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Versions: 00 rfc2420                                     Standards Track
Network Working Group                                         H. Kummert
Internet-draft                                               Nentec GmbH
Expires: January 1998                                          July 1997

             The PPP Triple-DES Encryption Protocol (3DESE)

Status of this Memo

   This document is a submission to the Point-to-Point Protocol Working
   Group of the Internet Engineering Task Force (IETF).  Comments should
   be submitted to the ietf-ppp@merit.edu mailing list.

   Distribution of this memo is unlimited.

   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
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   To learn the current status of any Internet-Draft, please check the
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   ftp.isi.edu (US West Coast).


   The Point-to-Point Protocol (PPP) [1] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.

   The PPP Encryption Control Protocol (ECP) [2] provides a method to
   negotiate and utilize encryption protocols over PPP encapsulated

   This document provides specific details for the use of the Triple-DES
   standard (3DES) [6] for encrypting PPP encapsulated packets.

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INTERNET DRAFT         PPP Triple-DES Encryption               July 1997

   Table of contents

   1.   Introduction ..............................................    2
   1.1  Algorithm .................................................    3
   1.2  Keys ......................................................    3
   2.   3DESE Configuration Option for ECP ........................    4
   3.   Packet format for 3DESE ...................................    4
   4.   Encryption ................................................    5
   4.1  Padding ...................................................    6
   4.2  Recovery after packet loss ................................    6
   5.   Security considerations ...................................    7
   6.   References ................................................    7
   7.   Acknowledgements ..........................................    7
   8.   Author's address ..........................................    8
   9.   Expiration date of this draft .............................    8

1. Introduction

   The purpose of encrypting packets exchanged between two PPP implemen-
   tations is to attempt to insure the privacy of communication con-
   ducted via the two implementations. There exists a large variety of
   encryption algorithms, where one is the DES algorithm. The DES
   encryption algorithm is a well studied, understood and widely imple-
   mented encryption algorithm.  Triple-DES means that this algorithm is
   applied three times on the data to be encrypted before it is sent
   over the line. The variant used is the DES-EDE3-CBC, which is
   described in more detail in the text. It was also chosen to be
   applied in the security section of IP [5].

   This document shows how to send via the Triple-DES algorithm
   encrypted packets over a point-to-point-link. It lies in the context
   of the generic PPP Encryption Control Protocol [2].

   Because of the use of the CBC-mode a sequence number is provided to
   ensure the right order of transmitted packets. So lost packets can be

   The padding section reflects the result of the discussion on this
   topic on the ppp mailing list.

   In this document, the key words "MUST", "SHOULD", and "recommended"
   are to be interpreted as described in [3].

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1.1  Algorithm

   The DES-EDE3-CBC algorithm is a simple variant of the DES-CBC algo-
   rithm.  In DES-EDE3-CBC, an Initialization Vector (IV) is XOR'd with
   the first 64-bit (8 octet) plaintext block (P1).  The keyed DES func-
   tion is iterated three times, an encryption (E) followed by a decryp-
   tion (D) followed by an encryption (E), and generates the ciphertext
   (C1) for the block. Each iteration uses an independent key: k1, k2
   and k3.

   For successive blocks, the previous ciphertext block is XOR'd with
   the current 8-octet plaintext block (Pi). The keyed DES-EDE3 encryp-
   tion function generates the ciphertext (Ci) for that block.

                      P1             P2                 Pi
                      |              |                  |
               IV--->(X)    +------>(X)      +-------->(X)
                      v     |        v       |          v
                   +-----+  |     +-----+    |       +-----+
               k1->|  E  |  | k1->|  E  |    :   k1->|  E  |
                   +-----+  |     +-----+    :       +-----+
                      |     |        |       :          |
                      v     |        v       :          v
                   +-----+  ^     +-----+    ^       +-----+
               k2->|  D  |  | k2->|  D  |    |   k2->|  D  |
                   +-----+  |     +-----+    |       +-----+
                      |     |        |       |          |
                      v     |        v       |          v
                   +-----+  |     +-----+    |       +-----+
               k3->|  E  |  | k3->|  E  |    |   k3->|  E  |
                   +-----+  |     +-----+    |       +-----+
                      |     |        |       |          |
                      +---->+        +------>+          +---->
                      |              |                  |
                      C1             C2                 Ci

   To decrypt, the order of the functions is reversed: decrypt with k3,
   encrypt with k2, decrypt with k1, and XOR with the previous cipher-
   text block.

   When all three keys (k1, k2 and k3) are the same, DES-EDE3-CBC is
   equivalent to DES-CBC.

1.2  Keys

   The secret DES-EDE3 key shared between the communicating parties is
   effectively 168-bits long.  This key consists of three independent

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   56-bit quantities used by the DES algorithm.  Each of the three 56-
   bit subkeys is stored as a 64-bit (8 octet) quantity, with the least
   significant bit of each octet used as a parity bit.

   When configuring keys for 3DESE those with incorrect parity or so-
   called weak keys [6] SHOULD be rejected.

2.  3DESE Configuration Option for ECP


        The ECP 3DESE Configuration Option indicates that the issuing
        implementation is offering to employ this specification for
        decrypting communications on the link, and may be thought of as
        a request for its peer to encrypt packets in this manner.  The
        ECP 3DESE Configuration Option has the following fields, which
        are transmitted from left to right:

               Figure 1:  ECP 3DESE Configuration Option

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        |     Type      |    Length     |         Initial Nonce ...


             tbd, to indicate the 3DESE protocol.



        Initial Nonce

             This field is an 8 byte quantity which is used by the peer
             implementation to encrypt the first packet transmitted
             after the sender reaches the opened state. To guard against
             replay attacks, the implementation SHOULD offer a different
             value during each ECP negotiation.

3.  Packet format for 3DESE


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        The 3DESE packets that contain the encrypted payload have the
        following fields:

               Figure 2:  3DESE Encryption Protocol Packet Format

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        |    Address    |    Control    |     0000      |  Protocol ID  |
        | Seq. No. High | Seq. No. Low  |        Ciphertext ...

        Address and Control

             These fields MUST be present unless the PPP Address and
             Control Field Compression option (ACFC) has been nego-

        Protocol ID

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

        Sequence Number

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


             The generation of this data is described in the next sec-

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.

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   The encryption algorithm is generally to pad the Protocol and Infor-
   mation 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.

   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 recom-
   mended 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.

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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

   Security issues are the primary subject of this draft. 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, Daydreamer, July 1994.

   [2]  Meyer, G., "The PPP Encryption Protocol", RFC 1968, Spider Sys-
        tems, June 1996.

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

   [4]  Meyer, G., Sklower, K. "The PPP DES Encryption Protocol (DESE)",
        RFC 1969, Spider Systems, June 1996.

   [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.

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7.  Acknowledgements

   Much portions of this document was taken from [4] and [5]. Bill Simp-
   son gave useful hints on the initial revision.

8. Author's  Address:

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

   Phone:  +49 721 9495 0
   E-mail: kummert@nentec.de

9. Expiration date of this draft

   January 23, 1998

Kummert                                                         [Page 8]