Customer-managed encryption keys

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This page discusses customer-managed encryption keys and how they are used in Cloud Storage. For other encryption options, see Data Encryption Options.

Overview

As an additional layer on top of Google-managed encryption keys, you can choose to use keys generated by Cloud Key Management Service. Such keys are known as customer-managed encryption keys. If you use a customer-managed encryption key, your encryption keys are stored within Cloud KMS. The project that holds your encryption keys can then be independent from the project that contains your buckets, thus allowing for better separation of duties.

When is the key used?

When you apply a customer-managed encryption key to an object, Cloud Storage uses the key when encrypting:

  • The object's data.
  • The object's CRC32C checksum.
  • The object's MD5 hash.

Cloud Storage uses standard server-side keys to encrypt the remaining metadata for the object, including the object's name. Thus, if you have sufficient permission, you can perform actions such as reading most metadata, listing objects, and deleting objects even after you've disabled or destroyed the associated customer-managed encryption key.

Service accounts

Encryption and decryption with customer-managed encryption keys is accomplished using service accounts. Once you give your Cloud Storage service account access to an encryption key, that service account encrypts:

When adding or rewriting an object in Cloud Storage, if you have both a default key set on your bucket and a specific key included in your request, Cloud Storage uses the specific key to encrypt the object.

When a requester wants to read an object encrypted with a customer-managed encryption key, they simply access the object as they normally would. During such a request, the service account automatically decrypts the requested object as long as:

  • The service account still has permission to decrypt using the key.
  • You have not disabled or destroyed the key.

If one of these conditions is not met, the service account does not decrypt the data, and the request fails.

Restrictions

The following restrictions apply when using customer-managed encryption keys:

  • You cannot perform object composition when one or more of the source objects are encrypted with a customer-managed encryption key.

  • You cannot encrypt an object with a customer-managed encryption key by updating the object's metadata. Include the key as part of a rewrite of the object instead.

  • You must create the Cloud KMS key in the same location as the data you intend to encrypt. For example, if your bucket is located in US-EAST1, any Cloud KMS key encrypting objects in that bucket must also be created in US-EAST1. For available Cloud KMS locations, see Cloud KMS locations.

  • You cannot specify a customer-managed encryption key as part of a Storage Transfer Service transfer, and any such keys on source objects are not applied to the transferred objects. Set a default customer-managed key on your bucket prior to performing the transfer.

  • Cloud KMS encryption and decryption rates are subject to a quota.

Relation to customer-supplied encryption keys

In addition to customer-managed encryption, Cloud Storage offers Customer-Supplied Encryption Keys as a way of controlling your data encryption. You can encrypt different objects in a single bucket with different encryption methods, but note that:

  • A single object can only be encrypted by one of these methods at a time.

  • If you have a default customer-managed key set for your bucket and specify a customer-supplied key in a request, Cloud Storage uses the customer-supplied key to encrypt the object.

  • You can set a default customer-managed key on your bucket, but you cannot set a default customer-supplied key on your bucket.

Key management

This section discusses considerations when rotating keys, replacing keys, and disabling or destroying key versions.

Key rotation

Cloud KMS supports both automatic and manual key rotation to a new version. After rotating a key, Cloud Storage uses the new version for all operations that encrypt using the key, such as:

  • Object uploads when the destination bucket uses the key as its default encryption key.

  • Object upload, copy, and rewrite operations that specifically use the key in the operation.

Previous versions of the key are not disabled or destroyed, so Cloud Storage can still decrypt existing objects that were previously encrypted using those versions.

Key replacement

Use the following guidelines when replacing the key you use to encrypt Cloud Storage objects with a new key:

  1. Check your buckets to see which use the key as their default encryption key. For these buckets, replace the old key with a new key.

    This ensures that all objects written to the bucket use the new key going forward.

  2. Inspect your source code to understand which requests use the key in ongoing operations, such as setting bucket configurations and uploading, copying, or rewriting objects. Update these instances to use the new key.

  3. Check for objects, in all of your buckets, encrypted with the old key. Use the Rewrite Object method to re-encrypt each object with the new key.

  4. Disable all versions of the old key. After disabling old key versions, monitor client and service logs for operations that fail due to a version becoming unavailable.

Disabling or destroying a key version

  • When you disable or destroy a specific key version, you cannot decrypt any object that is currently encrypted with that key version.

    For example, you cannot download, copy, or rewrite the object, and attempting to do so results in an error.

    • If you disable a key version, you can re-enable it. Once re-enabled, you can access objects that were encrypted by that key version.

    • If you destroy a key version, downloads of objects encrypted with that version are never possible again.

    Before disabling or destroying a key version, you should identify all objects, in all buckets, that were encrypted using the specific key version. Once identified, use the Rewrite Object method to re-encrypt each object using a new key version, a whole new key, or server-side keys.

  • When you disable or destroy the primary version of a key, you cannot use the key for encryption until you have a new primary version. For example, without a primary version:

    • You cannot specify the key as part of an object upload, copy, or rewrite.

    • You cannot upload, copy, or rewrite objects to a bucket that has the key set as its default encryption key unless you specify a different, valid key as part of the operation.

    Once you have a primary version for your key, operations that use the key to encrypt objects succeed.

    Before disabling or destroying a key version that is the primary version of the key, you should first stop using it as the primary version. You can do so by either:

    • Replacing it with a new primary version, typically by performing a key rotation.
    • Removing instances where you use the key for encryption. When you do so, Cloud Storage uses server-side keys for encryption instead.

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