[ Thanks to Ramsai
for this link. ]
“All communication with SimpleDB or any of the Amazon web
services must be through either the SOAP interface or the
Query/ReST interface. The request messages sent through either of
these interfaces is digitally signed by the sending user in order
to ensure that the messages have not been tampered within transit,
and that they really originate from the sending user. Requests that
use the Query/ReST interface will use the access keys for signing
the request, whereas requests to the SOAP interface will use the
x.509 certificates.”