The Secure Shell (SSH) Protocol Architecture
RFC 4251

replay of data from prior sessions.  For example, the authentication
   protocol ([SSH-USERAUTH]) uses this to prevent replay of signatures
   from previous sessions.  Because public key authentication exchanges
   are cryptographically bound to the session (i.e., to the initial key
   exchange), they cannot be successfully replayed in other sessions.
   Note that the session id can be made public without harming the
   security of the protocol.

   If two sessions have the same session id (hash of key exchanges),
   then packets from one can be replayed against the other.  It must be
   stressed that the chances of such an occurrence are, needless to say,
   minimal when using modern cryptographic methods.  This is all the
   more true when specifying larger hash function outputs and DH
   parameters.

   Replay detection using monotonically increasing sequence numbers as
   input to the MAC, or HMAC in some cases, is described in [RFC2085],
   [RFC2246], [RFC2743], [RFC1964], [RFC2025], and [RFC4120].  The
   underlying construct is discussed in [RFC2104].  Essentially, a
   different sequence number in each packet ensures that at least this
   one input to the MAC function will be unique and will provide a
   nonrecurring MAC output that is not predictable to an attacker.  If
   the session stays active long enough, however, this sequence number
   will wrap.  This event may provide an attacker an opportunity to
   replay a previously recorded packet with an identical sequence number
   but only if the peers have not rekeyed since the transmission of the
   first packet with that sequence number.  If the peers have rekeyed,
   then the replay will be detected since the MAC check will fail.  For
   this reason, it must be emphasized that peers MUST rekey before a
   wrap of the sequence numbers.  Naturally, if an attacker does attempt
   to replay a captured packet before the peers have rekeyed, then the
   receiver of the duplicate packet will not be able to validate the MAC
   and it will be discarded.  The reason that the MAC will fail is
   because the receiver will formulate a MAC based upon the packet
   contents, the shared secret, and the expected sequence number.  Since
   the replayed packet will not be using that expected sequence number
   (the sequence number of the replayed packet will have already been
   passed by the receiver), the calculated MAC will not match the MAC
   received with the packet.

9.3.4.  Man-in-the-middle

   This protocol makes no assumptions or provisions for an
   infrastructure or means for distributing the public keys of hosts.
   It is expected that this protocol will sometimes be used without
   first verifying the association between the server host key and the
   server host name.  Such usage is vulnerable to man-in-the-middle
   attacks.  This section describes this and encourages administrators

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   and users to understand the importance of verifying this association
   before any session is initiated.

   There are three cases of man-in-the-middle attacks to consider.  The
   first is where an attacker places a device between the client and the
   server before the session is initiated.  In this case, the attack
   device is trying to mimic the legitimate server and will offer its
   public key to the client when the client initiates a session.  If it
   were to offer the public key of the server, then it would not be able
   to decrypt or sign the transmissions between the legitimate server
   and the client unless it also had access to the private key of the
   host.  The attack device will also, simultaneously to this, initiate
   a session to the legitimate server, masquerading itself as the
   client.  If the public key of the server had been securely
   distributed to the client prior to that session initiation, the key
   offered to the client by the attack device will not match the key
   stored on the client.  In that case, the user SHOULD be given a
   warning that the offered host key does not match the host key cached
   on the client.  As described in Section 4.1, the user may be free to
   accept the new key and continue the session.  It is RECOMMENDED that
   the warning provide sufficient information to the user of the client
   device so the user may make an informed decision.  If the user
   chooses to continue the session with the stored public key of the
   server (not the public key offered at the start of the session), then
   the session-specific data between the attacker and server will be
   different between the client-to-attacker session and the attacker-
   to-server sessions due to the randomness discussed above.  From this,
   the attacker will not be able to make this attack work since the
   attacker will not be able to correctly sign packets containing this
   session-specific data from the server, since he does not have the
   private key of that server.

