# Amazon KMS key management
<a name="Overview.Encryption.Keys"></a>

 Amazon RDS automatically integrates with [Amazon Key Management Service (Amazon KMS)](https://docs.amazonaws.cn/kms/latest/developerguide/) for key management. Amazon RDS uses envelope encryption. For more information about envelope encryption, see [ Envelope encryption](https://docs.amazonaws.cn/kms/latest/developerguide/concepts.html#enveloping) in the *Amazon Key Management Service Developer Guide*. 

You can use two types of Amazon KMS keys to encrypt your DB instances . 
+ If you want full control over a KMS key, you must create a *customer managed key*. For more information about customer managed keys, see [Customer managed keys](https://docs.amazonaws.cn/kms/latest/developerguide/concepts.html#customer-cmk) in the *Amazon Key Management Service Developer Guide*. 
+  *Amazon managed keys* are KMS keys in your account that are created, managed, and used on your behalf by an Amazon service that is integrated with Amazon KMS. By default, the RDS Amazon managed key ( `aws/rds`) is used for encryption. You can't manage, rotate, or delete the RDS Amazon managed key. For more information about Amazon managed keys, see [Amazon managed keys](https://docs.amazonaws.cn/kms/latest/developerguide/concepts.html#aws-managed-cmk) in the *Amazon Key Management Service Developer Guide*. 

To manage KMS keys used for Amazon RDS encrypted DB instances , use the [Amazon Key Management Service (Amazon KMS)](https://docs.amazonaws.cn/kms/latest/developerguide/) in the [Amazon KMS console](https://console.amazonaws.cn/kms), the Amazon CLI, or the Amazon KMS API. To view audit logs of every action taken with an Amazon managed or customer managed key, use [Amazon CloudTrail](https://docs.amazonaws.cn/awscloudtrail/latest/userguide/). For more information about key rotation, see [Rotating Amazon KMS keys](https://docs.amazonaws.cn/kms/latest/developerguide/rotate-keys.html). 

## Authorizing use of a customer managed key
<a name="Overview.Encryption.Keys.Authorizing"></a>

When RDS uses a customer managed key in cryptographic operations, it acts on behalf of the user who is creating or changing the RDS resource.

To create an RDS resource using a customer managed key, a user must have permissions to call the following operations on the customer managed key:
+  `kms:CreateGrant` 
+  `kms:DescribeKey` 

You can specify these required permissions in a key policy, or in an IAM policy if the key policy allows it.

**Important**  
When you use explicit deny statements for all resources (\$1) in Amazon KMS key policies with managed services like Amazon RDS, you must specify a condition to allow the resource owning account. Operations might fail without this condition, even if the deny rule includes exceptions for your IAM user.

**Tip**  
To follow the principle of least privilege, do not allow full access to `kms:CreateGrant`. Instead, use the [kms:ViaService condition key](https://docs.amazonaws.cn/kms/latest/developerguide/policy-conditions.html#conditions-kms-via-service) to allow the user to create grants on the KMS key only when the grant is created on the user's behalf by an Amazon service.

You can make the IAM policy stricter in various ways. For example, if you want to allow the customer managed key to be used only for requests that originate in RDS , use the [ kms:ViaService condition key](https://docs.amazonaws.cn/kms/latest/developerguide/policy-conditions.html#conditions-kms-via-service) with the `rds.<region>.amazonaws.com` value. Also, you can use the keys or values in the [Amazon RDS encryption context](#Overview.Encryption.Keys.encryptioncontext) as a condition for using the customer managed key for encryption.

For more information, see [Allowing users in other accounts to use a KMS key](https://docs.amazonaws.cn/kms/latest/developerguide/key-policy-modifying-external-accounts.html) in the *Amazon Key Management Service Developer Guide* and [Key policies in Amazon KMS](https://docs.amazonaws.cn/kms/latest/developerguide/key-policies). 

## Amazon RDS encryption context
<a name="Overview.Encryption.Keys.encryptioncontext"></a>

When RDS uses your KMS key, or when Amazon EBS uses the KMS key on behalf of RDS , the service specifies an [encryption context](https://docs.amazonaws.cn/kms/latest/developerguide/concepts.html#encrypt_context). The encryption context is [additional authenticated data](https://docs.amazonaws.cn/crypto/latest/userguide/cryptography-concepts.html#term-aad) (AAD) that Amazon KMS uses to ensure data integrity. When an encryption context is specified for an encryption operation, the service must specify the same encryption context for the decryption operation. Otherwise, decryption fails. The encryption context is also written to your [Amazon CloudTrail](http://www.amazonaws.cn/cloudtrail/) logs to help you understand why a given KMS key was used. Your CloudTrail logs might contain many entries describing the use of a KMS key, but the encryption context in each log entry can help you determine the reason for that particular use.

At minimum, Amazon RDS always uses the DB instance ID for the encryption context, as in the following JSON-formatted example:

```
{ "aws:rds:db-id": "db-CQYSMDPBRZ7BPMH7Y3RTDG5QY" }
```

This encryption context can help you identify the DB instance for which your KMS key was used.

When your KMS key is used for a specific DB instance and a specific Amazon EBS volume, both the DB instance ID and the Amazon EBS volume ID are used for the encryption context, as in the following JSON-formatted example:

```
{
  "aws:rds:db-id": "db-BRG7VYS3SVIFQW7234EJQOM5RQ",
  "aws:ebs:id": "vol-ad8c6542"
}
```