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Versions: 00 01 02 03 04 rfc2829                                        
Network Working Group                                            M. Wahl
INTERNET-DRAFT                              Innosoft International, Inc.
                                                           H. Alvestrand
                                                              MaXware AS
                                                               J. Hodges
                                                     Stanford University
                                                         RL "Bob" Morgan
                                                     Stanford University
Expires in six months from                                  July 8, 1998


                      Authentication Methods for LDAP
                   <draft-ietf-ldapext-authmeth-02.txt>

1. 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 and may be updated, replaced, or obsoleted by other
   documents at any time.  It is inappropriate to use Internet-Drafts
   as reference material or to cite them other than as "work in
   progress."

   To view the entire list of current Internet-Drafts, please check
   the "1id-abstracts.txt" listing contained in the Internet-Drafts
   Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
   (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au
   (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu
   (US West Coast).

2. Abstract

   This document specifies particular combinations of security
   mechanisms which are required and recommended in LDAP [1]
   implementations.

3. Introduction

   LDAP version 3 is a powerful access protocol for directories.

   It offers means of searching, fetching and manipulating directory
   content, and ways to access a rich set of security functions.

   In order to function for the best of the Internet, it is vital
   that these security functions be interoperable; therefore there
   has to be a minimum subset of security functions that is common to
   all implementations that claim LDAPv3 conformance.



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   The basic threats to an LDAP directory service are:

     (1)   Unauthorized access to data via data-fetching operations,

     (2)   Unauthorized access to reusable client authentication
           information by monitoring others' access,

     (3)   Unauthorized access to data by monitoring others' access,

     (4)   Unauthorized modification of data,

     (5)   Unauthorized modification of configuration,

     (6)   Unauthorized or excessive use of resources (denial of
           service), and

     (7)   Spoofing of directory: Tricking a client into believing
           that information came from the directory when in fact it
           did not, either by modifying data in transit or misdirecting
           the client's connection.

   Threats (1), (4), (5) and (6) are due to hostile clients.  Threats
   (2), (3) and (7) are due to hostile agents on the path between client
   and server, or posing as a server.

   The LDAP protocol suite can be protected with the following
   security mechanisms:

     (1)   Client authentication by means of the SASL mechanism set,
           possibly backed by the TLS credentials exchange mechanism,

     (2)   Client authorization by means of access control based on
           the requestor's authenticated identity,

     (3)   Data integrity protection by means of the TLS protocol or
           data-integrity SASL mechanisms,

     (4)   Protection against snooping by means of the TLS protocol
           or data-encrypting SASL mechanisms,

     (5)   Resource limitation by means of administrative limits on
           service controls, and

     (6)   Server authentication by means of the TLS protocol or SASL
           mechanism.

   At the moment, imposition of access controls is done by means
   outside the scope of the LDAP protocol.

   In this document, the term "user" represents any application which
   is an LDAP client using the directory to retrieve or store information.

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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC 2119 [3].

4.  Deployment scenarios

   The following scenarios are typical for LDAP directories, and have
   different security requirements. (In the following, "sensitive"
   means data that will cause real damage to the owner if revealed;
   there may be data that is protected but not sensitive)

     (1)   A read-only directory, containing no sensitive data,
           accessible to "anyone", and TCP connection hijacking
           or IP spoofing is not a problem.  This directory requires
           no security functions except the service limits.

     (2)   A read-only directory containing no sensitive data; read
           access is granted based on identity.  TCP connection
           hijacking is not currently a problem. This scenario requires
           a secure authentication function.

     (3)   A read-only directory containing no sensitive data; and
           the client needs to ensure that the directory data is
           authenticated by the server not and modified while being
           returned from the server.

     (4)   A read-write directory, containing no sensitive data; read
           access is available to "anyone", update access to properly
           authorized persons.  TCP connection hijacking is not
           currently a problem.  This scenario requires a secure
           authentication function.

     (5)   A directory containing sensitive data.  This scenario
           requires session confidentiality protection AND secure
           authentication.

   Other scenarios are possible.

