25 keyAlphabet[i]:- 26
		
	Next
		
	text = (text.Replace(" ", "") ).ToLower()
	
	For Local i:Int = 1 To text.length
	
		If vigenere
				
			encrypted:+ Chr(keyAlphabet[Asc(Mid(text, i, 1) ) - 97] + 97 )
		
		Else If Not vigenere
		
			encrypted:+ Chr(keyAlphabet[Asc(Mid(text, i, 1) ) - 96] + 97 )
		
		End If
	Next
		
	Return encrypted
	
End Function

Function DecryptCaesarCypher:String(text:String, key:String, vigenere:Int = False)
	
	Local keyAlphabet:Int[26], posKey:Int
	Local decrypted:String
	
	key = key.ToLower() 
	
	posKey = (Asc(key) - 97) Mod 25
	
	For Local i:Int = 0 To 25
		
		keyAlphabet[i] = i - posKey
		If keyAlphabet[i] < 0 keyAlphabet[i]:+ 26
		
	Next
	
	text = (text.Replace(" ", "") ).ToLower()
	
	For Local i:Int = 1 To text.length

		If vigenere
		
			decrypted:+ Chr(keyAlphabet[Asc(Mid(text, i, 1) ) - 96] + 96 )
			
		Else If Not vigenere
		
			decrypted:+ Chr(keyAlphabet[Asc(Mid(text, i, 1) ) - 97] + 96 )
			
		End If
			
	Next
	
	Return decrypted	
	
End Function

Function EncryptVigenereCypher:String(text:String, key:String, autokey:Int)
	
	Local textKey:String
	Local encrypted:String
	
	If autokey
	
		textKey:+ key + Mid(text, 1, text.length - key.length)
	
	Else If Not autokey
	
		For Local i:Int = 1 To (text.length / key.length)
		
			textKey:+ key
		
		Next
		
		textKey:+ Mid(key, 1, text.length Mod key.length)
		
	End If
	
	For Local i:Int = 1 To text.length
		
		encrypted:+ EncryptCaesarCypher(Mid(text, i, 1), Mid(textKey, i, 1), True)
		
	Next
	
	Return encrypted
	
End Function

Function DecryptVigenereCypher:String(text:String, key:String)
	
	Local textKey:String
	Local decrypted:String
	
	For Local i:Int = 1 To (text.length / key.length)
		
		textKey:+ key
		
	Next
		
	textKey:+ Mid(key, 1, text.length Mod key.length)
	
	For Local i:Int = 1 To text.length
		
		decrypted:+ DecryptCaesarCypher(Mid(text, i, 1), Mid(textKey, i, 1), True)
		
	Next
	
	Return decrypted	

End Function

SuperStrict

Include "VigenereCypher.bmx"

Graphics 300, 300, 0, 60

Global CaesarEncrypted:String, VigenereEncrypted:String
Global CaesarDecrypted:String, VigenereDecrypted:String

'Benutzt die Beispiele von http://de.wikipedia.org/wiki/Verschiebechiffre und http://de.wikipedia.org/wiki/Polyalphabetische_Substitution

CaesarEncrypted = EncryptCaesarCypher("caesar", "C")
VigenereEncrypted = EncryptVigenereCypher("geheimnis", "AKEY", False) ' False für autokey

CaesarDecrypted = DecryptCaesarCypher(CaesarEncrypted, "C")
VigenereDecrypted = DecryptVigenereCypher(VigenereEncrypted, "AKEY")

Repeat
	
	Cls
	
		DrawText "Caesar ver- und entschluesselt:", 10, 100
		DrawText CaesarEncrypted, 10, 120
		DrawText CaesarDecrypted, 10, 140
		
		DrawText "Vigenere ver- und entschluesselt:", 10, 200
		DrawText VigenereEncrypted, 10, 220
		DrawText VigenereDecrypted, 10, 240
		
	Flip
	
Until KeyHit(KEY_ESCAPE)