Description
For this assignment you will be writing Haskell source code that can be used by the Glasgow Haskell Compiler (in interactive mode)
to encrypt and decrypt Vigenère ciphers (http://en.wikipedia.org/wiki/Vigenere_cipher). You can assume that your programs are
only expected for work with uppercase letters. You may not use any functions that you do not write yourself, except for the built-in
length function and the ord and chr functions defined below (for converting between characters and there corresponding ASCII
codes).
ord :: Char -> Int
ord c = fromEnum c
chr :: Int -> Char
chr i = (toEnum i)::Char
1. Write a function (with a type definition) that takes a single character argument and produces a single integer return value. If the
character is an uppercase letter, this function should return an integer between 0 and 25 inclusively, corresponding to the index
of that letter in the English alphabet. To clarify, calling the function with the argument “A” should result in the integer 0, calling
the function with the argument “B” should result in the integer 1, etc. For any other input, the function should return the
integer 25. You must use guards to write this function – do not use the if-then syntax.
2. Write a function (with a type definition) that takes two strings (i.e., lists of characters) k and p, and returns a string that is
composed of the characters of k repeated until a string that is the same length as p has been created. To clarify, if your function
was named foo, then foo “KEY” “MESSAGE” should return “KEYKEYK”. (This is the approach by which the keyword of a Vigenère
cipher is made compatible with the length of the message you wish to encrypt.)
3. Using the functions you have created for questions 1 and 2 above, write an encryption function (with a type definition) that
takes two strings – a keyword and a plaintext message to be encrypted – and returns a string of text that has been encrypted
with the Vigenère cipher corresponding to that keyword. Write a complementary decryption function that takes two strings – a
keyword and a ciphertext (i.e. encrypted) message to be decrypted – and returns a string of text that has been decrypted. (This
will, naturally, require that you research the Vigenère ciphers, and for that you are assigned to read the description that can be
found at http://en.wikipedia.org/wiki/Vigenere_cipher#Description.)
4. If your encryption and decryption functions have been implemented correctly, you can make the assertion that the a decryption
function call can be used to reverse an encryption function call (if applied with the same key, of course). Use this fact to design a
property test that can be used (with Test.QuickCheck) to test your program. To accomplish this you will probably need to write a
function that removes all the non-uppercase letters from a string (to ensure that the strings randomly generated by quickCheck
are viable) – this is not the only approach but it is probably simplest.
