Tiny crypto (C# version of https://jsfiddle.net/i_e_b/4rqo268h/)

TinyCrypto.cs

    [Test]
    public void tiny_crypto()
    {
        var key = new byte[] { 55, 167, 44, 86, 217 };
        
        var secretMessage = "This is the input text";
        var input = Encoding.UTF8.GetBytes(secretMessage);
        
        var encrypted = Encode(input, key);
        
        var result = Decode(encrypted, key);
        
        var recovered = Encoding.UTF8.GetString(result);
        Assert.That(recovered, Is.EqualTo(secretMessage));
    }

    private static Random rnd = new();

    public static byte[] Encode(byte[] input, byte[] key)
    {
        const int mod = 251;
        
        var result = new byte[input.Length + 1];
        var keyL = key.Length;
        var s = rnd.Next(0, 255);
        result[0] = (byte)(s ^ key[0]);
        for (int i = 0; i < input.Length; i++)
        {
            s = (s * (i + 1)) % mod;
            var code = s ^ key[i % keyL] ^ input[i];
            result[i + 1] = (byte)code;
        }

        return result;
    }
    public static byte[] Decode(byte[] input, byte[] key)
    {
        const int mod = 251;
        
        var keyL = key.Length;
        var result = new byte[input.Length - 1];
        var s = input[0] ^ key[0];
        for (int i = 0; i < result.Length; i++)
        {
            s = (s * (i + 1)) % mod;
            var decode = s ^ key[i%keyL] ^ input[i+1];
            result[i] = (byte)decode;
        }
        return result;
    }