Network Working Group                                        R. Atkinson
Request for Comments: 1825                     Naval Research Laboratory
Category: Standards Track                                    August 1995


            Security Architecture for the Internet Protocol

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.

1. INTRODUCTION

   This memo describes the security mechanisms for IP version 4 (IPv4)
   and IP version 6 (IPv6) and the services that they provide.  Each
   security mechanism is specified in a separate document.  This
   document also describes key management requirements for systems
   implementing those security mechanisms.  This document is not an
   overall Security Architecture for the Internet and is instead focused
   on IP-layer security.

1.1 Technical Definitions

   This section provides a few basic definitions that are applicable to
   this document.  Other documents provide more definitions and
   background information [VK83, HA94].

   Authentication
           The property of knowing that the data received is the same as
           the data that was sent and that the claimed sender is in fact
           the actual sender.

   Integrity
           The property of ensuring that data is transmitted from source
           to destination without undetected alteration.

   Confidentiality
           The property of communicating such that the intended
           recipients know what was being sent but unintended
           parties cannot determine what was sent.

   Encryption
           A mechanism commonly used to provide confidentiality.

   Non-repudiation
           The property of a receiver being able to prove that the sender
           of some data did in fact send the data even though the sender
           might later desire to deny ever having sent that data.

   SPI
           Acronym for "Security Parameters Index".  An unstructured
           opaque index which is used in conjunction with the
           Destination Address to identify a particular Security
           Association.

   Security Association
           The set of security information relating to a given network
           connection or set of connections.  This is described in
           detail below.

   Traffic Analysis
           The analysis of network traffic flow for the purpose of
           deducing information that is useful to an adversary.
           Examples of such information are frequency of transmission,
           the identities of the conversing parties, sizes of packets,
           Flow Identifiers used, etc. [Sch94].

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.

1.3 Typical Use

   There are two specific headers that are used to provide security
   services in IPv4 and IPv6.  These headers are the "IP Authentication
   Header (AH)" [Atk95a] and the "IP Encapsulating Security Payload
   (ESP)" [Atk95b] header.  There are a number of ways in which these IP
   security mechanisms might be used.  This section describes some of
   the more likely uses.  These descriptions are not complete or
   exhaustive.  Other uses can also be envisioned.

   The IP Authentication Header is designed to provide integrity and
   authentication without confidentiality to IP datagrams.  The lack of
   confidentiality ensures that implementations of the Authentication
   Header will be widely available on the Internet, even in locations
   where the export, import, or use of encryption to provide
   confidentiality is regulated.  The Authentication Header supports
   security between two or more hosts implementing AH, between two or
   more gateways implementing AH, and between a host or gateway
   implementing AH and a set of hosts or gateways.  A security gateway
   is a system which acts as the communications gateway between external
   untrusted systems and trusted hosts on their own subnetwork.  It also
   provides security services for the trusted hosts when they
   communicate with the external untrusted systems.  A trusted
   subnetwork contains hosts and routers that trust each other not to
   engage in active or passive attacks and trust that the underlying
   communications channel (e.g., an Ethernet) isn't being attacked.

   In the case where a security gateway is providing services on behalf
   of one or more hosts on a trusted subnet, the security gateway is
   responsible for establishing the security association on behalf of
   its trusted host and for providing security services between the
   security gateway and the external system(s).  In this case, only the
   gateway need implement AH, while all of the systems behind the
   gateway on the trusted subnet may take advantage of AH services
   between the gateway and external systems.

   A security gateway which receives a datagram containing a recognised
   sensitivity label, for example IPSO [Ken91], from a trusted host
   should take that label's value into consideration when
   creating/selecting an Security Association for use with AH between
   the gateway and the external destination.  In such an environment, a
   gateway which receives a IP packet containing the IP Encapsulating
   Security Payload (ESP) should add appropriate authentication,
   including implicit (i.e., contained in the Security Association used)
   or explicit label information (e.g., IPSO), for the decrypted packet
   that it forwards to the trusted host that is the ultimate
   destination.  The IP Authentication Header should always be used on
   packets containing explicit sensitivity labels to ensure end-to-end

   label integrity.  In environments using security gateways, those
   gateways MUST perform address-based IP packet filtering on
   unauthenticated packets purporting to be from a system known to be
   using IP security.

