IAM Policy for KMS-Encrypted Remote Terraform State in S3

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "s3:GetObject",
                "s3:PutObject",
                "s3:DeleteObject"
            ],
            "Resource": [
                "arn:aws:s3:::<bucket name>/*",
                "arn:aws:s3:::<bucket name>"
            ]
        },
        {
            "Effect": "Allow",
            "Action": [
                "kms:Encrypt",
                "kms:Decrypt",
                "kms:GenerateDataKey"
            ],
            "Resource": [
                "<arn of KMS key>"
            ]
        }
    ]
}

Don’t forget to update the KMS Key Policy, too. I spent a bit of time trying to figure out why it wasn’t working, until CloudTrail helpfully told me that the kms:GenerateDataKey permission was also required. Turn it on today, even if you don’t need the auditing. It’s an excellent permissions debugging tool.