MFA Auth Receipt

MFA Auth Receipt


To provide proper usable multi-factor authentication (MFA) support in OpenStack we need to return an auth receipt to the user representing partial authentication, which can be returned to Keystone as part of a challenge response process.

Problem Description

As of Ocata Keystone supports setting auth rules on a given user and potentially requiring them to auth with multiple methods. This is a good implementation of MFA, but the problem rests in a user or service knowing ahead of time how they need to auth. Most standard MFA systems are challenge response based. You auth with one of your factors, then are given a receipt of that partial authentication, which you can return along with the missing pieces.

Keystone needs such a mechanism because without it we can’t realistically consume MFA anywhere else in OpenStack in a manner that makes sense for users.

The original proposal for MFA in Keystone actually included this concept but was never implemented: multi-factor-authn

The concept of a half-token was discussed, but it never really got implemented in a way we could use for MFA.

Proposed Change

The proposal is to add a new mechanism which returns a receipt to the user on a partially successful auth attempt. This would in essence be very similar to a token, and ideally use a similar system for storage (fernet, jwt) to keep internal logic consistent and reusable.

When a user successfully authenticates with one of their many auth rules, we can infer that they are who they say they are, and then return to them a receipt that contains information about missing auth rules.

If a user attempts auth with multiple methods and most fail but at least one succeeds, we do not return a receipt, but we do include json error data as to what methods failed, and which did not.

Using a receipt to get a new receipt with extra auth methods should ideally be possible. If you need three auth methods to get a token, you should be able to add to your receipt until it can become a token.

This receipt would allow no API interaction, but it would be proof that certain auth methods have been completed. The expiry for the receipt would default to a fairly short time-frame (5 minutes).

Receipt Response

The return code for this would be 401 as the user is still unauthorized, with the response header containing the receipt id, and the body containing information about the receipt which they need to continue the auth process.

In addition this response would be explicitly setup to only occur when a user with auth rules successfully auths with one of the methods in any of their auth rules.

The receipt id (the encrypted payload) would be included in the header of the 401 response in the same fashion as a token is but under the header OPENSTACK-AUTH-RECEIPT.

An example response:

    "receipt": {
        "methods": [
        "user_id": "423f19a4ac1e4f48bbb4180756e6eb6c",
        "expires_at": "2015-11-06T15:32:17.893769Z"
    "required_auth_methods": [
        ['password', 'totp'],
        ['password', 'option2'],
        ['password', 'option3', 'option4']

required_auth_methods would be a list of rules that include the successfully authenticated methods and not a list of all of the user’s auth rules. The methods in the receipt do not entirely have to be all from the same auth rule, the receipt contains valid methods, and what rules they could apply to, so it’s an intersection test.

Another example response:

    "receipt": {
        "methods": [
            "password", "option3"
        "user_id": "423f19a4ac1e4f48bbb4180756e6eb6c",
        "expires_at": "2015-11-06T15:32:17.893769Z"
    "required_auth_methods": [
        ['password', 'option2'],
        ['option3', 'option4']

In this above case, the user now has two optional MFA paths off the same receipt despite there being no overlap between the two auth rules.

Receipt payload

The receipt will not include scope data as it isn’t relevant to it. A user will supply scope when using the receipt and trying to get a token, Keystone will not look at the receipt for scope. The only reason to potentially introduce scope to the receipt is if and when we introduce auth rules based on scope.

The actual provider for the receipt should be configurable in the same fashion as token, and similar types (fernet, jwt). In the case of fernet the key repository should ideally be able to be shared with the token provider, but configuration should be possible to split the repo. This minimises potential pain for deployers and gives them time before they need to add a second key repository.

Ideally any code we can share with the token providers we do, and move as much as makes sense into common utils. Much like tokens, we even handle key rotation in the same way. Accept receipts encrypted by the current and last key but issue new ones with the current key.

The receipt data itself (before encryption) would likely be:

    "methods": [
    "user_id": "423f19a4ac1e4f48bbb4180756e6eb6c",
    "issued_at": "2015-11-06T14:32:17.893797Z",

methods in the above payload being the valid and already satisfied auth methods for that receipt.

