rfc1826.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

4. CALCULATION OF THE AUTHENTICATION DATA

   The authentication data carried by the IP Authentication Header is
   usually calculated using a message digest algorithm (for example,
   MD5) either encrypting that message digest or keying the message
   digest directly [Riv92].  Only algorithms that are believed to be
   cryptographically strong one-way functions should be used with the IP
   Authentication Header.

   Because conventional checksums (e.g., CRC-16) are not
   cryptographically strong, they MUST NOT be used with the
   Authentication Header.

   When processing an outgoing IP packet for Authentication, the first
   step is for the sending system to locate the appropriate Security
   Association.  All Security Associations are unidirectional.  The
   selection of the appropriate Security Association for an outgoing IP
   packet is based at least upon the sending userid and the Destination
   Address.  When host-oriented keying is in use, all sending userids
   will share the same Security Association to a given destination.
   When user-oriented keying is in use, then different users or possibly
   even different applications of the same user might use different
   Security Associations.  The Security Association selected will



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   indicate which algorithm, algorithm mode, key, and other security
   properties apply to the outgoing packet.

   Fields which NECESSARILY are modified during transit from the sender
   to the receiver (e.g., TTL and HEADER CHECKSUM for IPv4 or Hop Limit
   for IPv6) and whose value at the receiver are not known with
   certainty by the sender are included in the authentication data
   calculation but are processed specially.  For these fields which are
   modified during transit, the value carried in the IP packet is
   replaced by the value zero for the purpose of the authentication
   calculation.  By replacing the field's value with zero rather than
   omitting these fields, alignment is preserved for the authentication
   calculation.

   The sender MUST compute the authentication over the packet as that
   packet will appear at the receiver.  This requirement is placed in
   order to allow for future IP optional headers which the receiver
   might not know about but the sender necessarily knows about if it is
   including such options in the packet.  This also permits the
   authentication of data that will vary in transit but whose value at
   the final receiver is known with certainty by the sender in advance.

   The sender places the calculated message digest algorithm output into
   the Authentication Data field within the Authentication Header.  For
   purposes of Authentication Data computation, the Authentication Data
   field is considered to be filled with zeros.

   The IPv4 "TIME TO LIVE" and "HEADER CHECKSUM" fields are the only
   fields in the IPv4 base header that are handled specially for the
   Authentication Data calculation.  Reassembly of fragmented packets
   occurs PRIOR to processing by the local IP Authentication Header
   implementation.  The "more" bit is of course cleared upon reassembly.
    Hence, no other fields in the IPv4 header will vary in transit from
   the perspective of the IP Authentication Header implementation.  The
   "TIME TO LIVE" and "HEADER CHECKSUM" fields of the IPv4 base header
   MUST be set to all zeros for the Authentication Data calculation.
   All other IPv4 base header fields are processed normally with their
   actual contents.  Because IPv4 packets are subject to intermediate
   fragmentation in routers, it is important that the reassembly of IPv4
   packets be performed prior to the Authentication Header processing.
   IPv4 Implementations SHOULD use Path MTU Discovery when the IP
   Authentication Header is being used [MD90].  For IPv4, not all
   options are openly specified in a RFC, so it is not possible to
   enumerate in this document all of the options that might normally be
   modified during transit.  The IP Security Option (IPSO) MUST be
   included in the Authentication Data calculation whenever that option
   is present in an IP datagram [Ken91].  If a receiving system does not
   recognise an IPv4 option that is present in the packet, that option



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   is included in the Authentication Data calculation.  This means that
   any IPv4 packet containing an IPv4 option that changes during transit
   in a manner not predictable by the sender and which IPv4 option is
   unrecognised by the receiver will fail the authentication check and
   consequently be dropped by the receiver.