   The IP Encapsulating Security Payload (ESP) is designed to provide
   integrity, authentication, and confidentiality to IP datagrams
   [Atk95b]. The ESP supports security between two or more hosts
   implementing ESP, between two or more gateways implementing ESP, and
   between a host or gateway implementing ESP and a set of hosts and/or
   gateways.  A security gateway is a system which acts as the
   communications gateway between external untrusted systems and trusted
   hosts on their own subnetwork and provides security services for the
   trusted hosts when they communicate with external untrusted systems.
   A trusted subnetwork contains hosts and routers that trust each other
   not to engage in active or passive attacks and trust that the
   underlying communications channel (e.g., an Ethernet) isn't being
   attacked.  Trusted systems always should be trustworthy, but in
   practice they often are not trustworthy.

   Gateway-to-gateway encryption is most valuable for building private
   virtual networks across an untrusted backbone such as the Internet.
   It does this by excluding outsiders.  As such, it is often not a
   substitute for host-to-host encryption, and indeed the two can be and
   often should be used together.

   In the case where a security gateway is providing services on behalf
   of one or more hosts on a trusted subnet, the security gateway is
   responsible for establishing the security association on behalf of
   its trusted host and for providing security services between the
   security gateway and the external system(s).  In this case, only the
   gateway need implement ESP, while all of the systems behind the
   gateway on the trusted subnet may take advantage of ESP services
   between the gateway and external systems.

   A gateway which receives a datagram containing a recognised
   sensitivity label from a trusted host should take that label's value
   into consideration when creating/selecting a Security Association for
   use with ESP between the gateway and the external destination.  In
   such an environment, a gateway which receives a IP packet containing
   the ESP should appropriately label the decrypted packet that it
   forwards to the trusted host that is the ultimate destination.  The
   IP Authentication Header should always be used on packets containing
   explicit sensitivity labels to ensure end-to-end label integrity.

   If there are no security gateways present in the connection, then two
   end systems that implement ESP may also use it to encrypt only the
   user data (e.g., TCP or UDP) being carried between the two systems.
   ESP is designed to provide maximum flexibility so that users may
   select and use only the security that they desire and need.

   Routing headers for which integrity has not been cryptographically
   protected SHOULD be ignored by the receiver.  If this rule is not
   strictly adhered to, then the system will be vulnerable to various
   kinds of attacks, including source routing attacks [Bel89] [CB94]
   [CERT95].

   While these documents do not specifically discuss IPv4 broadcast,
   these IP security mechanisms MAY be used with such packets.  Key
   distribution and Security Association management are not trivial for
   broadcast applications.  Also, if symmetric key algorithms are used
   the value of using cryptography with a broadcast packet is limited
   because the receiver can only know that the received packet came from
   one of many systems knowing the correct key to use.

1.4 Security Associations

   The concept of a "Security Association" is fundamental to both the IP
   Encapsulating Security Payload and the IP Authentication Header.  The
   combination of a given Security Parameter Index (SPI) and Destination
   Address uniquely identifies a particular "Security Association".  An
   implementation of the Authentication Header or the Encapsulating
   Security Payload MUST support this concept of a Security Association.
   An implementation MAY also support other parameters as part of a
   Security Association.  A Security Association normally includes the
   parameters listed below, but might include additional parameters as
   well:

   - Authentication algorithm and algorithm mode being used with
     the IP Authentication Header [REQUIRED for AH implementations].

   - Key(s) used with the authentication algorithm in use with
     the Authentication Header [REQUIRED for AH implementations].

   - Encryption algorithm, algorithm mode, and transform being
     used with the IP Encapsulating Security Payload [REQUIRED for
     ESP implementations].

