KMIP

The Key Management Interoperability Protocol was standardised by OASIS in 2010, after every storage vendor had invented its own way of talking to its own key server. The client — a disk array, a database, a backup server — sends binary messages encoded as TTLV (tag, type, length, value) over mutually authenticated TLS, usually on port 5696.

Where PKCS#11 is a local API to an HSM, KMIP is a network protocol to a key manager, which in turn often has an HSM behind it. The promise was that you could switch vendors without rewriting the integration. In practice each vendor supports its own subset of the spec, so interoperability is still checked at annual plugfests.