When I say "key" I mean the blob that gets stored, but I may be misremembering or misusing S3 terms... it was large amounts of DNA sequencing data, and one of the first tasks was to add S3 support for indexed reads to our internal HTSlib fork, and since then somebody else's implementation has been added to the library. In any case, I quickly forgot about most of the details of S3 when I no longer had to deal with it directly...