Meta server: encrypt all synchronous replication chain session keys with the corresponding chunk server key, master's key as replication chain transport isn't used for it. This is needed in order to allow transmission of these keys on a possibly clear text channel, and limit the keys exposure in general only the party that the key is created for. Change chunk allocate request, and lease renew response formats to simplify parsing and updating chunk access and chunk server access tokens in the case where only chunk access tokens need to be updated with the lease renew. Implement optional transmission chunk server access tokens and session keys with write lease renew to gracefully handle possible chunk server access tokens expiration.