Vigenère Cipher

Background

First described by Giovan Battista Bellaso in 1553, the cipher is easy to understand and implement, but it resisted all attempts to break it until 1863, three centuries later. This earned it the description le chiffre indéchiffrable (French for 'the indecipherable cipher'). Many people have tried to implement encryption schemes that are essentially Vigenère ciphers. In 1863, Friedrich Kasiski was the first to publish a general method of deciphering Vigenère ciphers. In the 19th century the scheme was misattributed to Blaise de Vigenère (1523–1596), and so acquired its present name. (thx wiki). Hack it here

The Vigenère cipher is "related" to the Caesar cipher, but because it uses a number of substitutions (2 or more), it is technically a "POLYALPHABETIC Cipher.

The "key" is a phrase that both the sender and receiver knows.

The "Tableau" (2 character lookup table) that a Vigenère cipher uses can be envisaged as follows:

Example:

Vigenère Encryption (1 key)example

ClearText:	W	E		A	T	T	A	C	K		A	T		D	A	W	N
Key:	A	L	E	R	T	A	L	E	R	T	A	L	E	R	T	A	L
CipherText	W	P		R	M	Y	L	G	B		A	E		U	T	W	Y

As keylength goes up, the number of possible substitutions needed to be brute-force checked skyrockets:

stolen from Tutorials Point

Code

Let's try it out:

Key:
Message:

Algorithm

#Thanks Greg Egan for the Pseud starter
BEGIN
	SET cryptText=''
	SET keyIndex = 0
	SET alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
	INPUT mode
	INPUT upcase(message)
	INPUT upcase(key)
	SET keyLength = length of key

	FOR each char in message:
		IF char not in alphabet THEN
			CALCULATE cryptText += char
		ELSE
			SET letterPos = position of char in alphabet
 
			SET keyChar = key[keyIndex]
			SET keyPos = position of keyChar in alphabet

			IF mode = 'encrypt' THEN
				CALCULATE newIndex = (letterPos + keyPos) mod 26
			ELSE
				CALCULATE newIndex = (letterPos - keyPos) mod 26
			ENDIF

			CALCULATE keyIndex = (keyIndex + 1) mod keyLength		
			SET cryptText += alphabet[newIndex]
		ENDIF
	ENDFOR

	PRINT cryptText
END

Variations

It seems both the Gronsfeld and Beaufort ciphers are variations of Vigenère - read a paper I stole from https://www.nku.edu/~christensen/092hnr304%20Gronsfeld%20and%20Beaufort.pdf about these variations