There are two main problems related to pinning techniques. The first one is related to the process
of bootstrapping the trust procedures, how we decide which associations are established.
These associations can be set the first encounter with the host in a Trust-On-First-Use basis
(TOFU), or can be defined by a list that is shipped with the application. The second one is
the need for maintenance of the secure associations database, which is the secure creation of
new associations and the revocation of existing ones if needed. Currently, there the two main
proposals for certificate pinning are the Trust Assertion for Certificate Keys (TACK) Internet
Draft [9] and the Public Key Pinning Extension for HTTP [10] promoted by Google.