Subversion Repositories openrisc_2011-10-31

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.SH

\&Remote Procedure Calls: Protocol Specification

.LP

.NH 0

\&Status of this Memo

.LP

Note: This chapter specifies a protocol that Sun Microsystems, Inc.,

and others are using.

It has been designated RFC1050 by the ARPA Network

Information Center.

.LP

.NH 1

\&Introduction

.LP

This chapter specifies  a  message protocol  used in implementing

Sun's Remote Procedure Call (RPC) package.  (The message protocol is

specified with the External Data Representation (XDR) language.

See the

.I "External Data Representation Standard: Protocol Specification"

for the details.  Here, we assume that  the  reader is familiar

with XDR and do not attempt to justify it or its uses).  The paper

by Birrell and Nelson [1]  is recommended as an  excellent background

to  and justification of RPC.

.NH 2

\&Terminology

.LP

This chapter discusses servers, services, programs, procedures,

clients, and versions.  A server is a piece of software where network

services are implemented.  A network service is a collection of one

or more remote programs.  A remote program implements one or more

remote procedures; the procedures, their parameters, and results are

documented in the specific program's protocol specification (see the

\fIPort Mapper Program Protocol\fP\, below, for an example).  Network

clients are pieces of software that initiate remote procedure calls

to services.  A server may support more than one version of a remote

program in order to be forward compatible with changing protocols.

.LP

For example, a network file service may be composed of two programs.

One program may deal with high-level applications such as file system

access control and locking.  The other may deal with low-level file

IO and have procedures like "read" and "write".  A client machine of

the network file service would call the procedures associated with

the two programs of the service on behalf of some user on the client

machine.

.NH 2

\&The RPC Model

.LP

The remote procedure call model is similar to the local procedure

call model.  In the local case, the caller places arguments to a

procedure in some well-specified location (such as a result

register).  It then transfers control to the procedure, and

eventually gains back control.  At that point, the results of the

procedure are extracted from the well-specified location, and the

caller continues execution.

.LP

The remote procedure call is similar, in that one thread of control

logically winds through two processes\(emone is the caller's process,

the other is a server's process.  That is, the caller process sends a

call message to the server process and waits (blocks) for a reply

message.  The call message contains the procedure's parameters, among

other things.  The reply message contains the procedure's results,

among other things.  Once the reply message is received, the results

of the procedure are extracted, and caller's execution is resumed.

.LP

On the server side, a process is dormant awaiting the arrival of a

call message.  When one arrives, the server process extracts the

procedure's parameters, computes the results, sends a reply message,

and then awaits the next call message.

.LP

Note that in this model, only one of the two processes is active at

any given time.  However, this model is only given as an example.

The RPC protocol makes no restrictions on the concurrency model

implemented, and others are possible.  For example, an implementation

may choose to have RPC calls be asynchronous, so that the client may

do useful work while waiting for the reply from the server.  Another

possibility is to have the server create a task to process an

incoming request, so that the server can be free to receive other

requests.

.NH 2

\&Transports and Semantics

.LP

The RPC protocol is independent of transport protocols.  That is, RPC

does not care how a message is passed from one process to another.

The protocol deals only with specification and interpretation of

messages.

.LP

It is important to point out that RPC does not try to implement any

kind of reliability and that the application must be aware of the

type of transport protocol underneath RPC.  If it knows it is running

on top of a reliable transport such as TCP/IP[6], then most of the

work is already done for it.  On the other hand, if it is running on

top of an unreliable transport such as UDP/IP[7], it must implement

is own retransmission and time-out policy as the RPC layer does not

provide this service.

.LP

Because of transport independence, the RPC protocol does not attach

specific semantics to the remote procedures or their execution.

Semantics can be inferred from (but should be explicitly specified

by) the underlying transport protocol.  For example, consider RPC

running on top of an unreliable transport such as UDP/IP.  If an

application retransmits RPC messages after short time-outs, the only

thing it can infer if it receives no reply is that the procedure was

executed zero or more times.  If it does receive a reply, then it can

infer that the procedure was executed at least once.