   - Key(s) used with the encryption algorithm in use with the
     Encapsulating Security Payload [REQUIRED for ESP implementations].

   - Presence/absence and size of a cryptographic synchronisation or
     initialisation vector field for the encryption algorithm [REQUIRED
     for ESP implementations].

   - Authentication algorithm and mode used with the ESP transform
     (if any is in use) [RECOMMENDED for ESP implementations].

   - Authentication key(s) used with the authentication algorithm
     that is part of the ESP transform (if any) [RECOMMENDED for
     ESP implementations].

   - Lifetime of the key or time when key change should occur
     [RECOMMENDED for all implementations].

   - Lifetime of this Security Association [RECOMMENDED for all
     implementations].

   - Source Address(es) of the Security Association, might be a
     wildcard address if more than one sending system shares the
     same Security Association with the destination [RECOMMENDED
     for all implementations].

   - Sensitivity level (for example, Secret or Unclassified)
     of the protected data [REQUIRED for all systems claiming
     to provide multi-level security, RECOMMENDED for all other systems].

   The sending host uses the sending userid and Destination Address to
   select an appropriate Security Association (and hence SPI value).
   The receiving host uses the combination of SPI value and Destination
   Address to distinguish the correct association.  Hence, an AH
   implementation will always be able to use the SPI in combination with
   the Destination Address to determine the security association and
   related security configuration data for all valid incoming packets.
   When a formerly valid Security Association becomes invalid, the
   destination system(s) SHOULD NOT immediately reuse that SPI value and
   instead SHOULD let that SPI value become stale before reusing it for
   some other Security Association.

   A security association is normally one-way.  An authenticated
   communications session between two hosts will normally have two
   Security Parameter Indexes in use (one in each direction).  The
   combination of a particular Security Parameter Index and a particular
   Destination Address uniquely identifies the Security Association.
   The Destination Address may be a unicast address or a multicast group
   address.

   The receiver-orientation of the Security Association implies that, in
   the case of unicast traffic, the destination system will normally
   select the SPI value.  By having the destination select the SPI
   value, there is no potential for manually configured Security
   Associations that conflict with automatically configured (e.g., via a
   key management protocol) Security Associations.  For multicast
   traffic, there are multiple destination systems but a single
   destination multicast group, so some system or person will need to
   select SPIs on behalf of that multicast group and then communicate
   the information to all of the legitimate members of that multicast
   group via mechanisms not defined here.

   Multiple senders to a multicast group MAY use a single Security
   Association (and hence Security Parameter Index) for all traffic to
   that group.  In that case, the receiver only knows that the message
   came from a system knowing the security association data for that
   multicast group.  A receiver cannot generally authenticate which
   system sent the multicast traffic when symmetric algorithms (e.g.,
   DES, IDEA) are in use.  Multicast traffic MAY also use a separate
   Security Association (and hence SPI) for each sender to the multicast
   group .  If each sender has its own Security Association and
   asymmetric algorithms are used, then data origin authentication is
   also a provided service.

2. DESIGN OBJECTIVES

   This section describes some of the design objectives of this security
   architecture and its component mechanisms.  The primary objective of
   this work is to ensure that IPv4 and IPv6 will have solid
   cryptographic security mechanisms available to users who desire
   security.

   These mechanisms are designed to avoid adverse impacts on Internet
   users who do not employ these security mechanisms for their traffic.
   These mechanisms are intended to be algorithm-independent so that the
   cryptographic algorithms can be altered without affecting the other
   parts of the implementation.  These security mechanisms should be
   useful in enforcing a variety of security policies.

   Standard default algorithms (keyed MD5, DES CBC) are specified to
   ensure interoperability in the global Internet.  The selected default
   algorithms are the same as the standard default algorithms used in
   SNMPv2 [GM93].

