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AWS Key Management Service
Developer Guide

AWS Key Management Service Concepts

Learn the basic terms and concepts in AWS Key Management Service (AWS KMS) and how they work together to help protect your data.

Customer Master Keys

The primary resources in AWS KMS are customer master keys (CMKs). You can use a CMK to encrypt and decrypt up to 4 kilobytes (4096 bytes) of data. Typically, you use CMKs to generate, encrypt, and decrypt the data keys that you use outside of AWS KMS to encrypt your data.

AWS KMS stores, tracks, and protects your CMKs. When you want to use a CMK, you access it through AWS KMS. CMKs never leave AWS KMS unencrypted. This strategy differs from data keys that AWS KMS returns to you, optionally in plaintext. AWS KMS does not store, manage, or track your data keys.

There are two types of CMKs in AWS accounts:

  • Customer managed CMKs are CMKs that you create, manage, and use. This includes enabling and disabling the CMK, rotating its cryptographic material, and establishing the IAM policies and key policies that govern access to the CMK, as well as using the CMK in cryptographic operations. You can allow an AWS service to use a customer managed CMK on your behalf, but you retain control of the CMK.

     

  • AWS managed CMKs are CMKs in your account that are created, managed, and used on your behalf by an AWS service that is integrated with AWS KMS. This CMK is unique to your AWS account and region. Only the service that created the AWS managed CMK can use it.

     

    You can recognize AWS managed CMKs because their aliases have the format aws/service-name, such as aws/redshift. Typically, a service creates its AWS managed CMK in your account when you set up the service or the first time you use the CMK.

The AWS services that integrate with AWS KMS can use it in many different ways. Some services create AWS managed CMKs in your account. Other services require that you specify a customer managed CMK that you have created. And, others support both types of CMKs to allow you the ease of an AWS managed CMK or the control of a customer-managed CMK.

Data Keys

Data keys are encryption keys that you can use to encrypt data, including large amounts of data and other data encryption keys. You can use your AWS KMS CMKs to generate, encrypt, and decrypt data keys, but AWS KMS does not store, manage, or track your data keys, and AWS KMS cannot use a data key to encrypt data for you. You must use and manage data keys inside your application.

When you make a GenerateDataKey API request, AWS KMS gets the data key from the CMK you specify. The CMK returns a plaintext copy of the data key and a copy encrypted under the CMK. Use the plaintext data key to encrypt data and then remove it from memory as soon as possible. You must store and manage the encrypted data key so you can decrypt your data. Typically, users store the encrypted data key with the encrypted data.

To decrypt data, pass the encrypted data key in a Decrypt API request. AWS KMS uses your CMK to decrypt the data key and then it returns the plaintext data key. You use the plaintext data key to decrypt your data and then remove the plaintext data key from memory as soon as possible.

Envelope Encryption

AWS KMS uses envelope encryption to protect data. Envelope encryption is the practice of encrypting plaintext data with a unique data key, and then encrypting the data key with a key encryption key (KEK). You might choose to encrypt the KEK with another KEK, and so on, but eventually you must have a master key. The master key is an unencrypted (plaintext) key with which you can decrypt one or more other keys.

Envelope encryption offers several benefits:

  • Protecting data keys

    When you encrypt a data key, you don't have to worry about where to store the encrypted data key, because the security of that data key is inherently protected by encryption. You can safely store the encrypted data key alongside the encrypted data.

  • Encrypting the same data under multiple master keys

    Encryption operations can be time consuming, particularly when the data being encrypted are large objects. Instead of reencrypting raw data multiple times with different keys, you can reencrypt only the data keys that protect the raw data.

  • Combining the strengths of multiple algorithms

    In general, symmetric key algorithms are faster and produce smaller ciphertexts than public key algorithms, but public key algorithms provide inherent separation of roles and easier key management. You might want to combine the strengths of each.

The following image provides an overview of envelope encryption. In this scenario, the data key is encrypted with a single KEK, which is the master key. In AWS KMS, your CMK represents the master key.


        Envelope encryption

Encryption Context

All AWS KMS cryptographic operations accept an optional set of key–value pairs that can contain additional contextual information about the data. This set of key–value pairs is called encryption context. When encryption context is used with an encryption operation, the encryption context for the corresponding decryption operation must match for the decryption to succeed. Encryption context is not secret. Encryption context is logged, and you can use it for auditing and when controlling access to AWS KMS API operation. For more information, see Encryption Context.

Key Policies

When you create a CMK, you choose who can manage or use that CMK. These permissions are contained in a document called the key policy. You can use the key policy to add, remove, or modify permissions at any time for a customer-managed CMK, but you cannot edit the key policy for an AWS-managed CMK. For more information, see Authentication and Access Control for AWS KMS.

Grants

A grant is another mechanism for providing permissions, an alternative to the key policy. You can use grants to give long-term access that allows AWS principals to use your customer-managed CMKs. For more information, see Using Grants.

Grant Tokens

When you create a grant, the permissions specified in the grant might not take effect immediately due to eventual consistency. If you need to mitigate the potential delay, use the grant token that you receive in the response to your CreateGrant API request. You can pass the grant token with some AWS KMS API requests to make the permissions in the grant take effect immediately. The following AWS KMS API operations accept grant tokens:

A grant token is not a secret. The grant token contains information about who the grant is for and therefore who can use it to cause the grant's permissions to take effect more quickly.

Auditing CMK Usage

You can use AWS CloudTrail to audit key usage. CloudTrail creates log files that contain a history of AWS API calls and related events for your account. These log files include all AWS KMS API requests made with the AWS Management Console, AWS SDKs, and command line tools, as well as those made through integrated AWS services. You can use these log files to get information about when the CMK was used, the operation that was requested, the identity of the requester, the IP address that the request came from, and so on. For more information, see Logging AWS KMS API Calls and the AWS CloudTrail User Guide.

Key Management Infrastructure

A common practice in cryptography is to encrypt and decrypt with a publicly available and peer-reviewed algorithm such as AES (Advanced Encryption Standard) and a secret key. One of the main problems with cryptography is that it's very hard to keep a key secret. This is typically the job of a key management infrastructure (KMI). AWS KMS operates the KMI for you. AWS KMS creates and securely stores your master keys, called CMKs. For more information about how AWS KMS operates, see the AWS Key Management Service Cryptographic Details whitepaper.