RFC1827 - IP Encapsulating Security Payload (ESP)

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　　Network Working Group R. Atkinson
Request for Comments: 1827 Naval Research Laboratory
Category: Standards Track August 1995

IP Encapsulating Security Payload (ESP)

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.

ABSTRACT

This document describes the IP Encapsulating Security Payload (ESP).
ESP is a mechanism for providing integrity and confidentiality to IP
datagrams. In some circumstances it can also provide authentication
to IP datagrams. The mechanism works with both IPv4 and IPv6.

1. INTRODUCTION

ESP is a mechanism for providing integrity and confidentiality to IP
datagrams. It may also provide authentication, depending on which
algorithm and algorithm mode are used. Non-repudiation and
protection from traffic analysis are not provided by ESP. The IP
Authentication Header (AH) might provide non-repudiation if used with
certain authentication algorithms [Atk95b]. The IP Authentication
Header may be used in conjunction with ESP to provide authentication.
Users desiring integrity and authentication without confidentiality
should use the IP Authentication Header (AH) instead of ESP. This
document assumes that the reader is familiar with the related
document "IP Security Architecture", which defines the overall
Internet-layer security architecture for IPv4 and IPv6 and provides
important background for this specification [Atk95a].

1.1 Overview

The IP Encapsulating Security Payload (ESP) seeks to provide
confidentiality and integrity by encrypting data to be protected and
placing the encrypted data in the data portion of the IP
Encapsulating Security Payload. Depending on the user's security
requirements, this mechanism may be used to encrypt either a
transport-layer segment (e.g., TCP, UDP, ICMP, IGMP) or an entire IP
datagram. Encapsulating the protected data is necessary to provide
confidentiality for the entire original datagram.

Use of this specification will increase the IP protocol processing
costs in participating systems and will also increase the
communications latency. The increased latency is primarily due to
the encryption and decryption required for each IP datagram
containing an Encapsulating Security Payload.

In Tunnel-mode ESP, the original IP datagram is placed in the
encrypted portion of the Encapsulating Security Payload and that
entire ESP frame is placed within a datagram having unencrypted IP
headers. The information in the unencrypted IP headers is used to
route the secure datagram from origin to destination. An unencrypted
IP Routing Header might be included between the IP Header and the
Encapsulating Security Payload.

In Transport-mode ESP, the ESP header is inserted into the IP
datagram immediately prior to the transport-layer protocol header
(e.g., TCP, UDP, or ICMP). In this mode bandwidth is conserved
because there are no encrypted IP headers or IP options.

In the case of IP, an IP Authentication Header may be present as a
header of an unencrypted IP packet, as a header after the IP header
and before the ESP header in a Transport-mode ESP packet, and also as
a header within the encrypted portion of a Tunnel-mode ESP packet.
When AH is present both in the cleartext IP header and also inside a
Tunnel-mode ESP header of a single packet, the unencrypted IPv6
Authentication Header is primarily used to provide protection for the
contents of the unencrypted IP headers and the encrypted
Authentication Header is used to provide authentication only for the
encrypted IP packet. This is discussed in more detail later in this
document.

The Encapsulating Security Payload is structured a bit differently
than other IP payloads. The first component of the ESP payload
consist of the unencrypted field(s) of the payload. The second
component consists of encrypted data. The field(s) of the
unencrypted ESP header inform the intended receiver how to properly
decrypt and process the encrypted data. The encrypted data component
includes protected fields for the security protocol and also the
encrypted encapsulated IP datagram.

The concept of a "Security Association" is fundamental to ESP. It is
described in detail in the companion document "Security Architecture
for the Internet Protocol" which is incorporated here by reference
[Atk95a]. Implementors should read that document before reading this
one.

1.2 Requirements Terminology

In this document, the words that are used to define the significance
of each particular requirement are usually capitalised. These words
are:

- MUST

This word or the adjective "REQUIRED" means that the item is an
absolute requirement of the specification.

- SHOULD

This word or the adjective "RECOMMENDED" means that there might
exist valid reasons in particular circumstances to ignore this
item, but the full implications should be understood and the case
carefully weighed before taking a different course.

- MAY

This word or the adjective "OPTIONAL" means that this item is
truly optional. One vendor might choose to include the item
because a particular marketplace requires it or because it
enhances the product, for example; another vendor may omit the
same item.