.LP

A server may wish to remember previously granted requests from a

client and not regrant them in order to insure some degree of

execute-at-most-once semantics.  A server can do this by taking

advantage of the transaction ID that is packaged with every RPC

request.  The main use of this transaction is by the client RPC layer

in matching replies to requests.  However, a client application may

choose to reuse its previous transaction ID when retransmitting a

request.  The server application, knowing this fact, may choose to

remember this ID after granting a request and not regrant requests

with the same ID in order to achieve some degree of

execute-at-most-once semantics.  The server is not allowed to examine

this ID in any other way except as a test for equality.

.LP

On the other hand, if using a reliable transport such as TCP/IP, the

application can infer from a reply message that the procedure was

executed exactly once, but if it receives no reply message, it cannot

assume the remote procedure was not executed.  Note that even if a

connection-oriented protocol like TCP is used, an application still

needs time-outs and reconnection to handle server crashes.

.LP

There are other possibilities for transports besides datagram- or

connection-oriented protocols.  For example, a request-reply protocol

such as VMTP[2] is perhaps the most natural transport for RPC.

.SH

.I

NOTE:  At Sun, RPC is currently implemented on top of both TCP/IP

and UDP/IP transports.

.LP

.NH 2

\&Binding and Rendezvous Independence

.LP

The act of binding a client to a service is NOT part of the remote

procedure call specification.  This important and necessary function

is left up to some higher-level software.  (The software may use RPC

itself\(emsee the \fIPort Mapper Program Protocol\fP\, below).

.LP

Implementors should think of the RPC protocol as the jump-subroutine

instruction ("JSR") of a network; the loader (binder) makes JSR

useful, and the loader itself uses JSR to accomplish its task.

Likewise, the network makes RPC useful, using RPC to accomplish this

task.

.NH 2

\&Authentication

.LP

The RPC protocol provides the fields necessary for a client to

identify itself to a service and vice-versa.  Security and access

control mechanisms can be built on top of the message authentication.

Several different authentication protocols can be supported.  A field

in the RPC header indicates which protocol is being used.  More

information on specific authentication protocols can be found in the

\fIAuthentication Protocols\fP\,

below.

.KS

.NH 1

\&RPC Protocol Requirements

.LP

The RPC protocol must provide for the following:

.IP  1.

Unique specification of a procedure to be called.

.IP  2.

Provisions for matching response messages to request messages.

.KE

.IP  3.

Provisions for authenticating the caller to service and vice-versa.

.LP

Besides these requirements, features that detect the following are

worth supporting because of protocol roll-over errors, implementation

bugs, user error, and network administration:

.IP  1.

RPC protocol mismatches.

.IP  2.

Remote program protocol version mismatches.

.IP  3.

Protocol errors (such as misspecification of a procedure's parameters).

.IP  4.

Reasons why remote authentication failed.

.IP  5.

Any other reasons why the desired procedure was not called.

.NH 2

\&Programs and Procedures

.LP

The RPC call message has three unsigned fields:  remote program

number, remote program version number, and remote procedure number.

The three fields uniquely identify the procedure to be called.

Program numbers are administered by some central authority (like

Sun).  Once an implementor has a program number, he can implement his

remote program; the first implementation would most likely have the

version number of 1.  Because most new protocols evolve into better,

stable, and mature protocols, a version field of the call message

identifies which version of the protocol the caller is using.

Version numbers make speaking old and new protocols through the same

server process possible.

.LP

The procedure number identifies the procedure to be called.  These

numbers are documented in the specific program's protocol

specification.  For example, a file service's protocol specification

may state that its procedure number 5 is "read" and procedure number

12 is "write".

