How AWS CloudTrail uses AWS KMS - AWS Key Management Service
Understanding when your KMS key is used

How AWS CloudTrail uses AWS KMS

You can use AWS CloudTrail to record AWS API calls and other activity for your AWS account and to save the recorded information to log files in an Amazon Simple Storage Service (Amazon S3) bucket that you choose. By default, the log files that CloudTrail puts in your S3 bucket are encrypted using server-side encryption with Amazon S3–managed encryption keys (SSE-S3). But you can choose instead to use server-side encryption with a KMS key (SSE-KMS). To learn how to encrypt your CloudTrail log files with AWS KMS, see Encrypting CloudTrail Log Files with AWS KMS keys (SSE-KMS) in the AWS CloudTrail User Guide.

Important

AWS CloudTrail and Amazon S3 support only symmetric AWS KMS keys. You cannot use an asymmetric KMS key to encrypt your CloudTrail Logs. For help determining whether a KMS key is symmetric or asymmetric, see Identifying asymmetric KMS keys.

You do not pay a key usage charge when CloudTrail reads or writes log files encrypted with an SSE-KMS key. However, you pay a key usage charge when you access CloudTrail log files encrypted with an SSE-KMS key. For information about AWS KMS pricing, see AWS Key Management Service Pricing. For information about CloudTrail pricing, see AWS CloudTrail pricing and Managing costs in the AWS CloudTrail User Guide.

Understanding when your KMS key is used

Encrypting CloudTrail log files with AWS KMS builds on the Amazon S3 feature called server-side encryption with an AWS KMS key (SSE-KMS). To learn more about SSE-KMS, see How Amazon Simple Storage Service (Amazon S3) uses AWS KMS in this guide or Protecting data using server-side encryption with KMS keys (SSE-KMS) in the Amazon Simple Storage Service User Guide.

When you configure AWS CloudTrail to use SSE-KMS to encrypt your log files, CloudTrail and Amazon S3 use your AWS KMS keys when you perform certain actions with those services. The following sections explain when and how those services can use your KMS key, and provide additional information that you can use to validate this explanation.

You configure CloudTrail to encrypt log files with your AWS KMS key

When you update your CloudTrail configuration to use your KMS key, CloudTrail sends a GenerateDataKey request to AWS KMS to verify that the KMS key exists and that CloudTrail has permission to use it for encryption. CloudTrail does not use the resulting data key.

The GenerateDataKey request includes the following information for the encryption context:

  • The Amazon Resource Name (ARN) of the CloudTrail trail

  • The ARN of the S3 bucket and path where the CloudTrail log files are delivered

The GenerateDataKey request results in an entry in your CloudTrail logs similar to the following example. When you see a log entry like this one, you can determine that CloudTrail ( ) called the AWS KMS ( ) GenerateDataKey operation ( ) for a specific trail ( ). AWS KMS created the data key under a specific KMS key ( ).

Note

You might need to scroll to the right to see some of the callouts in the following example log entry.

{
  "eventVersion": "1.02",
  "userIdentity": {
    "type": "IAMUser",
    "principalId": "AIDACKCEVSQ6C2EXAMPLE",
    "arn": "arn:aws:iam::086441151436:user/AWSCloudTrail",
    "accountId": "086441151436",
    "accessKeyId": "AKIAI44QH8DHBEXAMPLE",
    "userName": "AWSCloudTrail",
    "sessionContext": {"attributes": {
      "mfaAuthenticated": "false",
      "creationDate": "2015-11-11T21:15:33Z"
    }},
    "invokedBy": "internal.amazonaws.com"
  },
  "eventTime": "2015-11-11T21:15:33Z",
  "eventSource": "kms.amazonaws.com",
  "eventName": "GenerateDataKey",
  "awsRegion": "us-west-2",
  "sourceIPAddress": "internal.amazonaws.com",
  "userAgent": "internal.amazonaws.com",
  "requestParameters": {
    "keyId": "arn:aws:kms:us-west-2:111122223333:alias/ExampleAliasForCloudTrailKMS key",
    "encryptionContext": {
      "aws:cloudtrail:arn": "arn:aws:cloudtrail:us-west-2:111122223333:trail/Default",
      "aws:s3:arn": "arn:aws:s3:::example-bucket-for-CT-logs/AWSLogs/111122223333/"
    },
    "keySpec": "AES_256"
  },
  "responseElements": null,
  "requestID": "581f1f11-88b9-11e5-9c9c-595a1fb59ac0",
  "eventID": "3cdb2457-c035-4890-93b6-181832b9e766",
  "readOnly": true,
  "resources": [{
    "ARN": "arn:aws:kms:us-west-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab",
    "accountId": "111122223333"
  }],
  "eventType": "AwsServiceEvent",
  "recipientAccountId": "111122223333"
}

CloudTrail puts a log file into your S3 bucket

Each time CloudTrail puts a log file into your S3 bucket, Amazon S3 sends a GenerateDataKey request to AWS KMS on behalf of CloudTrail. In response to this request, AWS KMS generates a unique data key and then sends Amazon S3 two copies of the data key, one in plaintext and one that is encrypted with the specified KMS key. Amazon S3 uses the plaintext data key to encrypt the CloudTrail log file and then removes the plaintext data key from memory as soon as possible after use. Amazon S3 stores the encrypted data key as metadata with the encrypted CloudTrail log file.

