AUTHSRV(6) AUTHSRV(6)
NAME
authsrv, p9any, p9sk1, dp9ik - authentication protocols
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 four values important to authentica-
tion; a 56-bit DES key, a 128-bit AES key, a 28-byte authen-
tication ID, and a 48-byte authentication domain name. The
ID is a user name and identifies who is currently responsi-
ble 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, factotum(4) prompts for user name and
password. The user name becomes the terminal's authentica-
tion ID. The password is converted using passtokey (see
authsrv(2)) into a 56-bit DES and 128-bit AES keys and saved
in memory. The authentication domain is set to the null
string. If possible, factotum validates the key with the AS
before saving it. For Internet machines the correct AS to
ask is found using dhcpd(8).
When a CPU or file server boots, factotum 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 seman-
tics 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:
Ks server's host ID's key
Kc client's host ID's key
Kn a nonce key created for a ticket (key)
K{m} message m encrypted with key K
CHc an 8-byte random challenge from a client (chal)
CHs an 8-byte random challenge from a server (chal)
IDs server's ID (authid)
DN server's authentication domain name (authdom)
IDc client's ID (hostid, cuid)
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IDr client's desired ID on server (uid, suid)
YAc client → AS DH public key
YBc AS → client DH public key
YAs server → AS DH public key
YBs AS → server DH public key
RNc client's 32-byte random string
RNs server's 32-byte random string
The parenthesized names are the ones used in the Ticketreq
and Ticket structures in <authsrv.h>.
The message type constants AuthTreq, AuthChal, AuthPass,
AuthOK, AuthErr, AuthMod, AuthApop, AuthOKvar, AuthChap,
AuthMSchap, AuthCram, AuthVNC, and AuthPAK (type) are
defined in <authsrv.h>, as are the encrypted message types
AuthTs, AuthAs, AuthAc, AuthTp, and AuthHr (num).
Ticket Service
When a client and server wish to authenticate to each other,
they do so using tickets issued by the AS. Obtaining tick-
ets from the AS is the client's responsibility.
The protocol to obtain a ticket pair is:
C→A: AuthTreq, IDs, DN, CHs, IDc, IDr
A→C: AuthOK, Kc{AuthTc, CHs, IDc, IDr, Kn}, Ks{AuthTs, CHs,
IDc, IDr, Kn}
The two tickets are identical except for their type fields
and the keys with which they are encrypted. The client and
server can each decrypt one of the tickets, establishing a
shared secret Kn.
The tickets can be viewed as a statement by the AS that ``a
client possessing the Kn key is allowed to authenticate as
IDr.''
The presence of the server challenge CHs in the ticket
allows the server to verify the freshness of the ticket
pair.
The AS sets the IDr in the tickets to the requested IDr only
if IDc is allowed to speak for (q.v.) IDr. If not, the AS
sets IDr to the empty string.
If the users IDc or IDs do not exist, the AS silently gener-
ates one-time random keys to use in place of Kc or Ks, so
that clients cannot probe the AS to learn whether a user
name is valid.
P9sk1
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The Plan 9 shared key protocol p9sk1 allows a client and
server to authenticate each other. The protocol is:
C→S: CHc
The client starts by sending a random challenge to the
server.
S→C: AuthTreq, IDs, DN, CHs, -, -
The server replies with a ticket request giving its id
and authentication domain along with its own random
challenge.
C→S: Ks{AuthTs, CHs, IDc, IDr, Kn}, Kn{AuthAc, CHs}
The client adds IDc and IDr to the ticket request and
obtains a ticket pair from the AS as described above.
The client relays the server's ticket along with an
authenticator, the AuthAc message. The authenticator
proves to the server that the client knows Kn and is
therefore allowed to authenticate as IDr. (The inclu-
sion of CHs in the authenticator avoids replay
attacks.)
S→C: Kn{AuthAs, CHc}
The server replies with its own authenticator, proving
to the client that it also knows Kn and therefore Ks .
The 64-bit shared secret Kn is used as the session secret.
Password authenticated key exchange
Initially, the server and client keys Ks and Kc were equiva-
lent to the password derived 56-bit DES keys, which made the
encrypted tickets subject to offline dictionary attacks and
provided too small a key space against brute force attacks
on current hardware.
The AuthPAK protocol is used to establish new 256-bit random
keys with the AS for Ks and Kc before each ticket request on
the connection.
The protocol is based on SPAKE2EE, where a hash of the
user's secret is used to encypt the public keys of a
Elliptic-Curve Diffie-Hellman key exchange. The user's ID
and 128-bit AES key is hashed and mapped (using Elligator2)
into two curve points PM and PN, called the pakhash. Both
sides generate a random number xa/xb and make the public
keys YA/YB as: YA=xa*G+PM, YB=xb*G+PN. After the public
keys have been exchanged, each side calculates the shared
secret as: Z=xa*(YB-PN)=xb*(YA-PM). The shared secret Z is
then hashed with the transmitted public keys YA|YB producing
the 256-bit pakkey.
