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

User · Answer

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.

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

[c#] Calculate MD5 checksum for a file

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