Restrict IOT publish topic policy

Question

I'm using flutter/dart (mqtt_client / https://pub.dev/packages/mqtt_client) to send an AWS IOT MQTT messages over websockets and I'd like to restrict an IAM user to only specific topics that a user is allowed to Publish messages only to their specific topic.  I've attempted to add some restricted policies, but the application will fail with little information on the client side.  Also, in Cloud Watch, I don't see any specific errors.

Here's some example topics:

`arn:aws:iot:us-east-2:1234567890:topic/action_request/ASDF1234`

`arn:aws:iot:us-east-2:1234567890:topic/action_request/ASDF5678`

So, I want to add the proper JSON policy attached to the IAM user and they only have access to ASDF1234

All of my publish topics are patterned like the above. For now, I'm focusing on restricting the Publish endpoints and then working others like Subscribe.

I've tried numerous different policies like below. Also with adding some wildcards to no success on the client side.  They look right, but I'm not sure if there's indirectly other publish topics that are used internally within MQTT that's causing the failures or maybe just my syntax.

Another thought is if I add a condition that would allow only the above endpoint and no others.
https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_elements_condition_operators.html

```
{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": [
                "iot:Receive",
                "iot:ListNamedShadowsForThing",
                "iot:Subscribe",
                "iot:Connect",
                "iot:GetThingShadow",
                "iot:DeleteThingShadow",
                "iot:UpdateThingShadow"
            ],
            "Resource": "*"
        },
        {
            "Sid": "VisualEditor1",
            "Effect": "Allow",
            "Action": "iot:Publish",
            "Resource": "arn:aws:iot:us-east-2:1234567890:topic/*/ASDF1234*"
        }
    ]
}
```

Answer

Lets try to take it step by step.
First, the policy you required to use for your Thing is [IoT Policy](https://docs.aws.amazon.com/iot/latest/developerguide/iot-policies.html), and not IAM policy, which is for IAM identities (users, user groups, and roles).
The policy should be attached to a certificate. Cartificate should be attached to a Thing.
I'd suggest to start by enabling logging in the 'Settings' page of the AWS IoT Console, and using overly permissive IoT Policy(1) until you make it work. Once you succeed to connect and subscribe/publish, modify the IoT policy to include what you need.

(1) Not for production

```
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "iot:*"
      ],
      "Resource": "*"
    }
  ]
}
```

Restrict IOT publish topic policy

Relevant content