   This document does not describe the requirements for use of LDAP
   in physically protected networks; this is concerned with LDAP used
   on the Internet.

5.  Authentication and Authorization:  Definitions and Concepts

   This section defines basic terms, concepts, and interrelationships
   regarding authentication, authorization, credentials, and identity.
   These concepts are used in describing how various security
   approaches are utilized in client authentication and authorization.




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5.1.  Access Control Policy

   An access control policy is a set of rules defining the protection
   of resources, generally in terms of the capabilities of persons or
   other entities accessing those resources.  A common expression of an
   access control policy is an access control list.  Security objects
   and mechanisms, such as those described here, enable the expression of
   access control policies and their enforcement.  Access control
   policies are typically expressed in terms of access control attributes
   as described below.

5.2.  Access Control Factors

   A request, when it is being processed by a server, may be associated
   with a wide variety of security-related factors (section 4.2 of [1]).
   The server uses these factors to determine whether and how to process
   the request.  These are called access control factors (ACFs).  They
   might include source IP address, encryption strength, the type of
   operation being requested, time of day, etc.  Some factors may be
   specific to the request itself, others may be associated with the
   connection via which the request is transmitted, others (e.g. time of
   day) may be "environmental".

   Access control policies are expressed in terms of access control
   factors.  E.g., a request having ACFs i,j,k can perform operation Y
   on resource Z. The set of ACFs that a server makes available for such
   expressions is implementation-specific.

5.3.  Authentication, Credentials, Identity

   Authentication credentials are the evidence supplied by one party to
   another, asserting the identity of the supplying party (e.g. a user)
   who is attempting to establish an association with the other party
   (typically a server).  Authentication is the process of generating,
   transmitting, and verifying these credentials and thus the identity
   they assert.  An authentication identity is the name presented in a
   credential.

   There are many forms of authentication credentials -- the form used
   depends upon the particular authentication mechanism negotiated by the
   parties.  For example: X.509 certificates, Kerberos tickets, simple
   identity and password pairs.  Note that an authentication mechanism may
   constrain the form of authentication identities used with it.

5.4.  Authorization Identity

   An authorization identity is one kind of access control factor.  It is
   the name of the user or other entity that requests that operations be
   performed.  Access control policies are often expressed in terms of
   authorization identities; e.g., entity X can perform operation Y on
   resource Z.

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   The authorization identity bound to an association is often exactly the
   same as the authentication identity presented by the client, but it may
   be different.  SASL allows clients to specify an authorization identity
   distinct from the authentication identity asserted by the client's
   credentials.  This permits agents such as proxy servers to authenticate
   using their own credentials, yet request the access privileges of the
   identity for which they are proxying [SASL].  Also, the form of
   authentication identity supplied by a service like TLS may not
   correspond to the authorization identities used to express a server's
   access control  policy, requiring a server-specific mapping to be done.
   The method by which a server composes and validates an authorization
   identity from the authentication credentials supplied by a client is
   implementation-specific.

6. Required security mechanisms

   It is clear that allowing any implementation, faced with the above
   requirements, to pick and choose among the possible alternatives
   is not a strategy that is likely to lead to interoperability. In
   the absence of mandates, clients will be written that do not
   support any security function supported by the server, or worse,
   support only mechanisms like cleartext passwords that provide
   clearly inadequate security.

   Active intermediary attacks are the most difficult for an attacker
   to perform, and for an implementation to protect against.  Methods
   that protect only against hostile client and passive eavesdropping
   attacks are useful in situations where the cost of protection
   against active intermediary attacks is not justified based on the
   perceived risk of active intermediary attacks.

   Given the presence of the Directory, there is a strong desire to
   see mechanisms where identities take the form of a Distinguished
   Name and authentication data can be stored in the directory; this
   means that either this data is useless for faking authentication
   (like the Unix "/etc/passwd" file format used to be), or its
   content is never passed across the wire unprotected - that is,
   it's either updated outside the protocol or it is only updated in
   sessions well protected against snooping.  It is also desirable
   to allow authentication methods to carry authorization identities
   based on existing forms of user identities for backwards compatibility
   with non-LDAP-based authentication services.

