Added Vigenere Cipher script in Python by Theekshani

Encryption_Algorithms/vigenere_cipher_theekshani.py

@@ -0,0 +1,37 @@
+# vigenere_cipher_theekshani.py
+# A basic implementation of the Vigenère Cipher
+
+def encrypt_vigenere(plaintext, key):
+    encrypted = ""
+    key_index = 0
+
+    for char in plaintext:
+        if char.isalpha():
+            shift = ord(key[key_index % len(key)].lower()) - ord('a')
+            base = ord('A') if char.isupper() else ord('a')
+            encrypted += chr((ord(char) - base + shift) % 26 + base)
+            key_index += 1
+        else:
+            encrypted += char
+    return encrypted
+
+def decrypt_vigenere(ciphertext, key):
+    decrypted = ""
+    key_index = 0
+
+    for char in ciphertext:
+        if char.isalpha():
+            shift = ord(key[key_index % len(key)].lower()) - ord('a')
+            base = ord('A') if char.isupper() else ord('a')
+            decrypted += chr((ord(char) - base - shift) % 26 + base)
+            key_index += 1
+        else:
+            decrypted += char
+    return decrypted
+
+# Example usage
+plaintext = "HELLO WORLD"
+key = "KEY"
+cipher = encrypt_vigenere(plaintext, key)
+print("Encrypted:", cipher)
+print("Decrypted:", decrypt_vigenere(cipher, key))