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In the light of extending our eKMS support it will be helpful to have a tighter use of the word "KMS". KMS should refer to the actual component that manages keys. The keyservice, also called KMS in the constellation code, does not manage keys itself. It talks to a KMS backend, which in turn does the actual key management.
42 lines
1.8 KiB
Markdown
42 lines
1.8 KiB
Markdown
# Key derivation
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To derive disk-encryption keys and other secret material in a Constellation Kubernetes cluster, we use [HKDF](https://datatracker.ietf.org/doc/html/rfc5869).
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To derive a key, HKDF depends on four parameters: *input keying material (IKM)*, *salt*, *info*, and a hash function.
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The parameters *salt* and *info* are optional.
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Key derivation is performed in two steps:
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1. [Extract](https://datatracker.ietf.org/doc/html/rfc5869#section-2.2)
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A pseudorandom key is generated from the secret *IKM* and an optional independent *salt*.
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2. [Expand](https://datatracker.ietf.org/doc/html/rfc5869#section-2.3)
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The pseudorandom key generated by the extract phase is used to derive keys.
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Using the optional *info* parameter multiple different keys can be created from the key created in the extract phase.
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## Hash
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We use SHA-256 as the hash function.
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## Initial Key Material
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Constellation's master secret is used as *IKM* for HKDF.
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The master secret is either provided by the admin, or a randomly generated 32 byte value.
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It is supplied to the cluster during `constellation init` and in the following used by [Constellation's keyservice](../keyservice/) and managed by Kubernetes as a [secret](https://kubernetes.io/docs/concepts/configuration/secret/).
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## Salt
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To salt the extract phase of HKDF, Constellation uses a randomly generated 32 Byte value during `constellation init`.
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The *salt* is returned to the user for data recovery, and kept in the cluster as a Kubernetes secret.
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## Info
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The *info* field is used to add context-specific information to a derived key.
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This allows using the same *IKM* to derive multiple different keys.
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### Disk-encryption key
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For disk-encryption keys, *info* is set to the string `key-` appended with the lower-case LUKS2 UUID of the disk.
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