UhoProtocol.cs

using System;

namespace UhoProtocol
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(Uho("Hello, World!"));
            Console.WriteLine(Ohu(Uho("Hello, World!")));
        }

        static unsafe string Uho(string value)
        {
            var length = value.Length * 8;
            var result = stackalloc char[length];
            for (var i = 0; i < value.Length; ++i)
            {
                var x = (byte)value[i];
                for (var j = 0; j < 8; ++j)
                {
                    var isTrue = (x & (1 << j)) > 0;
                    result[i * 8 + j] = isTrue ? 'ウ' : 'ホ';
                }
            }
            return "ウホ" + new string(result, 0, length);
        }

        static string Ohu(string value)
        {
            var valueSpan = value.AsSpan(2);
            var result = new char[valueSpan.Length / 8];
            for (var i = 0; i < result.Length; ++i)
            {
                var x = new byte();
                for (var j = 0; j < 8; ++j)
                {
                    if (valueSpan[i * 8 + j] == 'ウ')
                    {
                        x |= (byte)(1 << j);
                    }
                }
                result[i] = (char)x;
            }
            return new string(result);
        }
    }
}