   The IPv6 "HOP LIMIT" field is the only field in the IPv6 base header
   that is handled specially for Authentication Data calculation.  The
   value of the HOP LIMIT field is zero for the purpose of
   Authentication Data calculation.  All other fields in the base IPv6
   header MUST be included in the Authentication Data calculation using
   the normal procedures for calculating the Authentication Data.  All
   IPv6 "OPTION TYPE" values contain a bit which MUST be used to
   determine whether that option data will be included in the
   Authentication Data calculation.  This bit is the third-highest-order
   bit of the IPv6 OPTION TYPE field. If this bit is set to zero, then
   the corresponding option is included in the Authentication Data
   calculation.  If this bit is set to one, then the corresponding
   option is replaced by all zero bits of the same length as the option
   for the purpose of the Authentication Data calculation.  The IPv6
   Routing Header "Type 0" will rearrange the address fields within the
   packet during transit from source to destination.  However, this is
   not a problem because the contents of the packet as it will appear at
   the receiver are known to the sender and to all intermediate hops.
   Hence, the IPv6 Routing Header "Type 0" is included in the
   Authentication Data calculation using the normal procedure.

   Upon receipt of a packet containing an IP Authentication Header, the
   receiver first uses the Destination Address and SPI value to locate
   the correct Security Association.  The receiver then independently
   verifies that the Authentication Data field and the received data
   packet are consistent.  Again, the Authentication Data field is
   assumed to be zero for the sole purpose of making the authentication
   computation.  Exactly how this is accomplished is algorithm
   dependent.  If the processing of the authentication algorithm
   indicates the datagram is valid, then it is accepted.  If the
   algorithm determines that the data and the Authentication Header do
   not match, then the receiver SHOULD discard the received IP datagram
   as invalid and MUST record the authentication failure in the system
   log or audit log.  If such a failure occurs, the recorded log data
   MUST include the SPI value, date/time received, clear-text Sending
   Address, clear-text Destination Address, and (if it exists) the
   clear-text Flow ID.  The log data MAY also include other information
   about the failed packet.







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5. CONFORMANCE REQUIREMENTS

   Implementations that claim conformance or compliance with this
   specification MUST fully implement the header described here, MUST
   support manual key distribution for use with this option, MUST comply
   with all requirements of the "Security Architecture for the Internet
   Protocol" [Atk95a], and MUST support the use of keyed MD5 as
   described in the companion document entitled "IP Authentication using
   Keyed MD5" [MS95].  Implementations MAY also implement other
   authentication algorithms.  Implementors should consult the most
   recent version of the "IAB Official Standards" RFC for further
   guidance on the status of this document.

6. SECURITY CONSIDERATIONS

   This entire RFC discusses an authentication mechanism for IP.  This
   mechanism is not a panacea to the several security issues in any
   internetwork, however it does provide a component useful in building
   a secure internetwork.

   Users need to understand that the quality of the security provided by
   this specification depends completely on the strength of whichever
   cryptographic algorithm has been implemented, the strength of the key
   being used, the correctness of that algorithm's implementation, upon
   the security of the key management mechanism and its implementation,
   and upon the correctness of the IP Authentication Header and IP
   implementations in all of the participating systems. If any of these
   assumptions do not hold, then little or no real security will be
   provided to the user.  Implementors are encouraged to use high
   assurance methods to develop all of the security relevant parts of
   their products.

   Users interested in confidentiality should consider using the IP
   Encapsulating Security Payload (ESP) instead of or in conjunction
   with this specification [Atk95b].  Users seeking protection from
   traffic analysis might consider the use of appropriate link
   encryption.  Description and specification of link encryption is
   outside the scope of this note [VK83].  Users interested in combining
   the IP Authentication Header with the IP Encapsulating Security
   Payload should consult the IP Encapsulating Security Payload
   specification for details.

   One particular issue is that in some cases a packet which causes an
   error to be reported back via ICMP might be so large as not to
   entirely fit within the ICMP message returned.  In such cases, it
   might not be possible for the receiver of the ICMP message to
   independently authenticate the portion of the returned message.  This
   could mean that the host receiving such an ICMP message would either



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   trust an unauthenticated ICMP message, which might in turn create
   some security problem, or not trust and hence not react appropriately
   to some legitimate ICMP message that should have been reacted to.  It
   is not clear that this issue can be fully resolved in the presence of
   packets that are the same size as or larger than the minimum IP MTU.
   Similar complications arise if an encrypted packet causes an ICMP
   error message to be sent and that packet is truncated.

