[+] A4 Add starter files

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+"""CSC110 Fall 2021 Assignment 4, Part 4: Two New Cryptosystems
+
+Instructions (READ THIS FIRST!)
+===============================
+Implement each of the functions in this file. As usual, do not change any function headers
+or preconditions. You do NOT need to add doctests.
+
+You may create some additional helper functions to help break up your code into smaller parts.
+
+Copyright and Usage Information
+===============================
+
+This file is provided solely for the personal and private use of students
+taking CSC110 at the University of Toronto St. George campus. All forms of
+distribution of this code, whether as given or with any changes, are
+expressly prohibited. For more information on copyright for CSC110 materials,
+please consult our Course Syllabus.
+
+This file is Copyright (c) 2021 David Liu, Mario Badr, and Tom Fairgrieve.
+"""
+
+
+################################################################################
+# Task 1 - The Grid Transpose Cryptosystem
+################################################################################
+def grid_encrypt(k: int, plaintext: str) -> str:
+    """Encrypt the given plaintext using the grid cryptosystem.
+
+    Preconditions:
+        - k >= 1
+        - len(plaintext) % k == 0
+        - plaintext != ''
+
+    >>> grid_encrypt(8, 'DAVID AND MARIO TEACH COMPUTER SCIENCE!!')
+    'DDTMCA EPIVMAUEIACTNDRHEC I REAOC !N OS!'
+    """
+
+
+def grid_decrypt(k: int, ciphertext: str) -> str:
+    """Decrypt the given ciphertext using the grid cryptosystem.
+
+    Preconditions:
+        - k >= 1
+        - len(ciphertext) % k == 0
+        - ciphertext != ''
+
+    >>> grid_decrypt(8, 'DDTMCA EPIVMAUEIACTNDRHEC I REAOC !N OS!')
+    'DAVID AND MARIO TEACH COMPUTER SCIENCE!!'
+    """
+
+
+def plaintext_to_grid(k: int, plaintext: str) -> list[list[str]]:
+    """Return the grid with k columns from the given plaintext.
+
+    Preconditions:
+        - k >= 1
+        - len(plaintext) % k == 0
+        - plaintext != ''
+    """
+
+
+def grid_to_ciphertext(grid: list[list[str]]) -> str:
+    """Return the ciphertext corresponding to the given grid.
+
+    Preconditions:
+        - grid != []
+        - grid[0] != []
+        - all({len(row1) == len(row2) for row1 in grid for row2 in grid})
+    """
+
+
+def ciphertext_to_grid(k: int, ciphertext: str) -> list[list[str]]:
+    """Return the grid corresponding to the given ciphertext.
+
+    Note that this grid should be the one that is used to generate the ciphertext.
+
+    Preconditions:
+        - k >= 1
+        - len(ciphertext) % k == 0
+        - ciphertext != ''
+    """
+
+
+def grid_to_plaintext(grid: list[list[str]]) -> str:
+    """Return the plaintext message corresponding to the given grid.
+
+
+    Preconditions:
+        - grid != []
+        - grid[0] != []
+        - all({len(row1) == len(row2) for row1 in grid for row2 in grid})
+    """
+
+
+################################################################################
+# Task 2 - Breaking The Grid Transpose Cryptosystem
+################################################################################
+def grid_break(ciphertext: str, candidates: set[str]) -> set[int]:
+    """Return the set of possible secret keys that decrypt the given ciphertext into a message
+    that contains at least one of the candidate words.
+
+    >>> candidate_words = {'DAVID', 'MINE'}
+    >>> grid_break('DDTMCA EPIVMAUEIACTNDRHEC I REAOC !N OS!', candidate_words) == {8, 10}
+    True
+    """
+
+
+def run_example_break(ciphertext_file: str, candidates: set[str]) -> list[str]:
+    """Return a list of possible plaintexts for the ciphertext found in the given file.
+
+    Based on the A4 directory structure, you can call this function like this:
+        >>> possible_plaintexts = run_example_break('ciphertexts/grid_ciphertext1.txt', {'climate'})
+    """
+    with open(ciphertext_file, encoding='utf-8') as f:
+        ciphertext = f.read()
+
+    # (Not to be handed in) Try completing this function by calling grid_break and returning a
+    # list of the possible plaintext messages.
+
+    return [ciphertext] + list(candidates)  # This is a dummy line, please replace it!
+
+
+################################################################################
+# Task 3 - The Permuted Grid Transpose Cryptosystem
+################################################################################
+def permutation_grid_encrypt(k: int, perm: list[int], plaintext: str) -> str:
+    """Encrypt the given plaintext using the grid cryptosystem.
+
+    Preconditions:
+        - k >= 1
+        - len(plaintext) % k == 0
+        - sorted(perm) == list(range(0, k))
+        - plaintext != ''
+
+    >>> permutation_grid_encrypt(8, [0, 1, 2, 3, 4, 5, 6, 7],
+    ...                          'DAVID AND MARIO TEACH COMPUTER SCIENCE!!')
+    'DDTMCA EPIVMAUEIACTNDRHEC I REAOC !N OS!'
+    >>> permutation_grid_encrypt(8, [3, 2, 5, 0, 7, 1, 6, 4],
+    ...                          'DAVID AND MARIO TEACH COMPUTER SCIENCE!!')
+    'IACTNVMAUE I REDDTMCN OS!A EPIAOC !DRHEC'
+    """
+
+
+def permutation_grid_decrypt(k: int, perm: list[int], ciphertext: str) -> str:
+    """Return the grid corresponding to the given ciphertext.
+
+    Note that this grid should be the one that is used to generate the ciphertext.
+
+    Preconditions:
+        - k >= 1
+        - len(ciphertext) % k == 0
+        - sorted(perm) == list(range(0, k))
+        - ciphertext != ''
+
+    >>> permutation_grid_decrypt(8, [0, 1, 2, 3, 4, 5, 6, 7],
+    ...                          'DDTMCA EPIVMAUEIACTNDRHEC I REAOC !N OS!')
+    'DAVID AND MARIO TEACH COMPUTER SCIENCE!!'
+    >>> permutation_grid_decrypt(8, [3, 2, 5, 0, 7, 1, 6, 4],
+    ...                          'IACTNVMAUE I REDDTMCN OS!A EPIAOC !DRHEC')
+    'DAVID AND MARIO TEACH COMPUTER SCIENCE!!'
+    """
+
+
+if __name__ == '__main__':
+    # When you are ready to check your work with python_ta, uncomment the following lines.
+    # (Delete the "#" and space before each line.)
+    # IMPORTANT: keep this code indented inside the "if __name__ == '__main__'" block
+    # Leave this code uncommented when you submit your files.
+    #
+    # import python_ta
+    # python_ta.check_all(config={
+    #     'extra-imports': ['python_ta.contracts'],
+    #     'allowed-io': ['run_example_break'],
+    #     'max-line-length': 100,
+    #     'disable': ['R1705', 'C0200']
+    # })
+
+    import python_ta.contracts
+    python_ta.contracts.check_all_contracts()
+
+    import doctest
+    doctest.testmod()