The pakkey is then used in place of Ks and Kc to
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authenticate and encrypt tickets from the AS using
Chacha20/Poly1305 AEAD for the next following request made
on the connection.
The protocol (for AuthTreq) to establish keys Ks and Kc with
the AS for IDs and IDc is:
C→A: AuthPAK, IDs, DN, CHs, IDc, IDr, YAs, YAc
A→C: AuthOK, YBs, YBc
The protocol (for AuthApop, AuthChap...) to establish a sin-
gle server key Ks for IDs:
C→A: AuthPAK, -, DN, CHs, IDs, IDc, YAs
A→C: AuthOK, YBs
The protocol (for AuthPass) to establish a single client key
Kc for IDc:
C→A: AuthPAK, -, -, CHc, -, IDc, YAc
A→C: AuthOK, YBc
Dp9ik
The dp9ik protocol is an extended version of p9sk1 that adds
the random strings RNc and RNs in the authenticator messages
for the session key derivation and uses the password authen-
ticated key exchange as described above to derive the ticket
encryption keys Ks and Kc:
C→S: CHc
The client starts by sending a random challenge to the
server.
S→C: AuthPAK, IDs, DN, CHs, -, -, YAs
The server generates a new public key YAs and replies
with a AuthPAK request giving its IDs and authentica-
tion domain DNs along with its own random challenge CHs
and its public key YAs.
C→S: YBs, Ks{AuthTs, CHs, IDc, IDr, Kn}, Kn{AuthAc, CHs,
RNc}
The client generates its own public key YAc and adds it
along with IDc and IDr to the AuthPAK request and
obtains the public keys YBs and YBc from the AS
response. At this point, client and AS have completed
their authenticated key exchange and derive Kc as
described above. Then the client requests a ticket
pair using the same message but with AuthPAK type
changed to AuthTreq. It decrypts his ticket with Kc
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extracting the shared secret Kn. The client relays the
server's YBs and ticket along with an authenticator,
the AuthAc message. The server finishes his authenti-
cated key exchange using YBs and derives Ks to decrypt
his ticket to extract the shared secret Kn. When the
decryption of the clients authenticator using Kn is
successfull then this proves to the server that the
client knows Kn and is therefore allowed to authenti-
cate as IDr. The random string RNc is used in the
derivation of the session secret.
S→C: Kn{AuthAs, CHc, RNs}
The server replies with its own authenticator, proving
to the client that it also knows Kn and contributes its
random string RNs for the session secret.
The 2048-bit session secret is derived with HKDF-SHA256
hashing the concatenated random strings RNc|RNs with the the
shared secret key Kn.
P9any
P9any is the standard Plan 9 authentication protocol. It
consists of a negotiation to determine a common protocol,
followed by the agreed-upon protocol.
The negotiation protocol is:
S→C: proto@authdom proto@authdom ...
C→S: proto dom
Each message is a NUL-terminated UTF string. The server
begins by sending a list of proto, authdom pairs it is will-
ing to use. The client responds with its choice.
A second version of this protocol exists (indicated by the
v.2 prefix before the list) where the server sends an
explicit confirmation with a OK message before the agreed-
upon protocol starts.
S→C: v.2 proto@authdom proto@authdom ...
C→S: proto dom
S→C: OK
The p9any protocol is the protocol used by all Plan 9 ser-
vices. The file server runs it over special authentication
files (see fauth(2) and attach(5)). Other services, such as
cpu(1), exportfs(4) and tlssrv(8) run p9any over the network
and then use the session secret to derive an ssl(3) or
tls(3) key to encrypt the rest of their communications.
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Password Change
Users connect directly to the AS to change their passwords.
The protocol is:
C→A: AuthPass, -, -, CHc, -, IDc
The client sends a password change ticket request.
A→C: Kc{AuthTp, CHc, IDc, IDc, Kn}
The server responds with a ticket containing the key Kn
encrypted with the client's key Kc
C→A: Kn{AuthPass, old, new, changesecret, secret}
The client decrypts the ticket using the old password
and then sends back an encrypted password request
(Passwordreq structure) containing the old password and
the new password. If changesecret is set, the AS also
changes the user's secret, the password used for non-
Plan 9 authentications.
A→C: AuthOK or AuthErr, 64-byte error message
The AS responds with simply AuthOK or with AuthErr fol-
lowed by a 64-byte error message.
Authentication Database
An ndb(2) database file /lib/ndb/auth exists for the AS.