   The second case that should be considered is similar to the first
   case in that it also happens at the time of connection, but this case
   points out the need for the secure distribution of server public
   keys.  If the server public keys are not securely distributed, then
   the client cannot know if it is talking to the intended server.  An
   attacker may use social engineering techniques to pass off server
   keys to unsuspecting users and may then place a man-in-the-middle
   attack device between the legitimate server and the clients.  If this
   is allowed to happen, then the clients will form client-to-attacker
   sessions, and the attacker will form attacker-to-server sessions and
   will be able to monitor and manipulate all of the traffic between the
   clients and the legitimate servers.  Server administrators are
   encouraged to make host key fingerprints available for checking by
   some means whose security does not rely on the integrity of the
   actual host keys.  Possible mechanisms are discussed in Section 4.1
   and may also include secured Web pages, physical pieces of paper,

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   etc.  Implementers SHOULD provide recommendations on how best to do
   this with their implementation.  Because the protocol is extensible,
   future extensions to the protocol may provide better mechanisms for
   dealing with the need to know the server's host key before
   connecting.  For example, making the host key fingerprint available
   through a secure DNS lookup, or using Kerberos ([RFC4120]) over
   GSS-API ([RFC1964]) during key exchange to authenticate the server
   are possibilities.

   In the third man-in-the-middle case, attackers may attempt to
   manipulate packets in transit between peers after the session has
   been established.  As described in Section 9.3.3, a successful attack
   of this nature is very improbable.  As in Section 9.3.3, this
   reasoning does assume that the MAC is secure and that it is
   infeasible to construct inputs to a MAC algorithm to give a known
   output.  This is discussed in much greater detail in Section 6 of
   [RFC2104].  If the MAC algorithm has a vulnerability or is weak
   enough, then the attacker may be able to specify certain inputs to
   yield a known MAC.  With that, they may be able to alter the contents
   of a packet in transit.  Alternatively, the attacker may be able to
   exploit the algorithm vulnerability or weakness to find the shared
   secret by reviewing the MACs from captured packets.  In either of
   those cases, an attacker could construct a packet or packets that
   could be inserted into an SSH stream.  To prevent this, implementers
   are encouraged to utilize commonly accepted MAC algorithms, and
   administrators are encouraged to watch current literature and
   discussions of cryptography to ensure that they are not using a MAC
   algorithm that has a recently found vulnerability or weakness.

   In summary, the use of this protocol without a reliable association
   of the binding between a host and its host keys is inherently
   insecure and is NOT RECOMMENDED.  However, it may be necessary in
   non-security-critical environments, and will still provide protection
   against passive attacks.  Implementers of protocols and applications
   running on top of this protocol should keep this possibility in mind.

9.3.5.  Denial of Service

   This protocol is designed to be used over a reliable transport.  If
   transmission errors or message manipulation occur, the connection is
   closed.  The connection SHOULD be re-established if this occurs.
   Denial of service attacks of this type (wire cutter) are almost
   impossible to avoid.

   In addition, this protocol is vulnerable to denial of service attacks
   because an attacker can force the server to go through the CPU and
   memory intensive tasks of connection setup and key exchange without
   authenticating.  Implementers SHOULD provide features that make this

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   more difficult, for example, only allowing connections from a subset
   of clients known to have valid users.

9.3.6.  Covert Channels

   The protocol was not designed to eliminate covert channels.  For
   example, the padding, SSH_MSG_IGNORE messages, and several other
   places in the protocol can be used to pass covert information, and
   the recipient has no reliable way of verifying whether such
   information is being sent.

9.3.7.  Forward Secrecy

   It should be noted that the Diffie-Hellman key exchanges may provide
   perfect forward secrecy (PFS).  PFS is essentially defined as the
   cryptographic property of a key-establishment protocol in which the
   compromise of a session key or long-term private key after a given
   session does not cause the compromise of any earlier session
   [ANSI-T1.523-2001].  SSH sessions resulting from a key exchange using
   the diffie-hellman methods described in the section Diffie-Hellman
   Key Exchange of [SSH-TRANS] (including "diffie-hellman-group1-sha1"
   and "diffie-hellman-group14-sha1") are secure even if private
   keying/authentication material is later revealed, but not if the
   session keys are revealed.  So, given this definition of PFS, SSH
   does have PFS.  However, this property is not commuted to any of the
   applications or protocols using SSH as a transport.  The transport
   layer of SSH provides confidentiality for password authentication and
   other methods that rely on secret data.

