Added Play Fair Cipher

Ciphers/play_fair_cipher.py

@@ -0,0 +1,98 @@
+import string
+
+
+def get_pairs_of_text(text):
+    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):
+    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):
+    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):
+    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):
+    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):
+    all_elements = []
+    elements = list(key + string.ascii_uppercase.replace('J', ''))
+    for el in elements:
+        if not el in all_elements:
+            all_elements.append(el)
+
+    KEY = []
+    for i in range(0, 25, 5):
+        KEY.append(all_elements[i:i+5])
+
+    return KEY
+
+
+def playfair(text, key):
+    print('Original Text: ', text)
+    enc = encrypt(text, key=key)
+    print('Encrypted Text: ', enc)
+    dec = decrypt(enc, key=key)
+    print('Decrypted Text: ', dec)
+
+
+if __name__ == '__main__':
+    text = 'BALLOON'
+    key = 'BALLOON'
+    playfair(text, key=create_key(key))