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rfc:rfc2695

Network Working Group A. Chiu Request for Comments: 2695 Sun Microsystems Category: Informational September 1999

               Authentication Mechanisms for ONC RPC

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1999).  All Rights Reserved.

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.

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 security mechanisms.

Chiu Informational [Page 1] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

Table of Contents

    1. Introduction ............................................... 2
    2. Diffie-Hellman Authentication .............................. 2
    2.1 Naming .................................................... 3
    2.2 DH Authentication Verifiers ............................... 3
    2.3 Nicknames and Clock Synchronization ....................... 5
    2.4 DH Authentication Protocol Specification .................. 5
    2.4.1 The Full Network Name Credential and Verifier (Client) .. 6
    2.4.2 The Nickname Credential and Verifier (Client) ........... 8
    2.4.3 The Nickname Verifier (Server) .......................... 9
    2.5 Diffie-Hellman Encryption ................................. 9
    3. Kerberos-based Authentication ............................. 10
    3.1 Naming ................................................... 11
    3.2 Kerberos-based Authentication Protocol Specification ..... 11
    3.2.1 The Full Network Name Credential and Verifier (Client) . 12
    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 ......... 15
    4. Security Considerations ................................... 16
    5. REFERENCES ................................................ 16
    6. AUTHOR'S ADDRESS .......................................... 17
    7. FULL COPYRIGHT STATEMENT ...................................18

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].

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).

Chiu Informational [Page 2] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

 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 are 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

Chiu Informational [Page 3] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

 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.
 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.

Chiu Informational [Page 4] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

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.
 A client may also get an 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:

Chiu Informational [Page 5] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

 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:
 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

Chiu Informational [Page 6] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

 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                           */
 };
 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.

Chiu Informational [Page 7] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

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:
 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.

Chiu Informational [Page 8] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

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.

Chiu Informational [Page 9] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

 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 are used in the DH
 Authentication scheme.  The least and most significant bits of each
 byte of the conversation key are unused.

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

Chiu Informational [Page 10] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

 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
    };

Chiu Informational [Page 11] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

 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:
 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

Chiu Informational [Page 12] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

 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
 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.

Chiu Informational [Page 13] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

 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 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

Chiu Informational [Page 14] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

 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).

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. Security Considerations

 The DH authentication mechanism and the Kerberos V4 authentication
 mechanism are described in this document only for informational
 purposes.
 In addition to the weakness pointed out earlier in this document (see
 WARNING on page 1), the two security mechanisms described herein lack
 the support for integrity and privacy data protection. It is strongly
 recommended that new applications use more secure mechanisms, and
 that existing applications migrate to more robust mechanisms.
 The RPCSEC_GSS ONC RPC security flavor, specified in RFC 2203, allows
 applications built on top of RPC to access security mechanisms that
 adhere to the GSS-API specification.  It provides a GSS-API based
 security framework that allows for strong security mechanisms.  RFC
 1964 describes the Kerberos Version 5 GSS-API security mechanism
 which provides integrity and privacy, in addition to authentication.
 RFC 2623 [14] describes how Kerberos V5 is pluggued into RPCSEC_GSS,
 and how the Version 2 and Version 3 of the NFS protocol use Kerberos
 V5 via RPCSEC_GSS. The RPCSEC_GSS/GSS-API/Kerberos-V5 stack provides
 a robust security mechanism for applications that require strong
 protection.

5. REFERENCES

 [1]  Srinivasan, R., "Remote Procedure Call Protocol Version 2", RFC
      1831, August 1995.
 [2]  Srinivasan, R., "XDR: External Data Representation Standard",
      RFC 1832, August 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, 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",
      December 1987.

Chiu Informational [Page 16] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

 [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 C. Neuman, "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.
 [11] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964,
      June 1996.
 [12] Linn, J., "Generic Security Service Application Program
      Interface, Version 2", RFC 2078, January 1997.
 [13] Eisler, M., Chiu, A., and Ling, L., "RPCSEC_GSS Protocol
      Specification", RFC 2203, September 1997.
 [14] Eisler, M., "NFS Version 2 and Version 3 Security Issues and the
      NFS Protocol's Use of RPCSEC_GSS and Kerberos V5", RFC 2623,
      June 1999.

6. AUTHOR'S ADDRESS

 Alex Chiu
 Sun Microsystems, Inc.
 901 San Antonio Road
 Palo Alto, CA 94303
 Phone: +1 (650) 786-6465
 EMail: alex.chiu@Eng.sun.com

Chiu Informational [Page 17] RFC 2695 Authentication Mechanisms for ONC RPC September 1999

7. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

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