   At the moment, only implementations using public key cryptography
   satisfy the requirement that data stored in the directory be
   insufficient for faking authentication.






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   Therefore, the following implementation conformance requirements
   are in place:

     (1)   For a read-only, public directory, anonymous authentication,
           described in section 7, can be used.

     (2)   Implementations providing password-based authenticated access
           MUST support authentication using CRAM-MD5, as described in
           section 8.1.  This provides client authentication with
           protection against passive eavesdropping attacks, but does
           not provide protection against active intermediary attacks.

     (3)   For a directory needing session protection and
           authentication, The Start TLS extended operation, and either
           the simple authentication choice or the SASL EXTERNAL
           mechanism, are to be used together.  Implementations SHOULD
           support authentication with a password as described in
           section 8.2, and SHOULD support authentication with a
           certificate as described in section 9.1.  Together, these
           can provide integrity and disclosure protection of
           transmitted data, and authentication of client and server,
           including protection against active intermediary attacks.

7. Anonymous authentication

   Directory operations which modify entries or access protected
   attributes or entries generally require client authentication.
   Clients which do not intend to perform any of these operations
   typically use anonymous authentication.

   LDAP implementations MUST support anonymous authentication, as
   defined in section 7.1.

   LDAP implementations MAY support anonymous authentication with TLS,
   as defined in section 7.2.

   While there MAY be access control restrictions to prevent access to
   directory entries, an LDAP server MUST allow an anonymously-bound
   client to retrieve the supportedSASLMechanisms attribute of the root
   DSE.

   An LDAP server MAY use other information about the client provided
   by the lower layers or external means to grant or deny access even
   to anonymously authenticated clients.

7.1. Anonymous authentication procedure

   An LDAP client which has not successfully completed a bind operation
   on a connection is anonymously authenticated.



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   An LDAP client MAY also specify anonymous authentication in a bind
   request by using a zero-length OCTET STRING with the simple
   authentication choice.

7.2. Anonymous authentication and TLS

   An LDAP client MAY use the Start TLS operation [5] to negotiate the
   use of TLS security [6].  If the client has not bound beforehand,
   then until the client uses the EXTERNAL SASL mechanism to negotiate
   the recognition of the client's certificate, the client is
   anonymously authenticated.

   Recommendations on TLS ciphersuites is given in section 12.

   An LDAP server which requests that clients provide their certificate
   during TLS negotiation MAY use a local security policy to determine
   whether to successfully complete TLS negotiation if the client did not
   present a certificate which could be validated.

8. Password-based authentication

   LDAP implementations MUST support authentication with a password using
   the CRAM-MD5 mechanism for password protection, as defined in section
   8.1.

   LDAP implementations SHOULD support authentication with the "simple"
   password choice when the connection is protected against eavesdropping
   using TLS, as defined in section 8.2.

   LDAP implementations MAY also support authentication with the
   "CRAM-MD5" authentication choice when the connection is protected
   using TLS, as defined in section 8.3.

8.1. CRAM-MD5 authentication

   A client which has a password available to the directory (e.g. stored
   in the userPassword attribute of the client's directory entry) MAY
   authenticate to the directory by performing a protected password
   bind sequence based on the CRAM-MD5 mechanism [4].

   An LDAP client may determine whether the server supports this
   mechanism by performing a search request on the root DSE, requesting
   the supportedSASLMechanisms attribute, and checking whether the
   string "CRAM-MD5" is present as a value of this attribute.

   In the first stage of authentication, the client sends a bind
   request in which the version number is 3, the name field is the name
   of the user's entry (if known to the client), the authentication choice
   is sasl, the sasl mechanism name is "CRAM-MD5", and the credentials
   are absent.  The client then waits for a response from the server to
   this request.

