Primeなんとか

Program.cs

using System;
using System.IO;
using System.Runtime.InteropServices;

namespace PrimeBitmap
{
    class Program
    {
        static void Main(string[] args)
        {
            foreach (var s in args)
            {
                var ext = Path.GetExtension(s);
                if (ext?.ToLower() == @".bmp")
                {
                    Extract(s);
                }
                else
                {
                    Camouflage(s);
                }
            }
        }

        static void Extract(string path)
        {
            using (var rfs = new FileStream(path, FileMode.Open))
            using (var wfs = new FileStream(path.Substring(0, path.Length - 4), FileMode.Create))
            {
                rfs.Seek(Marshal.SizeOf(typeof(BITMAPFILEHEADER)) + Marshal.SizeOf(typeof(BITMAPINFOHEADER)), SeekOrigin.Begin);
                rfs.CopyTo(wfs);
            }
        }

        static void Camouflage(string path)
        {
            var payloadSize = (long)Math.Ceiling((double)(new FileInfo(path).Length) / 3);
            var headerSize = Marshal.SizeOf(typeof (BITMAPFILEHEADER)) + Marshal.SizeOf(typeof (BITMAPINFOHEADER));
            var fileSize =  headerSize + payloadSize;
            var bfh = new BITMAPFILEHEADER
            {
                bfType = 0x4d42,
                bfSize = (uint)fileSize,
                bfOffBits = (uint)headerSize
            };

            var size = GetSize(payloadSize);
            var bih = new BITMAPINFOHEADER
            {
                biSize = 40,
                biWidth = size.width,
                biHeight = size.height,
                biPlanes = 1,
                biBitCount = 24,
                biCompression = 0,
                biSizeImage = 0,
                biXPelsPerMeter = 0,
                biYPelsPerMeter = 0,
                biClrUsed = 0,
                biClrImportant = 0
            };

            using (var rfs = new FileStream(path, FileMode.Open))
            using (var wfs = new FileStream(path + @".bmp", FileMode.Create))
            {
                var bytes = new byte[Marshal.SizeOf(bfh)];
                var gch = GCHandle.Alloc(bytes, GCHandleType.Pinned);
                Marshal.StructureToPtr(bfh, gch.AddrOfPinnedObject(), false);
                gch.Free();
                wfs.Write(bytes, 0, bytes.Length);

                bytes = new byte[Marshal.SizeOf(bih)];
                gch = GCHandle.Alloc(bytes, GCHandleType.Pinned);
                Marshal.StructureToPtr(bih, gch.AddrOfPinnedObject(), false);
                gch.Free();
                wfs.Write(bytes, 0, bytes.Length);

                rfs.CopyTo(wfs);
            }
        }

        static Size GetSize(long num)
        {
            var i = 1L;
            var w = 1;
            var h = 1;
            while (i < num)
            {
                if (w + 1 <= h + 1)
                {
                    w++;
                }
                else
                {
                    h++;
                }
                i = w * h;
            }

            return new Size { width = w, height = h };
        }

        [StructLayout(LayoutKind.Sequential, Pack = 1)]
        struct BITMAPFILEHEADER
        {
            public ushort bfType;
            public uint bfSize;
            public ushort bfReserved1;
            public ushort bfReserved2;
            public uint bfOffBits;
        }

        [StructLayout(LayoutKind.Sequential, Pack = 1)]
        struct BITMAPINFOHEADER
        {
            public uint biSize;
            public int biWidth;
            public int biHeight;
            public ushort biPlanes;
            public ushort biBitCount;
            public uint biCompression;
            public uint biSizeImage;
            public int biXPelsPerMeter;
            public int biYPelsPerMeter;
            public uint biClrUsed;
            public uint biClrImportant;
        }

        struct Size
        {
            public int width;
            public int height;
        }
    }
}