ONC RPC Working Group Alex Chiu
INTERNET-DRAFT Sun Microsystems
Category: Informational April 15, 1998
Authentication Mechanisms for ONC RPC
draft-ietf-oncrpc-auth-05.txt
ABSTRACT
This document describes two authentication mechanisms created by Sun
Microsystems that are commonly used in conjunction with the ONC Remote
Procedure Call (ONC RPC Version 2) protocol.
STATUS OF THIS MEMO
This document is an Internet-Draft. Internet-Drafts are working documents
of the Internet Engineering Task Force (IETF), its areas, and its working
groups. Note that other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months.
This Internet-Draft expires on October 14, 1998. Internet-Drafts may be
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To view the entire list of current Internet-Drafts, please check
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(US West Coast).
Distribution of this memo is unlimited.
WARNING
The DH authentication as defined in Section 2 in this document refers to
the authentication mechanism with flavor AUTH_DH currently implemented in
ONC RPC. It uses the underlying Diffie-Hellman algorithm for key exchange.
The DH authentication defined in this document is flawed due to the
selection of a small prime for the BASE field (Section 2.5). To avoid the
flaw a new DH authentication mechanism could be defined with a larger
prime. However, the new DH authentication would not be interoperable with
the existing DH authentication.
As illustrated in [10], a large number of attacks are possible on ONC RPC
system services that use non-secure authentication mechanisms. Other
secure authentication mechanisms need to be developed for ONC RPC. RFC
2203 describes the RPCSEC_GSS ONC RPC security flavor, a secure
authentication mechanism that enables RPC protocols to use Generic Security
Service Application Program Interface (RFC 2078) to provide security
services, integrity and privacy, that are independent of the underlying
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security mechanisms.
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CONTENTS
1. Introduction 4
2. Diffie-Hellman Authentication 5
2.1 Naming 5
2.2 DH Authentication Verifiers 5
2.3 Nicknames and Clock Synchronization 6
2.4 DH Authentication Protocol Specification 7
2.4.1 The Full Network Name Credential and Verifier (Client) 8
2.4.2 The Nickname Credential and Verifier (Client) 9
2.4.3 The Nickname Verifier (Server) 10
2.5 Diffie-Hellman Encryption 10
3. Kerberos-based Authentication 12
3.1 Naming 12
3.2 Kerberos-based Authentication Protocol Specification 12
3.2.1 The Full Network Name Credential and Verifier (Client) 13
3.2.2 The Nickname Credential and Verifier (Client) 14
3.2.3 The Nickname Verifier (Server) 15
3.2.4 Kerberos-specific Authentication Status Values 16
4. REFERENCES 17
5. AUTHOR'S ADDRESS 17
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1. Introduction
The ONC RPC protocol provides the fields necessary for a client to identify
itself to a service, and vice-versa, in each call and reply message.
Security and access control mechanisms can be built on top of this message
authentication. Several different authentication protocols can be
supported.
This document specifies two authentication protocols created by Sun
Microsystems that are commonly used Diffie-Hellman (DH) authentication and
Kerberos (Version 4) based authentication.
As a prerequisite to reading this document, the reader is expected to be
familiar with [1] and [2]. This document uses terminology and definitions
from [1] and [2].
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2. Diffie-Hellman Authentication
System authentication (defined in [1]) suffers from some problems. It is
very unix oriented, and can be easily faked (there is no attempt to provide
cryptographically secure authentication).
DH authentication was created to address these problems. However, it has
been compromised [9] due to the selection of a small length for the prime
in the ONC RPC implementation. While the information provided here will be
useful for implementors to ensure interoperability with existing
applications that use DH authentication, it is strongly recommended that
new applications use more secure authentication, and that existing
applications that currently use DH authentication migrate to more robust
authentication mechanisms.
2.1 Naming
The client is addressed 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 client. The server is not allowed to
interpret the contents of the client's name in any other way except to
identify the client. Thus, netnames should be unique for every client in
the Internet.
It is up to each operating system's implementation of DH 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(tm) 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(tm) 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". The first field is actually a naming method
rather than an operating system name. It 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.
2.2 DH Authentication Verifiers
Unlike System authentication, DH 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 within an accepted range relative to
the current time, 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.
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The conversation key is a DES [5] key which the client generates and passes
to the server in the first RPC call of a session. The conversation key is
encrypted using a public key scheme in this first transaction. The
particular public key scheme used in DH authentication is Diffie-Hellman
[3] with 192-bit keys. The details of this encryption method are described
later.
The client and the server need the same notion of the current time in order
for all of this to work, perhaps by using the Network Time Protocol [4].