.LP

Just as remote program protocols may change over several versions,

the actual RPC message protocol could also change.  Therefore, the

call message also has in it the RPC version number, which is always

equal to two for the version of RPC described here.

.LP

The reply message to a request  message  has enough  information to

distinguish the following error conditions:

.IP  1.

The remote implementation of RPC does speak protocol version 2.

The lowest and highest supported RPC version numbers are returned.

.IP  2.

The remote program is not available on the remote system.

.IP  3.

The remote program does not support the requested version number.

The lowest and highest supported remote program version numbers are

returned.

.IP  4.

The requested procedure number does not exist.  (This is usually a

caller side protocol or programming error.)

.IP  5.

The parameters to the remote procedure appear to be garbage from the

server's point of view.  (Again, this is usually caused by a

disagreement about the protocol between client and service.)

.NH 2

\&Authentication

.LP

Provisions for authentication of caller to service and vice-versa are

provided as a part of the RPC protocol.  The call message has two

authentication fields, the credentials and verifier.  The reply

message has one authentication field, the response verifier.  The RPC

protocol specification defines all three fields to be the following

opaque type:

.DS

.ft CW

.vs 11

enum auth_flavor {

    AUTH_NULL        = 0,

    AUTH_UNIX        = 1,

    AUTH_SHORT       = 2,

    AUTH_DES         = 3

    /* \fIand more to be defined\fP */

};

struct opaque_auth {

    auth_flavor flavor;

    opaque body<400>;

};

.DE

.LP

In simple English, any

.I opaque_auth

structure is an

.I auth_flavor

enumeration followed by bytes which are  opaque to the RPC protocol

implementation.

.LP

The interpretation and semantics  of the data contained  within the

authentication   fields  is specified  by  individual,  independent

authentication  protocol specifications.   (See

\fIAuthentication Protocols\fP\,

below, for definitions of the various authentication protocols.)

.LP

If authentication parameters were   rejected, the  response message

contains information stating why they were rejected.

.NH 2

\&Program Number Assignment

.LP

Program numbers are given out in groups of

.I 0x20000000

(decimal 536870912) according to the following chart:

.TS

box tab (&) ;

lfI lfI

rfL cfI .

Program Numbers&Description

_

.sp .5

20000000 - 3fffffff&Defined by user

40000000 - 5fffffff&Transient

60000000 - 7fffffff&Reserved

80000000 - 9fffffff&Reserved

a0000000 - bfffffff&Reserved

c0000000 - dfffffff&Reserved

e0000000 - ffffffff&Reserved

.TE

.LP

The first group is a range of numbers administered by Sun

Microsystems and should be identical for all sites.  The second range

is for applications peculiar to a particular site.  This range is

intended primarily for debugging new programs.  When a site develops

an application that might be of general interest, that application

should be given an assigned number in the first range.  The third

group is for applications that generate program numbers dynamically.

The final groups are reserved for future use, and should not be used.

.NH 2

\&Other Uses of the RPC Protocol

.LP

The intended use of this protocol is for calling remote procedures.

That is, each call message is matched with a response message.

However, the protocol itself is a message-passing protocol with which

other (non-RPC) protocols can be implemented.  Sun currently uses, or

perhaps abuses, the RPC message protocol for the following two

(non-RPC) protocols:  batching (or pipelining) and broadcast RPC.

These two protocols are discussed but not defined below.

.NH 3

\&Batching

.LP

Batching allows a client to send an arbitrarily large sequence of

call messages to a server; batching typically uses reliable byte

stream protocols (like TCP/IP) for its transport.  In the case of

batching, the client never waits for a reply from the server, and the

server does not send replies to batch requests.  A sequence of batch

calls is usually terminated by a legitimate RPC in order to flush the

pipeline (with positive acknowledgement).

.NH 3

\&Broadcast RPC

.LP

In broadcast RPC-based protocols, the client sends a broadcast packet

to the network and waits for numerous replies.  Broadcast RPC uses

unreliable, packet-based protocols (like UDP/IP) as its transports.