3. IP-LAYER SECURITY MECHANISMS

   There are two cryptographic security mechanisms for IP.  The first is
   the Authentication Header which provides integrity and authentication
   without confidentiality [Atk95a].  The second is the Encapsulating
   Security Payload which always provides confidentiality, and
   (depending on algorithm and mode) might also provide integrity and
   authentication [Atk95b].  The two IP security mechanisms may be used
   together or separately.

   These IP-layer mechanisms do not provide security against a number of
   traffic analysis attacks.  However, there are several techniques
   outside the scope of this specification (e.g., bulk link encryption)
   that might be used to provide protection against traffic analysis
   [VK83].

3.1 AUTHENTICATION HEADER

   The IP Authentication Header holds authentication information for its
   IP datagram [Atk95a].  It does this by computing a cryptographic
   authentication function over the IP datagram and using a secret
   authentication key in the computation.  The sender computes the
   authentication data prior to sending the authenticated IP packet.
   Fragmentation occurs after the Authentication Header processing for
   outbound packets and prior to Authentication Header processing for
   inbound packets.  The receiver verifies the correctness of the
   authentication data upon reception.  Certain fields which must change
   in transit, such as the "TTL" (IPv4) or "Hop Limit" (IPv6) field,
   which is decremented on each hop, are omitted from the authentication
   calculation.  However the omission of the Hop Limit field does not
   adversely impact the security provided.  Non-repudiation might be
   provided by some authentication algorithms (e.g., asymmetric
   algorithms when both sender and receiver keys are used in the
   authentication calculation) used with the Authentication Header, but
   it is not necessarily provided by all authentication algorithms that
   might be used with the Authentication Header.  The default
   authentication algorithm is keyed MD5, which, like all symmetric
   algorithms, cannot provide non-repudiation by itself.
   Confidentiality and traffic analysis protection are not provided by
   the Authentication Header.

   Use of the Authentication Header 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 calculation of the authentication data by the sender and the
   calculation and comparison of the authentication data by each
   receiver for each IP datagram containing an Authentication Header
   (AH).

   The Authentication Header provides much stronger security than exists
   in most of the current Internet and should not affect exportability
   or significantly increase implementation cost.  While the
   Authentication Header might be implemented by a security gateway on
   behalf of hosts on a trusted network behind that security gateway,
   this mode of operation is not encouraged.  Instead, the
   Authentication Header should be used from origin to final
   destination.

   All IPv6-capable hosts MUST implement the IP Authentication Header
   with at least the MD5 algorithm using a 128-bit key.  IPv4-systems
   claiming to implement the Authentication Header MUST implement the IP
   Authentication Header with at least the MD5 algorithm using a 128-bit
   key [MS95].  An implementation MAY support other authentication
   algorithms in addition to keyed MD5.

3.2 ENCAPSULATING SECURITY PAYLOAD

   The IP Encapsulating Security Payload (ESP) is designed to provide
   integrity, authentication, and confidentiality to IP datagrams
   [Atk95b].  It does this by encapsulating either an entire IP datagram
   or only the upper-layer protocol (e.g., TCP, UDP, ICMP) data inside
   the ESP, encrypting most of the ESP contents, and then appending a
   new cleartext IP header to the now encrypted Encapsulating Security
   Payload.  This cleartext IP header is used to carry the protected
   data through the internetwork.

3.2.1 Description of the ESP Modes

   There are two modes within ESP.  The first mode, which is known as
   Tunnel-mode, encapsulates an entire IP datagram within the ESP
   header.  The second mode, which is known as Transport-mode,
   encapsulates an upper-layer protocol (for example UDP or TCP) inside
   ESP and then prepends a cleartext IP header.

3.2.2 Usage of ESP

   ESP works between hosts, between a host and a security gateway, or
   between security gateways. This support for security gateways permits
   trustworthy networks behind a security gateway to omit encryption and
   thereby avoid the performance and monetary costs of encryption, while
   still providing confidentiality for traffic transiting untrustworthy
   network segments.  When both hosts directly implement ESP and there
   is no intervening security gateway, then they may use the Transport-
   mode (where only the upper layer protocol data (e.g., TCP or UDP) is
   encrypted and there is no encrypted IP header).  This mode reduces
   both the bandwidth consumed and the protocol processing costs for
   users that don't need to keep the entire IP datagram confidential.

