Network Working Group K. Sklower
Request for Comments: 1969 University of California, Berkeley
Category: Informational G. Meyer
Spider Systems
June 1996

The PPP DES Encryption Protocol (DESE)

Status of This Memo

This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.

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 DES
standard [5, 6] for encrypting PPP encapsulated packets.

Acknowledgements

The authors extend hearty thanks to Fred Baker of Cisco for helpful
improvements to the clarity of the document.

Table of Contents

1. IntrodUCtion ................................................ 2
1.1. Motivation ................................................ 2
1.2. Conventions ............................................... 2
2. General Overview ............................................ 2
3. Structure of This Specification ............................. 3
4. DESE Configuration Option for ECP ........................... 4
5. Packet Format for DESE ...................................... 5
6. Encryption .................................................. 6
6.1. Padding Considerations .................................... 6
6.2. Generation of the Ciphertext .............................. 7
6.3. Retrieval of the Plaintext ................................ 8
6.4. Recovery after Packet Loss ................................ 8
7. MRU Considerations .......................................... 8
8. Security Considerations ..................................... 9

9. References .................................................. 9
10. Authors' Addresses ......................................... 10
11. EXPiration Date of this Draft .............................. 10

1. Introduction

1.1. Motivation

The purpose of this memo is two-fold: to show how one specifies the
necessary details of a "data" or "bearer" protocol given the context
of the generic PPP Encryption Control Protocol, and also to provide
at least one commonly-understood means of secure data transmission
between PPP implementations.

The DES encryption algorithm is a well studied, understood and widely
implemented encryption algorithm. The DES cipher was designed for
efficient implementation in hardware, and consequently may be
relatively expensive to implement in software. However, its
pervasiveness makes it seem like a reasonable choice for a "model"
encryption protocol.

Source code implementing DES in the "Electronic Code Book Mode" can
be found in [7]. US export laws forbid the inclusion of
compilation-ready source code in this document.

1.2. Conventions

The following language conventions are used in the items of
specification in this document:

o MUST, SHALL or MANDATORY -- the item is an absolute requirement
of the specification.

o SHOULD or RECOMMENDED -- the item should generally be followed
for all but exceptional circumstances.

o MAY or OPTIONAL -- the item is truly optional and may be
followed or ignored according to the needs of the implementor.

2. General Overview

The purpose of encrypting packets exchanged between two PPP
implementations is to attempt to insure the privacy of communication
conducted via the two implementations. The encryption process
depends on the specification of an encryption algorithm and a shared
secret (usually involving at least a key) between the sender and
receiver.

Generally, the encryptor will take a PPP packet including the
protocol field, apply the chosen encryption algorithm, place the
resulting cipher text (and in this specification, an explicit
sequence number) in the information field of another PPP packet. The
decryptor will apply the inverse algorithm and interpret the
resulting plain text as if it were a PPP packet which had arrived
directly on the interface.

The means by which the secret becomes known to both communicating
elements is beyond the scope of this document; usually some form of
manual configuration is involved. Implementations might make use of
PPP authentication, or the EndPoint Identifier Option described in
PPP Multilink [3], as factors in selecting the shared secret. If the
secret can be deduced by analysis of the communication between the
two parties, then no privacy is guaranteed.

While the US Data Encryption Standard (DES) algorithm [5, 6] provides
multiple modes of use, this specification selects the use of only one
mode in conjunction with the PPP Encryption Control Protol (ECP): the
Cipher Block Chaining (CBC) mode. In addition to the US Government
publications cited above, the CBC mode is also discussed in [7],
although no C source code is provided for it per se.

The initialization vector for this mode is deduced from an explicit
64-bit nonce, which is exchanged in the clear during the negotiation
phase. The 56-bit key required by all DES modes is established as a
shared secret between the implementations.

One reason for choosing the chaining mode is that it is generally
thought to require more computation resources to deduce a 64 bit key
used for DES encryption by analysis of the encrypted communication
stream when chaining mode is used, compared with the situation where
each block is encrypted separately with no chaining. Further, if
chaining is not used, even if the key is never deduced, the
communication may be subject to replay attacks.

