For more information regarding how to get your Canonical User Id, please read the following documentation @ http://satrangtech.com/dragdropuploadtoamazons3/HowToGetCanonicalUserId.pdf.
PS: View Drag, Drop & Upload Files to Amazon S3, a Salesforce AppExchange app for moving files from Salesforce to Amazon S3: https://appexchange.salesforce.com/appxListingDetail?listingId=a0N3000000B5ZPqEAN