Encoder/decoder for byte array into string with 0-9,a-v

BytesEncoder.cs

using System;
using System.Diagnostics;
using System.Text;

class Program
{
    static void Main(string[] args)
    {
        var rnd = new Random(1);
        for (var i = 0; i < 5000; i++)
        {
            int len = rnd.Next(1000);
            var buf1 = new byte[len];
            rnd.NextBytes(buf1);
            var str = Encode(buf1);
            var buf2 = Decode(str);

            if (buf1.Length != buf2.Length)
            {
                Debug.Assert(buf1.Length % 5 != 0);
                Debug.Assert(buf1.Length == buf2.Length - 1);
                Debug.Assert(buf2[buf2.Length - 1] == 0);
                for (var j = 0; j < buf1.Length; j++)
                    Debug.Assert(buf1[j] == buf2[j]);
            }
            else Debug.Assert(buf1.Length % 5 == 0);
            for (var j = 0; j < buf1.Length; j++)
                Debug.Assert(buf1[j] == buf2[j]);
        }
    }

    static string Encode(byte[] bytes)
    {
        if (bytes == null || bytes.Length == 0)
            return string.Empty;

        var ret = new StringBuilder((int)Math.Ceiling(bytes.Length * 8 / 5f));
        bool cont = true;
        for (int i = -1, rest = 0; cont; )
        {
            int append;
            if (rest == 0)
                if (bytes.Length == ++i) break;
                else append = bytes[i] >> (rest = 3);
            else if (rest < 5)
            {
                rest = 5 - rest;
                append = bytes[i] << rest;
                if (bytes.Length != ++i)
                    append |= bytes[i] >> (rest = 8 - rest);
                else
                    cont = false;
            }
            else
                append = bytes[i] >> (rest -= 5);

            append &= 31;
            ret.Append((char)(append < 10 ? '0' + append : 'a' - 10 + append));
        }
        return ret.ToString();
    }

    static byte[] Decode(string value)
    {
        if (value == null || value.Length == 0)
            return new byte[0];

        var ret = new byte[(int)Math.Ceiling(value.Length * 5 / 8f)];
        var rest = 0;
        var i = -1;
        foreach (var ch in value)
        {
            var append = ch <= '9' ? ch - '0' : ch - 'a' + 10;
            if (rest == 0)
                ret[++i] = (byte)(append << (rest = 3));
            else if (rest < 5)
            {
                ret[i] |= (byte)(append >> (rest = 5 - rest));
                ret[++i] = (byte)(append << (rest = 8 - rest));
            }
            else
                ret[i] |= (byte)(append << (rest -= 5));
        }

        return ret;
    }
}