Managing Amazon SNS encryption keys and costs - Amazon Simple Notification Service
Estimating AWS KMS costsConfiguring AWS KMS permissionsAWS KMS errors

Managing Amazon SNS encryption keys and costs

The following sections provide information about working with keys managed in AWS Key Management Service (AWS KMS).

Note

Amazon SNS only supports symmetric encryption KMS keys. You cannot use any other type of KMS key to encrypt your service resources. For help determining whether a KMS key is a symmetric encryption key, see Identifying asymmetric KMS keys.

Estimating AWS KMS costs

To predict costs and better understand your AWS bill, you might want to know how often Amazon SNS uses your AWS KMS key.

Note

Although the following formula can give you a very good idea of expected costs, actual costs might be higher because of the distributed nature of Amazon SNS.

To calculate the number of API requests (R) per topic, use the following formula:

R = B / D * (2 * P)

B is the billing period (in seconds).

D is the data key reuse period (in seconds—Amazon SNS reuses a data key for up to 5 minutes).

P is the number of publishing principals that send to the Amazon SNS topic.

The following are example calculations. For exact pricing information, see AWS Key Management Service Pricing.

Example 1: Calculating the number of AWS KMS API calls for 1 publisher and 1 topic

This example assumes the following:

  • The billing period is January 1-31 (2,678,400 seconds).

  • The data key reuse period is 5 minutes (300 seconds).

  • There is 1 topic.

  • There is 1 publishing principal.

2,678,400 / 300 * (2 * 1) = 17,856

Example 2: Calculating the number of AWS KMS API calls for multiple publishers and 2 topics

This example assumes the following:

  • The billing period is February 1-28 (2,419,200 seconds).

  • The data key reuse period is 5 minutes (300 seconds).

  • There are 2 topics.

  • The first topic has 3 publishing principals.

  • The second topic has 5 publishing principals.

(2,419,200 / 300 * (2 * 3)) + (2,419,200 / 300 * (2 * 5)) = 129,024

Configuring AWS KMS permissions

Before you can use SSE, you must configure AWS KMS key policies to allow encryption of topics and encryption and decryption of messages. For examples and more information about AWS KMS permissions, see AWS KMS API Permissions: Actions and Resources Reference in the AWS Key Management Service Developer Guide. For details on how to set up an Amazon SNS topic with server-side encryption, see Additional information.

Note

You can also manage permissions for symmetric encryption KMS keys using IAM policies. For more information, see Using IAM Policies with AWS KMS.

While you can configure global permissions to send to and receive from Amazon SNS, AWS KMS requires explicitly naming the full ARN of KMSs in specific regions in the Resource section of an IAM policy.

You must also ensure that the key policies of the AWS KMS key allow the necessary permissions. To do this, name the principals that produce and consume encrypted messages in Amazon SNS as users in the KMS key policy.

Alternatively, you can specify the required AWS KMS actions and KMS ARN in an IAM policy assigned to the principals that publish and subscribe to receive encrypted messages in Amazon SNS. For more information, see Managing Access to AWS KMS in the AWS Key Management Service Developer Guide.

If selecting a customer-managed key for your Amazon SNS topic and you are using aliases to control access to KMS keys using IAM policies or KMS key policies with the condition key kms:ResourceAliases, ensure that the customer-managed key that is selected also has an alias associated. For more information on using alias to control access to KMS keys, see Using aliases to control access to KMS keys in the AWS Key Management Service Developer Guide.

Allow a user to send messages to a topic with SSE

The publisher must have the kms:GenerateDataKey* and kms:Decrypt permissions for the AWS KMS key.

