RFC2420 - The PPP Triple-DES Encryption Protocol (3DESE)

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　　Network Working Group H. Kummert
Request for Comments: 2420 Nentec GmbH
Category: Standards Track September 1998

The PPP Triple-DES Encryption Protocol (3DESE)

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

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

Abstract

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
links.

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

Table of Contents

1. Introduction .............................................. 2
1.1 Algorithm ................................................. 2
1.2 Keys ...................................................... 3
2. 3DESE Configuration Option for ECP ........................ 3
3. Packet format for 3DESE ................................... 4
4. Encryption ................................................ 5
4.1 Padding ................................................... 5
4.2 Recovery after packet loss ................................ 6
5. Security Considerations ................................... 6
6. References ................................................ 7
7. Acknowledgements .......................................... 7
8. Author's Address .......................................... 7
9. Full Copyright Statement .................................. 8

1. Introduction

The purpose of encrypting packets exchanged between two PPP
implementations is to attempt to insure the privacy of communication
conducted 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
implemented 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
detected.

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].

1.1 Algorithm

The DES-EDE3-CBC algorithm is a simple variant of the DES-CBC
algorithm. In DES-EDE3-CBC, an Initialization Vector (IV) is XOR'd
with the first 64-bit (8 octet) plaintext block (P1). The keyed DES
function is iterated three times, an encryption (E) followed by a
decryption (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
encryption 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
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

Description

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 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

2, to indicate the 3DESE protocol.

Length

10

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

Description

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

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.

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

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, RFC1661, 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, RFC2119, March 1997.

[4] Sklower, K., and G. Meyer, "The PPP DES Encryption Protocol,
Version 2 (DESE-bis)", RFC2419, 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: +49 721 9495 0
EMail: kummert@nentec.de

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
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followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
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