   Active attacks are now widely known to exist in the Internet [CER95].
   The presence of active attacks means that unauthenticated source
   routing, either unidirectional (receive-only) or with replies
   following the original received source route represents a significant
   security risk unless all received source routed packets are
   authenticated using the IP Authentication Header or some other
   cryptologic mechanism.  It is noteworthy that the attacks described
   in [CER95] include a subset of those described in [Bel89].

   The use of IP tunneling with AH creates multiple pairs of endpoints
   that might perform AH processing.  Implementers and administrators
   should carefully consider the impacts of tunneling on authenticity of
   the received tunneled packets.

ACKNOWLEDGEMENTS

   This document benefited greatly from work done by Bill Simpson, Perry
   Metzger, and Phil Karn to make general the approach originally
   defined by the author for SIP, SIPP, and finally IPv6.

   The basic concept here is derived in large part from the SNMPv2
   Security Protocol work described in [GM93].  Steve Bellovin, Steve
   Deering, Frank Kastenholz, Dave Mihelcic, and Hilarie Orman provided
   thoughtful critiques of early versions of this note.  Francis Dupont
   discovered and pointed out the security issue with ICMP in low IP MTU
   links that is noted just above.

REFERENCES

   [Atk95a] Atkinson, R., "Security Architecture for the Internet
            Protocol", RFC 1825, NRL, August 1995.

   [Atk95b] Atkinson, R., "IP Encapsulating Security Payload", RFC 1827,
            NRL, August 1995.

   [Bel89] Steven M. Bellovin, "Security Problems in the TCP/IP Protocol
           Suite", ACM Computer Communications Review, Vol. 19, No. 2,
           March 1989.





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   [BCCH94] Braden, R., Clark, D., Crocker, S., and C. Huitema, "Report
            of IAB Workshop on Security in the Internet Architecture",
            RFC 1636, USC/Information Sciences Institute, MIT, Trusted
            Information Systems, INRIA, June 1994, pp. 21-34.

   [CER95] Computer Emergency Response Team (CERT), "IP Spoofing Attacks
           and Hijacked Terminal Connections", CA-95:01, January 1995.
           Available via anonymous ftp from info.cert.org in
           /pub/cert_advisories.

   [GM93]  Galvin J., and K. McCloghrie, "Security Protocols for
           version 2 of the Simple Network Management Protocol
           (SNMPv2)", RFC 1446, Trusted Information Systems, Hughes LAN
           Systems, April 1993.

   [Hin94] Bob Hinden (Editor), Internet Protocol version 6 (IPv6)
           Specification, Work in Progress, October 1994.

   [Ken91] Kent, S., "US DoD Security Options for the Internet Protocol",
           RFC 1108, BBN Communications, November 1991.

   [Kno93] Knowles, Stev, "IESG Advice from Experience with Path MTU
           Discovery", RFC 1435, FTP Software, March 1993.

   [MS95]  Metzger, P., and W. Simpson, "IP Authentication with Keyed
           MD5", RFC 1828, Piermont, Daydreamer, August 1995.

   [MD90]  Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
           DECWRL, Stanford University, November 1990.

   [STD-2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
           RFC 1700, USC/Information Sciences Institute, October 1994.

   [Riv92] Rivest, R., "MD5 Digest Algorithm", RFC 1321, MIT and RSA Data
           Security, Inc., April 1992.

   [VK83]  V.L. Voydock & S.T. Kent, "Security Mechanisms in High-level
           Networks", ACM Computing Surveys, Vol. 15, No. 2, June 1983.













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DISCLAIMER

   The views and specification here are those of the author and are not
   necessarily those of his employer.  The Naval Research Laboratory has
   not passed judgement on the merits, if any, of this work.  The author
   and his employer specifically disclaim responsibility for any
   problems arising from correct or incorrect implementation or use of
   this specification.

AUTHOR INFORMATION

   Randall Atkinson
   Information Technology Division
   Naval Research Laboratory
   Washington, DC 20375-5320
   USA

   Phone:  [phone]
   Fax:    [phone]
   EMail:  [e-mail]































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