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.
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();
}
}
}
BitConverter.ToString(md5.ComputeHash(stream)).Replace("-","").ToLower(); –
Lail .Replace("-", String.Empty) is a better approach. I went through a one hour debug session because I get wrong results when comparing a user input to the file hash. –
Strophe "" instead of string.Empty? –
Doble "" and compare the hash to another string (user input) it is not equal. I compare the strings with ordinal equals. Do you know why this happening? –
Strophe "" instead of string.Empty. It's absolutely fine to use "". I suggest you ask another question with details, if you can still reproduce the problem. –
Doble BufferedStream, and I could just hash the first x number of bytes instead of the "first page" –
Vasques 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));
}
}
}
using blocks would be useful, because opening a file is more probably going to fail. Fail early/fast approach saves you the resources needed to create (and destroy) the MD5 instance in such scenarios. Also you can omit the braces of the first using and save a level of indentation without losing readability. –
Pissarro 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.
public static String GetHash<T>(this Stream stream) where T : HashAlgorithm, new() { StringBuilder sb = new StringBuilder(); using (T crypt = new T()) { byte[] hashBytes = crypt.ComputeHash(stream); foreach (byte bt in hashBytes) { sb.Append(bt.ToString("x2")); } } return sb.ToString(); } –
Pulque (T) HashAlgorithm.Create(typeof(T).Name) and removing the new() constraint. For my implementation, I also changed it so the parameter is this byte[] resource and putting the stream in the method with using var stream = new MemoryStream(resource). You'll then only need to tell the compiler that crypt isn't null. –
Infectious 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));
}
}
ReadAllBytes is that it loads the whole file into a single array. That doesn't work at all for files larger than 2 GiB and puts a lot of pressure on the GC even for medium sized files. Jon's answer is only slightly more complex, but doesn't suffer from these problems. So I prefer his answer over yours. –
Tensity usings after each other with out the first curly braces using (var md5 = MD5.Create()) using (var stream = File.OpenRead(filename)) gives you one using per line without unnecessary indentation. –
Essieessinger using directive." was not really a good reason to read everything into memory. The more effective approach is to stream in the data into ComputeHash, and if possible using should only be used, but I can totally understand if you want to avoid the extra level of indentation. –
Essieessinger I know that I am late to party but performed test before actually implement the solution.
I did perform test against inbuilt MD5 class and also md5sum.exe. In my case inbuilt class took 13 second where md5sum.exe too around 16-18 seconds in every run.
DateTime current = DateTime.Now;
string file = @"C:\text.iso";//It's 2.5 Gb file
string output;
using (var md5 = MD5.Create())
{
using (var stream = File.OpenRead(file))
{
byte[] checksum = md5.ComputeHash(stream);
output = BitConverter.ToString(checksum).Replace("-", String.Empty).ToLower();
Console.WriteLine("Total seconds : " + (DateTime.Now - current).TotalSeconds.ToString() + " " + output);
}
}
For dynamically-generated PDFs. The creation date and modified dates will always be different.
You have to remove them or set them to a constant value.
Then generate md5 hash to compare hashes.
You can use PDFStamper to remove or update dates.
In addition to the methods answered above if you're comparing PDFs you need to amend the creation and modified dates or the hashes won't match.
For PDFs generated with QuestPdf youll need to override the CreationDate and ModifiedDate in the Document Metadata.
public class PdfDocument : IDocument
{
...
DocumentMetadata GetMetadata()
{
return new()
{
CreationDate = DateTime.MinValue,
ModifiedDate = DateTime.MinValue,
};
}
...
}
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