The GenerateDataKey request includes the following information for the encryption context:

Each GenerateDataKey request results in an entry in your CloudTrail logs similar to the following example. When you see a log entry like this one, you can determine that CloudTrail ( ) called the AWS KMS ( ) GenerateDataKey operation ( ) for a specific trail ( ) to protect a specific log file ( ). AWS KMS created the data key under the specified KMS key ( ), shown twice in the same log entry.

Note

You might need to scroll to the right to see some of the callouts in the following example log entry.

{
  "eventVersion": "1.02",
  "userIdentity": {
    "type": "AssumedRole",
    "principalId": "AROACKCEVSQ6C2EXAMPLE:i-34755b85",
    "arn": "arn:aws:sts::086441151436:assumed-role/AWSCloudTrail/i-34755b85",
    "accountId": "086441151436",
    "accessKeyId": "AKIAI44QH8DHBEXAMPLE",
    "sessionContext": {
      "attributes": {
        "mfaAuthenticated": "false",
        "creationDate": "2015-11-11T20:45:25Z"
      },
      "sessionIssuer": {
        "type": "Role",
        "principalId": "AROACKCEVSQ6C2EXAMPLE",
        "arn": "arn:aws:iam::086441151436:role/AWSCloudTrail",
        "accountId": "086441151436",
        "userName": "AWSCloudTrail"
      }
    },
    "invokedBy": "internal.amazonaws.com"
  },
  "eventTime": "2015-11-11T21:15:58Z",
  "eventSource": "kms.amazonaws.com",
  "eventName": "GenerateDataKey",
  "awsRegion": "us-west-2",
  "sourceIPAddress": "internal.amazonaws.com",
  "userAgent": "internal.amazonaws.com",
  "requestParameters": {
    "encryptionContext": {
      "aws:cloudtrail:arn": "arn:aws:cloudtrail:us-west-2:111122223333:trail/Default",
      "aws:s3:arn": "arn:aws:s3:::example-bucket-for-CT-logs/AWSLogs/111122223333/CloudTrail/us-west-2/2015/11/11/111122223333_CloudTrail_us-west-2_20151111T2115Z_7JREEBimdK8d2nC9.json.gz"
    },
    "keyId": "arn:aws:kms:us-west-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab",
    "keySpec": "AES_256"
  },
  "responseElements": null,
  "requestID": "66f3f74a-88b9-11e5-b7fb-63d925c72ffe",
  "eventID": "7738554f-92ab-4e27-83e3-03354b1aa898",
  "readOnly": true,
  "resources": [{
    "ARN": "arn:aws:kms:us-west-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab",
    "accountId": "111122223333"
  }],
  "eventType": "AwsServiceEvent",
  "recipientAccountId": "111122223333"
}

You get an encrypted log file from your S3 bucket

Each time you get an encrypted CloudTrail log file from your S3 bucket, Amazon S3 sends a Decrypt request to AWS KMS on your behalf to decrypt the log file's encrypted data key. In response to this request, AWS KMS uses your KMS key to decrypt the data key and then sends the plaintext data key to Amazon S3. Amazon S3 uses the plaintext data key to decrypt the CloudTrail log file and then removes the plaintext data key from memory as soon as possible after use.

The Decrypt request includes the following information for the encryption context:

Each Decrypt request results in an entry in your CloudTrail logs similar to the following example. When you see a log entry like this one, you can determine that a user in your AWS account ( ) called the AWS KMS ( ) Decrypt operation ( ) for a specific trail ( ) and a specific log file ( ). AWS KMS decrypted the data key under a specific KMS key ( ).

Note

You might need to scroll to the right to see some of the callouts in the following example log entry.

{
  "eventVersion": "1.02",
  "userIdentity": {
    "type": "IAMUser",
    "principalId": "AIDACKCEVSQ6C2EXAMPLE",
    "arn": "arn:aws:iam::111122223333:role/cloudtrail-admin",
    "accountId": "111122223333",
    "accessKeyId": "AKIAIOSFODNN7EXAMPLE",
    "userName": "cloudtrail-admin",
    "sessionContext": {"attributes": {
      "mfaAuthenticated": "false",
      "creationDate": "2015-11-11T20:48:04Z"
    }},
    "invokedBy": "signin.amazonaws.com"
  },
  "eventTime": "2015-11-11T21:20:52Z",
  "eventSource": "kms.amazonaws.com",
  "eventName": "Decrypt",
  "awsRegion": "us-west-2",
  "sourceIPAddress": "internal.amazonaws.com",
  "userAgent": "internal.amazonaws.com",
  "requestParameters": {
    "encryptionContext": {
      "aws:cloudtrail:arn": "arn:aws:cloudtrail:us-west-2:111122223333:trail/Default",
      "aws:s3:arn": "arn:aws:s3:::example-bucket-for-CT-logs/AWSLogs/111122223333/CloudTrail/us-west-2/2015/11/11/111122223333_CloudTrail_us-west-2_20151111T2115Z_7JREEBimdK8d2nC9.json.gz"
    }
  },
  "responseElements": null,
  "requestID": "16a0590a-88ba-11e5-b406-436f15c3ac01",
  "eventID": "9525bee7-5145-42b0-bed5-ab7196a16daa",
  "readOnly": true,
  "resources": [{
    "ARN": "arn:aws:kms:us-west-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab",
    "accountId": "111122223333"
  }],
  "eventType": "AwsApiCall",
  "recipientAccountId": "111122223333"
}