Servers that support broadcast protocols only respond when the

request is successfully processed, and are silent in the face of

errors.  Broadcast RPC uses the Port Mapper RPC service to achieve

its semantics.  See the \fIPort Mapper Program Protocol\fP\, below,

for more information.

.KS

.NH 1

\&The RPC Message Protocol

.LP

This section defines the RPC message protocol in the XDR data

description language.  The message is defined in a top-down style.

.ie t .DS

.el .DS L

.ft CW

enum msg_type {

        CALL  = 0,

        REPLY = 1

};

.ft I

/*

* A reply to a call message can take on two forms:

* The message was either accepted or rejected.

*/

.ft CW

enum reply_stat {

        MSG_ACCEPTED = 0,

        MSG_DENIED   = 1

};

.ft I

/*

* Given that a call message was accepted,  the following is the

* status of an attempt to call a remote procedure.

*/

.ft CW

enum accept_stat {

        SUCCESS       = 0, /* \fIRPC executed successfully       \fP*/

        PROG_UNAVAIL  = 1, /* \fIremote hasn't exported program  \fP*/

        PROG_MISMATCH = 2, /* \fIremote can't support version #  \fP*/

        PROC_UNAVAIL  = 3, /* \fIprogram can't support procedure \fP*/

        GARBAGE_ARGS  = 4  /* \fIprocedure can't decode params   \fP*/

};

.DE

.ie t .DS

.el .DS L

.ft I

/*

* Reasons why a call message was rejected:

*/

.ft CW

enum reject_stat {

        RPC_MISMATCH = 0, /* \fIRPC version number != 2          \fP*/

        AUTH_ERROR = 1    /* \fIremote can't authenticate caller \fP*/

};

.ft I

/*

* Why authentication failed:

*/

.ft CW

enum auth_stat {

        AUTH_BADCRED      = 1,  /* \fIbad credentials \fP*/

        AUTH_REJECTEDCRED = 2,  /* \fIclient must begin new session \fP*/

        AUTH_BADVERF      = 3,  /* \fIbad verifier \fP*/

        AUTH_REJECTEDVERF = 4,  /* \fIverifier expired or replayed  \fP*/

        AUTH_TOOWEAK      = 5   /* \fIrejected for security reasons \fP*/

};

.DE

.KE

.ie t .DS

.el .DS L

.ft I

/*

* The  RPC  message:

* All   messages  start with   a transaction  identifier,  xid,

* followed  by a  two-armed  discriminated union.   The union's

* discriminant is a  msg_type which switches to  one of the two

* types   of the message.   The xid  of a \fIREPLY\fP  message always

* matches  that of the initiating \fICALL\fP   message.   NB: The xid

* field is only  used for clients  matching reply messages with

* call messages  or for servers detecting  retransmissions; the

* service side  cannot treat this id  as any type   of sequence

* number.

*/

.ft CW

struct rpc_msg {

        unsigned int xid;

        union switch (msg_type mtype) {

                case CALL:

                        call_body cbody;

                case REPLY:

                        reply_body rbody;

        } body;

};

.DE

.ie t .DS

.el .DS L

.ft I

/*

* Body of an RPC request call:

* In version 2 of the  RPC protocol specification, rpcvers must

* be equal to 2.  The  fields prog,  vers, and proc specify the

* remote program, its version number, and the  procedure within

* the remote program to be called.  After these  fields are two

* authentication  parameters: cred (authentication credentials)

* and verf  (authentication verifier).  The  two authentication

* parameters are   followed by  the  parameters  to  the remote

* procedure,  which  are specified  by  the  specific   program

* protocol.

*/

.ft CW

struct call_body {

        unsigned int rpcvers;  /* \fImust be equal to two (2) \fP*/

        unsigned int prog;

        unsigned int vers;

        unsigned int proc;

        opaque_auth cred;

        opaque_auth verf;

        /* \fIprocedure specific parameters start here \fP*/

};

.DE

.ie t .DS

.el .DS L

.ft I

/*

* Body of a reply to an RPC request:

* The call message was either accepted or rejected.

