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58 lines
4.6 KiB
Markdown
58 lines
4.6 KiB
Markdown
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# Encrypted persistent storage
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Confidential VMs provide runtime memory encryption to protect data in use.
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In the context of Kubernetes, this is sufficient for the confidentiality and integrity of stateless services.
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Consider a front-end web server, for example, that keeps all connection information cached in main memory.
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No sensitive data is ever written to an insecure medium.
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However, many real-world applications need some form of state or data-lake service that's connected to a persistent storage device and requires encryption at rest.
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As described in [Use persistent storage](../workflows/storage.md), cloud service providers (CSPs) use the container storage interface (CSI) to make their storage solutions available to Kubernetes workloads.
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These CSI storage solutions often support some sort of encryption.
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For example, Google Cloud [encrypts data at rest by default](https://cloud.google.com/security/encryption/default-encryption), without any action required by the customer.
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## Cloud provider-managed encryption
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CSP-managed storage solutions encrypt the data in the cloud backend before writing it physically to disk.
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In the context of confidential computing and Constellation, the CSP and its managed services aren't trusted.
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Hence, cloud provider-managed encryption protects your data from offline hardware access to physical storage devices.
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It doesn't protect it from anyone with infrastructure-level access to the storage backend or a malicious insider in the cloud platform.
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Even with "bring your own key" or similar concepts, the CSP performs the encryption process with access to the keys and plaintext data.
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In the security model of Constellation, securing persistent storage and thereby data at rest requires that all cryptographic operations are performed inside a trusted execution environment.
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Consequently, using CSP-managed encryption of persistent storage usually isn't an option.
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## Constellation-managed encryption
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Constellation provides CSI drivers for storage solutions in all major clouds with built-in encryption support.
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Block storage provisioned by the CSP is [mapped](https://guix.gnu.org/manual/en/html_node/Mapped-Devices.html) using the [dm-crypt](https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/dm-crypt.html), and optionally the [dm-integrity](https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/dm-integrity.html), kernel modules, before it's formatted and accessed by the Kubernetes workloads.
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All cryptographic operations happen inside the trusted environment of the confidential Constellation node.
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Note that for integrity-protected disks, [volume expansion](https://kubernetes.io/blog/2018/07/12/resizing-persistent-volumes-using-kubernetes/) isn't supported.
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By default the driver uses data encryption keys (DEKs) issued by the Constellation [*KMS*](components.md#kms).
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The DEKs are in turn derived from the Constellation's key encryption key (KEK), which is directly derived from the [master secret](keys.md#master-secret).
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This is the recommended mode of operation, and also requires the least amount of setup by the cluster administrator.
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Alternatively, the driver can be configured to use a key management system to store and access KEKs and DEKs.
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Refer to [keys and cryptography](keys.md) for more details on key management in Constellation.
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Once deployed and configured, the CSI driver ensures transparent encryption and integrity of all persistent volumes provisioned via its storage class.
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Data at rest is secured without any additional actions required by the developer.
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## Cryptographic algorithms
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This section gives an overview of the libraries, cryptographic algorithms, and their configurations, used in Constellation's CSI drivers.
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### dm-crypt
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To interact with the dm-crypt kernel module, Constellation uses [libcryptsetup](https://gitlab.com/cryptsetup/cryptsetup/).
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New devices are formatted as [LUKS2](https://gitlab.com/cryptsetup/LUKS2-docs/-/tree/master) partitions with a sector size of 4096 bytes.
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The used key derivation function is [Argon2id](https://datatracker.ietf.org/doc/html/rfc9106) with the [recommended parameters for memory-constrained environments](https://datatracker.ietf.org/doc/html/rfc9106#section-7.4) of 3 iterations and 64 MiB of memory, utilizing 4 parallel threads.
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For encryption Constellation uses AES in XTS-Plain64. The key size is 512 bit.
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### dm-integrity
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To interact with the dm-integrity kernel module, Constellation uses [libcryptsetup](https://gitlab.com/cryptsetup/cryptsetup/).
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When enabled, the used data integrity algorithm is [HMAC](https://datatracker.ietf.org/doc/html/rfc2104) with SHA256 as the hash function.
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The tag size is 32 Bytes.
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