Appendix A. Out-of-Band Key-Pinned Privacy Profile Example

This section presents an example of how the out-of-band key-pinned privacy profile could work in practice based on a minimal pin set (two pins).

A DNS client system is configured with an out-of-band key-pinned privacy profile from a network service, using a pin set containing two pins. Represented in HTTP Public Key Pinning (HPKP) [RFC7469] style, the pins are:

pin-sha256="FHkyLhvI0n70E47cJlRTamTrnYVcsYdjUGbr79CfAVI="
pin-sha256="dFSY3wdPU8L0u/8qECuz5wtlSgnorYV2f66L6GNQg6w="

The client also configures the IP addresses of its expected DNS server: perhaps 192.0.2.3 and 2001:db8::2:4.

The client connects to one of these addresses on TCP port 853 and begins the TLS handshake: negotiation of TLS 1.2 with a Diffie-Hellman key exchange. The server sends a certificate message with a list of three certificates (A, B, and C) and signs the ServerKeyExchange message correctly with the public key found in certificate A.

The client now takes the SHA-256 digest of the SPKI in cert A and compares it against both pins in the pin set. If either pin matches, the verification is successful; the client continues with the TLS connection and can make its first DNS query.

If neither pin matches the SPKI of cert A, the client verifies that cert A is actually issued by cert B. If it is, it takes the SHA-256 digest of the SPKI in cert B and compares it against both pins in the pin set. If either pin matches, the verification is successful. Otherwise, it verifies that B was issued by C and then compares the pins against the digest of C's SPKI.

If none of the SPKIs in the cryptographically valid chain of certs match any pin in the pin set, the client closes the connection with an error and marks the IP address as failed.