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Plan 9's auth() function - partialy secure distributed computing
Forwarded message:
>From [email protected] Tue Jan 13 17:51:44 1998
Date: Tue, 13 Jan 1998 17:51:42 -0600
From: Jim Choate <[email protected]>
Message-Id: <[email protected]>
X-within-URL: http://plan9.bell-labs.com/magic/man2html/6/auth
To: [email protected]
Subject: auth
[search] [index] delim $$ define lbr ' roman "{" ' define rbr ' roman
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NAME
ticket - authentication service
DESCRIPTION
This manual page describes the protocols used to authorize
connections, confirm the identities of users and machines, and
maintain the associated databases. The machine that provides
these services is called the authentication server (AS). The AS
may be a stand-alone machine or a general-use machine such as a
CPU server. The network database ndb(6) holds for each public
machine, such as a CPU server or file server, the name of the
authentication server that machine uses.
Each machine contains three values important to authentication;
a 56-bit DES key, a 28-byte authentication ID, and a 48-byte
authentication domain name. The ID is a user name and
identifies who is currently responsible for the kernel running
on that machine. The domain name identifies the machines across
which the ID is valid. Together, the ID and domain name
identify the owner of a key.
When a terminal boots, the user is prompted for user name and
password. The user name becomes the terminal's authentication
ID. The password is converted using passtokey (see auth(2))
into a 56-bit DES key and saved as the machine's key. The
authentication domain is set to the null string. If possible,
the terminal validates the key with the AS before saving it.
For Internet machines the correct AS to ask is found using
bootp(8). For Datakit machines the AS is a system called
p9auth on the same Datakit node as the file server the
terminal booted from.
When a CPU or file server boots, it reads the key, ID, and
domain name from non-volatile RAM. This allows servers to
reboot without operator intervention.
The details of any authentication are mixed with the semantics
of the particular service they are authenticating so we
describe them one case at a time. The following definitions
will be used in the descriptions:
$CH sub c$
an 8-byte random challenge from a client
$CH sub s$
an 8-byte random challenge from a server
$K sub s$
server's key
$K sub c$
client's key
$K sub n$
a nonce key created for a ticket
$K lbr m rbr$
message $m$ encrypted with key $K$
$ID sub s$
server's ID
$DN sub s$
server's authentication domain name
$ID sub c$
client's ID
$UID sub c$
user's name on the client
$UID sub s$
user's name on the server
A number of constants defined in auth.h are also used:
AuthTreq, AuthChal, AuthOK, AuthErr, AuthTs, AuthTc, AuthAs,
and AuthAc.
File Service
File service sessions are long-lived connections between a
client host and a file server. Processes belonging to different
users share the session. Whenever a user process on the client
mounts a file server (see bind(2)), it must authenticate
itself. There are four players in an authentication: the
server, the client kernel, the user process on the client, and
the authentication server. The goal of the authentication
protocol is to convince the server that the client may validly
speak for the user process.
To reduce the number of messages for each authentication,
common information is exchanged once at the beginning of the
session within a session message (see attach(5)):
Client->Server
Tsession($CH sub c$)
Server->Client
Rsession(${CH sub s},~{ID sub s},~{DN sub s}$)
Each time a user mounts a file server connection, an attach
message is sent identifying/authenticating the user:
Client->Server
Tattach($K sub s lbr AuthTs, ~ {CH sub s},~{UID sub c}, ~
{UID sub s}, ~ K sub n rbr , ~ {K sub n} lbr AuthAc, ~
{CH sub s}, count rbr )$
Server->Client
Rattach($ K sub n lbr AuthAs,~{CH sub c},~count rbr$)
The part of the attach request encrypted with $Ksubs$ is called
a ticket. Since it is encrypted in the server's secret key,
this message is guaranteed to have originated on the AS. The
part encrypted with the $K sub n$ found in the ticket is called
an authenticator. The authenticator is generated by the client
kernel and guarantees that the ticket was not stolen. The count
is incremented with each mount to make every authenticator
unique, thus foiling replay attacks. The server is itself
authenticated by the authenticator it sends as a reply to the
attach.
Tickets are created by the AS at the request of a user process.
The AS contains a database of which $ID sub c$'s may speak for
which $UID sub c$'s. If the $ID sub c$ may speak for the $UID
sub c$, two tickets are returned.
UserProc->AS
$AuthTreq, ~ CH sub s , ~ ID sub s , ~ DN sub s , ~ ID
sub c , ~ UID sub c$
AS->UserProc
$AuthOK, ~ K sub c lbr AuthTc, ~ CH sub s , ~ UID sub c ,
~ UID sub s , ~ K sub n rbr , ~ K sub s lbr AuthTs, ~ CH
sub s , ~ UID sub c , ~ UID sub s , ~ K sub n rbr$
Otherwise an error message is returned.
