How Amazon CloudTrail uses Amazon KMS - Amazon CloudTrail
Understanding when your KMS key is used for your trail
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How Amazon CloudTrail uses Amazon KMS

This section describes how Amazon KMS works with a CloudTrail trail that is encrypted with an SSE-KMS key.

Important

Amazon CloudTrail and Amazon S3 support only symmetric Amazon 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 Identify different key types in the Amazon Key Management Service Developer Guide.

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 Amazon KMS pricing, see Amazon Key Management Service Pricing. For information about CloudTrail pricing, see Amazon CloudTrail pricing.

Understanding when your KMS key is used for your trail

Encrypting CloudTrail log files with Amazon KMS builds on the Amazon S3 feature called server-side encryption with an Amazon KMS key (SSE-KMS). To learn more about SSE-KMS, see Using server-side encryption with Amazon KMS keys (SSE-KMS) in the Amazon Simple Storage Service User Guide.

When you configure Amazon CloudTrail to use SSE-KMS to encrypt your log files, CloudTrail and Amazon S3 use your Amazon 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 Amazon KMS key

When you update your CloudTrail configuration to use your KMS key, CloudTrail sends a GenerateDataKey request to Amazon 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 Amazon KMS GenerateDataKey operation for a specific trail. Amazon KMS created the data key under a specific KMS key.

{
    "eventVersion": "1.09",
    "userIdentity": {
        "type": "AWSService",
        "invokedBy": "cloudtrail.amazonaws.com"
    },
    "eventTime": "2024-12-06T20:14:46Z",
    "eventSource": "kms.amazonaws.com",
    "eventName": "GenerateDataKey",
    "awsRegion": "us-east-1",
    "sourceIPAddress": "cloudtrail.amazonaws.com",
    "userAgent": "cloudtrail.amazonaws.com",
    "requestParameters": {
        "keySpec": "AES_256",
        "keyId": "arn:aws:kms:us-east-1:123456789012:key/example1-6736-4661-bf00-exampleeb770",
        "encryptionContext": {
            "aws:cloudtrail:arn": "arn:aws:cloudtrail:us-east-1:123456789012:trail/management-events",
            "aws:s3:arn": "arn:aws:s3:::amzn-s3-demo-logging-bucket-123456789012-9af1fb49/AWSLogs/123456789012/CloudTrail/us-east-1/2024/12/06/123456789012_CloudTrail_us-east-1_20241206T2010Z_TO50OLMG1hIQ1png.json.gz"
        }
    },
    "responseElements": null,
    "requestID": "a0555e85-7e8a-4765-bd8f-2222295558e1",
    "eventID": "e4f3557e-7dbd-4e37-a00a-d86c137d1111",
    "readOnly": true,
    "resources": [
        {
            "accountId": "123456789012",
            "type": "AWS::KMS::Key",
            "ARN": "arn:aws:kms:us-east-1:123456789012:key/example1-6736-4661-bf00-exampleeb770"
         }],
    "eventType": "AwsApiCall",
    "managementEvent": true,
    "recipientAccountId": "123456789012",
    "sharedEventID": "ce71d6be-0846-498e-851f-111a1af9078f",
    "eventCategory": "Management"
}

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 Amazon KMS on behalf of CloudTrail. In response to this request, Amazon 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 Amazon KMS GenerateDataKey operation for a specific trail to protect a specific log file. Amazon KMS created the data key under the specified KMS key, shown twice in the same log entry.