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   The server shall generate a challenge and return a bind response in
   which the resultCode is saslBindInProgress, and the serverSaslCreds
   field is present.  The contents of the serverSaslCreds string is
   the challenge, which is not base64 encoded.  An example challenge is
   "<1896.697170952@postoffice.reston.mci.net>".  Note that in this
   stage of the mechanism, the server need not access the user's password.
   The server will save the challenge internally, associated with the
   connection, until the next stage of the bind operation is completed.
   The challenge string MUST NOT be reused.

   Upon receipt of the challenge, the client will generate the response
   digest value, which is a string of 32 hexadecimal digits.  An
   example digest derived from the above challenge and the password
   "tanstaaftanstaaf" is "b913a602c7eda7a495b4e6e7334d3890". The client
   will send a bind request, with a different message id, in which the
   version number is 3, the name field is the name of the user's entry
   (if known), the authentication choice is sasl, the sasl mechanism name
   is "CRAM-MD5", and the credentials field contains a concatenation of
   the user's authorization identity (of the authzId form defined in
   section 11), a space character (ASCII 32), and the digest value. An
   example of the credentials field would be:

   dn: cn=J Smith\, Esq.,dc=acme,dc=com b913a602c7eda7a495b4e6e7334d3890

   The client then will wait for another response from the server.

   If the server performs password authentication based on the
   userPassword attribute, the server will then, for each value of the
   userPassword attribute in the named user's entry, generate the digest
   value itself, and compare the result with the client's presented
   digest.  A similar process can be used by servers which maintain
   password through other means.

   If there is a match, then the server will respond with resultCode
   success, otherwise the server will respond with resultCode
   invalidCredentials.  The serverSaslCreds field will be absent.

   The server will delete the challenge from memory when the SASL
   negotiation completes, or if the client does not complete the SASL
   negotiation, as challenge strings MUST never be used twice.  A client
   MUST NOT send more than one bind request containing response digest
   values in which the same challenge string was used.  If a client
   wishes to change authentication, it MUST start from the beginning
   and request a new challenge.

8.2. "simple" authentication choice under encryption

   A user who has a directory entry containing a userPassword attribute
   MAY authenticate to the directory by performing a simple password
   bind sequence following the negotiation of a TLS ciphersuite
   providing connection confidentiality [6].

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   The client will use the Start TLS operation [5] to negotiate the
   use of TLS security [6] on the connection to the LDAP server.  The
   client need not have bound to the directory beforehand.

   For this authentication procedure to be successful, the client and
   server MUST negotiate a ciphersuite which contains a bulk encryption
   algorithm of appropriate strength.  Recommendations on cipher
   suites are given in section 12.

   Following the successful completion of TLS negotiation, the client
   MUST send an LDAP bind request with the version number of 3, the
   name field containing the name of the user's entry, and the "simple"
   authentication choice, containing a password.

   The server will, for each value of the userPassword attribute in
   the named user's entry, compare these for case-sensitive equality
   with the client's presented password.  If there is a match, then the
   server will respond with resultCode success, otherwise the server will
   respond with resultCode invalidCredentials.

8.3. CRAM-MD5 authentication choice under encryption

   It is also possible to perform CRAM-MD5 authentication following
   the negotiation of TLS.  The client and server need not negotiate a
   ciphersuite which provides confidentiality if the only service
   required is data integrity.

9. Certificate-based authentication

   LDAP implementations SHOULD support authentication via a client
   certificate in TLS, as defined in section 9.1.

9.1. Certificate-based authentication with TLS

   A user who has a public/private key pair in which the public key has
   been signed by a Certification Authority may use this key pair to
   authenticate to the directory server if the user's certificate is
   requested by the server.  The user's certificate subject field
   SHOULD be the name of the user's directory entry, and the
   Certification Authority must be sufficiently trusted by the
   directory server to have issued the certificate in order that the
   server can process the certificate.  The means by which servers
   validate certificate paths is outside the scope of this document.

   A server MAY support mappings for certificates in which the subject
   field name is different from the name of the user's directory entry.
   A server which supports mappings of names MUST be capable of being
   configured to support certificates for which no mapping is required.




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   The client will use the Start TLS operation [5] to negotiate the
   use of TLS security [6] on the connection to the LDAP server.  The
   client need not have bound to the directory beforehand.

