C# - Decode a X-VR-SPAMCAUSE mail header

spamcausedecoder.cs

using System;

public class Program
{
	/// <summary>
	/// Decode a X-VR-SPAMCAUSE mail header
	/// </summary>
	/// <param name="encodedSpamcause">The X-VR-SPAMCAUSE content to decode</param>
	/// <returns>The decoded spam cause</returns>
	public static string Decode(string encodedSpamcause)
	{
		// based upon the python version by @DoubleYouEl
		// https://gist.github.com/DoubleYouEl/e3de97293ce3d5452b3be7a336a06ad7
		string result = String.Empty;
		const byte x = (byte)'x';
		const byte g = (byte)'g';
		
		for (int i = 0; i < encodedSpamcause.Length; i += 2)
		{
			byte first  = (byte)encodedSpamcause[i];
			byte second = (byte)encodedSpamcause[i+1];
			if((i/2) % 2 == 0)
			{
				byte tmp = first;
				first = second;
				second = tmp;
			}
			var offset = 16 * (g - first);
			var ch = first + second - x - offset;
			result += (char)ch;
		}
		return result;
	}
  
	/// <summary>
	/// Encode a X-VR-SPAMCAUSE mail header
	/// </summary>
	/// <param name="spamcause">The X-VR-SPAMCAUSE content to encode</param>
	/// <returns>The encoded spam cause</returns>
	public static string Encode(string spamcause)
	{
		const byte b = (byte)'b';
		const byte x = (byte)'x';
		const byte g = (byte)'g';
		string result = String.Empty;
		
		for(int i = 0; i < spamcause.Length; ++i)
		{
			var c = (byte)spamcause[i];
			var first = b + c/17;
			var offset = 16 * (g - first);
			var second = c - first + x + offset;
			if(i%2 == 0)
			{
				var tmp = first;
				first = second;
				second = tmp;
			}
			result += (char)first;
			result += (char)second;
		}
		return result;
	}
	
	public static void Main()
	{
		string encodedSpamcause = "gggruggvucftvghtrhho";
		string decoded = Decode(encodedSpamcause);
		Console.WriteLine(decoded);
		string reencoded = Encode(decoded);
		if(!reencoded.Equals(encodedSpamcause))
		{
			Console.WriteLine("Error in encoding!");
		}
	}
}