Skip to content

Latest commit

 

History

History

1_LanceAndJanice

Folders and files

NameName
Last commit message
Last commit date

parent directory

..
README.md
README.md
 
 
__init__.py
__init__.py
 
 
solution.py
solution.py
 
 

README.md

Lance and Janice: Decoding encrypted messages using the Atbash Cipher

The problem

With messages in this secret code, every lowercase letter [a...z] is replaced with the corresponding one in [z...a], while every other character (including uppercase letters and punctuation) is left untouched. That is, 'a' becomes 'z', 'b' becomes 'y', 'c' becomes 'x', etc.

For instance, the word vmxibkgrlm, when decoded, would become encryption.

Solution

This is an example of the Atbash Cipher. The Atbash Cipher is a very simple substitution cipher that is sometimes called mirror code. It is believed to be the first cipher ever used. According to Wikipedia, Atbash is a monoalphabetic substitution cipher originally used to encrypt the Hebrew alphabet.

To use Atbash, you simply reverse the alphabet, so a becomes z, b becomes y and so on. The name derives from the first, last, second, and second to last Hebrew letters (Aleph–Taw–Bet–Shin). It can be modified for use with any known writing system with a standard collating order. Due to the fact that there is only one way to perform this, the Atbash cipher provides no communications security, as it lacks any sort of key.

Implementation

The modern English alphabet is a Latin alphabet consisting of 26 letters, each having an upper- and lower-case form. The solution only needs to decipher lowercase letters. Anything else in the ciphertext remains unchanged. Given the decimal code for a is 97 in Unicode, and z is 122, we can use that information to iterate over the ciphertext to determine if each character is in the range [a...z], or [97...122].

def solution(s):
    """
    Take in a string and returns the deciphered string.
    
    :param s: a cipher text string 
    :return: the deciphered string
    """
    if len(s) == 0:
        return ""

    max_supported_size = 1000000    # Arbitrarily limiting input string to 1MB for illustrative purposes

    if len(s) > max_supported_size:
        return ""

    cipher_text = list(s)
    plain_text = ""

    a = ord('a')    # unicode code for 'a' is 97
    z = ord('z')    # unicode code for 'z' is 122
    alphabet_size = z-a+1     # assuming lowercase alphabet in range 97...122

    for character in cipher_text:

        if ord(character) in range(a, z+1):   # if the unicode equivalent of the character is in the specified range
            character = chr(a + (alphabet_size - (ord(character) - a))-1)    # decode the input character
        elif character not in range(a, z+1):
            character = character
        else:
            character = ' '
        plain_text = plain_text + character

    return str(plain_text)

Unit Tests

These are the 2 test cases provided by Google.

Input:
solution.solution("wrw blf hvv ozhg mrtsg'h vkrhlwv?")
Output:
    did you see last night's episode?

Input:
solution.solution("Yvzs! I xzm'g yvorvev Lzmxv olhg srh qly zg gsv xlolmb!!")
Output:
    Yeah! I can't believe Lance lost his job at the colony!!

There are also a bunch of hidden tests that the code must pass, but they don't tell you what those tests are. This is the set of unit test I built up along the way.

class DecipherTests(unittest.TestCase):

    def test_1(self):
        cipher = "wrw blf hvv ozhg mrtsg'h vkrhlwv?"
        expected = "did you see last night's episode?"

        deciphered = solution(cipher)
        self.assertEqual(expected, deciphered)

    def test_2(self):
        cipher = "Yvzs! I xzm'g yvorvev Lzmxv olhg srh qly zg gsv xlolmb!!"
        expected = "Yeah! I can't believe Lance lost his job at the colony!!"

        deciphered = solution(cipher)
        self.assertEqual(expected, deciphered)

    def test_2_modified_last_letter(self):
        cipher = "Yvzs! I xzm'g yvorvev Lzmxv olhg srh qly zg gsv xlolmB!!"
        expected = "Yeah! I can't believe Lance lost his job at the colonB!!"

        deciphered = solution(cipher)
        self.assertEqual(expected, deciphered)

    def test_encryption_word(self):
        cipher = "vmxibkgrlm"
        expected = "encryption"

        deciphered = solution(cipher)
        self.assertEqual(expected, deciphered)

    def test_empty_cipher(self):
        cipher = ""
        expected = ""

        deciphered = solution(cipher)
        self.assertEqual(expected, deciphered)

    def test_all_punctuation(self):
        cipher = "!@#%$__--+.,"
        expected = cipher

        deciphered = solution(cipher)
        self.assertEqual(expected, deciphered)

    def test_all_caps(self):
        cipher = "THIS IS A CAPS TEST"
        expected = "THIS IS A CAPS TEST"

        deciphered = solution(cipher)
        self.assertEqual(expected, deciphered)

    def test_input_too_large(self):
        cipher = ""
        for i in range(0, 1000001):
            cipher += "a"

        expected = ""

        deciphered = solution(cipher)
        self.assertEqual(expected, deciphered)

References