   In the TLS negotiation, the server MUST request a certificate.  The
   client will provide its certificate to the server, and MUST perform
   a private key-based encryption, proving it has it private key
   associated with the certificate.

   As deployments will require protection of sensitive data in transit,
   the client and server MUST negotiate a ciphersuite which contains a
   bulk encryption algorithm of appropriate strength.  Recommendations
   of cipher suites are given in section 12.

   The server MUST verify that the client's certificate is valid.
   The server will normally check that the certificate is issued by a
   known CA, and that none of the certificates on the client's
   certificate chain are invalid or revoked.  There are several
   procedures by which the server can perform these checks.

   Following the successful completion of TLS negotiation, the client
   will send an LDAP bind request with the SASL "EXTERNAL" mechanism.

10. Other mechanisms

   The LDAP "simple" authentication choice is not suitable for
   authentication on the Internet where there is no network or transport
   layer confidentiality.

   As LDAP includes a native anonymous and plaintext authentication
   methods, the "ANONYMOUS" and "PLAIN" SASL mechanisms are not used
   with LDAP.  If an authorization identity of a form different from
   a DN is requested by the client, the CRAM-MD5 mechanism can be used.

   The following SASL-based mechanisms are not considered in this
   document: KERBEROS_V4, GSSAPI and SKEY.

   The "EXTERNAL" SASL mechanism can be used to request the LDAP server
   make use of security credentials exchanged by a lower layer. If a
   TLS session has not been established between the client and server
   prior to making the SASL EXTERNAL Bind request and there is no other
   external source of authentication credentials (e.g.  IP-level
   security RFC 1825), or if, during the process of establishing the
   TLS session, the server did not request the client's authentication
   credentials, the SASL EXTERNAL bind MUST fail with a result code of
   inappropriateAuthentication. Any authentication identity and
   authorization identity, as well as TLS connection, which were in
   effect prior to making the Bind request, MUST remain in force.




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11. Authorization Identity

   The authorization identity is carried as part of the SASL credentials
   field in the LDAP Bind request and response.

   When the "EXTERNAL" mechanism is being negotiated, if the
   credentials field is present, it contains an authorization identity
   of the authzId form described below.

   When the "CRAM-MD5" mechanism is being negotiated, if the client's
   credentials field is present, it contains a concatenation of an
   authorization identity of the authzId form, a space character, and
   the digest string.

   Other mechanisms define the location of the authorization
   identity in the credentials field.

   The authorization identity is a string in the UTF-8 character set,
   corresponding to the following ABNF [7]:

   ; Specific predefined authorization (authz) id schemes are
   ; defined below -- new schemes may be defined in the future.

   authzId    = dnAuthzId / uAuthzId

   ; distinguished-name-based authz id.
   dnAuthzId  = "dn:" dn
   dn         = utf8string    ; with syntax defined in RFC 2253

   ; unspecified userid, UTF-8 encoded.
   uAuthzId   = "u:" userid
   userid     = utf8string    ; syntax unspecified

   A utf8string is defined to be the UTF-8 encoding of one or more
   ISO 10646 characters.

   All servers which support the storage of authentication credentials,
   such as passwords or certificates, in the directory MUST support the
   dnAuthzId choice.

   The uAuthzId choice allows for compatibility with client applications
   which wish to authenticate to a local directory but do not know their
   own Distinguished Name or have a directory entry.  The format of the
   string is defined as only a sequence of UTF-8 encoded ISO 10646
   characters, and further interpretation is subject to prior agreement
   between the client and server.

   For example, the userid could identify a user of a specific directory
   service, or be a login name or the local-part of an RFC 822 email
   address. In general a uAuthzId MUST NOT be assumed to be globally
   unique.

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   Additional authorization identity schemes MAY be defined in future
   versions of this document.