2. KEY MANAGEMENT

Key management is an important part of the IP security architecture.
However, a specific key management protocol is not included in this
specification because of a long history in the public literature of
subtle flaws in key management algorithms and protocols. IP tries to
decouple the key management mechanisms from the security protocol
mechanisms. The only coupling between the key management protocol
and the security protocol is with the Security Parameter Index (SPI),
which is described in more detail below. This decoupling permits
several different key management mechanisms to be used. More
importantly, it permits the key management protocol to be changed or
corrected without unduly impacting the security protocol
implementations. Thus, a key management protocol for IP is not
specified within this memo. The IP Security Architecture describes
key management in more detail and specifies the key management
requirements for IP. Those key management requirements are
incorporated here by reference [Atk95a].

The key management mechanism is used to negotiate a number of
parameters for each security association, including not only the keys
but other information (e.g., the cryptographic algorithms and modes,

security classification level, if any) used by the communicating
parties. The key management protocol implementation usually creates
and maintains a logical table containing the several parameters for
each current security association. An ESP implementation normally
needs to read that security parameter table to determine how to
process each datagram containing an ESP (e.g., which algorithm/mode
and key to use).

3. ENCAPSULATING SECURITY PAYLOAD SYNTAX

The Encapsulating Security Payload (ESP) may appear anywhere after
the IP header and before the final transport-layer protocol. The
Internet Assigned Numbers Authority has assigned Protocol Number 50
to ESP [STD-2]. The header immediately preceding an ESP header will
always contain the value 50 in its Next Header (IPv6) or Protocol
(IPv4) field. ESP consists of an unencrypted header followed by
encrypted data. The encrypted data includes both the protected ESP
header fields and the protected user data, which is either an entire
IP datagram or an upper-layer protocol frame (e.g., TCP or UDP). A
high-level diagram of a secure IP datagram follows.

|<-- Unencrypted -->|<---- Encrypted ------>|
+-------------+--------------------+------------+---------------------+
| IP Header | Other IP Headers | ESP Header | encrypted data |
+-------------+--------------------+------------+---------------------+

A more detailed diagram of the ESP Header follows below.

+-------------+--------------------+------------+---------------------+
| Security Association Identifier (SPI), 32 bits |
+=============+====================+============+=====================+
| Opaque Transform Data, variable length |
+-------------+--------------------+------------+---------------------+

Encryption and authentication algorithms, and the precise format of
the Opaque Transform Data associated with them are known as
"transforms". The ESP format is designed to support new transforms
in the future to support new or additional cryptographic algorithms.
The transforms are specified by themselves rather than in the main
body of this specification. The mandatory transform for use with IP
is defined in a separate document [KMS95]. Other optional transforms
exist in other separate specifications and additional transforms
might be defined in the future.

3.1 Fields of the Encapsulating Security Payload

The SPI is a 32-bit pseudo-random value identifying the security
association for this datagram. If no security association has been
established, the value of the SPI field shall be 0x00000000. An SPI
is similar to the SAID used in other security protocols. The name
has been changed because the semantics used here are not exactly the
same as those used in other security protocols.

The set of SPI values in the range 0x00000001 though 0x000000FF are
reserved to the Internet Assigned Numbers Authority (IANA) for future
use. A reserved SPI value will not normally be assigned by IANA
unless the use of that particular assigned SPI value is openly
specified in an RFC.

The SPI is the only mandatory transform-independent field.
Particular transforms may have other fields unique to the transform.
Transforms are not specified in this document.

3.2 Security Labeling with ESP

The encrypted IP datagram need not and does not normally contain any
explicit Security Label because the SPI indicates the sensitivity
level. This is an improvement over the current practices with IPv4
where an explicit Sensitivity Label is normally used with
Compartmented Mode Workstations and other systems requiring Security
Labels [Ken91] [DIA]. In some situations, users MAY choose to carry
explicit labels (for example, IPSO labels as defined by RFC-1108
might be used with IPv4) in addition to using the implicit labels
provided by ESP. Explicit label options could be defined for use
with IPv6 (e.g., using the IPv6 End-to-End Options Header or the IPv6
Hop-by-Hop Options Header). Implementations MAY support explicit
labels in addition to implicit labels, but implementations are not
required to support explicit labels. Implementations of ESP in
systems claiming to provide multi-level security MUST support
implicit labels.