AS->UserProc
$AuthErr$, 64-byte error string
The user passes both tickets to the client's kernel using the
fauth system call (see fsession(2)). The kernel decrypts the
ticket encrypted with $K sub c$. If $UID sub c$ matches the
user's login ID, the tickets are remembered for any subsequent
attaches by that user of that file server session. Otherwise,
the ticket is assumed stolen and an error is returned.
Remote Execution
A number of applications require a process on one machine to
start a process with the same user ID on a server machine.
Examples are cpu(1), rx (see con(1)), and exportfs(4). The
called process replies to the connection with a ticket request.
Server->UserProc
$AuthTreq, ~ CH sub s , ~ ID sub s , ~ DN sub s , ~ xxx,
~ xxx$
Here xxx indicates a field whose contents do not matter.
The calling process adds its machine's $ID sub c$ and its $UID
sub c$ to the request and follows the protocol outlined above
to get two tickets from the AS. The process passes the $K sub
s$ encrypted ticket plus an authenticator generated by
/dev/authenticator from the $K sub c$ ticket to the remote
server, which writes them to the kernel to set the user ID (see
cons(3)). The server replies with its own authenticator which
can be written to the kernel along with the $K sub c$ encrypted
ticket to confirm the server's identity (see cons(3)).
UserProc->Server
$ K sub s lbr AuthTs, ~ CH sub s , ~ UID sub c , ~ UID
sub s , ~ K sub n rbr , ~ K sub n lbr AuthAc, ~ CH sub s
, ~ 0 rbr $
Server->UserProc
$K sub n lbr AuthAs, ~ CH sub s , ~ 0 rbr$
Challenge Box
A user may also start a process on a CPU server from a non Plan
9 machine using commands such as con, telnet, or ftp (see
con(1) and ftpfs(4)). In these situations, the user can
authenticate using a hand-held DES encryptor. The telnet or FTP
daemon first sends a ticket request to the authentication
server. If the AS has keys for both the $ID sub c$ and $UID sub
c$ in the ticket request it returns a challenge as a
hexadecimal number.
Daemon->AS
$AuthChal, ~ CH sub c , ~ ID sub c , ~ DN sub s , ~ ID
sub c , ~ UID sub c $
AS->Daemon
$AuthOK$, 16-byte ASCII challenge
Otherwise, it returns a null-terminated 64-byte error string.
AS->Daemon
$AuthErr$, 64-byte error string
The daemon relays the challenge to the calling program, which
displays the challenge on the user's screen. The user encrypts
it and types in the result, which is relayed back to the AS.
The AS checks it against the expected response and returns
either a ticket or an error.
Daemon->AS
16-byte ASCII response
AS->Daemon
$AuthOK, ~ K sub c lbr AuthTs, ~ CH sub c , ~ UID sub c ,
~ UID sub c , ~ K sub n rbr$
or
AS->Daemon
$AuthErr$, 64-byte error string
Finally, the daemon passes the ticket to the kernel to set the
user ID (see cons(3)).
Password Change
Any user can change the key stored for him or her on the AS.
Once again we start by passing a ticket request to the AS. Only
the user ID in the request is meaningful. The AS replies with a
single ticket (or an error message) encrypted in the user's
personal key. The user encrypts both the old and new keys with
the $K sub n$ from the returned ticket and sends that back to
the AS. The AS checks the reply for validity and replies with
an AuthOK byte or an error message.
UserProc->AS
$AuthPass, ~ xxx, ~ xxx, ~ xxx, ~ xxx, ~ UID sub c$
AS->UserProc
$AuthOK, ~ K sub c lbr AuthTc, ~ xxx, ~ xxx, ~ xxx, ~ K
sub n rbr$
UserProc->AS
$K sub u lbr AuthPass, ~ roman "old password", ~ roman
"new password" rbr$
AS->UserProc
$AuthOK$
or
AS->UserProc
$AuthErr$, 64-byte error string
Data Base
An ndb(2) database file exists for the authentication server.
The attribute types used by the AS are hostid and uid. The
value in the hostid is a client host's ID. The values in the
uid pairs in the same entry list which users that host ID make
speak for. A uid value of * means the host ID may speak for all
users. A uid value of !user means the host ID may not speak for
user. For example:
hostid=bootes
uid=!sys uid=!adm uid=*
is interpreted as bootes may speak for any user except sys and
adm.
FILES
/lib/ndb/auth
database file
/lib/ndb/auth.*
hash files for /lib/ndb/auth
SEE ALSO
fsession(2), auth(2), cons(3), attach(5), auth(8)
Copyright � 1995 Lucent Technologies. All rights reserved.