12. TLS Ciphersuites

   The following ciphersuites defined in [6] MUST NOT be used for
   confidentiality protection of passwords or data:

         TLS_NULL_WITH_NULL_NULL
         TLS_RSA_WITH_NULL_MD5
         TLS_RSA_WITH_NULL_SHA

   The following ciphersuites defined in [6] can be cracked easily
   (less than a week of CPU time on a standard CPU in 1997).  The
   client and server SHOULD carefully consider the value of the
   password or data being protected before using these ciphersuites:

         TLS_RSA_EXPORT_WITH_RC2_40_MD5
         TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5
         TLS_RSA_EXPORT_WITH_DES40_CBC_SHA
         TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA
         TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA
         TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA
         TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
         TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
         TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA

   The following ciphersuites are vulnerable to man-in-the-middle
   attacks, and SHOULD NOT be used to protect passwords or sensitive
   data, unless the network configuration is such that the danger of
   a man-in-the-middle attack is tolerable:

         TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
         TLS_DH_anon_WITH_RC4_128_MD5
         TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA
         TLS_DH_anon_WITH_DES_CBC_SHA
         TLS_DH_anon_WITH_3DES_EDE_CBC_SHA

   The RECOMMENDED ciphersuite is TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA.

13. Security Considerations

   Security issues are discussed throughout this memo; the
   (unsurprising) conclusion is that mandatory security is important,
   and that session encryption is required when snooping is a
   problem.






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   Servers are encouraged to prevent DIT modifications by anonymous
   users. Servers may also wish to minimize denial of service attacks
   by timing out idle connections, and returning the unwillingToPerform
   result code rather than performing computationally expensive
   operations requested by unauthorized clients.

   A connection on which the client has not performed the Start TLS
   operation or negotiated a suitable SASL mechanism for connection
   integrity and encryption services is subject to man-in-the-middle
   attacks to view and modify information in transit.

   Additional security considerations relating to the CRAM-MD5
   mechanism can be found in [4], and security considerations relating
   to the EXTERNAL mechanism to negotiate TLS can be found in [2], [5]
   and [6].

14. Acknowledgements

   This document is a product of the LDAPEXT Working Group of the
   IETF.  The contributions of its members is greatly appreciated.

15. Bibliography

   [1] M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access
       Protocol (v3)", Dec. 1997, RFC 2251.

   [2] J. Myers, "Simple Authentication and Security Layer (SASL)",
       Oct. 1997, RFC 2222.

   [3] S. Bradner, "Key words for use in RFCs to Indicate Requirement
       Levels", RFC 2119.

   [4] J. Klensin, R. Catoe, P. Krumviede, "IMAP/POP AUTHorize
       Extension for Simple Challenge/Response", Sep. 1997, RFC 2195.

   [5] J. Hodges, RL Morgan, M. Wahl, "LDAPv3 Extension for Transport
       Layer Security", INTERNET DRAFT
       <draft-ietf-ldapext-ldapv3-tls-00.txt>.

   [6] T. Diers, C. Allen, "The TLS Protocol Version 1.0", Oct. 1997,
       INTERNET DRAFT <draft-ietf-tls-protocol-04.txt>.

   [7] D. Crocker, Ed., P. Overell, "Augmented BNF for Syntax
       Specifications: ABNF", RFC 2234.








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16. Authors Address

    Mark Wahl
    Innosoft International, Inc.
    8911 Capital of Texas Hwy, Suite 4140
    Austin, TX 78759
    USA

    Phone: +1 626 919 3600
    EMail:  Mark.Wahl@innosoft.com


    Harald Tveit Alvestrand
    EMail: Harald.Alvestrand@maxware.no


    Jeff Hodges
    Computing & Communication Services
    Stanford University
    Pine Hall
    241 Panama Street
    Stanford, CA 94305-4122
    USA

    Phone: +1-650-723-2452
    EMail: Jeff.Hodges@Stanford.edu


    RL "Bob" Morgan
    Computing & Communication Services
    Stanford University
    Pine Hall
    241 Panama Street
    Stanford, CA 94305-4122
    USA

    Phone: +1-650-723-9711
    EMail: Bob.Morgan@Stanford.edu














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INTERNET-DRAFT        Authentication Methods for LDAP         July 1998

Full Copyright Statement

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