Calculate MD5 checksum for a file

Question

I'm using iTextSharp to read the text from a PDF file. However, there are times I cannot extract text, because the PDF file is only containing images. I download the same PDF files everyday, and I want to see if the PDF has been modified. If the text and modification date cannot be obtained, is a MD5 checksum the most reliable way to tell if the file has changed?

If it is, some code samples would be appreciated, because I don't have much experience with cryptography.

Answer 1

I know this question was already answered, but this is what I use:

using (FileStream fStream = File.OpenRead(filename)) {
    return GetHash<MD5>(fStream)
}

Where GetHash:

public static String GetHash<T>(Stream stream) where T : HashAlgorithm {
    StringBuilder sb = new StringBuilder();

    MethodInfo create = typeof(T).GetMethod("Create", new Type[] {});
    using (T crypt = (T) create.Invoke(null, null)) {
        byte[] hashBytes = crypt.ComputeHash(stream);
        foreach (byte bt in hashBytes) {
            sb.Append(bt.ToString("x2"));
        }
    }
    return sb.ToString();
}

Probably not the best way, but it can be handy.

Answer 2

It's very simple using System.Security.Cryptography.MD5:

using (var md5 = MD5.Create())
{
    using (var stream = File.OpenRead(filename))
    {
        return md5.ComputeHash(stream);
    }
}

(I believe that actually the MD5 implementation used doesn't need to be disposed, but I'd probably still do so anyway.)

How you compare the results afterwards is up to you; you can convert the byte array to base64 for example, or compare the bytes directly. (Just be aware that arrays don't override Equals. Using base64 is simpler to get right, but slightly less efficient if you're really only interested in comparing the hashes.)

If you need to represent the hash as a string, you could convert it to hex using BitConverter:

static string CalculateMD5(string filename)
{
    using (var md5 = MD5.Create())
    {
        using (var stream = File.OpenRead(filename))
        {
            var hash = md5.ComputeHash(stream);
            return BitConverter.ToString(hash).Replace("-", "").ToLowerInvariant();
        }
    }
}

Answer 3

This is how I do it:

using System.IO;
using System.Security.Cryptography;

public string checkMD5(string filename)
{
    using (var md5 = MD5.Create())
    {
        using (var stream = File.OpenRead(filename))
        {
            return Encoding.Default.GetString(md5.ComputeHash(stream));
        }
    }
}

Answer 4

Here is a slightly simpler version that I found. It reads the entire file in one go and only requires a single using directive.

byte[] ComputeHash(string filePath)
{
    using (var md5 = MD5.Create())
    {
        return md5.ComputeHash(File.ReadAllBytes(filePath));
    }
}

Answer 5

And if you need to calculate the MD5 to see whether it matches the MD5 of an Azure blob, then this SO question and answer might be helpful: MD5 hash of blob uploaded on Azure doesnt match with same file on local machine