{
  "Statement": [{
    "Effect": "Allow",
    "Action": [
      "kms:GenerateDataKey*",
      "kms:Decrypt"
    ],
    "Resource": "arn:aws:kms:us-east-2:123456789012:key/1234abcd-12ab-34cd-56ef-1234567890ab"
  }, {
    "Effect": "Allow",
    "Action": [
      "sns:Publish"
    ],
    "Resource": "arn:aws:sns:*:123456789012:MyTopic"
  }]
}

Enable compatibility between event sources from AWS services and encrypted topics

Several AWS services publish events to Amazon SNS topics. To allow these event sources to work with encrypted topics, you must perform the following steps.

  1. Use a customer managed key. For more information, see Creating Keys in the AWS Key Management Service Developer Guide.

  2. To allow the AWS service to have the kms:GenerateDataKey* and kms:Decrypt permissions, add the following statement to the KMS policy.

    {
  "Statement": [{
    "Effect": "Allow",
    "Principal": {
      "Service": "service.amazonaws.com"
    },
    "Action": [
      "kms:GenerateDataKey*",
      "kms:Decrypt"
    ],
    "Resource": "*"
  }]
}
    Event source Service principal
    Amazon CloudWatch cloudwatch.amazonaws.com
    Amazon CloudWatch Events events.amazonaws.com
    AWS CodeCommit codecommit.amazonaws.com
    AWS Database Migration Service dms.amazonaws.com
    AWS Directory Service ds.amazonaws.com
    Amazon DynamoDB dynamodb.amazonaws.com
    Amazon Inspector inspector.amazonaws.com
    Amazon Redshift redshift.amazonaws.com
    Amazon RDS events.rds.amazonaws.com
    Amazon S3 Glacier glacier.amazonaws.com
    Amazon Simple Email Service ses.amazonaws.com
    Amazon Simple Storage Service s3.amazonaws.com
    AWS Snowball Edge importexport.amazonaws.com
    AWS Systems Manager Incident Manager

    AWS Systems Manager Incident Manager consists of two service principles:

    ssm-incidents.amazonaws.com; ssm-contacts.amazonaws.com
    Note

    Some Amazon SNS event sources require you to provide an IAM role (rather than the service principal) in the AWS KMS key policy:

  3. Add the aws:SourceAccount and aws:SourceArn condition keys to the KMS resource policy to further protect the KMS key from confused deputy attacks. Refer to service specific documentation list (above) for exact details in each case.

    Important

    Adding the aws:SourceAccount, aws:SourceArn, and aws:SourceOrgID to a AWS KMS policy is not supported for EventBridge-to-encrypted topics.

    {
  "Effect": "Allow",
  "Principal": {
    "Service": "service.amazonaws.com"
  },
  "Action": [
    "kms:GenerateDataKey*",
    "kms:Decrypt"
  ],
  "Resource": "*",
  "Condition": {
    "StringEquals": {
      "aws:SourceAccount": "customer-account-id"
    },
    "ArnLike": {
      "aws:SourceArn": "arn:aws:service:region:customer-account-id:resource-type:customer-resource-id"
    }
  }
}

  4. Enable SSE for your topic using your KMS.

  5. Provide the ARN of the encrypted topic to the event source.

AWS KMS errors

When you work with Amazon SNS and AWS KMS, you might encounter errors. The following list describes the errors and possible troubleshooting solutions.

KMSAccessDeniedException

The ciphertext references a key that doesn't exist or that you don't have access to.

HTTP Status Code: 400

KMSDisabledException

The request was rejected because the specified KMS isn't enabled.

HTTP Status Code: 400

KMSInvalidStateException

The request was rejected because the state of the specified resource isn't valid for this request. For more information, see Key states of AWS KMS keys in the AWS Key Management Service Developer Guide.

HTTP Status Code: 400

KMSNotFoundException

The request was rejected because the specified entity or resource can't be found.

HTTP Status Code: 400

KMSOptInRequired

The AWS access key ID needs a subscription for the service.

HTTP Status Code: 403

KMSThrottlingException

The request was denied due to request throttling. For more information about throttling, see Quotas in the AWS Key Management Service Developer Guide.

HTTP Status Code: 400