Sharing an AWS managed KMS key with another account

I have an s3 folder that has encrypted objects in it. It is encrypted with the AWS KMS managed keys, not a custom key. I need another AWS account to be able to copy files from this bucket. From what I can tell, I can't share this KMS key across accounts. I also can't easily change this bucket to use custom keys as there are implications with customers. Are there any know good work arounds here?

Solution

I did some experimentation on two of my accounts.

I found out that by using bucket policies, you can enable access to other account only for:

bucket/object with no encryption
AES256 encryption which is managed by AWS owned key.

However, if you use AWS Managed CMK, bucket policy is NOT suited.

Instead, you can enable access to your bucket and objects using cross-account roles. I verified that it works.

Account A

In the Acc A (one with the bucket with AWS-KMS encryption, I created a role called kmss3bucket:

trust policy - Acc B (which will assume the role and access the objects)
AmazonS3ReadOnlyAccess managed policy
inline policy for AWS/S3 KMS key:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": "kms:Decrypt",
            "Resource": "arn:aws:kms:us-east-1:xxxxxx:key/b811de9a-150a-4d89-8a9a-03e1b878737d"
        }
    ]
}

Accpunt B

A user in Acc B assumes the role, using STS:

aws sts assume-role --role-arn arn:aws:iam::xxxxx:role/kmss3bucket --role-session-name cross-account-s3

This will generate new temp aws credentials. Using the credentials, Acc B will be able to copy the objects from Acc A encrypted with AWS-KMS.