9.14. Deploying high-availability NBDE systems


Tang provides two methods for building high-availability deployments of Network-bound Disc Encryption (NBDE): client redundancy and key sharing.

Client redundancy (recommended)
Clients should be configured with the ability to bind to multiple Tang servers. In this setup, each Tang server has its own keys and clients can decrypt by contacting a subset of these servers. Clevis already supports this workflow through its sss plugin. Use this method for a secure high-availability deployment.
Key sharing
For redundancy purposes, more than one instance of Tang can be deployed. To set up a second or any subsequent instance, install the tang packages and copy the key directory to the new host by using rsync over SSH. This method of sharing keys increases the risk of key compromise and requires additional automation infrastructure.
High-available NBDE using Shamir’s Secret Sharing

Shamir’s Secret Sharing (SSS) is a cryptographic scheme that divides a secret into several unique parts. To reconstruct the secret, a number of parts is required. The number is called the threshold and SSS is also referred to as a thresholding scheme.

Clevis provides an implementation of SSS. It creates a key and divides it into several pieces. Each piece is encrypted using another pin including even SSS recursively. Additionally, you define the threshold t. If an NBDE deployment decrypts at least t pieces, then it recovers the encryption key and the decryption process succeeds. When Clevis detects a smaller number of parts than specified in the threshold, it prints an error message.

예 9.1. Redundancy with two Tang servers

The following command decrypts a LUKS-encrypted device when at least one of two Tang servers is available:

# clevis luks bind -d /dev/sda1 sss '{"t":1,"pins":{"tang":[{"url":"http://tang1.srv"},{"url":"http://tang2.srv"}]}}'

The previous command used the following configuration scheme:

{
    "t":1,
    "pins":{
        "tang":[
            {
                "url":"http://tang1.srv"
            },
            {
                "url":"http://tang2.srv"
            }
        ]
    }
}

In this configuration, the SSS threshold t is set to 1 and the clevis luks bind command successfully reconstructs the secret if at least one from two listed tang servers is available.

예 9.2. Shared secret on a Tang server and a TPM device

The following command successfully decrypts a LUKS-encrypted device when both the tang server and the tpm2 device are available:

# clevis luks bind -d /dev/sda1 sss '{"t":2,"pins":{"tang":[{"url":"http://tang1.srv"}], "tpm2": {"pcr_ids":"0,7"}}}'

The configuration scheme with the SSS threshold 't' set to '2' is now:

{
    "t":2,
    "pins":{
        "tang":[
            {
                "url":"http://tang1.srv"
            }
        ],
        "tpm2":{
            "pcr_ids":"0,7"
        }
    }
}

For details and more examples, see the High Availability section in the tang(8) man page, the Shamir’s Secret Sharing section in the clevis(1) man page, and the clevis-encrypt-sss(1) man page on your system.

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