   Of course, if the DH private parameters for the client and server are
   revealed, then the session key is revealed, but these items can be
   thrown away after the key exchange completes.  It's worth pointing
   out that these items should not be allowed to end up on swap space
   and that they should be erased from memory as soon as the key
   exchange completes.

9.3.8.  Ordering of Key Exchange Methods

   As stated in the section on Algorithm Negotiation of [SSH-TRANS],
   each device will send a list of preferred methods for key exchange.
   The most-preferred method is the first in the list.  It is
   RECOMMENDED that the algorithms be sorted by cryptographic strength,
   strongest first.  Some additional guidance for this is given in
   [RFC3766].

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9.3.9.  Traffic Analysis

   Passive monitoring of any protocol may give an attacker some
   information about the session, the user, or protocol specific
   information that they would otherwise not be able to garner.  For
   example, it has been shown that traffic analysis of an SSH session
   can yield information about the length of the password - [Openwall]
   and [USENIX].  Implementers should use the SSH_MSG_IGNORE packet,
   along with the inclusion of random lengths of padding, to thwart
   attempts at traffic analysis.  Other methods may also be found and
   implemented.

9.4.  Authentication Protocol

   The purpose of this protocol is to perform client user
   authentication.  It assumes that this runs over a secure transport
   layer protocol, which has already authenticated the server machine,
   established an encrypted communications channel, and computed a
   unique session identifier for this session.

   Several authentication methods with different security
   characteristics are allowed.  It is up to the server's local policy
   to decide which methods (or combinations of methods) it is willing to
   accept for each user.  Authentication is no stronger than the weakest
   combination allowed.

   The server may go into a sleep period after repeated unsuccessful
   authentication attempts to make key search more difficult for
   attackers.  Care should be taken so that this doesn't become a self-
   denial of service vector.

9.4.1.  Weak Transport

   If the transport layer does not provide confidentiality,
   authentication methods that rely on secret data SHOULD be disabled.
   If it does not provide strong integrity protection, requests to
   change authentication data (e.g., a password change) SHOULD be
   disabled to prevent an attacker from modifying the ciphertext without
   being noticed, or rendering the new authentication data unusable
   (denial of service).

   The assumption stated above, that the Authentication Protocol only
   runs over a secure transport that has previously authenticated the
   server, is very important to note.  People deploying SSH are reminded
   of the consequences of man-in-the-middle attacks if the client does
   not have a very strong a priori association of the server with the
   host key of that server.  Specifically, for the case of the
   Authentication Protocol, the client may form a session to a man-in-

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   the-middle attack device and divulge user credentials such as their
   username and password.  Even in the cases of authentication where no
   user credentials are divulged, an attacker may still gain information
   they shouldn't have by capturing key-strokes in much the same way
   that a honeypot works.

9.4.2.  Debug Messages

   Special care should be taken when designing debug messages.  These
   messages may reveal surprising amounts of information about the host
   if not properly designed.  Debug messages can be disabled (during
   user authentication phase) if high security is required.
   Administrators of host machines should make all attempts to
   compartmentalize all event notification messages and protect them
   from unwarranted observation.  Developers should be aware of the
   sensitive nature of some of the normal event and debug messages, and
   may want to provide guidance to administrators on ways to keep this
   information away from unauthorized people.  Developers should
   consider minimizing the amount of sensitive information obtainable by
   users during the authentication phase, in accordance with the local
   policies.  For this reason, it is RECOMMENDED that debug messages be
   initially disabled at the time of deployment and require an active
   decision by an administrator to allow them to be enabled.  It is also
   RECOMMENDED that a message expressing this concern be presented to
   the administrator of a system when the action is taken to enable
   debugging messages.

