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PAM Conversation for OTP/Two-Factor-Authentication

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Problem statement

Two-Factor-Authentication (2FA) typically uses a long term password or PIN and an One-Time-Password (OTP) which in general is generated by a small device. To be backward compatible and to allow 2FA in existing application both factors can be entered one after the other in a single password prompt. This single string is then evaluated by the authentication backend.

On desktop systems where a single password (One-Factor-Authentication -1FA) is used this password is often used for other purposes to improve the user experience. For example to

  • unlock a keyring
  • un-encrypt files or the complete home-directory
  • allow authentication even if the authentication backend is not reachable (offline authentication).

These convenience features are not available if 2FA is used with both factors combined in a single string because there is no general rule which would allow to split the long-term and the one-time part. Only the authentication backend knows how to handle it. To make these features possible again with 2FA the user must enter the two factors individually into two separate prompts. This design page will show how this can be achieved with the help of PAM and SSSD.

Use cases

Unlock user’s keyring

Assuming a user with 2FA enabled which log into a desktop session. If the two factors are entered into a single prompt the resulting string cannot be used as a password for the desktop keyring application because it will change at every login. To not create issues SSSD will automatically remove the password item from the PAM environment in this case (see #3329 for details).

If the two factors are entered separately the first factor, the long-term password, can but used as a password for the keyring application because changes will be rare. In this case SSS can put the long term password into the PAM environment so that the PAM modules of the keyring application can pick it up at login time so that there is no need to unlock the keyring in a separate step.

Offline-authentication

Assuming a user with 2FA enabled which log into a desktop session which currently does not have a connection to the central authentication server. If the two factors are entered into a single prompt the resulting string can only be processed by the central authentication server and a loss of connection will make authentication and login impossible. Even the SSSD offline-authentication feature won’t help because SSSD will only store a hash of the password used for the last successful authentication and compare it with the hash of the current password. Since the combined password will change at every login the current combined password cannot be validated against any previously used password.

If the two factors are entered separately SSSD can save a hash of the first factor and can compare the hashes for the first factor when offline to allow at least access to the local machine. It has to be noted that the even if krb5_store_password_if_offline is set to true SSSD will not try to get a TGT when going online again because in the general case the second factor (OTP) might be already invalid.

Both use-cases mentioned above might only be working if the first factor (long-term password) is sufficiently long. A 4-digit PIN used by some OTP systems is not secure enough to be uses as a password for a keyring or to allow local access.

Overview of the solution

One of the design principles of SSSD’s PAM module pam_sss was that it should not do any decisions on its own but let SSSD do them. As a consequence, pam_sss cannot decide which type of password prompt should be shown to the user but must ask SSSD first. Currently the first communication between pam_sss and SSSD’s PAM responder happens after the user entered the password. Hence a new request, a pre-authentication request, to the PAM responder must be added before the user is prompted for the password. The pam responder can then relay the request to a suitable backend where it can be evaluated which type of prompt should be shown to the user. The result can be send back to pam_sss in a PAM response message which is already use to return other types of messages to pam_sss. This message can be used to send additional hints like e.g. type or vendor of the expected OTP hardware token to the user.

Based on the response pam_sss will ask the user for a single password or for the two factors in individual prompts. If only the first factor is entered and the second is empty the input will be treated as a single password. This might happen is the user accidentally entered both factor together or if applications or protocols (ssh, ftp) are configured or can handle only a single password prompt.

To keep the delays due to the new request to a minimum pam_sss should only run it, if the backend really supports it. Additionally is should be possible to disable the pre-authentication request completely with a new option for pam_sss.

In addition to the authentication dialog the password change dialog should respect the splitting of the two factors as well.

Implementation details

Making sure the PAM calls SSSD the ask for credentials for SSSD users

PAM allows to configure multiple different authentication methods but ideally only ask the user once for a password (or other credentials). Typically the first configure authentication method will ask for a password and if the user is not know to the authentication method the password is passed on to the next authentication method. Obviously this only works well with a single type of credentials.

In our case we want to prompt the user differently depending on whether 1FA or 2FA is configured for the user. Typically pam_unix is the first authentication module to make sure the authentication of local users (especially root) is not affected by other modules. But since pam_unix does not know anything about SSSD users or 2FA we have to make sure that pam_unix will not ask for a password for SSSD users. Instead of putting pam_sss in front of pam_unix we would like to use pam_localuser to skip pam_unix for non-local users. A PAM auth configuration might look like this :

auth        required               pam_env.so
auth        [default=1 success=ok] pam_localuser.so
auth        sufficient             pam_unix.so nullok try_first_pass
auth        requisite              pam_succeed_if.so uid >= 1000 quiet_success
auth        sufficient             pam_sss.so
auth        required               pam_deny.so

If the user is in /etc/passwd pam_localuser will return success and pam_unix will be called. Otherwise the next entry (default=1) will be skipped which is pam_unix in this case. The next module for a user which does not come from /etc/passwd if pam_succeed_if. I think it is a good idea to keep the pam_succeed_if to keep the separation between local users (uid < 1000) and remote users (uid >= 1000).

For the time being we keep the PAM password section (for password changes) as is because it is already used to handle changing the long-term password.

