C# .NET 6 中的不同文件SHA256、SHA512、MD5生成完全相同的hash

Question

I have a strange issue. I have the following very basic code:

private async Task<string> _CalculateChecksum(Stream strm)
    {
        System.Security.Cryptography.MD5 sha = System.Security.Cryptography.MD5.Create();
        byte[] checksum = await sha.ComputeHashAsync(strm);
        //return BitConverter.ToString(checksum).Replace("-", "");
        return Convert.ToBase64String(checksum);
    }

I would like to calculate hash values for files. I tried it with SHA256 first. It doesn't matter if I change the hash algorithm from MD5 to either SHA256 or SHA512, the generated hash values for different files are always the same. Eg: one of test file's size is approx. 4. bigger than the other file. The hash is the same with all algorithms. I also tried using BitConverter.ToString instead of Convert.ToBase64String , but it does not matter because the content of checksum byte array is exactly the same.

I also tried using normal (non-async) method, but the result is the same.

Does anyone have any idea why is this happening?

Here is the code where the hashing part is called:

MemoryStream ms = new MemoryStream();
await file.CopyToAsync(ms);
string hash = await _CalculateChecksum(ms);

"file" is an IFormFile object which contains the correct value.

Answer 1

After the await file.CopyToAsync(ms) , the position of the memory stream is at its end. sha.ComputeHashAsync(strm) calculates the hash of the data after the current position, so basically that of an empty stream.

Try resetting the position of the stream to the start before the call to _CalculateChecksum:

MemoryStream ms = new MemoryStream();
await file.CopyToAsync(ms);
ms.Seek(0, SeekOrigin.Begin);
string hash = await _CalculateChecksum(ms);