AWS S3 ETag is not an ordinary MD5 hash (http://docs.aws.amazon.com/AmazonS3/latest/API/RESTCommonResponseHeaders.html).
The default chunk size is 8 MB used by the official aws cli tool.
$ aws s3api head-object --bucket chunj-ref --key vep86.tgz
{
"AcceptRanges": "bytes",
"ContentType": "application/x-tar",
"LastModified": "Wed, 24 May 2017 15:22:46 GMT",
"ContentLength": 17548232894,
"ETag": "\"fbf3e61bc72ad19cf4d6e90a6c174687-2092\"",
"Metadata": {}
}$ python compute_s3_etag.py
"fbf3e61bc72ad19cf4d6e90a6c174687-2092"