   ESP works with both unicast and multicast traffic.

3.2.3 Performance Impacts of ESP

   The encapsulating security approach used by ESP can noticeably impact
   network performance in participating systems, but use of ESP should
   not adversely impact routers or other intermediate systems that are
   not participating in the particular ESP association.  Protocol
   processing in participating systems will be more complex when
   encapsulating security is used, requiring both more time and more
   processing power.  Use of encryption 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.  The precise cost of
   ESP will vary with the specifics of the implementation, including the
   encryption algorithm, key size, and other factors.  Hardware
   implementations of the encryption algorithm are recommended when high
   throughput is desired.

   For interoperability throughout the worldwide Internet, all
   conforming implementations of the IP Encapsulating Security Payload
   MUST support the use of the Data Encryption Standard (DES) in
   Cipher-Block Chaining (CBC) Mode as detailed in the ESP
   specification.  Other confidentiality algorithms and modes may also
   be implemented in addition to this mandatory algorithm and mode.
   Export and use of encryption are regulated in some countries [OTA94].

3.3 COMBINING SECURITY MECHANISMS

   In some cases the IP Authentication Header might be combined with the
   IP Encapsulating Security Protocol to obtain the desired security
   properties.  The Authentication Header always provides integrity and
   authentication and can provide non-repudiation if used with certain
   authentication algorithms (e.g., RSA).  The Encapsulating Security
   Payload always provides integrity and confidentiality and can also
   provide authentication if used with certain authenticating encryption
   algorithms.  Adding the Authentication Header to a IP datagram prior
   to encapsulating that datagram using the Encapsulating Security
   Protocol might be desirable for users wishing to have strong
   integrity, authentication, confidentiality, and perhaps also for
   users who require strong non-repudiation.  When the two mechanisms
   are combined, the placement of the IP Authentication Header makes
   clear which part of the data is being authenticated.  Details on
   combining the two mechanisms are provided in the IP Encapsulating
   Security Payload specification [At94b].

3.4 OTHER SECURITY MECHANISMS

   Protection from traffic analysis is not provided by any of the
   security mechanisms described above.  It is unclear whether
   meaningful protection from traffic analysis can be provided
   economically at the Internet Layer and it appears that few Internet
   users are concerned about traffic analysis.  One traditional method
   for protection against traffic analysis is the use of bulk link
   encryption.  Another technique is to send false traffic in order to
   increase the noise in the data provided by traffic analysis.
   Reference [VK83] discusses traffic analysis issues in more detail.

4. KEY MANAGEMENT

   The Key Management protocol that will be used with IP layer security
   is not specified in this document.  However, because the key
   management protocol is coupled to AH and ESP only via the Security
   Parameters Index (SPI), we can meaningfully define AH and ESP without
   having to fully specify how key management is performed.  We envision
   that several key management systems will be usable with these
   specifications, including manual key configuration.  Work is ongoing
   within the IETF to specify an Internet Standard key management
   protocol.

   Support for key management methods where the key management data is
   carried within the IP layer is not a design objective for these IP-
   layer security mechanisms.  Instead these IP-layer security
   mechanisms will primarily use key management methods where the key
   management data will be carried by an upper layer protocol, such as
   UDP or TCP, on some specific port number or where the key management
   data will be distributed manually.

   This design permits clear decoupling of the key management mechanism
   from the other security mechanisms, and thereby permits one to
   substitute new and improved key management methods without having to
   modify the implementations of the other security mechanisms.  This
   separation of mechanism is clearly wise given the long history of
   subtle flaws in published key management protocols [NS78, NS81].
   What follows in this section is a brief discussion of a few
   alternative approaches to key management.  Mutually consenting
   systems may additionally use other key management approaches by
   private prior agreement.