This database maintains ``speaks for'' relationships, i.e.,
it lists which users may speak for other users when authen-
ticating. 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 may 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. This property is used heavily on CPU servers.
Foreign Protocols
The AS accepts ticket request messages of types other than
AuthTreq to allow users to authenticate using non-Plan 9
protocols. In these situations, the server communicates
directly with the AS. Some protocols must begin without
knowing the client's name. They ignore the client name in
the ticket request. All the protocols end with the AS send-
ing an AuthOK message containing a server ticket and authen-
ticator.
AuthOK messages always have a fixed but context-dependent
size. The occasional variable-length OK message starts with
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a AuthOKvar byte and a five-byte space-padded decimal length
of the data that follows.
Anywhere an AuthOK message is expected, a AuthErr message
may be substituted.
S→A: AuthChal, -, DN, CHs, IDs, IDc
A→S: AuthOK, challenge
S→A: response
A→S: AuthOK, Ks{AuthTs, CHs, IDc, IDc, Kn}, Kn{AuthAc, CHs}
This protocol allows the use of handheld authenticators
such as SecureNet keys and SecureID tokens in programs
such as telnetd and ftpd (see ipserv(8)).
Challenge and response are text strings, NUL -padded to
16 bytes (NETCHLEN). The challenge is a random five-
digit decimal number. When using a SecureNet key or
netkey (see passwd(1)), the response is an eight-digit
decimal or hexadecimal number that is an encryption of
the challenge using the user's DES key.
When using a SecureID token, the challenge is ignored.
The response is the user's PIN followed by the six-
digit number currently displayed on the token. In this
case, the AS queries an external RADIUS server to check
the response. Use of a RADIUS server requires an entry
in the authentication database. For example:
radius=server-name secret=xyzzy
uid=howard rid=trickey
uid=sape rid=smullender
In this example, the secret xyzzy is the hash key used
in talking to the RADIUS server. The uid/rid lines map
from Plan 9 user ids to RADIUS ids. Users not listed
are assumed to have the same id in both places.
S→A: AuthApop, -, DN, CHs, IDs, -
A→S: AuthOKvar, challenge
S→A: AuthApop, -, DN, CHs, IDs, IDc; hexadecimal MD5 check-
sum
A→S: AuthOK, Ks{AuthTs, CHs, IDc, IDc, Kn}, Kn{AuthAc, CHs}
This protocol implements APOP authentication (see
pop3(8)). After receiving a ticket request of type
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AuthApop, the AS generates a random challenge of the
form <random@domain>. The client then replies with a
new ticket request giving the user name followed by the
MD5 checksum of the challenge concatenated with the
user's secret. If the response is correct, the authen-
tication server sends back a ticket and authenticator.
If the response is incorrect, the client may repeat the
ticket request/MD5 checksum message to try again.
The AuthCram protocol runs identically to the AuthApop
protocol, except that the expected MD5 checksum is the
keyed MD5 hash using the user's secret as the key (see
hmac_md5 in sechash(2)).
S→A: AuthChap, -, DN, CHs, IDs, -
A→S: challenge
S→A: pktid, IDc, response
A→S: AuthOK, Ks{AuthTs, CHs, IDc, IDc, Kn}, Kn{AuthAc, CHs}
This protocol implements CHAP authentication (see
ppp(8)). The challenge is eight random bytes. The
response is a 16-byte MD5 checksum over the packet id,
user's secret, and challenge. The reply packet is
defined as OChapreply in <authsrv.h>.
S→A: AuthMSchap, -, DN, CHs, IDs, -
A→S: challenge
S→A: IDc, lm-response, nt-response
A→S: AuthOK, Ks{AuthTs, CHs, IDc, IDc, Kn}, Kn{AuthAc, CHs}
This protocol implements Microsoft's MS-CHAP authenti-
cation (see ppp(8)). The challenge is eight random
bytes. The two responses are Microsoft's LM and NT
hashes. Only the NT hash may be used to authenticate,
as the LM hash is considered too weak. The reply
packet is defined as OMSchapreply in <authsrv.h>.
S→A: AuthVNC, -, DN, CHs, IDs, IDc
A→S: AuthOKvar, challenge
S→A: response
A→S: AuthOK, Ks{AuthTs, CHs, IDc, IDc, Kn}, Kn{AuthAc, CHs}
This protocol implements VNC authentication (see vncs
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in vnc(1)). The challenge is 16 random bytes, and the
response is a DES ECB encryption of the challenge. The
method by which VNC converts the user's secret into a
DES key is weak, considering only the first eight bytes
of the secret.
FILES
/lib/ndb/auth database file
/lib/ndb/auth.* hash files for /lib/ndb/auth
SEE ALSO
auth(2), fauth(2), cons(3), attach(5), auth(8)
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