4 of the functions defined below are missing an implementation! Finish the following functions:

1. gen_consecutive_chars() 2. gen_key(password) 3. sub_decrypt(password, ciphertext) 4. vig_decrypt(key, message)

""" # 1. Implement gen_consecutive_chars() def gen_consecutive_chars(start=97, end=122): """Creates a string composed of all characters starting at unicode code point, start, up to and including unicode code point, end. The default values return lowercase 'a' through (and including) 'z'

gen_consecutive_chars() # --> 'abcdefghijklmnopqrstuvwxyz' gen_consecutive_chars(start=65, end=66) # --> 'AB'

:param start: the code point to start the sequence of characters :type start: int :param end: the code point of the last character included :type end: int :return: a string consisting of consecutive characters from start code point to end code point :rtype: str """ # implement this function! return ''

def remove_letters(letters, s): """Removes every character in letters from string, s. :param letters: string of characters to be removed :type letters: str :param s: string that characters will be removed from :type s: str :return: a new string with all characters in letters removed from s :rtype: str """ new_s = '' for ch in s: if ch not in letters: new_s += ch return new_s

def remove_duplicates(s): """Removes all duplicate characters in string, s :param s: string to remove duplicates from :type s: str :return: new string without duplicates :rtype: str """ new_s = '' for ch in s: if ch not in new_s: new_s += ch return new_s

# 2. Implement gen_key(password) def gen_key(password): """Generate a key from a password (as outlined in chapter 2 of our book): 1. remove duplicates from the password 2. split the alphabet into two parts by using the last letter of the password (with duplicates removed) 3. remove any letters in both parts of the alphabet that appear in the password 4. construct the key by joining the password with the first and second part of the alphabet

gen_key('topsecret') # --> 'topsecruvwxyzabdfghijklmnq'

:param password: the password used to generate a key :type password: str :return: a string 26 characters long, consisting of all letters from a - z :rtype: str """ # implement this function! (need a hint; check out the book) return ''

def sub_encrypt(password, message): """Encrypt a message using the substitution cipher. If a character is not in the key, the character remains unchanged. :param password: the password used to generate a key :type password: str :param message: the message to be encrypted :type message: str :return: the resulting ciphertext as a string :rtype: str """ key = gen_key(password) alpha = gen_consecutive_chars() ciphertext = '' for ch in message: try: ciphertext += key[alpha.index(ch)] except ValueError: ciphertext += ch return ciphertext

# 3. Implement sub_decrypt(password) def sub_decrypt(password, ciphertext): """Decrypt ciphertext using the substitution cipher. If a character is not in the key, the character remains unchanged. The ciphertext should be normalized to all lowercase letters.

s = 'le tge svhpbkeges. cyee ti bape.' sub_decrypt(s) # --> we are discovered. flee at once.

:param password: the password used to generate a key :type password: str :param ciphertext: the text to be decrypted :type ciphertext: str :return: the plain text that results from decrypting the ciphertext :rtype: str """ # implement this function! return ''

def vig_encrypt(key, message): """Encrypt a message using the vigenre cipher. Punctuation is preserved.

:param key: the key used for encrypting the message :type key: str :param message: the message to be encrypted :type message: str :return: the ciphertext, as a string, produced by encrypting the message :rtype: str """

cypher_text = '' alphabet = gen_consecutive_chars() key_len, alphabet_len = len(key), len(alphabet)

# or replace next two lines with... # for i, ch in enumerate(message): for i in range(len(message)): ch = message[i]

# based on key... what row (labeled by letters) in table will we # use? the example in the book shows a mapping of the key to a # message: # DAVINCIDAVINCIDAVINC # the eagle has landed # consequently, the row in we use for the first letter, t is D # (row_letter is D for first letter, t) row_letter = key[i % key_len].lower()

# calculate offset to simulate shifting letters for each row: # again, using the first letter t, and D as the row letter... # D is at position 3 of the alphabet, which means that the key # (the alphabet) is shifted by 3: defghijklmnopqrstuvwxyzabcd # ... let's save this shift in a variable called offset offset = alphabet.index(row_letter.lower())

try: # so now, we can translate our original character, ch, by # finding out where it is in the offset row represented by D. # simply add the offset to what ch's position would be in the # alphabet (so if ch is t and the key specifies that the # offset is 3, then the index of the character that t is # translated to 19 + 3)... other_index = (alphabet.index(ch) + offset) % alphabet_len

# if working with the letter and key mentioned above, t is # translated to the letter at index 22 of the alphabet: w cypher_text += alphabet[other_index]

except ValueError: cypher_text += ch

return cypher_text

# 4. Implement vig_decrypt(key, message) def vig_decrypt(key, message): """Decrypt a message that was encrypted using the vigenre cipher. Punctuation is preserved. The ciphertext should be normalized to all lowercase letters.

s = 'whz rcooe pnu oailrf' vig_decrypt(s) # --> 'the eagle has landed'

:param key: the key originally used to encrypt the message :type key: str :param message: the message to be decrypted (the ciphertext) :type message: str :return: the plain text produced by decrypting the ciphertext :rtype: str """ # implement this function! return ''

if __name__ == '__main__': print('try your functions here!')