On 02/18/2015 06:55 AM, Sumit Bose wrote:
Hi,
I have created a design page for the special prompting for long-term
password and one-time password at
https://fedorahosted.org/sssd/wiki/DesignDocs/PAMConversationForOTP
Nice write-up.
There are some typos but this is OK.
Questions are inline.
Comments and suggestions are welcome. Please note, there are two open
questions related to configuration.
For your convenience I added the text below as well.
bye,
Sumit
= PAM Conversation for OTP/Two-Factor-Authentication =
Related ticket(s):
*
https://fedorahosted.org/sssd/ticket/2335
=== 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 factor individually
into two separate prompts. This design page will show how this can be
achieved with the help of PAM and SSSD.
Short overview (up to a couple of sentences) of the change being
designed. Might include pointers to reference materials (such as MSDN
articles when designing an AD integration feature) or other information
required to understand what the change is about.
=== 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 #2287 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 can 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 ===
On of the design principals of SSSD's PAM module pam_sss was that it
should do any decisions on its own but let SSSD do this. 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 addition 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.
Keep 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 possilbe 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 structure 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 enable 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.
Why a file and not an SSSD option? You lost me here.
Also is it really a round trip to KDC or it is an LDAP lookup?
Are you planning to determine whether you can prompt by getting the
supported auth mechs for user from KDC?
I see that it would be a very rare case when different users of the same
system would have different prompts. It is possible but a corner case.
In this case it is fine to have a roundtrip.
But for the usual case when the system is either enabled for 2FA or not
I would rather have an override in sssd.conf saying always prompt for
two or for one since I know what environment the system is deployed in.
This way we save a trip and can always prompt the way configuration
prescribes.
Am I missing something?
==== 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 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.'''
We should open corresponding tickets for authconfig and make sure they
are addressed in the same time frame.
Should we also create tickets for ipa-client and realmd to say "this is
a client where there will be two prompts" and pass it through?
=== 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] ''minimal_password_length'' will let the
pam responder only safe 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.'''
IMO we would have a marker in the user entry whether this user has a
single factor or not based on his first authentication and apply this
min length only to users that cave 2FA enabled.
That would prevent a regression.
==== 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(a)redhat.com>
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--
Thank you,
Dmitri Pal
Sr. Engineering Manager IdM portfolio
Red Hat, Inc.