4. ENCAPSULATING SECURITY PROTOCOL PROCESSING

This section describes the steps taken when ESP is in use between two
communicating parties. Multicast is different from unicast only in
the area of key management (See the definition of the SPI, above, for
more detail on this). There are two modes of use for ESP. The first
mode, which is called "Tunnel-mode", encapsulates an entire IP
datagram inside ESP. The second mode, which is called "Transport-
Mode", encapsulates a transport-layer (e.g., UDP, TCP) frame inside
ESP. The term "Transport-mode" must not be misconstrued as
restricting its use to TCP and UDP. For example, an ICMP message MAY

be sent either using the "Transport-mode" or the "Tunnel-mode"
depending upon circumstance. ESP processing occurs prior to IP
fragmentation on output and after IP reassembly on input. This
section describes protocol processing for each of these two modes.

4.1 ESP in Tunnel-mode

In Tunnel-mode ESP, the ESP header follows all of the end-to-end
headers (e.g., Authentication Header, if present in cleartext) and
immediately precedes an tunnelled IP datagram.

The sender takes the original IP datagram, encapsulates it into the
ESP, uses at least the sending userid and Destination Address as data
to locate the correct Security Association, and then applies the
appropriate encryption transform. If host-oriented keying is in use,
then all sending userids on a given system will have the same
Security Association for a given Destination Address. If no key has
been established, then the key management mechanism is used to
establish an encryption key for this communications session prior to
the use of ESP. The (now encrypted) ESP is then encapsulated in a
cleartext IP datagram as the last payload. If strict red/black
separation is being enforced, then the addressing and other
information in the cleartext IP headers and optional payloads MAY be
different from the values contained in the (now encrypted and
encapsulated) original datagram.

The receiver strips off the cleartext IP header and cleartext
optional IP payloads (if any) and discards them. It then uses the
combination of Destination Address and SPI value to locate the
correct session key to use for this packet. It then decrypts the ESP
using the session key that was just located for this packet.

If no valid Security Association exists for this session (for
example, the receiver has no key), the receiver MUST discard the
encrypted ESP and the failure MUST be recorded in the system log or
audit log. This system log or audit log entry SHOULD include the SPI
value, date/time, cleartext Sending Address, cleartext Destination
Address, and the cleartext Flow ID. The log entry MAY also include
other identifying data. The receiver might not wish to react by
immediately informing the sender of this failure because of the
strong potential for easy-to-exploit denial of service attacks.

If decryption succeeds, the original IP datagram is then removed from
the (now decrypted) ESP. This original IP datagram is then processed
as per the normal IP protocol specification. In the case of system
claiming to provide multilevel security (for example, a B1 or
Compartmented Mode Workstation) additional appropriate mandatory
access controls MUST be applied based on the security level of the

receiving process and the security level associated with this
Security Association. If those mandatory access controls fail, then
the packet SHOULD be discarded and the failure SHOULD be logged using
implementation-specific procedures.

4.2 ESP in Transport-mode

In Transport-mode ESP, the ESP header follows the end-to-end headers
(e.g., Authentication Header) and immediately precedes a transport-
layer (e.g., UDP, TCP, ICMP) header.

The sender takes the original transport-layer (e.g., UDP, TCP, ICMP)
frame, encapsulates it into the ESP, uses at least the sending userid
and Destination Address to locate the appropriate Security
Association, and then applies the appropriate encryption transform.
If host-oriented keying is in use, then all sending userids on a
given system will have the same Security Association for a given
Destination Address. If no key has been established, then the key
management mechanism is used to establish a encryption key for this
communications session prior to the encryption. The (now encrypted)
ESP is then encapsulated as the last payload of a cleartext IP
datagram.

The receiver processes the cleartext IP header and cleartext optional
IP headers (if any) and temporarily stores pertinent information
(e.g., source and destination addresses, Flow ID, Routing Header).
It then decrypts the ESP using the session key that has been
established for this traffic, using the combination of the
destination address and the packet's Security Association Identifier
(SPI) to locate the correct key.

