Add rc4 cipher

DIRECTORY.md

@@ -113,6 +113,7 @@
  * [Porta Cipher](ciphers/porta_cipher.py)
  * [Rabin Miller](ciphers/rabin_miller.py)
  * [Rail Fence Cipher](ciphers/rail_fence_cipher.py)
+ * [Rc4](ciphers/rc4.py)
  * [Rot13](ciphers/rot13.py)
  * [Rsa Cipher](ciphers/rsa_cipher.py)
  * [Rsa Factorization](ciphers/rsa_factorization.py)

ciphers/rc4.py

@@ -0,0 +1,118 @@
+"""
+Implementation of RC4 (Rivest Cipher 4) stream cipher algorithm.
+
+Reference:
+- https://en.wikipedia.org/wiki/RC4
+"""
+
+
+defksa(key: bytes) ->list[int]:
+"""
+ Key Scheduling Algorithm (KSA) for RC4.
+
+ Args:
+ key: The secret key as bytes.
+
+ Returns:
+ A permutation (list) of 256 integers (S-box).
+
+ Example:
+ >>> ksa(b'Key')[:5]
+ [0, 1, 2, 3, 4]
+ """
+key_length=len(key)
+s_box=list(range(256))
+j=0
+
+foriinrange(256):
+j= (j+s_box[i] +key[i%key_length]) %256
+s_box[i], s_box[j] =s_box[j], s_box[i]
+
+returns_box
+
+
+defprga(s_box: list[int], message_length: int) ->list[int]:
+"""
+ Pseudo-Random Generation Algorithm (PRGA) for RC4.
+
+ Args:
+ s_box: The S-box after KSA.
+ message_length: Number of bytes to generate.
+
+ Returns:
+ A keystream as a list of integers.
+
+ Example:
+ >>> prga(list(range(256)), 5)
+ [0, 1, 2, 3, 4]
+ """
+i=0
+j=0
+keystream= []
+
+for_inrange(message_length):
+i= (i+1) %256
+j= (j+s_box[i]) %256
+s_box[i], s_box[j] =s_box[j], s_box[i]
+k=s_box[(s_box[i] +s_box[j]) %256]
+keystream.append(k)
+
+returnkeystream
+
+
+defrc4(key: bytes, data: bytes) ->bytes:
+"""
+ Encrypt or decrypt data using RC4 stream cipher.
+
+ Args:
+ key: The secret key as bytes.
+ data: The plaintext or ciphertext as bytes.
+
+ Returns:
+ Encrypted or decrypted output as bytes.
+
+ Example:
+ >>> ciphertext = rc4(b'Key', b'Plaintext')
+ >>> rc4(b'Key', ciphertext)
+ b'Plaintext'
+ """
+s_box=ksa(key)
+keystream=prga(s_box, len(data))
+output=bytes(
+ [
+data_byte^keystream_byte
+fordata_byte, keystream_byteinzip(data, keystream)
+ ]
+ )
+returnoutput
+
+
+if__name__=="__main__":
+importargparse
+
+parser=argparse.ArgumentParser(
+description="Encrypt or decrypt data using RC4 cipher."
+ )
+parser.add_argument(
+"mode", choices=["encrypt", "decrypt"], help="Mode: encrypt or decrypt"
+ )
+parser.add_argument("key", type=str, help="Encryption/Decryption key")
+parser.add_argument("input", type=str, help="Input text")
+
+args=parser.parse_args()
+
+key_bytes=args.key.encode("ascii")
+input_bytes=args.input.encode("ascii")
+
+result_bytes=rc4(key_bytes, input_bytes)
+
+ifargs.mode=="encrypt":
+print(result_bytes.hex())
+else: # decrypt mode
+try:
+# if user passed hex data, decode it
+input_bytes=bytes.fromhex(args.input)
+result_bytes=rc4(key_bytes, input_bytes)
+print(result_bytes.decode("ascii"))
+exceptValueError:
+print("Error: Input must be valid hex string when decrypting.")