AWS KMS: Key Management and Encryption Patterns

Part of the Cybersecurity Skills Guide — This article is one deep-dive in our complete guide series.

By HADESS Team | February 28, 2026 | Updated: February 28, 2026 | 5 min read

KMS handles cryptographic key management so you do not have to run your own HSMs or build key rotation logic. But “managed” does not mean you can ignore the details. The key policy, how you use envelope encryption, and your rotation strategy all determine whether your encryption actually protects anything.

Key Policies vs. IAM Policies

KMS keys have resource-based policies (key policies) that work alongside IAM policies. Unlike most AWS resources, the key policy is the primary access control mechanism. If the key policy does not grant access to the account root, IAM policies cannot grant access to the key — even AdministratorAccess will not work.

The default key policy grants the account root full access, enabling IAM policies to delegate access. Remove this default statement without explicit grants and you lock yourself out of the key.

Write explicit key policies with separate principals for key usage (kms:Encrypt, kms:Decrypt, kms:GenerateDataKey) and key administration (kms:CreateGrant, kms:ScheduleKeyDeletion).

Envelope Encryption

KMS has a 4 KB limit on direct encryption. For anything larger, use envelope encryption: KMS generates a data key (GenerateDataKey), you encrypt your data with the plaintext data key locally, then store the encrypted data key alongside the encrypted data. To decrypt, call KMS to decrypt the data key, then use the plaintext data key to decrypt the data.

Most AWS services (S3, EBS, RDS, DynamoDB) handle envelope encryption transparently. When you build encryption in application code, implement the envelope pattern yourself or use the AWS Encryption SDK.

Key Rotation

Enable automatic key rotation for symmetric KMS keys. AWS rotates the key material annually and keeps the old key material so existing ciphertexts can still be decrypted. The key ID and ARN do not change, so you do not need to update references in your application.

For asymmetric keys and imported key material, automatic rotation is not available. You need to create new keys and update your application to use them. Maintain a key alias (like alias/my-app-signing-key) that you update to point to the new key, so your application references the alias rather than a specific key ID.

Cross-Account Access

Grant cross-account access through the key policy by adding the other account’s root as a principal, then create IAM policies in the target account to delegate access to specific roles. Use kms:ViaService condition to ensure the key can only be used through a specific AWS service (like S3) from the other account, preventing direct API calls.

For organizations, use aws:PrincipalOrgID in key policies to allow access from any account in your organization without listing each account individually.

Monitoring Key Usage

Enable CloudTrail logging for KMS API calls. Monitor for DisableKey, ScheduleKeyDeletion, and PutKeyPolicy events — these are high-risk actions. Set up CloudWatch alarms for unexpected Decrypt calls, which could indicate data exfiltration.

Use KMS key grants instead of persistent IAM policies for temporary access. Grants can be revoked immediately and provide audit trail through CloudTrail.

Next Steps

• Assess your cryptography and key management skills with the assessment tool
• Explore encryption topics in the skills library
• Map your certification path with the certificate roadmap — AWS Security Specialty covers KMS extensively

Related Guides in This Series

• Active Directory Security: Attack Paths and Hardening — HADESS | 2026
• AWS ALB Security: TLS, Authentication, and Access Controls — HADESS | 2026
• AWS CloudTrail: Log Analysis and Security Monitoring — HADESS | 2026

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