9.4.3.  Local Security Policy

   The implementer MUST ensure that the credentials provided validate
   the professed user and also MUST ensure that the local policy of the
   server permits the user the access requested.  In particular, because
   of the flexible nature of the SSH connection protocol, it may not be
   possible to determine the local security policy, if any, that should
   apply at the time of authentication because the kind of service being
   requested is not clear at that instant.  For example, local policy
   might allow a user to access files on the server, but not start an
   interactive shell.  However, during the authentication protocol, it
   is not known whether the user will be accessing files, attempting to
   use an interactive shell, or even both.  In any event, where local
   security policy for the server host exists, it MUST be applied and
   enforced correctly.

   Implementers are encouraged to provide a default local policy and
   make its parameters known to administrators and users.  At the
   discretion of the implementers, this default policy may be along the
   lines of anything-goes where there are no restrictions placed upon
   users, or it may be along the lines of excessively-restrictive, in

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   which case, the administrators will have to actively make changes to
   the initial default parameters to meet their needs.  Alternatively,
   it may be some attempt at providing something practical and
   immediately useful to the administrators of the system so they don't
   have to put in much effort to get SSH working.  Whatever choice is
   made must be applied and enforced as required above.

9.4.4  Public Key Authentication

   The use of public key authentication assumes that the client host has
   not been compromised.  It also assumes that the private key of the
   server host has not been compromised.

   This risk can be mitigated by the use of passphrases on private keys;
   however, this is not an enforceable policy.  The use of smartcards,
   or other technology to make passphrases an enforceable policy is
   suggested.

   The server could require both password and public key authentication;
   however, this requires the client to expose its password to the
   server (see the section on Password Authentication below.)

9.4.5.  Password Authentication

   The password mechanism, as specified in the authentication protocol,
   assumes that the server has not been compromised.  If the server has
   been compromised, using password authentication will reveal a valid
   username/password combination to the attacker, which may lead to
   further compromises.

   This vulnerability can be mitigated by using an alternative form of
   authentication.  For example, public key authentication makes no
   assumptions about security on the server.

9.4.6.  Host-Based Authentication

   Host-based authentication assumes that the client has not been
   compromised.  There are no mitigating strategies, other than to use
   host-based authentication in combination with another authentication
   method.

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9.5.  Connection Protocol

9.5.1.  End Point Security

   End point security is assumed by the connection protocol.  If the
   server has been compromised, any terminal sessions, port forwarding,
   or systems accessed on the host are compromised.  There are no
   mitigating factors for this.

   If the client has been compromised, and the server fails to stop the
   attacker at the authentication protocol, all services exposed (either
   as subsystems or through forwarding) will be vulnerable to attack.
   Implementers SHOULD provide mechanisms for administrators to control
   which services are exposed to limit the vulnerability of other
   services.  These controls might include controlling which machines
   and ports can be targeted in port-forwarding operations, which users
   are allowed to use interactive shell facilities, or which users are
   allowed to use exposed subsystems.

9.5.2.  Proxy Forwarding

   The SSH connection protocol allows for proxy forwarding of other
   protocols such as SMTP, POP3, and HTTP.  This may be a concern for
   network administrators who wish to control the access of certain
   applications by users located outside of their physical location.
   Essentially, the forwarding of these protocols may violate site-
   specific security policies, as they may be undetectably tunneled
   through a firewall.  Implementers SHOULD provide an administrative
   mechanism to control the proxy forwarding functionality so that
   site-specific security policies may be upheld.

   In addition, a reverse proxy forwarding functionality is available,
   which, again, can be used to bypass firewall controls.

   As indicated above, end-point security is assumed during proxy
   forwarding operations.  Failure of end-point security will compromise
   all data passed over proxy forwarding.