If network time synchronization cannot be guaranteed, then the client can
determine the server's time before beginning the conversation using a time
request protocol.
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:
(1) the timestamp is greater than the one previously seen from the same
client. (2) the timestamp has not expired.
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 "ttl" (standing for
"time-to-live" - you can think of this as the lifetime for the client's
credential). The "ttl" is a number the client passes (encrypted) to the
server in its first transaction.
In the first transaction, the server checks only that the timestamp has not
expired. Also, as an added check, the client sends an encrypted item in
the first transaction known as the "ttl verifier" which must be equal to
the time-to-live minus 1, or the server will reject the credential. If
either check fails, the server rejects the credential with an
authentication status of AUTH_BADCRED, however if the timestamp is earlier
than the previous one seen, the server returns an authentication status of
AUTH_REJECTEDCRED.
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 timestamp it
received from the client, minus one second, encrypted with the conversation
key. If the client gets anything different than this, it will reject it,
returning an AUTH_INVALIDRESP authentication status to the user.
2.3 Nicknames and Clock Synchronization
After the first transaction, the server's DH 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.
The nickname could be an index into a table on the server which stores for
each client its netname, decrypted conversation key and ttl.
Though they originally were synchronized, the client's and server's clocks
can get out of synchronization again. When this happens the server returns
to the client an authentication status of AUTH_REJECTEDVERF at which point
the client should attempt to resynchronize.
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A client may also get a AUTH_BADCRED error when using a nickname that was
previously valid. 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 full name 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.
2.4 DH Authentication Protocol Specification
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.
The following definitions are used for describing the protocol:
enum authdh_namekind {
ADN_FULLNAME = 0,
ADN_NICKNAME = 1
};
typedef opaque des_block[8]; /* 64-bit block of encrypted data */
const MAXNETNAMELEN = 255; /* maximum length of a netname */
The flavor used for all DH authentication credentials and verifiers is
"AUTH_DH", with the numerical value 3. The opaque data constituting the
client credential encodes the following structure:
union authdh_cred switch (authdh_namekind namekind) {
case ADN_FULLNAME:
authdh_fullname fullname;
case ADN_NICKNAME:
authdh_nickname nickname;
};
The opaque data constituting a verifier that accompanies a client
credential encodes the following structure:
union authdh_verf switch (authdh_namekind namekind) {
case ADN_FULLNAME:
authdh_fullname_verf fullname_verf;
case ADN_NICKNAME:
authdh_nickname_verf nickname_verf;
};
The opaque data constituting a verifier returned by a server in response to
a client request encodes the following structure:
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struct authdh_server_verf;
These structures are described in detail below.
2.4.1 The Full Network Name Credential and Verifier (Client)
First, the client creates a conversation key for the session. Next, the
client fills out the following structure:
+---------------------------------------------------------------+
| timestamp | timestamp | | |
| seconds | micro seconds | ttl | ttl - 1 |
| 32 bits | 32 bits | 32 bits | 32 bits |
+---------------------------------------------------------------+
0 31 63 95 127
The fields are stored in XDR (external data representation) format. The
timestamp encodes the time since midnight, January 1, 1970. These 128 bits
of data are then encrypted in the DES CBC mode, using the conversation key
for the session, and with an initialization vector of 0. This yields:
+---------------------------------------------------------------+
| T | | |
| T1 T2 | W1 | W2 |
| 32 bits | 32 bits | 32 bits | 32 bits |
+---------------------------------------------------------------+
0 31 63 95 127
where T1, T2, W1, and W2 are all 32-bit quantities, and have some
correspondence to the original quantities occupying their positions, but
are now interdependent on each other for proper decryption. The 64 bit
sequence comprising T1 and T2 is denoted by T.
The full network name credential is represented as follows using XDR
notation:
struct authdh_fullname {
string name<MAXNETNAMELEN>; /* netname of client */
des_block key; /* encrypted conversation key */
opaque w1[4]; /* W1 */
};
The conversation key is encrypted using the "common key" using the ECB
mode. The common key is a DES key that is derived from the Diffie-Hellman
public and private keys, and is described later.
The verifier is represented as follows:
struct authdh_fullname_verf {
des_block timestamp; /* T (the 64 bits of T1 and T2) */
opaque w2[4]; /* W2 */
};
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Note that all of the encrypted quantities (key, w1, w2, timestamp) in the
above structures are opaque.
