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Has anybody seen my keys? A key-hierarchy strategy for rack-level security

2026-09-07 | original
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Why This Matters

This is an engineering deep-dive into how a rack-scale system builds a key hierarchy on top of a 'rack secret' reconstructed from K key shares, and why the choice between key derivation and key wrapping matters operationally. It matters because it shows the practical tradeoffs vendors face in securing data at rest across clustered hardware: derived keys avoid on-disk storage but cascade on rotation, while wrapped keys survive parent-key rotation but must be stored and replicated. For customers, this determines whether a compromised node or a cluster reconfiguration forces expensive re-encryption of bulk storage.

Key Takeaways
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Assuming a static cluster, we now have a rack secret that can be computed from K key shares and used to derive child keys. In the following example, K = 2 , just so we can keep the diagrams small.

Show Source | Mermaid

The rack secret protects every key in the system that is used after rack unlock. There are two mechanisms to get the next keys in the hierachy:

Key Derivation

Key Wrapping (Key Encryption)

Key Derivation we already use to derive any primary child keys from the rack secret. But it can also be used to derive keys from keys. Key wrapping is just taking a key and encrypting it. Each has their benefits and drawbacks.

Key derivation is nice because you never have to store the derived keys on disk. You can just regenerate them. The problem with it is that if the derived key changes any downstream derived keys will now change also change.

Key wrapping is nice because if the wrapper key is rotated, the downstream keys do not have to change. This is particularly useful for encrypting large amounts of data on storage. You don’t want to be forced to decrypt and then re-encrypt just because a parent key was rotated. The downside of key wrapping is that you now have to store the wrapped (encrypted) key on disk somewhere. If the same key is used in multiple places you have to replicate it.

For trust quorum, we have the particular problem that when nodes are added or removed we generate new shares. While it’s possible to maintain the same rack secret and key derivation with new shares, it is less secure over time because any single malicious node who retrieved the rack secret at one time and saved it can now recover any data on any existing drives or any data produced in the future. While this is always an issue, we prefer to allow the rotation of the rack secret in the case of a known compromise. We therefore always generate a new rack secret on every change of the trust quorum (reconfiguration), or other key-share rotation. This way, even if all existing data on the rack is compromised, at least no new new data will be compromised if the compromised sled is removed.

Right now what do we know about our security goals? We want to:

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Explore topics: key derivation key wrapping rack secret trust quorum key hierarchy

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