Why overwrite a file more than once to securely delete all traces of a file?

Question

Erasing programs such as Eraser recommend overwriting data maybe 36 times.

As I understand it all data is stored on a hard drive as 1s or 0s.

If an overwrite

Answer 1

Here's a Gutmann erasing implementation I put together. It uses the cryptographic random number generator to produce a strong block of random data.

public static void DeleteGutmann(string fileName)
{
    var fi = new FileInfo(fileName);

    if (!fi.Exists)
    {
        return;
    }

    const int GutmannPasses = 35;
    var gutmanns = new byte[GutmannPasses][];

    for (var i = 0; i < gutmanns.Length; i++)
    {
        if ((i == 14) || (i == 19) || (i == 25) || (i == 26) || (i == 27))
        {
            continue;
        }

        gutmanns[i] = new byte[fi.Length];
    }

    using (var rnd = new RNGCryptoServiceProvider())
    {
        for (var i = 0L; i < 4; i++)
        {
            rnd.GetBytes(gutmanns[i]);
            rnd.GetBytes(gutmanns[31 + i]);
        }
    }

    for (var i = 0L; i < fi.Length;)
    {
        gutmanns[4][i] = 0x55;
        gutmanns[5][i] = 0xAA;
        gutmanns[6][i] = 0x92;
        gutmanns[7][i] = 0x49;
        gutmanns[8][i] = 0x24;
        gutmanns[10][i] = 0x11;
        gutmanns[11][i] = 0x22;
        gutmanns[12][i] = 0x33;
        gutmanns[13][i] = 0x44;
        gutmanns[15][i] = 0x66;
        gutmanns[16][i] = 0x77;
        gutmanns[17][i] = 0x88;
        gutmanns[18][i] = 0x99;
        gutmanns[20][i] = 0xBB;
        gutmanns[21][i] = 0xCC;
        gutmanns[22][i] = 0xDD;
        gutmanns[23][i] = 0xEE;
        gutmanns[24][i] = 0xFF;
        gutmanns[28][i] = 0x6D;
        gutmanns[29][i] = 0xB6;
        gutmanns[30][i++] = 0xDB;
        if (i >= fi.Length)
        {
            continue;
        }

        gutmanns[4][i] = 0x55;
        gutmanns[5][i] = 0xAA;
        gutmanns[6][i] = 0x49;
        gutmanns[7][i] = 0x24;
        gutmanns[8][i] = 0x92;
        gutmanns[10][i] = 0x11;
        gutmanns[11][i] = 0x22;
        gutmanns[12][i] = 0x33;
        gutmanns[13][i] = 0x44;
        gutmanns[15][i] = 0x66;
        gutmanns[16][i] = 0x77;
        gutmanns[17][i] = 0x88;
        gutmanns[18][i] = 0x99;
        gutmanns[20][i] = 0xBB;
        gutmanns[21][i] = 0xCC;
        gutmanns[22][i] = 0xDD;
        gutmanns[23][i] = 0xEE;
        gutmanns[24][i] = 0xFF;
        gutmanns[28][i] = 0xB6;
        gutmanns[29][i] = 0xDB;
        gutmanns[30][i++] = 0x6D;
        if (i >= fi.Length)
        {
            continue;
        }

        gutmanns[4][i] = 0x55;
        gutmanns[5][i] = 0xAA;
        gutmanns[6][i] = 0x24;
        gutmanns[7][i] = 0x92;
        gutmanns[8][i] = 0x49;
        gutmanns[10][i] = 0x11;
        gutmanns[11][i] = 0x22;
        gutmanns[12][i] = 0x33;
        gutmanns[13][i] = 0x44;
        gutmanns[15][i] = 0x66;
        gutmanns[16][i] = 0x77;
        gutmanns[17][i] = 0x88;
        gutmanns[18][i] = 0x99;
        gutmanns[20][i] = 0xBB;
        gutmanns[21][i] = 0xCC;
        gutmanns[22][i] = 0xDD;
        gutmanns[23][i] = 0xEE;
        gutmanns[24][i] = 0xFF;
        gutmanns[28][i] = 0xDB;
        gutmanns[29][i] = 0x6D;
        gutmanns[30][i++] = 0xB6;
    }

    gutmanns[14] = gutmanns[4];
    gutmanns[19] = gutmanns[5];
    gutmanns[25] = gutmanns[6];
    gutmanns[26] = gutmanns[7];
    gutmanns[27] = gutmanns[8];

    Stream s;

    try
    {
        s = new FileStream(
            fi.FullName,
            FileMode.Open,
            FileAccess.Write,
            FileShare.None,
            (int)fi.Length,
            FileOptions.DeleteOnClose | FileOptions.RandomAccess | FileOptions.WriteThrough);
    }
    catch (UnauthorizedAccessException)
    {
        return;
    }
    catch (IOException)
    {
        return;
    }

    using (s)
    {
        if (!s.CanSeek || !s.CanWrite)
        {
            return;
        }

        for (var i = 0L; i < gutmanns.Length; i++)
        {
            s.Seek(0, SeekOrigin.Begin);
            s.Write(gutmanns[i], 0, gutmanns[i].Length);
            s.Flush();
        }
    }
}