However, if chaining is to extend beyond packet boundaries, both the
sender and receiver must agree on the order the packets were
encrypted. Thus, this specification provides for an explicit 16 bit
sequence number to sequence decryption of the packets. This mode of
operation even allows recovery from occasional packet loss; details
are also given below.

3. Structure of This Specification

The PPP Encryption Control Protocol (ECP), provides a framework for
negotiating parameters associated with encryption, such as choosing
the algorithm. It specifies the assigned numbers to be used as PPP

protocol numbers for the "data packets" to be carried as the
associated "data protocol", and describes the state machine.

Thus, a specification for use in that matrix need only describe any
additional configuration options required to specify a particular
algorithm, and the process by which one encrypts/decrypts the
information once the Opened state has been achieved.

4. DESE Configuration Option for ECP

Description

The ECP DESE 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 DESE Configuration Option has the following fields,
which are transmitted from left to right:

Figure 1: ECP DESE 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

1, to indicate the DESE 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. An

example might be to use the number of seconds since Jan
1st, 1970 (GMT/UT) in the upper 32 bits, and the current
number of nanoseconds relative to the last second mark in
the lower 32 bits.

Its formulaic role is described in the Encryption section
below.

5. Packet Format for DESE

Description

The DESE packets themselves have the following fields:

Figure 2: DES 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 that ciphertext includes headers for the
Multilink Protocol, and REQUIRES that the Individual Link
Encryption Control Protocol has reached the opened state.
The leading zero MAY be absent if the PPP Protocol Field
Compression option (PFC) has been 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.

6. 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 DES in Chaining Block Cipher mode with a 56-bit key K.

There are a lot of details concerning what constitutes the Protocol
and Information fields, in the presence or non-presence of Multilink,
and whether the ACFC and PFC options have been negotiated, and the
sort of padding chosen.

Regardless of whether ACFC has been negotiated on the link, the
sender applies the encryption procedure to only that portion of the
packet excluding the address and control field.

If the Multilink Protocol has been negotiated and encryption is to be
construed as being applied to each link separately, then the
encryption procedure is to be applied to the (possibly extended)
protocol and information fields of the packet in the Multilink
Protocol.

If the Multilink Protocol has been negotiated and encryption is to be
construed as being applied to the bundle, then the multilink
procedure is to be applied to the resulting DESE packets.

6.1. Padding Considerations

Since the DES algorithm operates on blocks of 8 octets, packets which
are of length not a multiple of 8 octets must be padded. This can be
injurious to the interpretation of some protocols which do not
contain an explicit length field in their protocol headers.
(Additional padding of the ciphered packet for the purposes of
transmission by HDLC hardware which requires an even number of bytes
should not be necessary since the information field will now be of
length a multiple of 8, and whether or not the packet is of even
length can be forced by use or absence of a leading zero in the

protocol field).

For protocols which do have an explicit length field, such as IP,
IPX, XNS, and CLNP, then padding may be accomplished by adding random
trailing garbage. Even when performing the Multilink protocol, if it
is only being applied to packets with explicit length fields, and if
care is taken so that all non-terminating fragments (i.e., those not
bearing the (E)nd bit) are of lengths divisible by 8; then no ill
effects will happen if garbage padding is applied only to terminating
fragments.

For certain cases, such as the PPP bridging protocol when the
trailing CRC is forwarded or when any bridging is being applied to
protocols not having explicit length fields, adding garbage changes
the interpretation of the packet. The self-describing padding option
[4] permits unambiguous removal of padded bytes; although it should
only be used when absolutely necessary as it may inadvertently
require adding as many as 8 octets to packets that could otherwise be
left unaltered.

Consider a packet, which by unlucky circumstance is already a
multiple of 8 octets, but terminates in the sequence 0x1, 0x2.
Self-describing padding would otherwise remove the trailing two
bytes. For purposes of coexistence with archaic HDLC chips where
it is necessary to transmit packets of even length, one would
normally only have to add an additional two octets (0x1, 0x2),
which could then be removed. However, since the packet was
initially a multiple of 8 bytes, an additional 8 bytes would need
to be added.

