Object Storage Service:Checking OSS MD5 consistency

To check whether an uploaded object in OSS is consistent with the local file, you can include the Content-MD5 header value in the upload request. When OSS receives the object, OSS compares the MD5 hash with the Content-MD5 header value. The object can be uploaded only when the MD5 hash is consistent with the Content-MD5 header value. This way, data consistency is ensured.

This section uses the message content "0123456789" as an example to show the correct and incorrect ways to calculate the Content-MD5 value.

• Correct calculation

  1. First, calculate the MD5 hash, which is a 128-bit binary array.

  2. Base64-encode the binary array, not the 32-character hexadecimal string.

  The following example uses Python:

  >>> import base64,hashlib
  >>> hash = hashlib.md5()
  >>> hash.update("0123456789")   # In Python 3, change this to hash.update(b"0123456789").
  >>> base64.b64encode(hash.digest())
  'eB5e********************'

  The hash.digest() method returns the 128-bit binary array.

  >>> hash.digest()
  'x\x1e^$]i\xb5f\x97\x9b\x86\xe2\x8d#\xf2\xc7'

• Example of an incorrect calculation

  Note

  A common mistake is to directly Base64-encode the 32-character hexadecimal string.

  # The hash.hexdigest() method calculates the visible 32-character hexadecimal string.
  >>> hash.hexdigest()
  '781e****************************'
  # The result of Base64-encoding the incorrect MD5 hash.
  >>> base64.b64encode(hash.hexdigest())
  'Nzgx****************************************'