9.5.3.  X11 Forwarding

   Another form of proxy forwarding provided by the SSH connection
   protocol is the forwarding of the X11 protocol.  If end-point
   security has been compromised, X11 forwarding may allow attacks
   against the X11 server.  Users and administrators should, as a matter
   of course, use appropriate X11 security mechanisms to prevent
   unauthorized use of the X11 server.  Implementers, administrators,
   and users who wish to further explore the security mechanisms of X11
   are invited to read [SCHEIFLER] and analyze previously reported

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   problems with the interactions between SSH forwarding and X11 in CERT
   vulnerabilities VU#363181 and VU#118892 [CERT].

   X11 display forwarding with SSH, by itself, is not sufficient to
   correct well known problems with X11 security [VENEMA].  However, X11
   display forwarding in SSH (or other secure protocols), combined with
   actual and pseudo-displays that accept connections only over local
   IPC mechanisms authorized by permissions or access control lists
   (ACLs), does correct many X11 security problems, as long as the
   "none" MAC is not used.  It is RECOMMENDED that X11 display
   implementations default to allow the display to open only over local
   IPC.  It is RECOMMENDED that SSH server implementations that support
   X11 forwarding default to allow the display to open only over local
   IPC.  On single-user systems, it might be reasonable to default to
   allow the local display to open over TCP/IP.

   Implementers of the X11 forwarding protocol SHOULD implement the
   magic cookie access-checking spoofing mechanism, as described in
   [SSH-CONNECT], as an additional mechanism to prevent unauthorized use
   of the proxy.

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10.  References

10.1.  Normative References

   [SSH-TRANS]        Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
                      (SSH) Transport Layer Protocol", RFC 4253, January
                      2006.

   [SSH-USERAUTH]     Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
                      (SSH) Authentication Protocol", RFC 4252, January
                      2006.

   [SSH-CONNECT]      Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
                      (SSH) Connection Protocol", RFC 4254, January
                      2006.

   [SSH-NUMBERS]      Lehtinen, S. and C. Lonvick, Ed., "The Secure
                      Shell (SSH) Protocol Assigned Numbers", RFC 4250,
                      January 2006.

   [RFC2119]          Bradner, S., "Key words for use in RFCs to
                      Indicate Requirement Levels", BCP 14, RFC 2119,
                      March 1997.

   [RFC2434]          Narten, T. and H. Alvestrand, "Guidelines for
                      Writing an IANA Considerations Section in RFCs",
                      BCP 26, RFC 2434, October 1998.

   [RFC3066]          Alvestrand, H., "Tags for the Identification of
                      Languages", BCP 47, RFC 3066, January 2001.

   [RFC3629]          Yergeau, F., "UTF-8, a transformation format of
                      ISO 10646", STD 63, RFC 3629, November 2003.

10.2.  Informative References

   [RFC0822]          Crocker, D., "Standard for the format of ARPA
                      Internet text messages", STD 11, RFC 822, August
                      1982.

   [RFC0854]          Postel, J. and J. Reynolds, "Telnet Protocol
                      Specification", STD 8, RFC 854, May 1983.

   [RFC1034]          Mockapetris, P., "Domain names - concepts and
                      facilities", STD 13, RFC 1034, November 1987.

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RFC 4251               SSH Protocol Architecture            January 2006

   [RFC1282]          Kantor, B., "BSD Rlogin", RFC 1282, December 1991.

   [RFC4120]          Neuman, C., Yu, T., Hartman, S., and K. Raeburn,
                      "The Kerberos Network Authentication Service
                      (V5)", RFC 4120, July 2005.

   [RFC1964]          Linn, J., "The Kerberos Version 5 GSS-API
                      Mechanism", RFC 1964, June 1996.

   [RFC2025]          Adams, C., "The Simple Public-Key GSS-API
                      Mechanism (SPKM)", RFC 2025, October 1996.

   [RFC2085]          Oehler, M. and R. Glenn, "HMAC-MD5 IP
                      Authentication with Replay Prevention", RFC 2085,
                      February 1997.

   [RFC2104]          Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
                      Keyed-Hashing for Message Authentication", RFC
                      2104, February 1997.

   [RFC2246]          Dierks, T. and C. Allen, "The TLS Protocol Version
                      1.0", RFC 2246, January 1999.

