There MUST be an RRSIG for each RRset using at least one DNSKEY of
each algorithm in the zone apex DNSKEY RRset. The apex DNSKEY RRset
itself MUST be signed by each algorithm appearing in the DS RRset
located at the delegating parent (if any).
Zone must be signed with at least one key of each algorithm that appears in the keyset. We should check the RRSIGs from the keyset and from the SOA record.
RFC 4035, section 2.2 (last paragraph):
Zone must be signed with at least one key of each algorithm that appears in the keyset. We should check the RRSIGs from the keyset and from the SOA record.