If no key exists for this session or the attempt to decrypt fails,
the encrypted ESP MUST be discarded and the failure MUST be recorded
in the system log or audit log. If such a failure occurs, the
recorded log data SHOULD include the SPI value, date/time received,
clear-text Sending Address, clear-text Destination Address, and the
Flow ID. The log data MAY also include other information about the
failed packet. If decryption does not work properly for some reason,
then the resulting data will not be parsable by the implementation's
protocol engine. Hence, failed decryption is generally detectable.

If decryption succeeds, the original transport-layer (e.g., UDP, TCP,
ICMP) frame is removed from the (now decrypted) ESP. The information
from the cleartext IP header and the now decrypted transport-layer
header is jointly used to determine which application the data should
be sent to. The data is then sent along to the appropriate
application as normally per IP protocol specification. In the case
of a system claiming to provide multilevel security (for example, a

B1 or Compartmented Mode Workstation), additional Mandatory Access
Controls MUST be applied based on the security level of the receiving
process and the security level of the received packet's Security
Association.

4.3. Authentication

Some transforms provide authentication as well as confidentiality and
integrity. When such a transform is not used, then the
Authentication Header might be used in conjunction with the
Encapsulating Security Payload. There are two different approaches
to using the Authentication Header with ESP, depending on which data
is to be authenticated. The location of the Authentication Header
makes it clear which set of data is being authenticated.

In the first usage, the entire received datagram is authenticated,
including both the encrypted and unencrypted portions, while only the
data sent after the ESP Header is confidential. In this usage, the
sender first applies ESP to the data being protected. Then the other
plaintext IP headers are prepended to the ESP header and its now
encrypted data. Finally, the IP Authentication Header is calculated
over the resulting datagram according to the normal method. Upon
receipt, the receiver first verifies the authenticity of the entire
datagram using the normal IP Authentication Header process. Then if
authentication succeeds, decryption using the normal IP ESP process
occurs. If decryption is successful, then the resulting data is
passed up to the upper layer.

If the authentication process were to be applied only to the data
protected by Tunnel-mode ESP, then the IP Authentication Header would
be placed normally within that protected datagram. However, if one
were using Transport-mode ESP, then the IP Authentication Header
would be placed before the ESP header and would be calculated across
the entire IP datagram.

If the Authentication Header is encapsulated within a Tunnel-mode ESP
header, and both headers have specific security classification levels
associated with them, and the two security classification levels are
not identical, then an error has occurred. That error SHOULD be
recorded in the system log or audit log using the procedures
described previously. It is not necessarily an error for an
Authentication Header located outside of the ESP header to have a
different security classification level than the ESP header's
classification level. This might be valid because the cleartext IP
headers might have a different classification level after the data
has been encrypted using ESP.

5. CONFORMANCE REQUIREMENTS

Implementations that claim conformance or compliance with this
specification MUST fully implement the header described here, MUST
support manual key distribution with this header, MUST comply with
all requirements of the "Security Architecture for the Internet
Protocol" [Atk95a], and MUST support the use of DES CBC as specified
in the companion document entitled "The ESP DES-CBC Transform"
[KMS95]. Implementors MAY also implement other ESP transforms.
Implementers should consult the most recent version of the "IAB
Official Standards" RFCfor further guidance on the status of this
document.

6. SECURITY CONSIDERATIONS

This entire document discusses a security mechanism for use with IP.
This mechanism is not a panacea, but it does provide an important
component useful in creating a secure internetwork.

Cryptographic transforms for ESP which use a block-chaining algorithm
and lack a strong integrity mechanism are vulnerable to a cut-and-
paste attack described by Bellovin and should not be used unless the
Authentication Header is always present with packets using that ESP
transform [Bel95].

Users need to understand that the quality of the security provided by
this specification depends completely on the strength of whichever
encryption algorithm has been implemented, the correctness of that
algorithm's implementation, upon the security of the key management
mechanism and its implementation, the strength of the key [CN94]
[Sch94, p233] and upon the correctness of the ESP 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. Use of high assurance
development techniques is recommended for the IP Encapsulating
Security Payload.

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.

If user-oriented keying is not in use, then the algorithm in use
should not be an algorithm vulnerable to any kind of Chosen Plaintext
attack. Chosen Plaintext attacks on DES are described in [BS93] and
[Mat94]. Use of user-oriented keying is recommended in order to
preclude any sort of Chosen Plaintext attack and to generally make
cryptanalysis more difficult. Implementations SHOULD support user-

oriented keying as is described in the IP Security Architecture
[Atk95a].

