One document matched: draft-ietf-ldapext-authmeth-02.txt
Differences from draft-ietf-ldapext-authmeth-01.txt
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
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To view the entire list of current Internet-Drafts, please check
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(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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Full Copyright Statement
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Wahl, Alvestrand, Hodges, Morgan Page 15
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