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The cryptographic mechanism to recreate the encryption key is based on the blinded key stored on the node and the private key of the involved Tang servers. To protect against the possibility of an attacker who has obtained both the Tang server private key and the node’s encrypted disk, periodic rekeying is advisable.

You must perform the rekeying operation for every node before you can delete the old key from the Tang server. The following sections provide procedures for rekeying and deleting old keys.

Backing up keys for a Tang server

The Tang server uses /usr/libexec/tangd-keygen to generate new keys and stores them in the /var/db/tang directory by default. To recover the Tang server in the event of a failure, back up this directory. The keys are sensitive and because they are able to perform the boot disk decryption of all hosts that have used them, the keys must be protected accordingly.

Procedure
  • Copy the backup key from the /var/db/tang directory to the temp directory from which you can restore the key.

Recovering keys for a Tang server

You can recover the keys for a Tang server by accessing the keys from a backup.

Procedure
  • Restore the key from your backup folder to the /var/db/tang/ directory.

    When the Tang server starts up, it advertises and uses these restored keys.

Rekeying Tang servers

This procedure uses a set of three Tang servers, each with unique keys, as an example.

Using redundant Tang servers reduces the chances of nodes failing to boot automatically.

Rekeying a Tang server, and all associated NBDE-encrypted nodes, is a three-step procedure.

Prerequisites
  • A working Network-Bound Disk Encryption (NBDE) installation on one or more nodes.

Procedure
  1. Generate a new Tang server key.

  2. Rekey all NBDE-encrypted nodes so they use the new key.

  3. Delete the old Tang server key.

    Deleting the old key before all NBDE-encrypted nodes have completed their rekeying causes those nodes to become overly dependent on any other configured Tang servers.