About disk encryption technology

Key escrow is the traditional system for storing cryptographic keys. The key server on the network stores the encryption key for a node with an encrypted boot disk and returns it when queried. The complexities around key management, transport encryption, and authentication do not make this a reasonable choice for boot disk encryption.

Although available in Fedora, key escrow-based disk encryption setup and management is a manual process and not suited to OKD automation operations, including automated addition of nodes, and currently not supported by OKD.

Trusted Platform Module (TPM) disk encryption is best suited for data centers or installations in remote protected locations. Full disk encryption utilities such as dm-crypt and BitLocker encrypt disks with a TPM bind key, and then store the TPM bind key in the TPM, which is attached to the motherboard of the node. The main benefit of this method is that there is no external dependency, and the node is able to decrypt its own disks at boot time without any external interaction.

TPM disk encryption protects against decryption of data if the disk is stolen from the node and analyzed externally. However, for insecure locations this may not be sufficient. For example, if an attacker steals the entire node, the attacker can intercept the data when powering on the node, because the node decrypts its own disks. This applies to nodes with physical TPM2 chips as well as virtual machines with Virtual Trusted Platform Module (VTPM) access.

Network-Bound Disk Encryption (NBDE) effectively ties the encryption key to an external server or set of servers in a secure and anonymous way across the network. This is not a key escrow, in that the nodes do not store the encryption key or transfer it over the network, but otherwise behaves in a similar fashion.

Clevis and Tang are generic client and server components that provide network-bound encryption. Fedora CoreOS (FCOS) uses these components in conjunction with Linux Unified Key Setup-on-disk-format (LUKS) to encrypt and decrypt root and non-root storage volumes to accomplish Network-Bound Disk Encryption.

When a node starts, it attempts to contact a predefined set of Tang servers by performing a cryptographic handshake. If it can reach the required number of Tang servers, the node can construct its disk decryption key and unlock the disks to continue booting. If the node cannot access a Tang server due to a network outage or server unavailability, the node cannot boot and continues retrying indefinitely until the Tang servers become available again. Because the key is effectively tied to the node’s presence in a network, an attacker attempting to gain access to the data at rest would need to obtain both the disks on the node, and network access to the Tang server as well.

The following figure illustrates the deployment model for NBDE.

The following figure illustrates NBDE behavior during a reboot.

Shamir’s secret sharing (sss) is a cryptographic algorithm to securely divide up, distribute, and re-assemble keys. Using this algorithm, OKD can support more complicated mixtures of key protection.

When you configure a cluster node to use multiple Tang servers, OKD uses sss to set up a decryption policy that will succeed if at least one of the specified servers is available. You can create layers for additional security. For example, you can define a policy where OKD requires both the TPM and one of the given list of Tang servers to decrypt the disk.