Replicating objects created with server-side encryption (SSE) using AWS KMS CMKs - Amazon Simple Storage Service

Replicating objects created with server-side encryption (SSE) using AWS KMS CMKs

By default, Amazon S3 doesn't replicate objects that are stored at rest using server-side encryption with customer master keys (CMKs) stored in AWS KMS. This section explains additional configuration that you add to direct Amazon S3 to replicate these objects.

Important

Replication of encrypted data is a server-side process that occurs entirely within Amazon S3. Replication does not support client-side encryption.

For an example with step-by-step instructions, see Replicating encrypted objects. For information about creating a replication configuration, see Replicating objects.

Specifying additional information in the replication configuration

In the replication configuration, you do the following:

  • In the Destination configuration, add the symmetric customer managed AWS KMS CMK that you want Amazon S3 to use to encrypt object replicas.

  • Explicitly opt in by enabling replication of objects encrypted using AWS KMS CMKs by adding the SourceSelectionCriteria element.

<ReplicationConfiguration> <Rule> ... <SourceSelectionCriteria> <SseKmsEncryptedObjects> <Status>Enabled</Status> </SseKmsEncryptedObjects> </SourceSelectionCriteria> <Destination> ... <EncryptionConfiguration> <ReplicaKmsKeyID>AWS KMS key ID for the AWS region of the destination bucket.</ReplicaKmsKeyID> </EncryptionConfiguration> </Destination> ... </Rule> </ReplicationConfiguration>
Important

The AWS KMS CMK must have been created in the same AWS Region as the destination buckets.

The AWS KMS CMK must be valid. The PUT Bucket replication API doesn't check the validity of AWS KMS CMKs. If you use an invalid CMK, you will receive the 200 OK status code in response, but replication fails.

The following example shows a replication configuration, which includes optional configuration elements.

<?xml version="1.0" encoding="UTF-8"?> <ReplicationConfiguration> <Role>arn:aws:iam::account-id:role/role-name</Role> <Rule> <ID>Rule-1</ID> <Priority>1</Priority> <Status>Enabled</Status> <DeleteMarkerReplication> <Status>Disabled</Status> </DeleteMarkerReplication> <Filter> <Prefix>Tax</Prefix> </Filter> <Destination> <Bucket>arn:aws:s3:::destination-bucket</Bucket> <EncryptionConfiguration> <ReplicaKmsKeyID>The AWS KMS key ID for the AWS region of the destination buckets (S3 uses it to encrypt object replicas).</ReplicaKmsKeyID> </EncryptionConfiguration> </Destination> <SourceSelectionCriteria> <SseKmsEncryptedObjects> <Status>Enabled</Status> </SseKmsEncryptedObjects> </SourceSelectionCriteria> </Rule> </ReplicationConfiguration>

This replication configuration has one rule. The rule applies to objects with the Tax key prefix. Amazon S3 uses the AWS KMS key ID to encrypt these object replicas.

Granting additional permissions for the IAM role

To replicate objects that are encrypted at rest under AWS Key Management Service (AWS KMS), grant the following additional permissions to the IAM role you specify in the replication configuration. You grant these permissions by updating the permission policy associated with the IAM role. Objects created with server-side encryption using customer-provided (SSE-C) encryption keys are not replicated.

  • s3:GetObjectVersionForReplication action for source objects – Allows Amazon S3 to replicate both unencrypted objects and objects created with server-side encryption using Amazon S3 managed encryption (SSE-S3) keys or CMKs stored in AWS KMS (SSE-KMS).

    Note

    We recommend that you use the s3:GetObjectVersionForReplication action instead of the s3:GetObjectVersion action because it provides Amazon S3 with only the minimum permissions necessary for replication. In addition, permission for the s3:GetObjectVersion action allows replication of unencrypted and SSE-S3-encrypted objects, but not of objects created using a CMK stored in AWS KMS.

  • kms:Decrypt and kms:Encrypt AWS KMS actions:

    • kms:Decrypt permissions for the AWS KMS CMK used to decrypt the source object

    • kms:Encrypt permissions for the AWS KMS CMK used to encrypt the object replica

We recommend that you restrict these permissions only to the destination buckets and objects using AWS KMS condition keys. The AWS account that owns the IAM role must have permissions for these AWS KMS actions (kms:Encrypt and kms:Decrypt) for AWS KMS CMKs listed in the policy. If the AWS KMS CMKs are owned by another AWS account, the CMK owner must grant these permissions to the AWS account that owns the IAM role. For more information about managing access to these CMKs, see Using IAM Policies with AWS KMS in the AWS Key Management Service Developer Guide.

Amazon S3 Bucket Keys and replication

To use replication with an S3 Bucket Key, the AWS KMS key policy for the CMK used to encrypt the object replica must include kms:Decrypt permissions for the calling principal. The call to kms:Decrypt verifies the integrity of the S3 Bucket Key before using it.

When an S3 Bucket Key is enabled for the source and destination bucket, the encryption context will be the bucket Amazon Resource Name (ARN) and not the object ARN, for example, arn:aws:s3:::bucket_ARN. You need to update your IAM policies to use the bucket ARN for the encryption context. However, if an S3 Bucket Key is only enabled on the destination bucket and not the source bucket, you don't need to update your IAM policies to use the bucket ARN for the encryption context.

The example below shows the encryption context with the bucket ARN.

