Added RC4 stream cipher implementation

Author: Charuvarthan-T

ciphers/rc4.py

@@ -0,0 +1,57 @@
+from typing import List
+
+class RC4:
+    def __init__(self, key: bytes):
+        self.key=key
+        self.s=self._ksa(key)
+
+    def _ksa(self, key: bytes) -> List[int]:
+        """Key Scheduling Algorithm (KSA)"""
+        key_length=len(key)
+        s=list(range(256))  
+        j=0
+        for i in range(256):
+            j=(j+s[i]+key[i%key_length])%256
+            s[i],s[j]=s[j],s[i]  
+        return s
+
+    def _prga(self) -> int:
+        """Pseudo-Random Generation Algorithm (PRGA)"""
+        i=0
+        j=0
+        while True:
+            i=(i+1)%256
+            j=(j+self.s[i])%256
+            self.s[i],self.s[j]=self.s[j],self.s[i]  
+            yield self.s[(self.s[i]+self.s[j])%256]  
+
+    def _reset_state(self):
+        """Reset state for each encryption/decryption."""
+        self.s=self._ksa(self.key)
+
+    def encrypt(self,plaintext:bytes)->bytes:
+        """Encrypt plaintext using RC4"""
+        self._reset_state()  
+        prga=self._prga()
+        return bytes([p^next(prga) for p in plaintext])
+
+    def decrypt(self,ciphertext:bytes)->bytes:
+        """Decrypt ciphertext using RC4 (Same as encryption)"""
+        return self.encrypt(ciphertext)  
+
+
+if __name__ == "__main__":
+    key=b"SecretKey"
+    rc4=RC4(key)
+    
+    plaintext=b"Hello, RC4 Cipher!"
+    print("Original:",plaintext)
+    
+    ciphertext=rc4.encrypt(plaintext)
+    print("Encrypted:",ciphertext)
+    
+    decrypted_text=rc4.decrypt(ciphertext)
+    print("Decrypted:",decrypted_text)
+
+    assert plaintext==decrypted_text, "Decryption failed"
+    print("Encryption and decryption successful.")