*/

.ft CW

union reply_body switch (reply_stat stat) {

        case MSG_ACCEPTED:

                accepted_reply areply;

        case MSG_DENIED:

                rejected_reply rreply;

} reply;

.DE

.ie t .DS

.el .DS L

.ft I

/*

* Reply to   an RPC request  that  was accepted  by the server:

* there could be an error even though the request was accepted.

* The first field is an authentication verifier that the server

* generates in order to  validate itself  to the caller.  It is

* followed by    a  union whose     discriminant  is   an  enum

* accept_stat.  The  \fISUCCESS\fP  arm of    the union  is  protocol

* specific.  The \fIPROG_UNAVAIL\fP, \fIPROC_UNAVAIL\fP, and \fIGARBAGE_ARGP\fP

* arms of the union are void.   The \fIPROG_MISMATCH\fP arm specifies

* the lowest and highest version numbers of the  remote program

* supported by the server.

*/

.ft CW

struct accepted_reply {

        opaque_auth verf;

        union switch (accept_stat stat) {

                case SUCCESS:

                        opaque results[0];

                        /* \fIprocedure-specific results start here\fP */

                case PROG_MISMATCH:

                        struct {

                                unsigned int low;

                                unsigned int high;

                        } mismatch_info;

                default:

.ft I

                        /*

                        * Void.  Cases include \fIPROG_UNAVAIL, PROC_UNAVAIL\fP,

                        * and \fIGARBAGE_ARGS\fP.

                        */

.ft CW

                        void;

        } reply_data;

};

.DE

.ie t .DS

.el .DS L

.ft I

/*

* Reply to an RPC request that was rejected by the server:

* The request  can   be rejected for   two reasons:  either the

* server   is not  running a   compatible  version  of the  RPC

* protocol    (\fIRPC_MISMATCH\fP), or    the  server   refuses    to

* authenticate the  caller  (\fIAUTH_ERROR\fP).  In  case of  an  RPC

* version mismatch,  the server returns the  lowest and highest

* supported    RPC  version    numbers.  In   case   of refused

* authentication, failure status is returned.

*/

.ft CW

union rejected_reply switch (reject_stat stat) {

        case RPC_MISMATCH:

                struct {

                        unsigned int low;

                        unsigned int high;

                } mismatch_info;

        case AUTH_ERROR:

                auth_stat stat;

};

.DE

.NH 1

\&Authentication Protocols

.LP

As previously stated, authentication parameters are opaque, but

open-ended to the rest of the RPC protocol.  This section defines

some "flavors" of authentication implemented at (and supported by)

Sun.  Other sites are free to invent new authentication types, with

the same rules of flavor number assignment as there is for program

number assignment.

.NH 2

\&Null Authentication

.LP

Often calls must be made where the caller does not know who he is or

the server does not care who the caller is.  In this case, the flavor

value (the discriminant of the \fIopaque_auth\fP's union) of the RPC

message's credentials, verifier, and response verifier is

.I AUTH_NULL .

The  bytes of the opaque_auth's body  are undefined.

It is recommended that the opaque length be zero.

.NH 2

\&UNIX Authentication

.LP

The caller of a remote procedure may wish to identify himself as he

is identified on a UNIX system.  The  value of the credential's

discriminant of an RPC call  message is

.I AUTH_UNIX .

The bytes of

the credential's opaque body encode the following structure:

.DS

.ft CW

struct auth_unix {

        unsigned int stamp;

        string machinename<255>;

        unsigned int uid;

        unsigned int gid;

        unsigned int gids<10>;

};

.DE

The

.I stamp

is an  arbitrary    ID which the  caller machine   may

generate.  The

.I machinename

is the  name of the  caller's machine (like  "krypton").  The

.I uid

is  the caller's effective user  ID.  The

.I gid

is  the caller's effective  group  ID.  The

.I gids

is  a

counted array of groups which contain the caller as  a member.  The

verifier accompanying the  credentials  should  be  of

.I AUTH_NULL

(defined above).

