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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 the following files:
rpc each open represents a new private channel to factotum
proto when read lists the protocols available
confirm for confiming 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 it
can also act as a client for other processes provided its user
id may speak for the other process’ user id (see Plan 9’s 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 a Plan 9 shared key protocol.
p9sk2 a variant of p9sk1.
p9cr a 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.
dsa DSA signatures, used by SSH
mschap a proprietary Microsoft protocol also used by PPP and PPTP.
rsa RSA encryption and signatures, used by SSH and TLS.
pass passwords in the clear.
vncvnc(1)’s challenge/response.
wep WEP passwords for wireless ethernet cards. The “Protocols”
section below describes these protocols in more detail.
The options to factotum 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 a 9psk1 key from NVRAM using readnvram
(see authsrv(3)), 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 time.
–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.
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. Here are some examples:
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proto=p9sk1 dom=avayalabs.com user=presotto !password=lucent
proto=apop server=mit.edu user=rsc !password=nerdsRus
proto=pass user=tb service=ssh !password=does.it.matter
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The “Protocols” section below describes the attributes specific
to each supported protocol.
All keys can have additional attibutes that act either as comments
or as selectors to distinguish them in the auth(3) 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. Common 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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speaksfor
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for authenticating processes whose user id does not match factotum’s.
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encrypt
decrypt
sign for cryptographically signing data
verify
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for verifying cryptographic signatures
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Whenever factotum runs as a server, it must have a p9sk1 key 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(3)) 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 mesages
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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debugtoggle debugging on and off, i.e., the debugging also turned
on by the –d option.
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,
generated by ssh_genkey (see ssh(1)).
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(3)).
The RPC protocol is normally embodied by one of the routines in
auth(3). 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 attibutes 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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read get 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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done authentication has succeeded, no further RPC’s are necessary
done haveai
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authentication has succeeded, an AuthInfo structure (see auth(3))
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 maximun 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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done see above
done haveai
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readhex, writehex
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like read and write, except that an ok response to readhex returns
the data encoded as a long hexadecimal string, and the argument
to writehex is expected to be a long hexadecimal string. These
are useful for manually debugging of binary protocols.
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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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attr retrieve 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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Protocols
Factotum supports many authentication types, each with its own
roles and required key attributes.
P9any, p9sk1, p9sk2, and p9cr are used to authenticate to Plan
9 systems; valid roles are client and server. All require proto=p9sk1
keys with user, dom (authentication domain), and !password attributes.
P9sk1 and p9sk2 are the Plan 9 shared-key authentication protocols.
P9sk2 is a deprecated form of p9sk1 that neglects to authenticate
the server.
P9any is a meta-protocol that negotiates a protocol (p9sk1 or
p9sk2) and an authentication domain and then invokes the given
protocol with a dom= attribute.
P9any, p9sk1, and p9sk2 are intended to be proxied via auth_proxy
(see auth(3)).
P9cr is a textual challenge-response protocol; roles are client
and server. It uses p9sk1 keys as described above. The protocol
with factotum is textual: client writes a user name, server responds
with a challenge, client writes a response, server responds with
ok or bad. Typically this information is wrapped in other protocols
before being sent over
the network.
Vnc is the challenge-response protocol used by vnc(1); valid roles
are client and server. The client protocol requires a proto=vnc
key with attribute !password. Conventionally, client keys also
have user and server attributes. The server protocol requires
a p9sk1 key as described above. The protocol with factotum is
the same as p9cr,
except that the challenge and response are not textual.
Apop and cram are challenge-response protocols typically used
to authenticate to mail servers. The client protocols require
proto=apop or proto=cram keys with user and !password attributes.
Conventionally, client keys also have server attributes. The server
protocol requires a p9sk1 key as described above. The protocol
with factotum is
textual: server writes a challenge of the form random@domain,
client responds with user name and then a hexadecimal response
(two separate writes), and then the server responds with ok or
bad.
Chap and mschap are challenge-response protocols used in PPP sessions;
valid roles are client and server. The client protocols require
proto=chap or proto=mschap keys with user and !password attributes.
Conventionally, client keys also have server attributes. The server
protocol requires a p9sk1 key as described above. The protocol
with factotum is: server writes an 8-byte binary challenge, client
responds with user name and then a Chapreply or MSchapreply structure
(defined in <auth.h> ).
Pass is a client-only protocol that hands out passwords from proto=pass
keys with user and !password attributes. The protocol is a single
read that returns a string: a space-separated quoted user name
and password that can be parsed with tokenize (see getfields(3)).
Conventionally, client keys have distinguishing attributes like
service and
server that can be specified in the start message to select a
key.
Wep is a client-only pseudo-protocol that initializes the encryption
key on a wireless ethernet device. It uses proto=wep keys with
!key1, !key2, or !key3 attributes. The protocol with factotum
is: the client writes a device name that must begin with #l. In
response, factotum opens the device’s control file, sets the wireless
secret using the key, and
turns on encryption. If the key has an essid attribute, factotum
uses it to set the wireless station ID.
Rsa is an implementation of the RSA protocol. Valid roles are
decrypt, encrypt, sign, and verify. Rsa uses proto=rsa keys with
ek and n attributes, large integers specifying the public half
of the key. If a key is to be used for decryption or signing,
then it must also have attributes !p, !q, !kp, !kq, !c2, and !dk
specifying the private half of the
key; see rsa(3). Conventionally, rsa keys also have service attributes
specifying the context in which the key is used: ssh (SSH version
1), ssh−rsa (SSH version 2), or tls (SSL and TLS). If an SSH key
has a comment attribute, that comment is presented to remote SSH
servers during key negotiation. The protocol for encryption (decryption)
is:
write the message, then read back the encrypted (decrypted) form.
The protocol for signing is: write a hash of the actual message,
then read back the signature. The protocol for verifying a signature
is: write the message hash, write the purported signature, then
read back ok or bad telling whether the signature could be verified.
The hash defaults to
SHA1 but can be specified by a hash attribute on the key. Valid
hash functions are md5 and sha1. The hash function must be known
to factotum because the signature encodes the type of hash used.
The encrypt and verify operations are included as a convenience;
factotum is not using any private information to perform them.
Dsa is an implementation of the NIST digital signature algorithm.
Valid roles are sign and verify. It uses proto=dsa keys with p,
q, alpha, and key attributes. If the key is to be used for signing,
it must also have a !secret attribute; see dsa(3). Conventionally,
dsa keys also have service attributes specifying the context in
which the key is
used: ssh−dss (SSH version 2) is the only one. If an SSH key has
a comment attribute, that comment is presented to SSH servers
during key negotiation. The protocol for signing and verifying
is the same as the RSA protocol. Unlike rsa, the dsa protocol
ignores the hash attribute; it always uses SHA1.
Httpdigest is a client-only MD5-based challenge-response protocol
used in HTTP; see RFC 2617. It uses proto=httpdigest keys with
user, realm, and !password attributes. The protocol with factotum
is textual: write the challenge, read the response. The challenge
is a string with three space-separated fields nonce, method, and
uri, parseable
with tokenize. The response is a hexadecimal string of length
32.
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