The fullname credential and its associated verifier together contain the
network name of the client, an encrypted conversation key, the ttl, a
timestamp, and a ttl verifier that is one less than the ttl. The ttl is
actually the lifetime for the credential. The server will accept the
credential if the current server time is "within" the time indicated in the
timestamp plus the ttl. Otherwise, the server rejects the credential with
an authentication status of AUTH_BADCRED. One way to insure that requests
are not replayed would be for the server to insist that timestamps are
greater than the previous one seen, unless it is the first transaction. If
the timestamp is earlier than the previous one seen, the server returns an
authentication status of AUTH_REJECTEDCRED.
The server returns a authdh_server_verf structure, which is described in
detail below. This structure contains a "nickname", which may be used for
subsequent requests in the current conversation.
2.4.2 The Nickname Credential and Verifier (Client)
In transactions following the first, the client may use the shorter
nickname credential and verifier for efficiency. First, the client fills
out the following structure:
+-------------------------------+
| timestamp | timestamp |
| seconds | micro seconds |
| 32 bits | 32 bits |
+-------------------------------+
0 31 63
The fields are stored in XDR (external data representation) format. These
64 bits of data are then encrypted in the DES ECB mode, using the
conversation key for the session. This yields:
+-------------------------------+
| (T1) | (T2) |
| T |
| 64 bits |
+-------------------------------+
0 31 63
The nickname credential is represented as follows using XDR notation:
struct authdh_nickname {
unsigned int nickname; /* nickname returned by server */
};
The nickname verifier is represented as follows using XDR notation:
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struct authdh_nickname_verf {
des_block timestamp; /* T (the 64 bits of T1 and T2) */
opaque w[4]; /* Set to zero */
};
The nickname credential may be reject by the server for several reasons.
An authentication status of AUTH_BADCRED indicates that the nickname is no
longer valid. The client should retry the request using the fullname
credential. AUTH_REJECTEDVERF indicates that the nickname verifier is not
valid. Again, the client should retry the request using the fullname
credential.
2.4.3 The Nickname Verifier (Server)
The server never returns a credential. It returns only one kind of
verifier, i.e., the nickname verifier. This has the following XDR
representation:
struct authdh_server_verf {
des_block timestamp_verf; /* timestamp verifier (encrypted) */
unsigned int nickname; /* new client nickname (unencrypted) */
};
The timestamp verifier is constructed in exactly the same way as the client
nickname credential. The server sets the timestamp value to the value the
client sent minus one second and encrypts it in DES ECB mode using the
conversation key. The server also sends the client a nickname to be used
in future transactions (unencrypted).
2.5 Diffie-Hellman Encryption
In this scheme, there are two constants "BASE" and "MODULUS" [3]. The
particular values Sun has chosen for these for the DH authentication
protocol are:
const BASE = 3;
const MODULUS = "d4a0ba0250b6fd2ec626e7efd637df76c716e22d0944b88b";
Note that the modulus is represented above as a hexadecimal string.
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:
PK(A) = ( BASE ** SK(A) ) mod MODULUS
PK(B) = ( BASE ** SK(B) ) mod MODULUS
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.
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A computes:
CK(A, B) = ( PK(B) ** SK(A)) mod MODULUS
while B computes:
CK(A, B) = ( PK(A) ** SK(B)) mod MODULUS
These two can be shown to be equivalent:
(PK(B) ** SK(A)) mod MODULUS = (PK(A) ** SK(B)) mod MODULUS
We drop the "mod MODULUS" parts and assume modulo arithmetic to simplify
things:
PK(B) ** SK(A) = PK(A) ** SK(B)
Then, replace PK(B) by what B computed earlier and likewise for PK(A).
(BASE ** SK(B)) ** SK(A) = (BASE ** SK(A)) ** SK(B)
which leads to:
BASE ** (SK(A) * SK(B)) = BASE ** (SK(A) * SK(B))
This common key CK(A, B) is not used to encrypt the timestamps used in the
protocol. Rather, it is used only to encrypt a conversation key which is
then used to encrypt the timestamps. The reason for doing this is to use
the common key as little as possible, for fear that it could be broken.
Breaking the conversation key is a far less damaging, since conversations
are relatively short-lived.
The conversation key is encrypted using 56-bit DES keys, yet the common key
is 192 bits. To reduce the number of bits, 56 bits are selected from the
common key as follows. The middle-most 8-bytes are selected from the common
key, and then parity is added to the lower order bit of each byte,
producing a 56-bit key with 8 bits of parity.
Only 48 bits of the 8-byte conversation key is used in the DH
Authentication scheme. The least and most significant bits of each byte of
the conversation key are unused.
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3. Kerberos-based Authentication
Conceptually, Kerberos-based authentication is very similar to DH
authentication. The major difference is, Kerberos-based authentication
takes advantage of the fact that Kerberos tickets have encoded in them the
client name and the conversation key. This RFC does not describe Kerberos
name syntax, protocols and ticket formats. The reader is referred to [6],
[7], and [8].
