Cis GCP Foundations 1.10

1.10 Ensure KMS Encryption Keys Are Rotated Within a Period of 90 Days (Automated)

Profile Applicability

• Level 1

Description

Google Cloud Key Management Service stores cryptographic keys in a hierarchical structure designed for useful and elegant access control management.

The format for the rotation schedule depends on the client library that is used. For the gcloud command-line tool, the next rotation time must be in ISO or RFC3339 format, and the rotation period must be in the form INTEGER[UNIT], where units can be one of seconds (s), minutes (m), hours (h) or days (d).

Rationale

Set a key rotation period and starting time. A key can be created with a specified rotation period, which is the time between when new key versions are generated automatically. A key can also be created with a specified next rotation time. A key is a named object representing a cryptographic key used for a specific purpose. The key material, the actual bits used for encryption, can change over time as new key versions are created.

A key is used to protect some corpus of data. A collection of files could be encrypted with the same key and people with decrypt permissions on that key would be able to decrypt those files. Therefore, it's necessary to make sure the rotation period is set to a specific time.

Impact

After a successful key rotation, the older key version is required in order to decrypt the data encrypted by that previous key version.

Audit Procedure

From Google Cloud Console

1. Go to Cryptographic Keys by visiting: https://console.cloud.google.com/security/kms
2. Click on each key ring, then ensure each key in the keyring has Next Rotation set for less than 90 days from the current date.

From Google Cloud CLI

1. Ensure rotation is scheduled by ROTATION_PERIOD and NEXT_ROTATION_TIME for each key:

gcloud kms keys list --keyring=<KEY_RING> --location=<LOCATION> --format=json'(rotationPeriod)'

Expected Result

Ensure outcome values for rotationPeriod and nextRotationTime satisfy the below criteria:

• rotationPeriod is <= 129600m
• rotationPeriod is <= 7776000s
• rotationPeriod is <= 2160h
• rotationPeriod is <= 90d
• nextRotationTime is <= 90days from current DATE

Remediation

From Google Cloud Console

1. Go to Cryptographic Keys by visiting: https://console.cloud.google.com/security/kms
2. Click on the specific key ring.
3. From the list of keys, choose the specific key and Click on Right side pop up the blade (3 dots).
4. Click on Edit rotation period.
5. On the pop-up window, Select a new rotation period in days which should be less than 90 and then choose Starting on date (date from which the rotation period begins).

From Google Cloud CLI

1. Update and schedule rotation by ROTATION_PERIOD and NEXT_ROTATION_TIME for each key:

gcloud kms keys update new --keyring=KEY_RING --location=LOCATION --next-rotation-time=NEXT_ROTATION_TIME --rotation-period=ROTATION_PERIOD

Default Value

By default, KMS encryption keys are rotated every 90 days.

Additional Information

• Key rotation does NOT re-encrypt already encrypted data with the newly generated key version. If you suspect unauthorized use of a key, you should re-encrypt the data protected by that key and then disable or schedule destruction of the prior key version.
• It is not recommended to rely solely on irregular rotation, but rather to use irregular rotation if needed in conjunction with a regular rotation schedule.

References

1. https://cloud.google.com/kms/docs/key-rotation#frequency_of_key_rotation
2. https://cloud.google.com/kms/docs/re-encrypt-data

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