Handling the two authentication factors

Both the wire protocol between pam_ssd and the pam responder and SSSD internally with the sss_auth_token struct handle the credentials as a blob with a length and a type. Currently the blob contains either the password or is NULL in case of no password (there is a special usage where it contains a Kerberos credential cache identification).

Adding the two authentication factor to those structures can be achieve without modifying them by using a new type for 2FA and creating a blob which starts with two 32bit unsigned integer value containing the size of the first and second authentication factor respectively followed by the first factor and finally the second factor. :

uint32_t uint32_t uint8_t[6] uint8_t[5]
0x06 0x06 abcdef 12345\0

As shown the first and second factor may or may not include a trailing \0 in the blob. But calls which decompose the blob into its component must assure that that there is a trailing \0 if strings are expected.

With this scheme only packaging and un-packaging the two factors has to be added to existing or new calls but all other internal handling like sending the data from the responder to the backends can be left unchanged.

Backends which should handle 2FA must be made aware of the new authentication token type.

The pre-authentication request

The pre-authentication request will follow the same path as the authentication request with an empty password and with type SSS_PAM_PREAUTH instead of SSS_PAM_AUTHENTICATE. It is up to the backend if and how this request will be handled. Currently only the IPA auth provider will support the pre-auth request in the sense that it can send different results base on the expected authentication type (1FA, 2FA) back to the client. Since the IPA provider will basically use the generic krb5 auth provider the krb5 auth provider will support the pre-auth request as well.

The IPA provider will send a back a PAM response of the type SSS_PAM_OTP_INFO in case of 2FA with optional token_id, vendor name and challenge so that pam_sss can give additional hints to the user and a unsigned 32bit integer value indication the type of the optional data. This indicator will make it more easy to add more data in future or just indicate that the user uses 2FA but the backend is offline.

If 2FA is not enabled for the user or errors occur just a PAM_SUCCESS will be returned. In this case pam_sss will just ask for a single password.

If the backend is offline the PAM responder will tell the client that only the first factor is needed for local authentication with the help of a special SSS_PAM_OTP_INFO message. To achieve this the type of the hashed authentication token in the cache must be saved. Additionally the length of the second factor should be saved in the cache to allow splitting a combined password which might be entered by the user accidentally or via services where special prompting might not be available like e.g. ssh. If the second factor varies in size this scheme will fail but saving the length of the first factor will make an offline attack against the hashed password much easier.

Since the pre-auth request is an additional round-trip from pam_sss to the KDC and back it might delay the logon process a bit. To avoid this in environments where only 1FA is used and option the pam_sss, disable_preauth, can disable the pre_auth request completely. Additionally I would suggest a more dynamic solution where is pre-auth request is only send if a special file, e.g. /var/lib/sssd/pubconf/do_pam_preauth, exits. The IPA provider can create this file at startup if 2FA is supported.

Special use of the first factor (long term password)

Cached password hash for offline-authentication

If authentication was successful, the cache_credentials option is set to true and the first factor has at least minimal_password_length SSSD will saved a hashed version of the first factor to the user’s cache entry as it is done for the 1FA password.

PAM

If authentication is successful and the forward_pass option is given for pam_sss the first factor will be saved in the PAM_AUTHTOK item so that other modules in the PAM stack can use it. QUESTION: shall we(authconfig) add forward_pass by default? Currently is it not.

Configuration changes

pam_sss

New options:

  • disable_preauth will unconditionally disable the pre-authentication request
  • use_2fa will always ask for two authentication factor, might be only useful for testing

sssd.conf

New option:

  • minimal_password_length will let the pam responder only save a hash of the password if it has a minimal length. Additionally it might indicate to pam_sss to remove passwords from the PAM environment which are shorter. Question: We can limit this option to the first factor of a 2FA authentication. Although it might be useful for 1FA passwords as well it might introduce a regression to existing installations.

Changing the first factor

It is already possible to change the long-term password (first factor), the current scheme will not be changed here.

How To Test

Prerequisites

Create an user with 2FA/OTP authentication as e.g. described in http://www.freeipa.org/page/V4/OTP#Configuration

Login prompt

If 2FA is enabled for a user there should be two separate prompts for the two authentication factors for services which can support special prompting. This includes e.g. gdm and su. ssh can only support this if ChallengeResponseAuthentication is enabled on the server side. Nevertheless even if ChallengeResponseAuthentication is not enabled ssh should allow login if both factors are given at the password prompt in a single string.

For users without 2FA the single password prompt should be seen.

# su - otpuser
First factor:
Second factor:
sh$

# su - user
Password:
sh$

If both factors are entered at the First factor prompt and the second factor prompt is empty, authentication should be successful but it cannot be expected that the user’s keyring is unlocked or that offline-authentication will be available.

Unlocking user’s keyring

If an otpuser logs in with gdm and enters the two authentication factors separately in the expected prompts the keyring of the user should be unlocked automatically and no additional password prompt should be seen after logging in.

Offline authentication

If an otpuser logs in with an application which supports special prompting, e.g. gdm or su, and the SSSD configuration option cache_credentials is set to True SSSD will save a hash of the first factor in the cache to allow offline authentication. If later on the system goes offline authentication should still be possible with the first authentication factor. Only a prompt for the first factor should be shown by application which supports special prompting.

Authors

Sumit Bose <sbose@redhat.com>