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Factotum is a user–level file system that acts as the authentication
agent for a user. It does so by managing a set of keys. A key
is a collection of information used to authenticate a particular
action. Stored as a list of attribute=value pairs, a key typically
contains a user, an authentication domain, a protocol, and some
secret data.
Factotum presents a two level directory. The first level contains
a single directory factotum, which in turn contains:
rpc each open represents a new private channel to factotum
proto when read lists the protocols available
confirm for confirming the use of key
needkey allows external programs to control the addition of new
keys
log a log of actions
ctl for maintaining keys; when read, it returns a list of keys.
For secret attributes, only the attribute name follow by a ? is
returned.
In any authentication, the caller typically acts as a client and
the callee as a server. The server determines the authentication
domain, sometimes after a negotiation with the client. Authentication
always requires the client to prove its identity to the server.
Under some protocols, the authentication is mutual. Proof is
accomplished using secret information kept by factotum in conjunction
with a cryptographic protocol.
Factotum can act in the role of client for any process possessing
the same user id as it. For select protocols such as p9sk1 and
dp9ik it can also act as a client for other processes provided
its user id may speak for the other process' user id (see authsrv(6)).
Factotum can act in the role of server for any process.
Factotum's structure is independent of any particular authentication
protocol. Factotum supports the following protocols:
p9any a metaprotocol used to negotiate which actual protocol to
use.
p9sk1 legacy Plan 9 shared key protocol described in authsrv(6)'s
``Ticket Service'' and ``P9sk1'' sections.
dp9ik extended version of p9sk1 that adds password bruteforce resistance
and forward secrecy (see authsrv(6)'s ``Password authenticated
key exchange'' and ``Dp9ik'' sections).
p9cr legacy Plan 9 protocol that can use either p9sk1 keys or SecureID
tokens.
apop the challenge/response protocol used by POP3 mail servers.
cram the challenge/response protocol also used by POP3 mail servers.
chap the challenge/response protocols used by PPP and PPTP.
mschap a proprietary Microsoft challenge/response protocol also
used by PPP, PPTP and CIFS.
mschapv2 version two of Microsofts challenge/response protocol
used by WPA.
mschap2 Microsofts NTLMv2 challenge/response protocol used by CIFS.
rsa RSA public key decryption, used by SSH and TLS.
pass passwords in the clear.
vncvnc(1)'s challenge/response.
wpapsk WPA passwords for wireless ethernet cards.
The options are:
–a supplies the address of the authentication server to use. Without
this option, it will attempt to find an authentication server
by querying the connection server, the file <mtpt>/ndb, and finally
the network database in /lib/ndb.
–m specifies the mount point to use, by default /mnt.
–s specifies the service name to use. Without this option, factotum
does not create a service file in /srv.
–D turns on 9P tracing, written to standard error.
–d turns on debugging, written to standard error.
–g causes the agent to prompt for the key, write it to the ctl file,
and exit. The agent will prompt for values for any of the attributes
ending with a question mark (?) and will append all the supplied
attribute = value pairs. See the section on key templates below.
–n don't look for a secstore.
–S indicates that the agent is running on a CPU server. On starting,
it will attempt to get p9sk1 and dp9ik keys from NVRAM using readnvram
(see authsrv(2)), prompting for anything it needs. It will never
subsequently prompt for a key that it doesn't have. This option
is typically used by the kernel at boot
k causes the NVRAM to be written. It is only valid with the –S option.
This option is typically used by the kernel at boot time.
–u causes the agent to prompt for user id and writes it to /dev/hostowner.
It is mutually exclusive with –k and –S. This option is typically
used by the kernel at boot time.
–p causes the agent not to mark itself `private' via proc(3), so
that it can be debugged. It is implied by –d.
Fgui is a graphic user interface for confirming key usage and
entering new keys. It hides the window in which it starts and
waits reading requests from confirm and needkey. For each requests,
it unhides itself and waits for user input. See the sections on
key confirmation and key prompting below.
Userpasswd queries and prints a cleartext user/password pair from
factotum for the proto=pass key tuple specified in fmt. This can
be used by shell scripts to do cleartext password authentication.
Key Tuples
A key tuple is a space delimited list of attribute=value pairs.
An attribute whose name begins with an exclamation point (!) does
not appear when reading the ctl file. The required attributes
depend on the authentication protocol.
Dp9ik, p9sk1 and p9cr all require a key with proto=dp9ik or proto=p9sk1,
a dom attribute identifying the authentication domain, a user
name valid in that domain, and either a !password or !hex attribute
specifying the password or hexadecimal secret to be used. Here
is an example:
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proto=dp9ik dom=9front user=glenda !password=secret
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Apop, cram, chap, and mschap, require a key with a proto attribute
whose value matches the protocol, in addition to server, user,
and !password attributes; e.g.
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proto=apop server=mit.edu user=rsc !password=nerdsRus
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Vnc is similar but does not require a user attribute.
Pass requires a key with proto=pass in addition to user and !password
attributes; e.g.
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proto=pass user=tb !password=does.it.matter
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Rsa requires a key with proto=rsa in addition to all the hex attributes
defining an RSA key: ek, n, !p, !q, !kp, !kq, !c2, and !dk. By
convention, programs using the RSA protocol also require a service
attribute set to ssh or tls.
All keys can have additional attributes that act either as comments
or as selectors to distinguish them in the auth(2) library calls.
The factotum owner can use any key stored by factotum. Any key
may have one or more owner attributes listing the users who can
use the key as though they were the owner. For example, the TLS
and SSH host keys on a server often have an attribute owner=*
to allow any user (and in particular, none) to run the
TLS or SSH server–side protocol.
