MD5 hash of blob uploaded on Azure doesnt match with same file on local machine

Question

I am currently working on uploading media on Azure Blob storage. All is working fine except when i try to macth the MD5 hash of uploaded media with the local file (exactly same one which was uploaded). Local file returns a byte array where are blob.Properties.ContentMD5 returns a string and both do not match.

Local MD5 hash: sÔ(F¦‚"“Db~[N

blob.Properties.ContentMD5: c9QoHkamgiKTRANifltOGQ==

Any possible way to match both these?

Solution

Here is a good article on how to calculate and check Blob MD5 checksums.

I have faced this before, and I don't know why, but you can'T just do md5.computeHash(fileBytes). For Azure Blobs, it uses the following path to get the hash:

// Validate MD5 Value
var md5Check = System.Security.Cryptography.MD5.Create();
md5Check.TransformBlock(retrievedBuffer, 0, retrievedBuffer.Length, null, 0);     
md5Check.TransformFinalBlock(new byte[0], 0, 0);

// Get Hash Value
byte[] hashBytes = md5Check.Hash;
string hashVal = Convert.ToBase64String(hashBytes);

and it works...

And yes, as Guarav already mentioned - MD5 hash is saved as base64 string.

Comments

[Gaurav Mantri]

Try to convert MD5 byte array into Base64 string and see if that matches.

[Salman]

Thanks. Your code returns the same string as blob.Properties.ContentMD5. However, i used the same code to convert the byte array from source file to Base64String and it works!

[Manfred]

Code example for a shorter version using the newer MD5.ComputeHash(Stream stream) can be found at stackoverflow.com/a/43647643/411428

[Myles]

It looks like that article is no longer available on the blog. Here is an archive copy from when this answer was posted.