How to get the md5sum of a file on Amazon's S3

Question

If I have existing files on Amazon's S3, what's the easiest way to get their md5sum without having to download the files?

Answer 1

AWS's documentation of ETag says:

The entity tag is a hash of the object. The ETag reflects changes only to the contents of an object, not its metadata. The ETag may or may not be an MD5 digest of the object data. Whether or not it is depends on how the object was created and how it is encrypted as described below:

• Objects created by the PUT Object, POST Object, or Copy operation, or through the AWS Management Console, and are encrypted by SSE-S3 or plaintext, have ETags that are an MD5 digest of their object data.
• Objects created by the PUT Object, POST Object, or Copy operation, or through the AWS Management Console, and are encrypted by SSE-C or SSE-KMS, have ETags that are not an MD5 digest of their object data.
• If an object is created by either the Multipart Upload or Part Copy operation, the ETag is not an MD5 digest, regardless of the method of encryption.

Reference: http://docs.aws.amazon.com/AmazonS3/latest/API/RESTCommonResponseHeaders.html

Answer 2

ETag does not seem to be MD5 for multipart uploads (as per Gael Fraiteur's comment). In these cases it contains a suffix of minus and a number. However, even the bit before the minus does not seem to be the MD5, even though it is the same length as an MD5. Possibly the suffix is the number of parts uploaded?