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.

Many of the concepts here are derived from or were influenced by the
US Government's SP3 security protocol specification, the ISO/IEC's
NLSP specification, or from the proposed swIPe security protocol
[SDNS89, ISO92a, IB93, IBK93, ISO92b]. The use of DES for
confidentiality is closely modeled on the work done for the SNMPv2
[GM93]. Steve Bellovin, Steve Deering, Dave Mihelcic, and Hilarie
Orman provided solid critiques of early versions of this memo.

REFERENCES

[Atk95a] Atkinson, R., "Security Architecture for the Internet
Protocol", RFC1825, NRL, August 1995.

[Atk95b] Atkinson, R., "IP Authentication Header", RFC1826, 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.

[Bel95] Steven M. Bellovin, Presentation at IP Security Working
Group Meeting, Proceedings of the 32nd Internet Engineering
Task Force, March 1995, Internet Engineering Task Force,
Danvers, MA.

[BS93] Eli Biham and Adi Shamir, "Differential Cryptanalysis of the
Data Encryption Standard", Springer-Verlag, New York, NY,
1993.

[CN94] John M. Carroll & Sri Nudiati, "On Weak Keys and Weak Data:
Foiling the Two Nemeses", Cryptologia, Vol. 18, No. 23,
July 1994. pp. 253-280

[CERT95] 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.

[DIA] US Defense Intelligence Agency (DIA), "Compartmented Mode
Workstation Specification", Technical Report
DDS-2600-6243-87.

[GM93] Galvin J., and K. McCloghrie, "Security Protocols for
version 2 of the Simple Network Management Protocol
(SNMPv2)", RFC1446, Trusted Information Systems, Hughes LAN
Systems, April 1993.

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

[IB93] John Ioannidis & Matt Blaze, "Architecture and Implementation
of Network-layer Security Under Unix", Proceedings of the USENIX
Security Symposium, Santa Clara, CA, October 1993.

[IBK93] John Ioannidis, Matt Blaze, & Phil Karn, "swIPe:
Network-Layer Security for IP", presentation at the Spring
1993 IETF Meeting, Columbus, Ohio.

[ISO92a] ISO/IEC JTC1/SC6, Network Layer Security Protocol, ISO-IEC
DIS 11577, International Standards Organisation, Geneva,
Switzerland, 29 November 1992.

[ISO92b] ISO/IEC JTC1/SC6, Network Layer Security Protocol, ISO-IEC
DIS 11577, Section 13.4.1, page 33, International Standards
Organisation, Geneva, Switzerland, 29 November 1992.

[Ken91] Kent, S., "US DoD Security Options for the Internet
Protocol", RFC1108, BBN Communications, November 1991.

[KMS95] Karn, P., Metzger, P., and W. Simpson, "The ESP DES-CBC
Transform", RFC1829, Qualcomm, Inc., Piermont, Daydreamer,
August 1995.

[Mat94] Matsui, M., "Linear Cryptanalysis method for DES Cipher",
Proceedings of Eurocrypt '93, Berlin, Springer-Verlag, 1994.

[NIST77] US National Bureau of Standards, "Data Encryption Standard",
Federal Information Processing Standard (FIPS) Publication
46, January 1977.

[NIST80] US National Bureau of Standards, "DES Modes of Operation"
Federal Information Processing Standard (FIPS) Publication
81, December 1980.

[NIST81] US National Bureau of Standards, "Guidelines for Implementing
and Using the Data Encryption Standard", Federal Information
Processing Standard (FIPS) Publication 74, April 1981.

[NIST88] US National Bureau of Standards, "Data Encryption Standard",
Federal Information Processing Standard (FIPS) Publication
46-1, January 1988.

[STD-2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
RFC1700, USC/Information Sciences Institute, October 1994.

[Sch94] Bruce Schneier, Applied Cryptography, John Wiley & Sons,
New York, NY, 1994. ISBN 0-471-59756-2

[SDNS89] SDNS Secure Data Network System, Security Protocol 3, SP3,
Document SDN.301, Revision 1.5, 15 May 1989, as published
in NIST Publication NIST-IR-90-4250, February 1990.

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'S ADDRESS

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

Phone: (202) 404-7090
Fax: (202) 404-7942

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