ACM Private Key Security
When you request a certificate, AWS Certificate Manager (ACM) generates a public/private key pair. For imported certificates, you generate the key pair. The public key becomes part of the certificate. ACM stores the certificate and its corresponding private key, and uses AWS Key Management Service (AWS KMS) to help protect the private key. The process works like this:
The first time you request or import a certificate in an AWS region, ACM creates an AWS-managed customer master key (CMK) in AWS KMS with the alias aws/acm. This CMK is unique in each AWS account and each AWS region.
ACM uses this CMK to encrypt the certificate's private key. ACM stores only an encrypted version of the private key (ACM does not store the private key in plaintext form). ACM uses the same CMK to encrypt the private keys for all certificates in a specific AWS account and a specific AWS region.
When you associate the certificate with an Elastic Load Balancing load balancer or an Amazon CloudFront distribution, ACM sends the certificate and the encrypted private key to the load balancer or distribution. You also implicitly create a grant in AWS KMS that allows the load balancer or distribution to use the CMK in AWS KMS to decrypt that specific certificate's private key. For more information about grants, see Using Grants in the AWS Key Management Service Developer Guide.
The load balancer or distribution uses the CMK in AWS KMS to decrypt the private key. Then the load balancer or distribution uses the certificate and the decrypted (plaintext) private key to establish secure communication channels (SSL/TLS sessions) with its clients.
When the certificate is disassociated from the load balancer or distribution, the grant (created at step 3) is retired. This means the load balancer or distribution can no longer use the CMK in AWS KMS to decrypt that specific certificate's private key.