AWS Secrets manager resource policy access for user

I'm trying to make an AWS Secrets Manager resource to be accesed only by certain user by writing a resource policy for the Secrets Manager but I can't make it work, I have tried a policy with Allow and Deny with Principal and NotPrincipal, a Deny policy with NotPrincipal and Condition, NotArnLike with aws:SourceArn. All this configs with the arn of the user arn:aws:iam::123456789012:user/fbuccioni.

My scenario is kinda root account, 2 devops with user/policy privileges to 3rd parties and need only the root account access to the secretsmanager:GetValue action. That's why I'm trying to securize the resource instead doing separate IAM identity based policies.

How can I make it work?

Is there a default Deny policy and I have to Allow? in the aws examples have an allow condition only.

To make it work I have to do several tests and research but finally I got the answer.

For IAM users

I start doing the tests without the root user, so I try with an IAM user, the policy doesn't work with Principal statement in any possibly value, I have to do a Condition to make it work:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Deny",
      "Principal": "*",
      "Action": [
        "secretsmanager:GetSecretValue",
        "secretsmanager:PutResourcePolicy",
        "secretsmanager:DeleteResourcePolicy"
      ],
      "Resource": "*",
      "Condition": {
        "StringNotLike": {
          "aws:userId": [
            "AIDA1EXAMPLE2USER3ID4",
            "012345678987"
          ]
        }
      }
    }
  ]
}

being AIDA1EXAMPLE2USER3ID4 the User ID and 012345678987 the account number ID, you can retrieve the UserID with the command:

aws sts get-caller-identity

For Root account

The root account have the superpower to overpass any policy or permission, you just have to lock for everything and voila.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Deny",
      "Principal": "*",
      "Action": [
        "secretsmanager:GetSecretValue",
        "secretsmanager:PutResourcePolicy",
        "secretsmanager:DeleteResourcePolicy"
      ],
      "Resource": "*"
    }
  ]
}