Minimal Privilege MSK SCRAM KMS Key policy

Question

We are using MSK SCRAM which registers Secrets Manager secrets for authentication. This does require a separate symmetric KMS key to be used with the secrets. The secrets are required to be of the form 'AmazonMSK_*'. Our Security is asking to get the access to this KMS key down to the minimum privilege in the associated KMS key policy.

It seems the recommendation is to use Condition variables. Link : https://docs.aws.amazon.com/kms/latest/developerguide/key-policy-services.html

I have tried to use this in this manner:

```
Effect: Allow
Action: 
- kms:CreateGrant
- kms:Encrypt
Resource: "*"
Principal: "*"
Condition: 
     StringLike:
         kms:EncryptionContext:aws:secretsmanager:arn: 
          - "arn.aws.secretsmanager:us-west-2:*:secret:AmazonMSK_*" 
```

but i get : "Access to KMS is not allowed"

a condition such as :
```
 Condition: 
     StringEquals:
         kms:ViaService: "secretsmanager.us-west-2.amazonaws.com"
```
works, but is not specific enough. Does someone know what EncryptionContext could be used for 
secretsmanager conditions?

Accepted Answer

I believe the primary problem with the key policy above is the key context that you are specifying. The key context used by AWS Secrets Manager to specify the specific secret is ***kms:EncryptionContext:SecretARN***, as described here -> https://docs.aws.amazon.com/secretsmanager/latest/userguide/security-encryption.html#security-encryption-encryption-context. Use **extreme caution** when updating the key policies so they don't become unmanageable -> https://docs.aws.amazon.com/kms/latest/developerguide/key-policy-default.html. Make sure you have a statement for key administration, in addition to, the key user policy that you are customizing. The policy statement for Amazon MSK should look something like this:

```
{
    "Sid": "AllowUseOfTheKeyForSecretsManager",
    "Effect": "Allow",
    "Principal": {
        "AWS": "arn:aws:iam:::role/"
    },
    "Action": [
        "kms:Encrypt",
        "kms:Decrypt",
        "kms:ReEncrypt*",
        "kms:GenerateDataKey*",
        "kms:DescribeKey"
    ],
    "Resource": "*",
    "Condition": {
        "StringLike": {
            "kms:EncryptionContext:SecretARN": "arn:aws:secretsmanager:::secret:AmazonMSK_*"
        }
    }
}
```

Minimal Privilege MSK SCRAM KMS Key policy

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