AWS KMS Multi-Region Keys: The Ultimate Guide

🔧 1. Disaster Recovery and Business Continuity

In a failover scenario, applications in a secondary region can immediately decrypt data that was encrypted in the primary region. This significantly reduces your Recovery Time Objective (RTO) as you don't need to decrypt and re-encrypt data with a new key in the disaster recovery region.

🔧 2. Global Data Management

For applications with a global user base, you can replicate encrypted data across regions. With Multi-Region Keys, user-facing applications in each region can decrypt this data locally, providing a better user experience with lower latency.

🔧 3. Active-Active Architectures

In an active-active setup, where your application is running simultaneously in multiple regions, Multi-Region Keys simplify your encryption and decryption workflows. Your application can use the same key ID to perform cryptographic operations in any of the active regions.

🔧 4. Distributed Signing Applications

For applications that require digital signatures across different regions, multi-region asymmetric signing keys ensure that you can generate identical signatures consistently, regardless of where the signing operation takes place.

🔧 Using the AWS CLI

Creating a Multi-Region Key using the AWS CLI is a two-step process: creating the primary key and then replicating it to other regions.

# Create the primary key in your primary region (e.g., us-east-1)
aws kms create-key --region us-east-1 --multi-region --description "My multi-region primary key" --tags TagKey=Project,TagValue=Phoenix

# Replicate the primary key to a replica region (e.g., eu-west-1)
# Replace 'mrk-1234...' with the ARN of your primary key
aws kms replicate-key --region eu-west-1 --key-id arn:aws:kms:us-east-1:123456789012:key/mrk-1234... --description "My multi-region replica key"

🔧 Using Terraform

For a more declarative and reproducible approach, you can use Terraform to manage your Multi-Region Keys.

provider "aws" {
  region = "us-east-1"
  alias  = "primary"
}

provider "aws" {
  region = "eu-west-1"
  alias  = "replica"
}

resource "aws_kms_key" "primary" {
  provider                = aws.primary
  description             = "Multi-Region primary key"
  deletion_window_in_days = 10
  multi_region            = true
  enable_key_rotation     = true
}

resource "aws_kms_replica_key" "replica" {
  provider        = aws.replica
  description     = "Multi-Region replica key"
  primary_key_arn = aws_kms_key.primary.arn
}

🔧 AccessDeniedException

Problem: You receive an "AccessDeniedException" when trying to use or manage a multi-region key.

Solutions:

• Check Key Policies: Ensure that the IAM user or role has the necessary permissions in the key policy of both the primary and the replica key you are trying to use.
• Verify IAM Policies: In addition to the key policy, check for any IAM policies attached to the principal that might be denying access.
• SCP Restrictions: If you are using AWS Organizations, check for any Service Control Policies (SCPs) that might be restricting KMS actions.

🔧 DisabledException

Problem: You cannot use a key because it is disabled.

Solution: Remember that enabling or disabling a multi-region key is an independent action. If you disable a primary key, its replicas are not automatically disabled. You must enable the specific key (primary or replica) in the region where you are performing the cryptographic operation.

🔧 Replication Failure

Problem: You are unable to replicate a primary key to a new region.

Solution:

• The IAM principal needs `kms:ReplicateKey` permission on the primary key and `kms:CreateKey` permission in the replica region.
• Ensure the target region is enabled for your account.