"kms:EncryptionContext:aws:s3:arn": [ "arn:aws:s3:::bucket_ARN" ]

For more information, see Encryption context and Changes to note before enabling an S3 Bucket Key.

Example policies - Using AWS KMS server-side-encryption (SSE-KMS) with replication

The following example IAM policies show statements for using AWS KMS server-side encryption with replication.

In this example, the encryption context is the object ARN. If you use SSE-KMS with an S3 Bucket Key enabled, you must use the bucket ARN as the encryption context. For more information, see Encryption context.

If the

Example Using AWS KMS server-side-encryption (SSE-KMS) – separate destination buckets

The following example policy shows statements for using AWS KMS with separate destination buckets.

{ "Version": "2012-10-17", "Statement": [{ "Action": ["kms:Decrypt"], "Effect": "Allow", "Resource": "List of AWS KMS key ARNs used to encrypt source objects.", "Condition": { "StringLike": { "kms:ViaService": "s3.source-bucket-region.amazonaws.com", "kms:EncryptionContext:aws:s3:arn": "arn:aws:s3:::source-bucket-name/key-prefix1/*" } } }, { "Action": ["kms:Encrypt"], "Effect": "Allow", "Resource": "AWS KMS key ARNs (for the AWS Region of the destination bucket 1). Used to encrypt object replicas created in destination bucket 1.", "Condition": { "StringLike": { "kms:ViaService": "s3.destination-bucket-1-region.amazonaws.com", "kms:EncryptionContext:aws:s3:arn": "arn:aws:s3:::destination-bucket-name-1/key-prefix1/*" } } }, { "Action": ["kms:Encrypt"], "Effect": "Allow", "Resource": "AWS KMS key ARNs (for the AWS Region of destination bucket 2). Used to encrypt object replicas created in destination bucket 2.", "Condition": { "StringLike": { "kms:ViaService": "s3.destination-bucket-2-region.amazonaws.com", "kms:EncryptionContext:aws:s3:arn": "arn:aws:s3:::destination-bucket-2-name/key-prefix1*" } } } ] }

Example Replicating objects created with server-side encryption using Amazon S3 managed encryption keys and CMKs stored in AWS KMS

The following is a complete IAM policy that grants the necessary permissions to replicate unencrypted objects, objects created with server-side encryption using Amazon S3 managed encryption keys and CMKs stored in AWS KMS.

Note

Objects created with server-side encryption using customer-provided (SSE-C) encryption keys are not replicated.

{ "Version":"2012-10-17", "Statement":[ { "Effect":"Allow", "Action":[ "s3:GetReplicationConfiguration", "s3:ListBucket" ], "Resource":[ "arn:aws:s3:::source-bucket" ] }, { "Effect":"Allow", "Action":[ "s3:GetObjectVersionForReplication", "s3:GetObjectVersionAcl" ], "Resource":[ "arn:aws:s3:::source-bucket/key-prefix1*" ] }, { "Effect":"Allow", "Action":[ "s3:ReplicateObject", "s3:ReplicateDelete" ], "Resource":"arn:aws:s3:::destination-bucket/key-prefix1*" }, { "Action":[ "kms:Decrypt" ], "Effect":"Allow", "Condition":{ "StringLike":{ "kms:ViaService":"s3.source-bucket-region.amazonaws.com", "kms:EncryptionContext:aws:s3:arn":[ "arn:aws:s3:::source-bucket-name/key-prefix1*" ] } }, "Resource":[ "List of AWS KMS key ARNs used to encrypt source objects." ] }, { "Action":[ "kms:Encrypt" ], "Effect":"Allow", "Condition":{ "StringLike":{ "kms:ViaService":"s3.destination-bucket-region.amazonaws.com", "kms:EncryptionContext:aws:s3:arn":[ "arn:aws:s3:::destination-bucket-name/prefix1*" ] } }, "Resource":[ "AWS KMS key ARNs (for the AWS Region of the destination buckets) to use for encrypting object replicas" ] } ] }

Granting additional permissions for cross-account scenarios

In a cross-account scenario, where source and destination buckets are owned by different AWS accounts, you can use a customer managed CMK to encrypt object replicas. However, the CMK owner must grant the source bucket owner permission to use the CMK.

To grant the source bucket owner permission to use the AWS KMS CMK (IAM console)

  1. Sign in to the AWS Management Console and open the AWS Key Management Service (AWS KMS) console at https://console.aws.amazon.com/kms.

  2. To change the AWS Region, use the Region selector in the upper-right corner of the page.

  3. To view the keys in your account that you create and manage, in the navigation pane choose Customer managed keys.

  4. Choose the CMK.

  5. Under General configuration, choose the Key policy tab.

  6. Choose Other AWS Accounts.

  7. Choose Add another AWS Account.

  8. In arn:aws:iam::, enter the source bucket account ID.

  9. Choose Save Changes.

To grant the source bucket owner permission to use the AWS KMS CMK (AWS CLI)

AWS KMS transaction limit considerations

When you add many new objects with AWS KMS encryption after enabling cross-region replication (CRR), you might experience throttling (HTTP 503 Slow Down errors). Throttling occurs when the number of AWS KMS transactions per second exceeds the current limit. For more information, see Limits in the AWS Key Management Service Developer Guide.

To request a limit increase, use Service Quotas. For more information, see Amazon Web Services Limits. If Service Quotas isn't supported in your Region, open an AWS Support case.