Great, now I can actually host my private yum/apt repository on S3. These are the different keys you can use in your bucket policy conditions:

aws:CurrentTime — For date/time conditions (see Date Conditions)

aws:SecureTransport — Boolean representing whether the request was sent using SSL (see Boolean Conditions)

aws:SourceIp — The requester's IP address, for use with IP address conditions (see IP Address)

aws:UserAgent — Information about the requester's client application, for use with string conditions (see String Conditions)

aws:EpochTime — Number of seconds since epoch.

aws:Referer — Same the the HTTP referer field.

(from http://docs.amazonwebservices.com/AmazonS3/latest/dev/Access...)