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Goals of our design

1) Once a user signs in using a credential, we want them to be able to stay signed in indefinitely. This holds true even if the client returns outside of any time window we might anticipate. To reauthenticate, we've been issuing a token for the purpose of re-acquiring a session when the user loses it.

2) We don't want the reauthentication token stored in plaintext on the server, since it is effectively a password.

3) The reauthentication token can be lost in transit back to the client, along with the session being returned. When this happens the client is in a "failed reauthentication" state and these improvements are primarily designed to ensure the client can recover from this state.

4) We want to invalidate the existing session when issuing a new session (on sign in). There are some alternatives for how we might accomplish this, but because it's related to sign in, it can be done on a later update (see "Addressing Concurrent Sign In Requests" below). 

In the current implementation, When a reauthentication request succeeds, but the client fails to get back the session, we create a new reauth token and store the old token in Redis. While the client can recover by resending the old reauth token again, and they will get the session, the session we send back does not include the new reauth token (we don't have it due to #2 above). We just return the old token in the session. As a result, at some point, that user will still have to authenticate when the cached reauthentication token expires from cache.

The proposed design would fix this.

Signing in

Signing in does not change:

...

Goals of our design

1) Once a user signs in using a credential, we want them to be able to stay signed in indefinitely. This holds true even if the client returns outside of any time window we might anticipate. To reauthenticate, we've been issuing a token for the purpose of re-acquiring a session when the user loses it.

2) We don't want the reauthentication token stored in plaintext on the server, since it is effectively a password.

3) The reauthentication token can be lost in transit back to the client, along with the session being returned. When this happens the client is in a "failed reauthentication" state and these improvements are primarily designed to ensure the client can recover from this state.

4) We want to invalidate the existing session when issuing a new session (on sign in). There are some alternatives for how we might accomplish this, but because it's related to sign in, it can be done on a later update (see "Addressing Concurrent Sign In Requests" below). 

In the current implementation, When a reauthentication request succeeds, but the client fails to get back the session, we create a new reauth token and store the old token in Redis. While the client can recover by resending the old reauth token again, and they will get the session, the session we send back does not include the new reauth token (we don't have it due to #2 above). We just return the old token in the session. As a result, at some point, that user will still have to authenticate when the cached reauthentication token expires from cache.

The proposed design would fix this.

Signing in

This doesn't change (it might change if we attempt to address concurrent sign in requests; see below).

  • User signs in
  • We create a new session, session token, reauthToken
  • We store the following Redis mappings:
    • sessionToken ↝ userId
    • userId ↝ session
  • return the session with the sessionToken and reauthToken in the session

Authenticating a request

This doesn't change (it might change if we attempt to address concurrent sign in requests; see below).

  • User makes request with a sessionToken;
  • Retrieve the userId with the sessionToken (if this fails return 404);
  • Retrieve the session with the userId (if this fails return 404);
  • Return the session with this session token (we do not store the reauthentication token in the session);

Reauthentication

This is changing so we'll include the success, failure, and concurrent scenarios:

Success

  • User reauthenticates with the reauthentication token
  • We retrieve the N most recent records by their creation date, hash the token by the algorithm in each record, and compare to the hashed records looking for a match. A match is an authentication success
  • If a session exists, we create a new session but keep the session token/internal session token;
  • If a session does not exist, a new session includes new session token/internal session token;
  • Persist a new record in the secrets table for the new reauthToken
  • We store the following Redis mappings:
    • sessionToken ↝ userId
    • userId ↝ session
  • return the session with the sessionToken and reauthToken in the session

Authenticating a request

This doesn't change (it might change if we attempt to address concurrent sign in requests; see below).

  • User makes request with a sessionToken
  • Retrieve the userId with the sessionToken (if this fails return 404)
  • Retrieve the session with the userId (if this fails return 404)
  • return the session with this session token and whatever reauthentication token is in the session

...

The session token is not rotated once it exists (the session contents are rebuilt but the session and internal session tokens are not changed unless the session doesn't exist n the first place because it has expired). If the session token rotated with each reauth request and we removed the validity of the last session token, concurrent reauthentication requests might capture the invalidated session token. The session token still expires after 12 hours regardless of how it is read or updated.

Failure

  • User reauthenticates with the reauthentication token
  • We retrieve the N most recent records by their creation date, hash the token by the algorithm in each record, and compare to the hashed records looking for a match. In this case, there is no match
  • We return a 404 to the user and we do not rotate the reauthentication tokens.

Concurrent Requests

  • User reauthenticates with the reauthentication tokenWe , then sends a second identical request;
  • For the first request, we retrieve the N most recent records by their creation date, hash the token by the algorithm in each record, and compare to the hashed records looking for a match. A match is an authentication success
  • If a session exists, we create a new session but keep the session token/internal session token;
  • If a session does not exist, a new session includes new session token/internal session token;
  • Persist a new record in the secrets table for the new reauthToken
  • We store the following Redis mappings:
    • sessionToken ↝ userId
    • userId ↝ session
  • return the session with the sessionToken and reauthToken in the session

...

  • authentication success;
  • a new session is prepared for the first request;
  • a new record in the secrets table is persisted for the first request;
  • We store the following Redis mappings for the first request:
    • sessionToken ↝ userId
    • userId ↝ session
  • We return the first request with this session prepared;
  • The second request, meanwhile, retrieves N most recent records which may or may not induce the new row in the secrets table created by the other request, but it will still include the desired reauth token row and should lead to another authentication success;
  • a new session is prepared and the session token from the first request is maintained;
  • We store the following mappings again, which should be identical except possibly for the session contents:
    • sessionToken ↝ userId
    • userId ↝ session
  • We return the second request with this session which looks similar to the first instance that was returned.

If the first request failed to return, and was followed up with a retry, the steps should look similar to this. We can adjust N to make reauthentication more or less robust to concurrent reauthentication requests (e.g. if we find out clients routinely send 5 at once, we could increase N, though not desirable).

Sign Out

  • Delete the userId ↝ session mapping
  • Delete the sessionToken ↝ userId mapping
  • Delete the reauth secret records for this user in the secrets table

Concurrency issues

There are some issue we have identified when the client makes multiple requests to reauthenticate:

...

Addressing Concurrent Sign In Requests

Concurrent sign ins should be rarer because they involve human intervention (enter credentials, click on a link), but could still theoretically happen. There are two approaches to dealing with this.

The simpler option would be to record the timestamp when a session token is created, and reuse that token for a grace period on subsequent or concurrent sign ins. This is the simplest approach.

A more complicated approach would be to issue a new token with each successful sign in. This would solve concurrent sign ins but and it would also allow for sign ins on multiple devices (which has been discussed as a capability for Bridge). The logic could be as follows:

  • issue new session token on each sign in, adding to a set of tokens in the session
  • on access
    • sessionToken ↝ userId
    • userId ↝ session
      • is token in session tokens set?
        • NO: not authenticated
        • YES: is there more than one token in session?
          • NO: return session
          • YES: replace set with a set consisting only of this token, write session to cache, return session

Note that with this approach, we can later allow multiple clients to authenticate simultaneously by not stripping out other session tokens. Each token has an expiry due to the first sessionToken ↝ userId lookup, independent of the session expiry. We might also want to tie these session tokens to something like a UA header or an IP address to make it harder to hijack them.