There are three types of queries to be made against the Verification service:
- submit a certificate;
- query a CN;
- query any duplicate CN for a given domain.

The biggest load on the Certificate verification service are the
queries keyed on CN. These queries will be done every time a user
comes across a new certificate. On a blog site where every blog and
comment is signed with the users’ private key, it means one lookup for
each new participant.

Most communities are relatively small, so while newcomer to a
community has to do a lot of lookups initially, that amount quickly
diminishes. A stable community has very little lookups to expect.

Another source of lookups is when one person wants to send a private
message to another. To protect agaist MitM, both sender and receiver
make two lookups of each others’ and their own CN, one lookup of each
before sending the first message, one lookup after receiving the first
reply. This suffices to establish to both parties that the CN is
unique. Both parties have effectively communicated their public keys
between them. They will remember this fact. From now on, there are no
more lookups for messages between these two.

This key-exchange dance with 4 lookups needs to be done separately for
each person one communicates with. Once this association is made,
there is no further need for key lookups. This lookup is cemented, no
matter if the CA signs another