Design and build a privately hosted Public Key Infrastructure

Highly available Certificate Authorities refer to online intermediate CAs, not Root CAs, which are kept offline and unavailable for use. See principle 10 for more detail on root CAs.

An intermediate certificate authority (CA) acts as a trusted component within a PKI, performing such critical functions as certificate issuing, renewal, and revocation. Failure or unavailability of these functions would cause disruption, so you should design your CA and its supporting components to be highly available.

Clustering should be used to prevent the failure of one system component bringing down the whole PKI.

Backup and restore

Consider a back up and restore plan for your CA in the case of failure, this is especially important if distributing a new CA certificate to end entities is difficult.

The backup should include the state (authentication, revocation, registration database etc.) and the private key. See principle 10.

Backing up private keys

Secure handling of private keys is an important part of the backup process. Do not store private keys unencrypted. Here are some example methods that could be used:

• HSM Backup - If a hardware security module is being used, use the HSM's secure private key backup mechanism. Beware that this mechanism is often proprietary so may only restore to an HSM from the same manufacturer.
• Key Splitting Algorithm - Use a key splitting algorithm to divide the keys into multiple shards, then require a number of shards to rebuild the key. For example, split a key into five shards and require three to rebuild it. The shards should be stored in multiple secure locations under the responsibility of different people. This method balances redundancy and security since, if one of the shards were to be compromised or corrupted, it would not affect the integrity of the overall backup. It is important to seek professional advice when considering key splitting as implementing this securely is difficult. Only use a key splitting algorithm in a trusted product or that's been developed by a trained cryptographic professional.
• Key Wrapping - Wrap (encrypt) a private key with a symmetric key before exporting it off the device. The symmetric key will still need to be appropriately physically secured.

Full system restore

There should be a clearly defined restore process for the whole PKI environment. This should be exercised on a regular basis.