srli · June 26, 2018 03:35 · Jun 26, 2018
diff --git a/caesar_cipher.py b/caesar_cipher.py
@@ -0,0 +1,93 @@
+import string
+import random
+from collections import Counter
+
+"""
+An easy implementation of the Caesar cipher
+
+Written by Sophie Li, 2018
+http://blog.justsophie.com/caesar-cipher-generator/
+"""
+
+def encode_string(input_string):
+    # Generate seed for shift
+    seed = random.randint(0, len(string.ascii_lowercase))
+
+    # Create cipher dictionary by shifting alphabet over by "seed" places
+    alphabet = list(string.ascii_lowercase)
+    alphabet_shifted = alphabet[seed:] + alphabet[:seed]
+    cipher_dict = dict(zip(alphabet, alphabet_shifted))
+
+    # Remove punctuation, spacing, and capitalization of input string
+    clean_text = input_string.translate(None, string.punctuation).replace(" ", "").lower()
+
+    # Encode string, adding a space every 4 letters for readability
+    cipher_text = ""
+    for ind, letter in enumerate(clean_text):
+        cipher_text += cipher_dict[letter]
+        if (ind+1)%4 == 0:
+            cipher_text += " "
+
+    return cipher_text
+
+"""
+There are a few ways to decode strings, but for this quick example, a
+frequency analysis can be a good starting point-- especially if your message is
+long. If the message is too short, there won't be enough data to do a good analysis.
+"""
+def cipher_freq_analysis(cipher_string):
+    # Calculate histogram of cipher text
+    cipher_hist = Counter(cipher_string.replace(" ", ""))
+
+    # Find alphabet positions of 4 most common letters
+    e_position = string.ascii_lowercase.index('e')
+    a_position = string.ascii_lowercase.index('a')
+    o_position = string.ascii_lowercase.index('o')
+    i_position = string.ascii_lowercase.index('i')
+
+    possible_seeds = []
+    for ind in range(1, 3):
+        common_letter = cipher_hist.most_common(ind)[0][0]
+
+        # Check distance between the cipher histogram and common letters
+        e_distance = string.ascii_lowercase.index(common_letter) - e_position
+        a_distance = string.ascii_lowercase.index(common_letter) - a_position
+        o_distance = string.ascii_lowercase.index(common_letter) - o_position
+        i_distance = string.ascii_lowercase.index(common_letter) - i_position
+
+        # Add these distances as possible seeds
+        possible_seeds += [e_distance, a_distance, o_distance, i_distance]
+
+    return possible_seeds
+
+def decode_string_by_seed(cipher_string, seed):
+    # Create cipher dictionary by shifting alphabet over by "seed" places
+    alphabet = list(string.ascii_lowercase)
+    alphabet_shifted = alphabet[seed:] + alphabet[:seed]
+    decode_dict = dict(zip(alphabet_shifted, alphabet))
+    decode_dict[" "] = " "
+
+    # Encode string, adding a space every 4 letters for readability
+    decoded_text = ""
+    for ind, letter in enumerate(cipher_string):
+        decoded_text += decode_dict[letter]
+
+    print decoded_text
+
+def decode_string(cipher_string):
+    possible_seeds = cipher_freq_analysis(cipher_string)
+
+    print "------"
+    print "Decoding string by possible seeds. CTRL+C to exit at any time"
+    for seed in possible_seeds:
+        print "------\n "
+        decode_string_by_seed(cipher_string, seed)
+        raw_input("")
+
+if __name__ == '__main__':
+    raw_string = "Hi there, this is a sample string to be encoded. It's pretty normal looking."
+
+    cipher_string = encode_string(raw_string)
+    print "Ecoded string: ", cipher_string
+
+    decode_string(cipher_string)
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