In TACK, clients are allowed to pin to a server-chosen signing key (TACK signing key, TSK),
which will be used to sign server’s TLS keys. Given that the actual TLS keys are not pinned,
the site is able to deploy different certificates and keys on different servers, without having the
clients to renew its pins. Also since pins are not based on CA keys, there is no need to trust in
CAs. TACK also defines a mechanism to activate pins. As part of the TLS handshake, a client
could request a compliant TACK server to send its TSK public key and signature. Once a client
has seen the same hostname-TSK pair multiple times, it could decide to activate a time-limited
pin for that pair. By time-limiting the pins, the potential impact of a bad pinning decision is
bounded. The specification also mentions that pins could be aggregated and shared through a
trusted third party but without defining either the infrastructure or the protocols required. This
proposal, while promising, is still in a very early stage and accordingly not suitable for use in
a production environment.