Fast file integrity verification

Question

Is there any fast algorithm that allows to compare two files (for verification purpose) without need to read the entire contents?

Answer 1

You could use a MD5 hash on both files and compare them that way. However it does technically read the whole file. You won't be able to have 100% certainty without checking I don't think.

In C# one would do this in the following way (sorry, you didn't mention a specific language):

protected string GetMD5HashFromFile(string fileName)
{
    byte[] retVal = { };

    using (FileStream file = new FileStream(fileName, FileMode.Open))
    using (MD5 md5 = new MD5CryptoServiceProvider())
    {
        retVal = md5.ComputeHash(file);
    }

    if (retVal.Length > 0)
    {
        StringBuilder sb = new StringBuilder();

        for (int i = 0; i < retVal.Length; i++)
        {
            sb.Append(retVal[i].ToString("x2"));
        }

        return sb.ToString();
    }
    else
    {
        return string.Empty;
    }
}

bool CompareFiles(string fileName1, string fileName2)
{
    return (GetMD5HashFromFile(fileName1) == GetMD5HashFromFile(fileName2));
}