3.1 Naming
A Kerberos name contains three parts. The first is the principal name,
which is usually a user's or service's name. The second is the instance,
which in the case of a user is usually NULL. Some users may have
privileged instances, however, such as root or admin. In the case of a
service, the instance is the name of the machine on which it runs; that is,
there can be an NFS service running on the machine ABC, which is different
from the NFS service running on the machine XYZ. The third part of a
Kerberos name is the realm. The realm corresponds to the Kerberos service
providing authentication for the principal. When writing a Kerberos name,
the principal name is separated from the instance (if not NULL) by a
period, and the realm (if not the local realm) follows, preceded by an
``@'' sign. The following are examples of valid Kerberos names:
billb
jis.admin
srz@lcs.mit.edu
treese.root@athena.mit.edu
3.2 Kerberos-based Authentication Protocol Specification
The Kerberos-based authentication protocol described is based on Kerberos
version 4.
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.
The following definitions are used for describing the protocol:
enum authkerb4_namekind {
AKN_FULLNAME = 0,
AKN_NICKNAME = 1
};
The flavor used for all Kerberos-based authentication credentials and
verifiers is "AUTH_KERB4", with numerical value 4. The opaque data
constituting the client credential encodes the following structure:
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union authkerb4_cred switch (authkerb4_namekind namekind) {
case AKN_FULLNAME:
authkerb4_fullname fullname;
case AKN_NICKNAME:
authkerb4_nickname nickname;
};
The opaque data constituting a verifier that accompanies a client
credential encodes the following structure:
union authkerb4_verf switch (authkerb4_namekind namekind) {
case AKN_FULLNAME:
authkerb4_fullname_verf fullname_verf;
case AKN_NICKNAME:
authkerb4_nickname_verf nickname_verf;
};
The opaque data constituting a verifier returned by a server in response to
a client request encodes the following structure:
struct authkerb4_server_verf;
These structures are described in detail below.
3.2.1 The Full Network Name Credential and Verifier (Client)
First, the client must obtain a Kerberos ticket from the Kerberos Server.
The ticket contains a Kerberos session key, which will become the
conversation key. Next, the client fills out the following structure:
+---------------------------------------------------------------+
| timestamp | timestamp | | |
| seconds | micro seconds | ttl | ttl - 1 |
| 32 bits | 32 bits | 32 bits | 32 bits |
+---------------------------------------------------------------+
0 31 63 95 127
The fields are stored in XDR (external data representation) format. The
timestamp encodes the time since midnight, January 1, 1970. "ttl" is
identical in meaning to the corresponding field in Diffie-Hellman
authentication: the credential "time-to-live" for the conversation being
initiated. These 128 bits of data are then encrypted in the DES CBC mode,
using the conversation key, and with an initialization vector of 0. This
yields:
+---------------------------------------------------------------+
| T | | |
| T1 T2 | W1 | W2 |
| 32 bits | 32 bits | 32 bits | 32 bits |
+---------------------------------------------------------------+
0 31 63 95 127
where T1, T2, W1, and W2 are all 32-bit quantities, and have some
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correspondence to the original quantities occupying their positions, but
are now interdependent on each other for proper decryption. The 64 bit
sequence comprising T1 and T2 is denoted by T.
The full network name credential is represented as follows using XDR
notation:
struct authkerb4_fullname {
opaque ticket<>; /* kerberos ticket for the server */
opaque w1[4]; /* W1 */
};
The verifier is represented as follows:
struct authkerb4_fullname_verf {
des_block timestamp; /* T (the 64 bits of T1 and T2) */
opaque w2[4]; /* W2 */
};
Note that all of the client-encrypted quantities (w1, w2, timestamp) in the
above structures are opaque. The client does not encrypt the Kerberos
ticket for the server.
The fullname credential and its associated verifier together contain the
Kerberos ticket (which contains the client name and the conversation key),
the ttl, a timestamp, and a ttl verifier that is one less than the ttl.
The ttl is actually the lifetime for the credential. The server will
accept the credential if the current server time is "within" the time
indicated in the timestamp plus the ttl. Otherwise, the server rejects the
credential with an authentication status of AUTH_BADCRED. One way to
insure that requests are not replayed would be for the server to insist
that timestamps are greater than the previous one seen, unless it is the
first transaction. If the timestamp is earlier than the previous one seen,
the server returns an authentication status of AUTH_REJECTEDCRED.
The server returns a authkerb4_server_verf structure, which is described in
detail below. This structure contains a "nickname", which may be used for
subsequent requests in the current session.