6.2. Generation of the Ciphertext

In this discussion, E[k] will denote the basic DES cipher determined
by a 56-bit key k acting on 64 bit blocks. and D[k] will denote the
corresponding decryption mechanism. The padded plaintext described
in the previous section then becomes a sequence of 64 bit blocks P[i]
(where i ranges from 1 to n). The circumflex character (^)
represents the bit-wise exclusive-or operation applied to 64-bit
blocks.

When encrypting the first packet to be transmitted in the opened
state let C[0] be the result of applying E[k] to the Initial Nonce
received in the peer's ECP DESE option; otherwise let C[0] be the
final block of the previously transmitted packet.

The ciphertext for the packet is generated by the iterative process

C[i] = E[k](P[i] ^ C[i-1])

for i running between 1 and n.

6.3. Retrieval of the Plaintext

When decrypting the first packet received in the opened state, let
C[0] be the result of applying E[k] to the Initial Nonce transmitted
in the ECP DESE option. The first packet will have sequence number
zero. For subsequent packets, let C[0] be the final block of the
previous packet in sequence space. Decryption is then accomplished
by

P[i] = C[i-1] ^ D[k](C[i]),

for i running between 1 and n.

6.4. 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 algorithm in the previous section requires C[0] for packet
N to be C[last] for packet N - 1, it will be impossible to decode
packet N. However, all packets N + 1 and following can be decoded 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).

7. MRU Considerations

Because padding can occur, and because there is an additional
protocol field in effect, implementations should take into account
the growth of the packets. As an example, if PFC had been
negotiated, and if the MRU before had been exactly a multiple of 8,
then the plaintext resulting combining a full sized data packets with
a one byte protocol field would require an additional 7 bytes of
padding, and the sequence number would be an additional 2 bytes so
that the information field in the DESE protocol is now 10 bytes
larger than that in the original packet. Because the convention is
that PPP options are independent of each other, negotiation of DESE
does not, by itself, automatically increase the MRU value.

8. Security Considerations

Security issues are the primary subject of this memo. This proposal
relies on exterior and unspecified methods for authentication and
retrieval of shared secrets.

It proposes no new technology for privacy, but merely describes a
convention for the application of the DES cipher to data transmission
between PPP implementation.

Any methodology for the protection and retrieval of shared secrets,
and any limitations of the DES cipher are relevant to the use
described here.

9. References

[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51,
RFC1661, Daydreamer, July 1994.

[2] Meyer, G., "The PPP Encryption Protocol", RFC1968, Spider
Systems, June 1996.

[3] Sklower, K., Lloyd, B., McGregor, G., and D. Carr, "The PPP
Multilink Protocol (MP)", RFC1717, UC Berkeley, November 1994.

[4] Simpson, W., Editor, "PPP LCP Extensions", RFC1570, Daydreamer,
January 1994.

[5] National Bureau of Standards, "Data Encryption Standard", FIPS
PUB 46 (January 1977).

[6] National Bureau of Standards, "DES Modes of Operation", FIPS PUB
81 (December 1980).

[7] Schneier, B., "Applied Cryptography - Protocols Algorithms, and
source code in C", John Wiley & Sons, Inc. 1994. There is an
errata associated with the book, and people can get a copy by
sending e-mail to schneier@counterpane.com.

10. Authors' Addresses

Keith Sklower
Computer Science Department
384 Soda Hall, Mail Stop 1776
University of California
Berkeley, CA 94720-1776

Phone: (510) 642-9587
EMail: sklower@CS.Berkeley.EDU

Gerry M. Meyer
Spider Systems
Stanwell Street
Edinburgh EH6 5NG
Scotland, UK

Phone: (UK) 131 554 9424
Fax: (UK) 131 554 0649
EMail: gerry@spider.co.uk