.LP

The value of the discriminant of  the response verifier received in

the  reply  message  from  the    server  may   be

.I AUTH_NULL

or

.I AUTH_SHORT .

In  the  case  of

.I AUTH_SHORT ,

the bytes of the response verifier's string encode an opaque

structure.  This new opaque structure may now be passed to the server

instead of the original

.I AUTH_UNIX

flavor credentials.  The server keeps a cache which maps shorthand

opaque structures (passed back by way of an

.I AUTH_SHORT

style response verifier) to the original credentials of the caller.

The caller can save network bandwidth and server cpu cycles by using

the new credentials.

.LP

The server may flush the shorthand opaque structure at any time.  If

this happens, the remote procedure call message will be rejected due

to an authentication error.  The reason for the failure will be

.I AUTH_REJECTEDCRED .

At this point, the caller may wish to try the original

.I AUTH_UNIX

style of credentials.

.KS

.NH 2

\&DES Authentication

.LP

UNIX authentication suffers from two major problems:

.IP  1.

The naming is too UNIX-system oriented.

.IP  2.

There is no verifier, so credentials can easily be faked.

.LP

DES authentication attempts to fix these two problems.

.KE

.NH 3

\&Naming

.LP

The first problem is handled by addressing the caller by a simple

string of characters instead of by an operating system specific

integer.  This string of characters is known as the "netname" or

network name of the caller.  The server is not allowed to interpret

the contents of the caller's name in any other way except to

identify the caller.  Thus, netnames should be unique for every

caller in the internet.

.LP

It is up to each operating system's implementation of DES

authentication to generate netnames for its users that insure this

uniqueness when they call upon remote servers.  Operating systems

already know how to distinguish users local to their systems.  It is

usually a simple matter to extend this mechanism to the network.

For example, a UNIX user at Sun with a user ID of 515 might be

assigned the following netname: "unix.515@sun.com".  This netname

contains three items that serve to insure it is unique.  Going

backwards, there is only one naming domain called "sun.com" in the

internet.  Within this domain, there is only one UNIX user with

user ID 515.  However, there may be another user on another

operating system, for example VMS, within the same naming domain

that, by coincidence, happens to have the same user ID.  To insure

that these two users can be distinguished we add the operating

system name.  So one user is "unix.515@sun.com" and the other is

"vms.515@sun.com".

.LP

The first field is actually a naming method rather than an

operating system name.  It just happens that today there is almost

a one-to-one correspondence between naming methods and operating

systems.  If the world could agree on a naming standard, the first

field could be the name of that standard, instead of an operating

system name.

.LP

.NH 3

\&DES Authentication Verifiers

.LP

Unlike UNIX authentication, DES authentication does have a verifier

so the server can validate the client's credential (and

vice-versa).  The contents of this verifier is primarily an

encrypted timestamp.  The server can decrypt this timestamp, and if

it is close to what the real time is, then the client must have

encrypted it correctly.  The only way the client could encrypt it

correctly is to know the "conversation key" of the RPC session.  And

if the client knows the conversation key, then it must be the real

client.

.LP

The conversation key is a DES [5] key which the client generates

and notifies the server of in its first RPC call.  The conversation

key is encrypted using a public key scheme in this first

transaction.  The particular public key scheme used in DES

authentication is Diffie-Hellman [3] with 192-bit keys.  The

details of this encryption method are described later.

.LP

The client and the server need the same notion of the current time

in order for all of this to work.  If network time synchronization

cannot be guaranteed, then client can synchronize with the server

before beginning the conversation, perhaps by consulting the

Internet Time Server (TIME[4]).