Any key may have a role attribute for restricting how it can be
used. If this attribute is missing, the key can be used in any
role. The possible values are:
client
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for authenticating outbound calls
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server
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for authenticating inbound calls
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speakfor
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for authenticating processes whose user id does not match factotum's.
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If a key has a disabled attribute (with any value), the key is
not used during any protocols. Factotum automatically marks keys
with disabled=by.factotum when they fail during certain authentication
protocols (in particular, the Plan 9 ones).
Whenever factotum runs as a server, it must have dp9ik or p9sk1
keys in order to communicate with the authentication server for
validating passwords and challenge/responses of other users.
Key Templates
Key templates are used by routines that interface to factotum
such as auth_proxy and auth_challenge (see auth(2)) to specify
which key and protocol to use for an authentication. Like a key
tuple, a key template is also a list of attribute=value pairs.
It must specify at least the protocol and enough other
attributes to uniquely identify a key, or set of keys, to use.
The keys chosen are those that match all the attributes specified
in the template. The possible attribute/value formats are:
attr=val The attribute attr must exist in the key and its value
must exactly match val
attr? The attribute attr must exist in the key but its value doesn't
matter.
attr The attribute attr must exist in the key with a null value
Key templates are also used by factotum to request a key either
via an RPC error or via the needkey interface. The possible attribute/value
formats are:
attr=val This pair must remain unchanged
attr? This attribute needs a value
attr The pair must remain unchanged
Control and Key Management
A number of messages can be written to the control file. The messages
are:
key attribute–value–list
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add a new key. This will replace any old key whose public, i.e.
non ! attributes, match.
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delkey attribute–value–list
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delete a key whose attributes match those given.
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debug
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toggle debugging on and off, i.e., the debugging also turned on
by the –d option.
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By default when factotum starts it looks for a secstore(1) account
on $auth for the user and, if one exists, prompts for a secstore
password in order to fetch the file factotum, which should contain
control file commands. An example would be
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key dom=x.com proto=p9sk1 user=boyd !hex=26E522ADE2BBB2A229
key proto=rsa service=ssh size=1024 ek=3B !dk=...
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where the first line sets a password for challenge/response authentication,
strong against dictionary attack by being a long random string,
and the second line sets a public/private keypair for ssh authentication.
Confirming key use
The confirm file provides a connection from factotum to a confirmation
server, normally the program auth/fgui. Whenever a key with the
confirm attribute is used, factotum requires confirmation of its
use. If no process has confirm opened, use of the key will be
denied. However, if the file is opened a request
can be read from it with the following format:
confirm tag=tagno <key template>
The reply, written back to confirm, consists of string:
tag=tagno answer=xxx
If xxx is the string yes then the use is confirmed and the authentication
will proceed. Otherwise, it fails.
Confirm is exclusive open and can only be opened by a process
with the same user id as factotum.
Prompting for keys
The needkey file provides a connection from factotum to a key
server, normally the program auth/fgui. Whenever factotum needs
a new key, it first checks to see if needkey is opened. If it
isn't, it returns a error to its client. If the file is opened
a request can be read from it with the following format:
needkey tag=tagno <key template>
It is up to the reader to then query the user for any missing
fields, write the key tuple into the ctl file, and then reply
by writing into the needkey file the string:
tag=tagno
Needkey is exclusive open and can only be opened by a process
with the same user id as factotum.
The RPC Protocol
Authentication is performed by
1) opening rpc
2) setting up the protocol and key to be used (see the start RPC
below),
3) shuttling messages back and forth between factotum and the other
party (see the read and write RPC's) until done
4) if successful, reading back an AuthInfo structure (see authsrv(2)).
The RPC protocol is normally embodied by one of the routines in
auth(2). We describe it here should anyone want to extend the
library.
An RPC consists of writing a request message to rpc followed by
reading a reply message back. RPC's are strictly ordered; requests
and replies of different RPC's cannot be interleaved. Messages
consist of a verb, a single space, and data. The data format depends
on the verb. The request verbs are:
start attribute–value–list
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start a new authentication. Attribute–value–pair–list must include
a proto attribute, a role attribute with value client or server,
and enough other attributes to uniquely identify a key to use.
A start RPC is required before any others. The possible replies
are:
ok start succeeded.
error string
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where string is the reason.
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readget data from factotum to send to the other party. The possible
replies are:
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ok read succeeded, this is zero length message.
ok data
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read succeeded, the data follows the space and is unformatted.
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doneauthentication has succeeded, no further RPC's are necessary
done haveai
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authentication has succeeded, an AuthInfo structure (see auth(2))
can be retrieved with an authinfo RPC
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phase string
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its not your turn to read, get some data from the other party
and return it with a write RPC.
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error string
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authentication failed, string is the reason.
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protocol not started
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a start RPC needs to precede reads and writes
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needkey attribute–value–list
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a key matching the argument is needed. This argument may be passed
as an argument to factotum –g in order to prompt for a key. After
that, the authentication may proceed, i.e., the read restarted.
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write data
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send data from the other party to factotum. The possible replies
are:
ok the write succeeded
needkey attribute–value–list
toosmall n
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the write is too short, get more data from the other party and
retry the write. n specifies the maximum total number of bytes.
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phase string
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its not your turn to write, get some data from factotum first.
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donesee above
done haveai
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authinfo
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retrieve the AuthInfo structure. The possible replies are:
ok data
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is a marshaled form of the AuthInfo structure.
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error string
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where string is the reason for the error.
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attrretrieve the attributes used in the start RPC. The possible
replies are:
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ok attribute–value–list
error string
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where string is the reason for the error.
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