Download backup

apiserver/backup: api endpoint for downloading a backup
Is this a standard header? If it isn't, then I would like us to be clear what sha algorithm we are using. So "X-Content-SHA1" or SHA256 or whatever.
some quick googling finds:
http://www.ietf.org/rfc/rfc3230.txt

Which seems to say: 
Digest: SHA=BASE64ENCODED

Also note: http://tools.ietf.org/html/rfc6648
Which says that using "X-" is deprecated, because it just ends up with the "official" names for working things having an X- in them until you break compatibility with everyone and introduce the non X- prefixed version.