CloudFront automatically adds this element to the response only if you've set up the distribution to serve private content with signed URLs. The element lists the key pair IDs that CloudFront is aware of for each trusted signer. The Signer child element lists the AWS account number of the trusted signer (or an empty Self element if the signer is you). The Signer element also includes the IDs of any active key pairs associated with the trusted signer's AWS account. If no KeyPairId element appears for a Signer, that signer can't create working signed URLs.