{
    "eventVersion": "1.09",
    "userIdentity": {
        "type": "AWSService",
        "invokedBy": "cloudtrail.amazonaws.com"
    },
    "eventTime": "2024-12-06T21:49:28Z",
    "eventSource": "kms.amazonaws.com",
    "eventName": "GenerateDataKey",
    "awsRegion": "us-east-1",
    "sourceIPAddress": "cloudtrail.amazonaws.com",
    "userAgent": "cloudtrail.amazonaws.com",
    "requestParameters": {
        "encryptionContext": {
            "aws:cloudtrail:arn": "arn:aws:cloudtrail:us-east-1::trail/insights-trail",
            "aws:s3:arn": "arn:aws:s3:::amzn-s3-demo-logging-bucket1-123456789012-7867ab0c/AWSLogs/123456789012/CloudTrail/us-east-1/2024/12/06/123456789012_CloudTrail_us-east-1_20241206T2150Z_hVXmrJzjZk2wAM2V.json.gz"
        },
        "keySpec": "AES_256",
        "keyId": "arn:aws:kms:us-east-1:123456789012:key/example9-16ef-48ba-9163-example67a5a"
    },
    "responseElements": null,
    "requestID": "11117d14-9232-414a-b3d1-01bab4dc9f99",
    "eventID": "999e9a50-512c-4e2a-84a3-111a5f511111",
    "readOnly": true,
    "resources": [
        {
            "accountId": "123456789012",
            "type": "AWS::KMS::Key",
            "ARN": "arn:aws:kms:us-east-1:123456789012:key/example9-16ef-48ba-9163-example67a5a"
        }
    ],
    "eventType": "AwsApiCall",
    "managementEvent": true,
    "recipientAccountId": "123456789012",
    "sharedEventID": "5e663acc-b7fd-4cdd-8328-0eff862952fa",
    "eventCategory": "Management"
}

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 Amazon KMS on your behalf to decrypt the log file's encrypted data key. In response to this request, Amazon 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 an assumed role called the Amazon KMS Decrypt operation for a specific trail and a specific log file. Amazon KMS decrypted the data key under a specific KMS key.

{
    "eventVersion": "1.09",
    "userIdentity": {
        "type": "AssumedRole",
        "principalId": "AIDACKCEVSQ6C2EXAMPLE",
        "arn": "arn:aws:sts::123456789012:assumed-role/Admin",
        "accountId": "123456789012",
        "accessKeyId": "AKIAIOSFODNN7EXAMPLE",
        "sessionContext": {
            "sessionIssuer": {
                "type": "Role",
                "principalId": "AIDACKCEVSQ6C2EXAMPLE",
                "arn": "arn:aws:iam::123456789012:role/Admin",
                "accountId": "123456789012",
                "userName": "Admin"
            },
            "attributes": {
                "creationDate": "2024-12-06T22:04:04Z",
                "mfaAuthenticated": "false"
            }
        },
        "invokedBy": "AWS Internal"
    },
    "eventTime": "2024-12-06T22:26:34Z",
    "eventSource": "kms.amazonaws.com",
    "eventName": "Decrypt",
    "awsRegion": "us-east-1",
    "sourceIPAddress": "AWS Internal",
    "userAgent": "AWS Internal",
    "requestParameters": {
        "encryptionContext": {
            "aws:cloudtrail:arn": "arn:aws:cloudtrail:us-east-1:123456789012:trail/insights-trail",
            "aws:s3:arn": "arn:aws:s3:::amzn-s3-demo-logging-bucket1-123456789012-7867ab0c/AWSLogs/123456789012/CloudTrail/us-east-1/2024/12/06/123456789012_CloudTrail_us-east-1_20241206T0000Z_aAAsHbGBdye3jp2R.json.gz"
        },
        "encryptionAlgorithm": "SYMMETRIC_DEFAULT"
    },
    "responseElements": null,
    "requestID": "1ab2d2d2-111a-2222-a59b-11a2b3832b53",
    "eventID": "af4d4074-2849-4b3d-1a11-a1aaa111a111",
    "readOnly": true,
    "resources": [
        {
            "accountId": "123456789012",
            "type": "AWS::KMS::Key",
            "ARN": "arn:aws:kms:us-east-1:123456789012:key/example9-16ef-48ba-9163-example67a5a"
        }
    ],
    "eventType": "AwsApiCall",
    "managementEvent": true,
    "recipientAccountId": "123456789012",
    "eventCategory": "Management",
    "sessionCredentialFromConsole": "true"
}