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42 lines
1.8 KiB
Markdown
42 lines
1.8 KiB
Markdown
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# 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 KMS](../kms/) 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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