There isn’t much data Keystone itself needs that it can’t get from database as long as it knows the user id. All we need to pass back is what methods have already been validated, when it was issued at (for expiry checking), and the user the receipt applies to (so we can match it with the user in the auth request).

Receipt processing

When auth is continued with a receipt a user must supply the receipt id in the OPENSTACK-AUTH-RECEIPT header. When Keystone sees an auth attempt with that header it will use the set receipt provider to decrypt the payload (id), and will then treat the methods as defined in the receipt as having been already satisfied as part of auth, and auth continues with the new user supplied methods.

If all given methods are valid, but all the total valid methods including those from the receipt still do not fulfill any of the auth rules, another receipt is returned.

If the receipt is expired, don’t even process the user supplied auth methods and fail right away.

A mismatch between receipt user_id and user_id in any of the methods is a failure.

A failure in any of the user supplied auth methods is a failure, and does not extend the receipt even if some new methods were valid, but should include failure details.

If a user supplies a method that was already satisfied in the receipt, the user supplied method takes precedence, even if it fails.

All of these failures return a 401.


In Ocata when the auth rules were introduced the alternative was put forward to simply change the error messages of the failed MFA auth to inform the user why their auth failed. Any services could also parse the errors and infer what the missing auth was and generate GUI or cli queries back to the user.

This doesn’t work for a few reasons. While we can structure the response to be parsable it still requires that for every auth attempt ALL methods are supplied. This means an MFA attempt for password+totp needs to cache the password to resubmit it when it is determined that totp is also needed.

For Horizon that doesn’t work. We can’t cache the password server-side, and we shouldn’t store it in a cookie or in browser really, and asking the user to supply their password again isn’t good UX.

This alternative also isn’t a real challenge response.

Security Impact

A user who authenticates with one successful method now knows what auth rules can be used for a given user. This isn’t particularly unsafe, as while they know the rules, they can’t auth with them. Additionally it is safe to assume that one successful auth method is enough to prove identity of the user, while not enough to satisfy authentication requirements.

If any method fails, we still return an error, and auth rules are only exposed if at least one method was valid.

An additional security concern is that a given receipt could be used to make multiple tokens in the short window it is valid. Given that users can already have multiple tokens, this is only a concern if a receipt is intercepted and completed by someone else. Given though that this receipt exists in the context of MFA, the second factor should limit this concern quite heavily. In reality this isn’t a problem unique to the auth receipt, but an eventual action we should take is to do some form of receipt revocations when turned into a token.

Notifications Impact

Notifications will continue to be issued for successful and fail authentication as today. Issuance of a receipt will not issue a notification.

We may add receipt notifications in the future.

Other End User Impact

The user may now process the returned data and utilize the receipt to authenticate in multiple steps.

It is important to note that this change does not affect anyone only doing auth with no auth rules set. The default auth use case does not change, only the MFA use case which is not yet fully implemented or used anyway.

Performance Impact

It is expected load and auth-processing-time will increase a small amount due added encryption and decryption of the receipt.

Other Deployer Impact

Depending on how we handle fernet/jwt key repositories, deployers may need to add a second key repository. Ideally we’d allow this to be configurable so at first they didn’t need to do this, but had the option to for added security.

Developer Impact

This would only affect authentication workflows in OpenStack and any code dealing with them. This change though will give us a good programmatic way to deal with MFA and will mean those tools can handle failures in a useful way.



Primary assignee:

Work Items

  1. Add support for the auth receipt to Keystone and change the auth plugin layer to return it on one successful auth method out of configured rules rather than an error, and make the auth layer able to process a receipt for continued auth.
  2. Add support to KeystoneAuth to handle this correctly and infer from this receipt what other methods are needed, as well as adding proper Multi-Method support into KeystoneAuth.
  3. Work with the CLI and Dashboard teams to build support in for MFA using the changes made to KeystoneAuth. (optional follow up)



Documentation Impact

Will require extra documentation for new features, but no existing non-MFA auth paths will change.

We need to add to the docs that MFA receipts also need encryption keys, and that by default they are shared with fernet. Will need notes added that fernet key rotation affect the receipts if their repository is shared.

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