3.2.2 The Nickname Credential and Verifier (Client)
In transactions following the first, the client may use the shorter
nickname credential and verifier for efficiency. First, the client fills
out the following structure:
+-------------------------------+
| timestamp | timestamp |
| seconds | micro seconds |
| 32 bits | 32 bits |
+-------------------------------+
0 31 63
The fields are stored in XDR (external data representation) format. These
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64 bits of data are then encrypted in the DES ECB mode, using the
conversation key for the session. This yields:
+-------------------------------+
| (T1) | (T2) |
| T |
| 64 bits |
+-------------------------------+
0 31 63
The nickname credential is represented as follows using XDR notation:
struct authkerb4_nickname {
unsigned int nickname; /* nickname returned by server */
};
The nickname verifier is represented as follows using XDR notation:
struct authkerb4_nickname_verf {
des_block timestamp; /* T (the 64 bits of T1 and T2) */
opaque w[4]; /* Set to zero */
};
The nickname credential may be reject by the server for several reasons.
An authentication status of AUTH_BADCRED indicates that the nickname is no
longer valid. The client should retry the request using the fullname
credential. AUTH_REJECTEDVERF indicates that the nickname verifier is not
valid. Again, the client should retry the request using the fullname
credential. AUTH_TIMEEXPIRE indicates that the session's Kerberos ticket
has expired. The client should initiate a new session by obtaining a new
Kerberos ticket.
3.2.3 The Nickname Verifier (Server)
The server never returns a credential. It returns only one kind of
verifier, i.e., the nickname verifier. This has the following XDR
representation:
struct authkerb4_server_verf {
des_block timestamp_verf; /* timestamp verifier (encrypted) */
unsigned int nickname; /* new client nickname (unencrypted) */
};
The timestamp verifier is constructed in exactly the same way as the client
nickname credential. The server sets the timestamp value to the value the
client sent minus one second and encrypts it in DES ECB mode using the
conversation key. The server also sends the client a nickname to be used
in future transactions (unencrypted).
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3.2.4 Kerberos-specific Authentication Status Values
The server may return to the client one of the following errors in the
authentication status field:
enum auth_stat {
...
/*
* kerberos errors
*/
AUTH_KERB_GENERIC = 8, /* Any Kerberos-specific error other
than the following */
AUTH_TIMEEXPIRE = 9, /* The client's ticket has expired */
AUTH_TKT_FILE = 10, /* The server was unable to find the
ticket file. The client should
create a new session by obtaining a
new ticket */
AUTH_DECODE = 11, /* The server is unable to decode the
authenticator of the client's ticket */
AUTH_NET_ADDR = 12 /* The network address of the client
does not match the address contained
in the ticket */
/* and more to be defined */
};
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4. REFERENCES
[1] Srinivasan, R., "Remote Procedure Call Protocol Version 2", RFC-1831,
Sun Microsystems, 1995.
[2] Srinivasan, R., "XDR: External Data Representation Standard",
RFC-1832, Sun Microsystems, 1995.
[3] Diffie & Hellman, "New Directions in Cryptography", IEEE
Transactions on Information Theory IT-22, November 1976.
[4] Mills, D., "Network Time Protocol (Version 3)", RFC-1305,
University of Delaware, March 1992.
[5] National Bureau of Standards, "Data Encryption Standard", Federal
Information Processing Standards Publication 46, January 1977.
[6] Miller, S., Neuman, C., Schiller, J., and J. Saltzer, "Section
E.2.1: Kerberos Authentication and Authorization System",
M.I.T. Project Athena, Cambridge, Massachusetts, December 21,
1987.
[7] Steiner, J., Neuman, C., and J. Schiller, "Kerberos: An
Authentication Service for Open Network Systems", pp. 191-202 in
Usenix Conference Proceedings, Dallas, Texas, February, 1988.
[8] Kohl, J. and Neuman, C., "The Kerberos Network Authentication
Service (V5)", RFC-1510, September 1993.
[9] La Macchia, B.A., and Odlyzko, A.M., "Computation of Discrete
Logarithms in Prime Fields", pp. 47-62 in "Designs, Codes and
Cryptography", Kluwer Academic Publishers, 1991.
[10] Cheswick, W.R., and Bellovin, S.M., "Firewalls and Internet Security,"
Addison-Wesley, 1995.
5. AUTHOR'S ADDRESS
Alex Chiu
Sun Microsystems, Inc.
901 San Antonio Road
Palo Alto, CA 94303
Phone: +1 (650) 786-6465
E-mail: alex.chiu@Eng.sun.com
Expires: October 14, 1998 Informational [Page 17]
