Wolfram Function Repository
Instantuse addon functions for the Wolfram Language
Function Repository Resource:
Decipher a Vigenèreenciphered string
ResourceFunction["VigenereDecipher"][string,key] deciphers string using the key key. 

ResourceFunction["VigenereDecipher"][string,{key_{1},key_{2},…}] repeatedly deciphers string using first key_{1}, then key_{2}, etc. 

ResourceFunction["VigenereDecipher"][{s_{1},s_{2},…},key] deciphers the strings s_{1},s_{2}, etc. using the key key. 

ResourceFunction["VigenereDecipher"][key] represents an operator form of ResourceFunction["VigenereDecipher"] that can be applied to an expression. 
Decipher a message using the key "LEMON":
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Decipher with multiple keys:
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Decipher a message twice, first using "GO" and then "CAT":
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Decipher multiple strings:
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Create an operator that deciphers strings with the key "sparks":
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Apply the operator to a string:
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If a single character is used, the Vigenère cipher is identical to a Caesar cipher:
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When deciphering with multiple keys, the keys can first be enciphered with each other to decipher the message with a single key. The length of the replacing key has to be the LCM of the lengths of each of the keys:
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This indeed matches with what you expect:
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The case of the original text is retained; the case of the key does not matter:
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VigenereDecipher is related to VigenereCipher:
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Spaces and the letters a and A in the key will not alter the enciphered message:
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The key should have valid letters and valid length:
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Two keys can "negate" each other:
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Enciphering the keys with each other gives a string with only As:
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