code: public static InputStream decrypt(InputStream inputStream, byte[] key) {
          checkNotNull(inputStream);
          checkNotNull(key);
          checkArgument(key.length >= 5 && key.length <= 256);
          StreamCipher rc4 = new RC4Engine();
          rc4.init(false, new KeyParameter(key));
          return new CipherInputStream(inputStream, rc4);
      }
docs: Decrypt input stream using RC4
  
  @param inputStream input stream to be decrypted
  @param key         rc4 key (40..2048 bits)
  @return decrypted input stream