Swift: Calculated md5 hash of image file doesn't match with terminal and other hash generator

Question

I want to calculate hash of an image, first I convert image to data and then with help of this function I will calculate hash of image file (Data), but the generated hash doesn't match with online generator and other language convertors like (Java), even I tried other libraries but i get same result, I think while Im converting to Data something happend to my file so hash doesn't match with other convertors.

but when i calculate a plain text hash, it matches with all online convertor and other languages convertor but its not the same with Image?

generated hash in terminal is different

thanks for any help

   func md5(url: URL) {
          let bufferSize = 1024*1024
          do {
            let file = try FileHandle.init(forReadingFrom: url)
              defer {
                  file.closeFile()
              }

              var context = CC_MD5_CTX.init()
              CC_MD5_Init(&context)
              while case let data = file.readData(ofLength: bufferSize), data.count > 0 {
                  data.withUnsafeBytes { (poiner) -> Void in
                      _ = CC_MD5_Update(&context, poiner, CC_LONG(data.count))
                  }
              }

                           // Calculate the MD5 summary
              var digest = Data(count: Int(CC_MD5_DIGEST_LENGTH))
              digest.withUnsafeMutableBytes { (pointer) -> Void in
                  _ = CC_MD5_Final(pointer, &context)
              }
              let result = digest.map { (byte) -> String in
                  String.init(format: "%02hhx", byte)
              }.joined()
              print("result: \(result)")
          } catch let error as Error {
              print("calculation error: \(error.localizedDescription)") // Where is the try, where is the error?
          }
      }

Answer 1

Ok, here is a little test. To do this I dragged a png and some random file into the project. They are highlighted in the image below.

Note that I added the files directly to the project, not to assets. That answer I linked to in the comments actually mentions that you can not get the image directly but, as you indicate, it is also saved as png data.

The project is a standard iOS SwiftUI app and I added the code below to the autogenerated ContentView.swift file.

import SwiftUI
import CommonCrypto

struct ContentView: View
{
    var body: some View
    {
        Text("Hello, world!")
            .padding()
            .onAppear
            {
                md5 ( url : Bundle.main.url(forResource: "t", withExtension: "png" ) )
                md5 ( url : Bundle.main.url(forResource: "something", withExtension: "ext" ) )
            }
    }
}

func md5 ( url : URL? ) -> Void
{
    if url == nil
    {
        print ( "Skipping empty url" )
        return
    }
    else
    {
        print ( "Summing \( url )" )
    }

    let bufferSize = 1024*1024
    do {
        let file = try FileHandle.init(forReadingFrom: url!)
        defer {
            file.closeFile()
        }
        
        var context = CC_MD5_CTX.init()
        CC_MD5_Init(&context)
        while case let data = file.readData(ofLength: bufferSize), data.count > 0 {
            data.withUnsafeBytes { (poiner) -> Void in
                _ = CC_MD5_Update(&context, poiner, CC_LONG(data.count))
            }
        }
        
        // Calculate the MD5 summary
        var digest = Data(count: Int(CC_MD5_DIGEST_LENGTH))
        digest.withUnsafeMutableBytes { (pointer) -> Void in
            _ = CC_MD5_Final(pointer, &context)
        }
        let result = digest.map { (byte) -> String in
            String.init(format: "%02hhx", byte)
        }.joined()
        print("result: \(result)")
    } catch let error as Error {
        print("calculation error: \(error.localizedDescription)") // Where is the try, where is the error?
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

This code is pretty much your MD5 function together with some code to test those files added to the project.

Now, the MD5 for the file works but it does not work for the image!

I presume Apple add some stuff to the image. I print the URL and checked - the image is a lot smaller than the original so it is probably some stripped down version with all or most of the metadata removed but this is just a guess.

Anyhow, I share your pain ... Why do these images differ - [img drawInRect:] problem here Apple changes an image that you draw in what appears to be an arbitrary way. I presume these kinds of tweaks are to improve UI or UX but personally I do not appreciate them.

Your md5 function works though fwiw. But you should be careful to test the same file as you do with the other MD5 functions. Thus you should not convert it in any way that would change even just one byte from the original. This is why pngData is a bad idea. But at the same time, I would expect if you copy it into your project that it would stay the same but apparently not.

EDIT

FWIW, if you rename the image to something without extension png and then add it to the project it stays the same. I duplicated and renamed that image to z.z and added that to the project and then the MD5 (and z.z) is the same as the original.