Create vernam_cipher.py (#10702)

* Create vernam_cipher.py

added vernam cipher

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>

ciphers/vernam_cipher.py

@@ -0,0 +1,42 @@
+def vernam_encrypt(plaintext: str, key: str) -> str:
+    """
+    >>> vernam_encrypt("HELLO","KEY")
+    'RIJVS'
+    """
+    ciphertext = ""
+    for i in range(len(plaintext)):
+        ct = ord(key[i % len(key)]) - 65 + ord(plaintext[i]) - 65
+        while ct > 25:
+            ct = ct - 26
+        ciphertext += chr(65 + ct)
+    return ciphertext
+
+
+def vernam_decrypt(ciphertext: str, key: str) -> str:
+    """
+    >>> vernam_decrypt("RIJVS","KEY")
+    'HELLO'
+    """
+    decrypted_text = ""
+    for i in range(len(ciphertext)):
+        ct = ord(ciphertext[i]) - ord(key[i % len(key)])
+        while ct < 0:
+            ct = 26 + ct
+        decrypted_text += chr(65 + ct)
+    return decrypted_text
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()
+
+    # Example usage
+    plaintext = "HELLO"
+    key = "KEY"
+    encrypted_text = vernam_encrypt(plaintext, key)
+    decrypted_text = vernam_decrypt(encrypted_text, key)
+    print("\n\n")
+    print("Plaintext:", plaintext)
+    print("Encrypted:", encrypted_text)
+    print("Decrypted:", decrypted_text)