   [RFC2410]          Glenn, R. and S. Kent, "The NULL Encryption
                      Algorithm and Its Use With IPsec", RFC 2410,
                      November 1998.

   [RFC2743]          Linn, J., "Generic Security Service Application
                      Program Interface Version 2, Update 1", RFC 2743,
                      January 2000.

   [RFC3766]          Orman, H. and P. Hoffman, "Determining Strengths
                      For Public Keys Used For Exchanging Symmetric
                      Keys", BCP 86, RFC 3766, April 2004.

   [RFC4086]          Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                      "Randomness Requirements for Security", BCP 106,
                      RFC 4086, June 2005.

   [FIPS-180-2]       US National Institute of Standards and Technology,
                      "Secure Hash Standard (SHS)", Federal Information
                      Processing Standards Publication 180-2, August
                      2002.

   [FIPS-186-2]       US National Institute of Standards and Technology,
                      "Digital Signature Standard (DSS)", Federal
                      Information Processing Standards Publication 186-
                      2, January 2000.

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   [FIPS-197]         US National Institute of Standards and Technology,
                      "Advanced Encryption Standard (AES)", Federal
                      Information Processing Standards Publication 197,
                      November 2001.

   [ANSI-T1.523-2001] American National Standards Institute, Inc.,
                      "Telecom Glossary 2000", ANSI T1.523-2001,
                      February 2001.

   [SCHNEIER]         Schneier, B., "Applied Cryptography Second
                      Edition:  protocols algorithms and source in code
                      in C", John Wiley and Sons, New York, NY, 1996.

   [SCHEIFLER]        Scheifler, R., "X Window System : The Complete
                      Reference to Xlib, X Protocol, Icccm, Xlfd, 3rd
                      edition.", Digital Press, ISBN 1555580882,
                      February 1992.

   [KAUFMAN]          Kaufman, C., Perlman, R., and M. Speciner,
                      "Network Security: PRIVATE Communication in a
                      PUBLIC World", Prentice Hall Publisher, 1995.

   [CERT]             CERT Coordination Center, The.,
                      "http://www.cert.org/nav/index_red.html".

   [VENEMA]           Venema, W., "Murphy's Law and Computer Security",
                      Proceedings of 6th USENIX Security Symposium, San
                      Jose CA
                      http://www.usenix.org/publications/library/
                      proceedings/sec96/venema.html, July 1996.

   [ROGAWAY]          Rogaway, P., "Problems with Proposed IP
                      Cryptography", Unpublished paper
                      http://www.cs.ucdavis.edu/~rogaway/ papers/draft-
                      rogaway-ipsec-comments-00.txt, 1996.

   [DAI]              Dai, W., "An attack against SSH2 protocol", Email
                      to the SECSH Working Group ietf-ssh@netbsd.org
                      ftp:// ftp.ietf.org/ietf-mail-archive/secsh/2002-
                      02.mail, Feb 2002.

   [BELLARE]          Bellaire, M., Kohno, T., and C. Namprempre,
                      "Authenticated Encryption in SSH: Fixing the SSH
                      Binary Packet Protocol", Proceedings of the 9th
                      ACM Conference on Computer and Communications
                      Security, Sept 2002.

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   [Openwall]         Solar Designer and D. Song, "SSH Traffic Analysis
                      Attacks", Presentation given at HAL2001 and
                      NordU2002 Conferences, Sept 2001.

   [USENIX]           Song, X.D., Wagner, D., and X. Tian, "Timing
                      Analysis of Keystrokes and SSH Timing Attacks",
                      Paper given at 10th USENIX Security Symposium,
                      2001.

Authors' Addresses

   Tatu Ylonen
   SSH Communications Security Corp
   Valimotie 17
   00380 Helsinki
   Finland

   EMail: ylo@ssh.com

   Chris Lonvick (editor)
   Cisco Systems, Inc.
   12515 Research Blvd.
   Austin  78759
   USA

   EMail: clonvick@cisco.com

Trademark Notice

   "ssh" is a registered trademark in the United States and/or other
   countries.

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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
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