Encryption Key Management is a major function of online systems that requires lots of organization and no one will ever notice your efforts if things go right. Yet, if you screw it up, the world will come crashing down. Enter, KMS and its good buddy CloudHSM.


KMS is a managed service that makes it easy for your to create, control, rotate and use encryption keys. For use with AWS services, KMS uses Symmetric encryption (AES-256). The Customer Master Key is the default key used by KMS to encrypt/decrypt the data key; the encrypted data key is stored with the data. Asymetric keys (RSA & ECC) are supported for users outside of AWS who can’t call KMS API

Service Features

  • Highly Performant - Very durable, low latency, and High Throughput
  • No direct access to key - must use KMS API to use
  • IAM Access controlled
  • Regional Independence - your key can’t travel the world…
  • Integrated with AWS CloudTrail & Cloudwatch; KMS keys are single region
  • Full AWS API, CLI and SDK support
  • pay per API call; can encrypt/decrypt up to 4KB in size

Types of Customer Master Keys (CMK)

type Can view can manage/rotate dedicated to my account Key Policy
Customer managed yes yes; optional yes yes
AWS Managed yes no; required yes read only; CloudTrail friendly
AWS owned CMK no no; varies no no


KMS Key Material Origin

One time definition of key material; three types:

  • EXTERNAL which means you imported/manage/rotate them, must be 256 Symmetric key and asymmetric isn’t supported), uses asymmetric key to encrypt the data for transport
  • AWS_KMS which, duh, means KMS will handle it
  • AWS_CLOUDHSM which uses HSM that requires at least 2 HSM deployed across 2 AZ.


Roles need access to the key to USE the key. Role, users and external users can be granted the ability to use the key. Keys are managed regionally not globally; see multi-region.

Unless the key policy explicitly allows it, you cannot use IAM policies to allow access to a KMS key.

Multi-region Key Approach

Primary key is created then replicated to other regions using the same ID; enables encryption and description being possible in different regions. Keys are NOT global; they are replicated and managed separately. Replicas can be promoted into primary for DR situations.


CloudHSM is a single tenant hardware security module that must in a VPC. There is very limited integration with AWS services with only S3 SSE-C, Oracle, MS SQL, and Redshift well supported. There is no AWS API into this thing… One big use case is SSL decryption. S3, KMS, and EBS encryption are possible but integration applications must be written. Support asymmetric and symmetric keys.

AWS does not have access to the contents of your module. If you loose the keys, they are lost if you did not have a copy.

CloudTrail integration is supported. Syslog is as well. IAM permissions only apply to the CRUD of the HSM cluster; management of the cluster is done by CloudHSM software.

CloudHSM would be great if you need FIPS 140-2 level 3 validation and don’t mind fixed cost of $5k per module. And you are responsible for the HA part so really you need 2 HSM… with a cost of $10k. And a $5k setup fee per region plus an hourly fee.

The HSM does work with peered VPC as well. A CloudHSM is accessed using an ENI in a public subnet of the VPC.

HSM or KMS ?

Overall, KMS is probably adequate unless additional protection is necessary for some applications and data that are subject to strict contractual or regulatory requirements for managing cryptographic keys, then HSM should be used. In detail:

  • normal, or moderate security requirements? KMS
  • Integrate w/ AWS services other than S3, EBS, EC2, Redshift & RDS? KMS
  • Low variable costs based on usage? KMS
  • Centralize Key Management onsite & AWS? HSM
  • FIPS 140-2 Level 1 or Level 2: KMS
  • FIPS 140-2 level 3? HSM
  • Complete Key Control? HSM
  • Offload SSL? HSM
  • SSL cert NOT on instances? HSM
  • High-performance in-VPC bulk? HSM using an on-device cryptographic user (CU)
  • PKCS#11, Java Cryptopgraphy extensions (JCE), Microsoft CryptoNG? HSM