Calculate the AWS S3 Checksum

s3HashCalculator.cs

public static async Task<string> HashOf(string filename, int chunkSizeInMb)
    {
      var returnMd5 = string.Empty;
      var chunkSize = chunkSizeInMb * 1024 * 1024;

      await Task.Run(() =>
      {
        using (var crypto = new System.Security.Cryptography.MD5CryptoServiceProvider())
        {
          var hashLength = crypto.HashSize / 8;

          using (var stream = File.OpenRead(filename))
          {
            if (chunkSize != 0 && stream.Length > chunkSize)
            {
              var chunkCount = (long)Math.Ceiling(stream.Length / (double)chunkSize);

              var hash = new byte[chunkCount * hashLength];
              var hashStream = new MemoryStream(hash);

              var nByteLeftToRead = stream.Length;
              while (nByteLeftToRead > 0)
              {
                int nByteCurrentRead = (int)Math.Min(nByteLeftToRead, chunkSize);
                byte[] buffer = new byte[nByteCurrentRead];
                nByteLeftToRead -= stream.Read(buffer, 0, nByteCurrentRead);
                byte[] tmpHash = crypto.ComputeHash(buffer);
                hashStream.Write(tmpHash, 0, hashLength);
              }

              returnMd5 = $"{BitConverter.ToString(crypto.ComputeHash(hash)).Replace("-", string.Empty).ToLower()}-{chunkCount}";
            }
            else
            {
              returnMd5 = BitConverter.ToString(crypto.ComputeHash(stream))
              .Replace("-", string.Empty)
              .ToLower();
            }

            stream.Close();
          }
        }
      });
      return returnMd5;
    }