.LP

The way a server determines if a client timestamp is valid is

somewhat complicated.  For any other transaction but the first, the

server just checks for two things:

.IP  1.

the timestamp is greater than the one previously seen from the

same client.

.IP  2.

the timestamp has not expired.

.LP

A timestamp is expired if the server's time is later than the sum

of the client's timestamp plus what is known as the client's

"window".  The "window" is a number the client passes (encrypted)

to the server in its first transaction.  You can think of it as a

lifetime for the credential.

.LP

This explains everything but the first transaction.  In the first

transaction, the server checks only that the timestamp has not

expired.  If this was all that was done though, then it would be

quite easy for the client to send random data in place of the

timestamp with a fairly good chance of succeeding.  As an added

check, the client sends an encrypted item in the first transaction

known as the "window verifier" which must be equal to the window

minus 1, or the server will reject the credential.

.LP

The client too must check the verifier returned from the server to

be sure it is legitimate.  The server sends back to the client the

encrypted timestamp it received from the client, minus one second.

If the client gets anything different than this, it will reject it.

.LP

.NH 3

\&Nicknames and Clock Synchronization

.LP

After the first transaction, the server's DES authentication

subsystem returns in its verifier to the client an integer

"nickname" which the client may use in its further transactions

instead of passing its netname, encrypted DES key and window every

time.  The nickname is most likely an index into a table on the

server which stores for each client its netname, decrypted DES key

and window.

.LP

Though they originally were synchronized, the client's and server's

clocks can get out of sync again.  When this happens the client RPC

subsystem most likely will get back

.I RPC_AUTHERROR

at which point it should resynchronize.

.LP

A client may still get the

.I RPC_AUTHERROR

error even though it is

synchronized with the server.  The reason is that the server's

nickname table is a limited size, and it may flush entries whenever

it wants.  A client should resend its original credential in this

case and the server will give it a new nickname.  If a server

crashes, the entire nickname table gets flushed, and all clients

will have to resend their original credentials.

.KS

.NH 3

\&DES Authentication Protocol (in XDR language)

.ie t .DS

.el .DS L

.ft I

/*

* There are two kinds of credentials: one in which the client uses

* its full network name, and one in which it uses its "nickname"

* (just an unsigned integer) given to it by the server.  The

* client must use its fullname in its first transaction with the

* server, in which the server will return to the client its

* nickname.  The client may use its nickname in all further

* transactions with the server.  There is no requirement to use the

* nickname, but it is wise to use it for performance reasons.

*/

.ft CW

enum authdes_namekind {

        ADN_FULLNAME = 0,

        ADN_NICKNAME = 1

};

.ft I

/*

* A 64-bit block of encrypted DES data

*/

.ft CW

typedef opaque des_block[8];

.ft I

/*

* Maximum length of a network user's name

*/

.ft CW

const MAXNETNAMELEN = 255;

.ft I

/*

* A fullname contains the network name of the client, an encrypted

* conversation key and the window.  The window is actually a

* lifetime for the credential.  If the time indicated in the

* verifier timestamp plus the window has past, then the server

* should expire the request and not grant it.  To insure that

* requests are not replayed, the server should insist that

* timestamps are greater than the previous one seen, unless it is

* the first transaction.  In the first transaction, the server

* checks instead that the window verifier is one less than the

* window.

*/

.ft CW

struct authdes_fullname {

string name;  /* \fIname of client \f(CW*/

des_block key;               /* \fIPK encrypted conversation key \f(CW*/

unsigned int window;         /* \fIencrypted window \f(CW*/

};

.ft I

/*

* A credential is either a fullname or a nickname

*/

.ft CW

union authdes_cred switch (authdes_namekind adc_namekind) {

        case ADN_FULLNAME:

                authdes_fullname adc_fullname;

        case ADN_NICKNAME:

                unsigned int adc_nickname;

};

.ft I

/*

* A timestamp encodes the time since midnight, January 1, 1970.

