Basic concepts - AWS Key Management Service

Basic concepts

Learning some basic terms and concepts will help you get the most out of AWS Key Management Service.

AWS KMS key
Note

AWS KMS is replacing the term customer master key (CMK) with AWS KMS key and KMS key. The concept has not changed. To prevent breaking changes, AWS KMS is keeping some variations of this term.

A logical key that represents the top of your key hierarchy. A KMS key is given an Amazon Resource Name (ARN) that includes a unique key identifier, or key ID. AWS KMS keys have three types:

  • Customer managed key – Customers create and control the lifecycle and key policies of customer managed keys. All requests made against these keys are logged as CloudTrail events.

  • AWS managed keys – AWS creates and controls the lifecycle and key policies of AWS managed keys, which are resources in a customer’s AWS account. Customers can view access policies and CloudTrail events for AWS managed keys, but cannot manage any aspect of these keys. All requests made against these keys are logged as CloudTrail events.

  • AWS owned keys – These keys are created and exclusively used by AWS for internal encryption operations across different AWS services. Customers do not have visibility into key policies or AWS owned key usage in CloudTrail.

Alias

A user-friendly name that is associated with a KMS key. The alias can be used interchangeably with key ID in many of the AWS KMS API operations.

Permissions

A policy attached to a KMS key that defines permissions on the key. The default policy allows any principals that you define, as well as allowing the AWS account root user to add IAM policies that reference the key.

Grants

The delegated permission to use a KMS key when the intended IAM principals or duration of usage is not known at the outset and therefore cannot be added to a key or IAM policy. One use of grants is to define scoped-down permissions for how an AWS service can use a KMS key. The service may need to use your key to do asynchronous work on your behalf on encrypted data in the absence of a direct-signed API call from you.

Data keys

Cryptographic keys generated on HSMs, protected by a KMS key. AWS KMS allows authorized entities to obtain data keys protected by a KMS key. They can be returned both as plaintext (unencrypted) data keys and as encrypted data keys. Data keys can be symmetric or asymmetric (with both the public and private portions returned).

Ciphertexts

The encrypted output of AWS KMS, sometimes referred to as customer ciphertext to eliminate confusion. Ciphertext contains encrypted data with additional information that identifies the KMS key to use in the decryption process. Encrypted data keys are one common example of ciphertext produced when using a KMS key, but any data under 4 KB in size can be encrypted under a KMS key to produce a ciphertext.

Encryption context

A key–value pair map of additional information that is associated with AWS KMS–protected information. AWS KMS uses authenticated encryption to protect data keys. The encryption context is incorporated into the AAD of the authenticated encryption in AWS KMS–encrypted ciphertexts. This context information is optional and not returned when requesting a key (or an encryption operation). But if used, this context value is required to successfully complete a decryption operation. An intended use of the encryption context is to provide additional authenticated information. This information can help you enforce policies and be included in the AWS CloudTrail logs. For example, you could use a key–value pair of {"key name":"satellite uplink key"} to name the data key. Subsequent use of the key creates an AWS CloudTrail entry that includes “key name”: “satellite uplink key.” This additional information can provide useful context to understand why a given KMS key was used.

Public key

When using asymmetric ciphers (RSA or elliptic curve), the public key is the “public component” of a public-private key pair. The public key can be shared and distributed to entities that need to encrypt data for the owner of the public-private key pair. For digital signature operations, the public key is used to verify the signature.

Private key

When using asymmetric ciphers (RSA or elliptic curve), the private key is the “private component” of a public-private key pair. The private key is used to decrypt data or create digital signatures. Similar to symmetric KMS keys, private keys are encrypted in HSMs. They are decrypted only into the short term memory of the HSM and only for the time needed to process your cryptographic request.