Added Play Fair Cipher

Ciphers/play_fair_cipher.py

@@ -0,0 +1,142 @@
+import string
+from pprint import pprint
+
+
+def get_pairs_of_text(text):
+    '''
+    This function generates pairs of elements
+    from the given text.
+    eg: BALLOON becomes BA LX LO ON
+    '''
+    pairs = []
+    i = 0
+
+    while i < len(text):
+        a = text[i]
+        try:
+            b = text[i+1]
+        except Exception as e:
+            pairs.append((a, 'X'))
+            break
+
+        if a == b:
+            pairs.append((a, 'X'))
+            i += 1
+        else:
+            pairs.append((a, b))
+            i += 2
+    return pairs
+
+
+def get_index(k, key):
+    '''
+    This function returns the index of the specified element
+    inside the key matrix.
+    '''
+    for i in range(5):
+        for j in range(5):
+            if key[i][j] == k:
+                return i, j
+
+
+def find_new_pairs(pair, key, encrypt):
+    '''
+    This is the function which returns a tuple consisting of new characters 
+    based on the pair given as input.
+    '''
+    a, b = pair
+    ai, aj = get_index(a, key)
+    bi, bj = get_index(b, key)
+
+    if ai == bi:
+        if encrypt: # move right
+            aj = (aj + 1) % 5
+            bj = (bj + 1) % 5
+        else: # move left
+            aj = (aj - 1) % 5
+            bj = (bj - 1) % 5
+    elif aj == bj:
+        if encrypt: # move down
+            ai = (ai + 1) % 5
+            bi = (bi + 1) % 5
+        else: # move up
+            ai = (ai - 1) % 5
+            bi = (bi - 1) % 5
+    else:
+        aj, bj = bj, aj
+
+    return (key[ai][aj], key[bi][bj])
+
+
+def encrypt(text, key):
+    '''
+    This is the main encrypt function
+    '''
+
+    # here we generate pairs of the original text
+    pairs = get_pairs_of_text(text)
+    new_pairs = []
+    for pair in pairs:
+        new_pairs.append(find_new_pairs(pair, key, encrypt=True))
+    return ''.join(a+b for a, b in new_pairs)
+
+
+def decrypt(text, key):
+    '''
+    This is the main decrypt function.
+    '''
+
+    # here we generate pairs of the cipher text
+    pairs = get_pairs_of_text(text)
+    new_pairs = []
+    for pair in pairs:
+        new_pairs.append(find_new_pairs(pair, key, encrypt=False))
+    return ''.join(a+b for a, b in new_pairs).replace('X', '')
+
+
+def create_key(key):
+    '''
+    This function creates a 5x5 matrix for the given key
+    '''
+    all_elements = []
+
+    # this list contains those elements which are not present in the key
+    list_of_non_keys = list(set(list(string.ascii_uppercase)) - set(list(key)))
+
+    # generating a list of 25 characters by removing the one which is not present in the key
+    elements = list(key + string.ascii_uppercase.replace(list_of_non_keys[0], ''))
+    for el in elements:
+        if not el in all_elements:
+            all_elements.append(el)
+
+    KEY = []
+    # adding the elements to the matrix
+    for i in range(0, 25, 5):
+        KEY.append(all_elements[i:i+5])
+
+    print("Key Matrix: ")
+    pprint(KEY)
+
+    return KEY
+
+
+def playfair(text, key):
+    print('Original Text: ', text)
+
+    # the encrypted function is passed the text and the key as parameters.
+    enc = encrypt(text, key=key)
+    print('Cipher Text: ', enc)
+
+    # the decrypt function is passed the cipher text and the key as parameters.
+    dec = decrypt(enc, key=key)
+    print('Decrypted Text: ', dec)
+
+
+if __name__ == '__main__':
+    text = 'theworkingbros'.replace(' ', '').upper()
+    key = 'risinglight'.replace(' ', '').upper()
+
+    # main calling function
+    playfair(text, key=create_key(key))
+
+# This code is contributed by Bhushan Borole