Tutorial 1 - The Golden Key

author:: begriffs

In Tutorial 0 - Get it Running we created a read-only API with a single endpoint to list todos. There are many directions we can go to make this API more interesting, but one good place to start would be allowing some users to change data in addition to reading it.

Step 1. Add a Trusted User

The previous tutorial created a web_anon role in the database with which to execute anonymous web requests. Let’s make a role called todo_user for users who authenticate with the API. This role will have the authority to do anything to the todo list.

-- run this in psql using the database created
-- in the previous tutorial

create role todo_user nologin;
grant todo_user to authenticator;

grant usage on schema api to todo_user;
grant all on api.todos to todo_user;
grant usage, select on sequence api.todos_id_seq to todo_user;

Step 2. Make a Secret

Clients authenticate with the API using JSON Web Tokens. These are JSON objects which are cryptographically signed using a password known to only us and the server. Because clients do not know the password, they cannot tamper with the contents of their tokens. PostgREST will detect counterfeit tokens and will reject them.

Let’s create a password and provide it to PostgREST. Think of a nice long one, or use a tool to generate it. Your password must be at least 32 characters long.

Note

Unix tools can generate a nice password for you:

# Allow "tr" to process non-utf8 byte sequences
export LC_CTYPE=C

# read random bytes and keep only alphanumerics
< /dev/urandom tr -dc A-Za-z0-9 | head -c32

Open the tutorial.conf (created in the previous tutorial) and add a line with the password:

# PASSWORD MUST BE AT LEAST 32 CHARS LONG
# add this line to tutorial.conf:

jwt-secret = "<the password you made>"

If the PostgREST server is still running from the previous tutorial, restart it to load the updated configuration file.

Step 3. Sign a Token

Ordinarily your own code in the database or in another server will create and sign authentication tokens, but for this tutorial we will make one “by hand.” Go to jwt.io and fill in the fields like this:

jwt.io interface — How to create a token at https://jwt.io

Remember to fill in the password you generated rather than the word “secret”. After you have filled in the password and payload, the encoded data on the left will update. Copy the encoded token.

Note

While the token may look well obscured, it’s easy to reverse engineer the payload. The token is merely signed, not encrypted, so don’t put things inside that you don’t want a determined client to see.

Step 4. Make a Request

Back in the terminal, let’s use curl to add a todo. The request will include an HTTP header containing the authentication token.

export TOKEN="<paste token here>"

curl http://localhost:3000/todos -X POST \
     -H "Authorization: Bearer $TOKEN"   \
     -H "Content-Type: application/json" \
     -d '{"task": "learn how to auth"}'

And now we have completed all three items in our todo list, so let’s set done to true for them all with a PATCH request.

curl http://localhost:3000/todos -X PATCH \
     -H "Authorization: Bearer $TOKEN"    \
     -H "Content-Type: application/json"  \
     -d '{"done": true}'

A request for the todos shows three of them, and all completed.

curl http://localhost:3000/todos

[
  {
    "id": 1,
    "done": true,
    "task": "finish tutorial 0",
    "due": null
  },
  {
    "id": 2,
    "done": true,
    "task": "pat self on back",
    "due": null
  },
  {
    "id": 3,
    "done": true,
    "task": "learn how to auth",
    "due": null
  }
]

Step 5. Add Expiration

Currently our authentication token is valid for all eternity. The server, as long as it continues using the same JWT password, will honor the token.

It’s better policy to include an expiration timestamp for tokens using the exp claim. This is one of two JWT claims that PostgREST treats specially.

Claim	Interpretation
`role`	The database role under which to execute SQL for API request
`exp`	Expiration timestamp for token, expressed in “Unix epoch time”

Note

Epoch time is defined as the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), January 1st 1970, minus the number of leap seconds that have taken place since then.

To observe expiration in action, we’ll add an exp claim of five minutes in the future to our previous token. First find the epoch value of five minutes from now. In psql run this:

select extract(epoch from now() + '5 minutes'::interval) :: integer;

Go back to jwt.io and change the payload to

{
  "role": "todo_user",
  "exp": 123456789
}

NOTE: Don’t forget to change the dummy epoch value 123456789 in the snippet above to the epoch value returned by the psql command.

Copy the updated token as before, and save it as a new environment variable.

export NEW_TOKEN="<paste new token>"

Try issuing this request in curl before and after the expiration time:

curl http://localhost:3000/todos \
     -H "Authorization: Bearer $NEW_TOKEN"

After expiration, the API returns HTTP 401 Unauthorized:

{
  "hint": null,
  "details": null,
  "code": "PGRST301",
  "message": "JWT expired"
}

Bonus Topic: Immediate Revocation

Even with token expiration there are times when you may want to immediately revoke access for a specific token. For instance, suppose you learn that a disgruntled employee is up to no good and his token is still valid.

To revoke a specific token we need a way to tell it apart from others. Let’s add a custom email claim that matches the email of the client issued the token.

Go ahead and make a new token with the payload

{
  "role": "todo_user",
  "email": "disgruntled@mycompany.com"
}

Save it to an environment variable:

export WAYWARD_TOKEN="<paste new token>"

PostgREST allows us to specify a stored procedure to run during attempted authentication. The function can do whatever it likes, including raising an exception to terminate the request.

First make a new schema and add the function:

create schema auth;
grant usage on schema auth to web_anon, todo_user;

create or replace function auth.check_token() returns void
  language plpgsql
  as $$
begin
  if current_setting('request.jwt.claims', true)::json->>'email' =
     'disgruntled@mycompany.com' then
    raise insufficient_privilege
      using hint = 'Nope, we are on to you';
  end if;
end
$$;

Next update tutorial.conf and specify the new function:

# add this line to tutorial.conf

db-pre-request = "auth.check_token"

Restart PostgREST for the change to take effect. Next try making a request with our original token and then with the revoked one.

# this request still works

curl http://localhost:3000/todos -X PATCH \
     -H "Authorization: Bearer $TOKEN"    \
     -H "Content-Type: application/json"  \
     -d '{"done": true}'

# this one is rejected

curl http://localhost:3000/todos -X PATCH      \
     -H "Authorization: Bearer $WAYWARD_TOKEN" \
     -H "Content-Type: application/json"       \
     -d '{"task": "AAAHHHH!", "done": false}'

The server responds with 403 Forbidden:

{
  "hint": "Nope, we are on to you",
  "details": null,
  "code": "42501",
  "message": "insufficient_privilege"
}