rfc2420.txt

来自「RFC 的详细文档！」· 文本 代码 · 共 452 行 · 第 1/2 页


RFC 2420               PPP Triple-DES Encryption          September 1998


              negotiated.

        Protocol ID

              The value of this field is 0x53 or 0x55; the latter
              indicates the use of the Individual Link Encryption
              Control Protocol and that the ciphertext contains a
              Multilink fragment.  Protocol Field Compression MAY be
              applied to the leading zero if negotiated.

        Sequence Number

              These 16-bit numbers are assigned by the encryptor
              sequentially starting with 0 (for the first packet
              transmitted once ECP has reached the opened state).

        Ciphertext

              The generation of this data is described in the next
              section.

4.  Encryption

   Once the ECP has reached the Opened state, the sender MUST NOT apply
   the encryption procedure to LCP packets nor ECP packets.

   If the async control character map option has been negotiated on the
   link, the sender applies mapping after the encryption algorithm has
   been run.

   The encryption algorithm is generally to pad the Protocol and
   Information fields of a PPP packet to some multiple of 8 bytes, and
   apply 3DES as described in section 1.1 with the three 56-bit keys k1,
   k2 and k3.

   The encryption procedure is only applied to that portion of the
   packet excluding the address and control field.

   When encrypting the first packet after ECP stepped into opened state
   the Initial Nonce is encrypted via 3DES-algorithm before its use.

4.1  Padding

   Since the 3DES algorithm operates on blocks of 8 octets, plain text
   packets which are of length not a multiple of 8 octets must be padded
   prior to encrypting.  If this padding is not removed after decryption
   this can be injurious to the interpretation of some protocols which
   do not contain an explicit length field in their protocol headers.



Kummert                     Standards Track                     [Page 5]

RFC 2420               PPP Triple-DES Encryption          September 1998


   Therefore all packets not already a multiple of eight bytes in length
   MUST be padded prior to encrypting using the unambiguous technique of
   Self Describing Padding with a Maximum Pad Value (MPV) of 8. This
   means that the plain text is padded with the sequence of octets 1, 2,
   3, .. 7 since its length is a multiple of 8 octets. Negotiation of
   SDP is not needed.  Negotiation of the LCP Self Describing Option may
   be negotiated independently to solve an orthogonal problem.

   Plain text which length is already a multiple of 8 octets may require
   padding with a further 8 octets (1, 2, 3 ... 8).  These additional
   octets MUST only be appended, if the last octet of the plain text had
   a value of 8 or less.

   When using Multilink and encrypting on individual links it is
   recommended that all non-terminating fragments have a length
   divisible by 8. So no additional padding is needed on those
   fragments.

   After the peer has decrypted the ciphertext, it strips off the Self
   Describing Padding octets to recreate the original plain text.  The
   peer SHOULD discard the frame if the octets forming the padding do
   not match the Self Describing Padding scheme just described.

   Note that after decrypting, only the content of the last byte needs
   to be examined to determine the presence or absence of a Self
   Described Pad.

4.2  Recovery after packet loss

   Packet loss is detected when there is a discontinuity in the sequence
   numbers of consecutive packets.  Suppose packet number N - 1 has an
   unrecoverable error or is otherwise lost, but packets N and N + 1 are
   received correctly.

   Since the previously described algorithm requires the last Ci of
   packet N - 1 to decrypt C1 of packet N, it will be impossible to
   decrypt packet N.  However, all packets N + 1 and following can be
   decrypted in the usual way, since all that is required is the last
   block of ciphertext of the previous packet (in this case packet N,
   which WAS received).

5.  Security Considerations

   This proposal is concerned with providing confidentiality solely.  It
   does not describe any mechanisms for integrity, authentication or
   nonrepudiation.  It does not guarantee that any message received has
   not been modified in transit through replay, cut-and-paste or active
   tampering.  It does not provide authentication of the source of any



Kummert                     Standards Track                     [Page 6]

RFC 2420               PPP Triple-DES Encryption          September 1998


   packet received, or protect against the sender of any packet denying
   its authorship.

   Security issues are the primary subject of this memo. This proposal
   relies on exterior and unspecified methods for retrieval of shared
   secrets.  It proposes no new technology for privacy, but merely
   describes a convention for the application of the 3DES cipher to data
   transmission between PPP implementations.  Any methodology for the
   protection and retrieval of shared secrets, and any limitations of
   the 3DES cipher are relevant to the use described here.

6.  References

   [1]  Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
        51, RFC 1661, July 1994.


   [2]  Meyer, G., "The PPP Encryption Control Protocol (ECP)", RFC
        1968, June 1996.

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

   [4]  Sklower, K., and G. Meyer, "The PPP DES Encryption Protocol,
        Version 2 (DESE-bis)", RFC 2419, September 1998.

   [5]  Doraswamy, N., Metzger, P., Simpson, W., "The ESP Triple DES
        Transform", Work in Progress, June 1997.

   [6]  Schneier, B., "Applied Cryptography", Second Edition, John Wiley
        & Sons, New York, NY, 1995, ISBN 0-471-12845-7.

7.  Acknowledgements

   Many portions of this document were taken from [4] and [5]. Bill
   Simpson gave useful hints on the initial revision.

8. Author's Address

   Holger Kummert
   Nentec Gesellschaft fuer Netzwerktechnologie
   76227 Karlsruhe, Killisfeldstr. 64, Germany

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






Kummert                     Standards Track                     [Page 7]

RFC 2420               PPP Triple-DES Encryption          September 1998


9.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























Kummert                     Standards Track                     [Page 8]