*/

.ft CW

struct timestamp {

        unsigned int seconds;    /* \fIseconds \fP*/

        unsigned int useconds;   /* \fIand microseconds \fP*/

};

.ft I

/*

* Verifier: client variety

* The window verifier is only used in the first transaction.  In

* conjunction with a fullname credential, these items are packed

* into the following structure before being encrypted:

*

* \f(CWstruct {\fP

*     \f(CWadv_timestamp;            \fP-- one DES block

*     \f(CWadc_fullname.window;      \fP-- one half DES block

*     \f(CWadv_winverf;              \fP-- one half DES block

* \f(CW}\fP

* This structure is encrypted using CBC mode encryption with an

* input vector of zero.  All other encryptions of timestamps use

* ECB mode encryption.

*/

.ft CW

struct authdes_verf_clnt {

        timestamp adv_timestamp;    /* \fIencrypted timestamp       \fP*/

        unsigned int adv_winverf;   /* \fIencrypted window verifier \fP*/

};

.ft I

/*

* Verifier: server variety

* The server returns (encrypted) the same timestamp the client

* gave it minus one second.  It also tells the client its nickname

* to be used in future transactions (unencrypted).

*/

.ft CW

struct authdes_verf_svr {

timestamp adv_timeverf;     /* \fIencrypted verifier      \fP*/

unsigned int adv_nickname;  /* \fInew nickname for client \fP*/

};

.DE

.KE

.NH 3

\&Diffie-Hellman Encryption

.LP

In this scheme, there are two constants,

.I BASE

and

.I MODULUS .

The

particular values Sun has chosen for these for the DES

authentication protocol are:

.ie t .DS

.el .DS L

.ft CW

const BASE = 3;

const MODULUS =

        "d4a0ba0250b6fd2ec626e7efd637df76c716e22d0944b88b"; /* \fIhex \fP*/

.DE

.ft R

The way this scheme works is best explained by an example.  Suppose

there are two people "A" and "B" who want to send encrypted

messages to each other.  So, A and B both generate "secret" keys at

random which they do not reveal to anyone.  Let these keys be

represented as SK(A) and SK(B).  They also publish in a public

directory their "public" keys.  These keys are computed as follows:

.ie t .DS

.el .DS L

.ft CW

PK(A) = ( BASE ** SK(A) ) mod MODULUS

PK(B) = ( BASE ** SK(B) ) mod MODULUS

.DE

.ft R

The "**" notation is used here to represent exponentiation.  Now,

both A and B can arrive at the "common" key between them,

represented here as CK(A, B), without revealing their secret keys.

.LP

A computes:

.ie t .DS

.el .DS L

.ft CW

CK(A, B) = ( PK(B) ** SK(A)) mod MODULUS

.DE

.ft R

while B computes:

.ie t .DS

.el .DS L

.ft CW

CK(A, B) = ( PK(A) ** SK(B)) mod MODULUS

.DE

.ft R

These two can be shown to be equivalent:

.ie t .DS

.el .DS L

.ft CW

(PK(B) ** SK(A)) mod MODULUS = (PK(A) ** SK(B)) mod MODULUS

.DE

.ft R

We drop the "mod MODULUS" parts and assume modulo arithmetic to

simplify things:

.ie t .DS

.el .DS L

.ft CW

PK(B) ** SK(A) = PK(A) ** SK(B)

.DE

.ft R

Then, replace PK(B) by what B computed earlier and likewise for

PK(A).

.ie t .DS

.el .DS L

.ft CW

((BASE ** SK(B)) ** SK(A) = (BASE ** SK(A)) ** SK(B)

.DE

.ft R

which leads to:

.ie t .DS

.el .DS L

.ft CW

BASE ** (SK(A) * SK(B)) = BASE ** (SK(A) * SK(B))

.DE

.ft R

This common key CK(A, B) is not used to encrypt the timestamps used