Allowing access to a KMS key from another account

Question

I am trying to allow the use of my KMS key from 'Account A' in 'Account B', but I seem to be missing a step.  Please let me know if you can see what I'm doing wrong.

The context is that I am sharing a DB snapshot, and the snapshot is encrypted using a CMK in Account A

What I have done:
Following this document as guidance: https://docs.aws.amazon.com/kms/latest/developerguide/key-policy-modifying-external-accounts.html
In Account A's KMS, I updated the key policy with the following:
`        {
            "Sid": "Allow an external account to use this KMS key",
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam:::root"
            },
            "Action": [
                "kms:Encrypt",
                "kms:Decrypt",
                "kms:ReEncrypt*",
                "kms:GenerateDataKey*",
                "kms:DescribeKey"
            ],
            "Resource": "*"
        }
`
This key policy does have more, but there are no deny statements in it.

And then in Account B I have created a new IAM policy with the following:
`
 {
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowUseOfKeyInAccountA",
            "Effect": "Allow",
            "Action": [
                "kms:Encrypt",
                "kms:Decrypt",
                "kms:ReEncrypt*",
                "kms:GenerateDataKey*",
                "kms:DescribeKey"
            ],
            "Resource": "arn:aws:kms:::key/"
        }
    ]
}
`
I attached this policy to the role I'm currently using and then logged out and back in again to the console.
Then I went to the snapshots shared with me and attempted to copy that snapshot that was shared from Account A.
When I try to save it off though, I get an error:

> The source snapshot KMS key [arn:aws:kms:::key/ matching the CMK ARN and policy above] does not exist, is not enabled or you do not have permissions to access it.

What step did I miss?

Accepted Answer

It looks like what I was missing was 
```
kms:CreateGrant
```
in my Account A CMK policy.  Once added, I was able to do my snapshot copy

Allowing access to a KMS key from another account

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