[Solved] Vigenere Cipher Black Hawk Down

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I think your calculation is wrong:

You currently have 

encryptedLetter = (letter - firstLetterOffset) + key[position % keyLength] % 26 + firstLetterOffset

by check the C operator precedence table we notice that % is evaluated before - or +, meaning that your code actually mean :

encryptedLetter = (letter - firstLetterOffset) + ( key[position % keyLength] % 26 ) + firstLetterOffset

Where you wanted :

encryptedLetter = ( (letter - firstLetterOffset) + key[position % keyLength] ) % 26 + firstLetterOffset

Conclusion : you need to put more parenthese to specify in which order you which to evaluate your expression.

In addition you took the letter number for the text character but not for the key !

Correct expression

encryptedLetter = ( (letter - firstLetterOffset) + key[position % keyLength] - firstLetterOffset ) % 26 + firstLetterOffset

Demonstration in javascript

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solved Vigenere Cipher Black Hawk Down