You can grant access permissions to buckets and objects within your Amazon S3 account to anyone with Canonical User ID. The canonical user id is specific to S3 and is a 64 character long hex string. In Bucket Explorer, you can use it as an alternative to Email ID when you share your bucket with any other user with her AWS registered EmailID.

Get the Canonical Details by just supplying the AWS Access Identifiers Access Key* Secret Key*

Use of AWS Canonical ID in Bucket Explorer