The GCP KMS Cross-Project Encryption detection rule is designed to identify potentially malicious activities involving Google Cloud services, particularly when a service account from one project utilizes a Key Management Service (KMS) encryption key from another project. This situation may indicate an attacker using their own KMS key to encrypt data in a victim's project, potentially hindering access to that data. The rule employs GCP audit logs to track KMS operations tied to the service account in question, facilitating forensic analysis to characterize encryption activities across project boundaries. The rule emphasizes investigating specific operations within a narrow time frame, cross-referencing logs to ascertain whether the detected behavior aligns with approved integrations, and identifying any affected data via Google Cloud Storage (GCS) operations.