4.1 Manual Key Distribution

   The simplest form of key management is manual key management, where a
   person manually configures each system with its own key and also with
   the keys of other communicating systems.  This is quite practical in

   small, static environments but does not scale.  It is not a viable
   medium-term or long-term approach, but might be appropriate and
   useful in many environments in the near-term.  For example, within a
   small LAN it is entirely practical to manually configure keys for
   each system.  Within a single administrative domain it is practical
   to configure keys for each router so that the routing data can be
   protected and to reduce the risk of an intruder breaking into a
   router.  Another case is where an organisation has an encrypting
   firewall between the internal network and the Internet at each of its
   sites and it connects two or more sites via the Internet.  In this
   case, the encrypting firewall might selectively encrypt traffic for
   other sites within the organisation using a manually configured key,
   while not encrypting traffic for other destinations.  It also might
   be appropriate when only selected communications need to be secured.

4.2 Some Existing Key Management Techniques

   There are a number of key management algorithms that have been
   described in the public literature.  Needham & Schroeder have
   proposed a key management algorithm which relies on a centralised key
   distribution system [NS78, NS81].  This algorithm is used in the
   Kerberos Authentication System developed at MIT under Project Athena
   [KB93].  Diffie and Hellman have devised an algorithm which does not
   require a centralised key distribution system [DH76].  Unfortunately,
   the original Diffie-Hellman technique is vulnerable to an active "man
   in the middle" attack [Sch93, p. 43-44].  However, this vulnerability
   can be mitigated by using signed keys to authentically bootstrap into
   the Diffie-Hellman exchange [Sch93, p. 45].

4.3 Automated Key Distribution

   Widespread deployment and use of IP security will require an
   Internet-standard scalable key management protocol.  Ideally such a
   protocol would support a number of protocols in the Internet protocol
   suite, not just IP security.  There is work underway within the IETF
   to add signed host keys to the Domain Name System [EK94] The DNS keys
   enable the originating party to authenticate key management messages
   with the other key management party using an asymmetric algorithm.
   The two parties would then have an authenticatible communications
   channel that could be used to create a shared session key using
   Diffie-Hellman or other means [DH76] [Sch93].

4.4 Keying Approaches for IP

   There are two keying approaches for IP.  The first approach, called
   host-oriented keying, has all users on host 1 share the same key for
   use on traffic destined for all users on host 2.  The second
   approach, called user-oriented keying, lets user A on host 1 have one

   or more unique session keys for its traffic destined for host 2; such
   session keys are not shared with other users on host1.  For example,
   user A's ftp session might use a different key than user A's telnet
   session.  In systems claiming to provide multi-level security, user A
   will typically have at least one key per sensitivity level in use
   (e.g., one key for UNCLASSIFIED traffic, a second key for SECRET
   traffic, and a third key for TOP SECRET traffic).  Similarly, with
   user-oriented keying one might use separate keys for information sent
   to a multicast group and control messages sent to the same multicast
   group.

   In many cases, a single computer system will have at least two
   mutually suspicious users A and B that do not trust each other.  When
   host-oriented keying is used and mutually suspicious users exist, it
   is sometimes possible for user A to determine the host-oriented key
   via well known methods, such as a Chosen Plaintext attack.  Once user
   A has improperly obtained the key in use, user A can then either read
   user B's encrypted traffic or forge traffic from user B.  When user-
   oriented keying is used, certain kinds of attack from one user onto
   another user's traffic are not possible.

   IP Security is intended to be able to provide Authentication,
   Integrity, and Confidentiality for applications operating on
   connected end systems when appropriate algorithms are in use.
   Integrity and Confidentiality can be provided by host-oriented keying
   when appropriate dynamic key management techniques and appropriate
   algorithms are in use.  However, authentication of principals using
   applications on end-systems requires that processes running
   applications be able to request and use their own Security
   Associations.  In this manner, applications can make use of key
   distribution facilities that provide authentication.

   Hence, support for user-oriented keying SHOULD be present in all IP
   implementations, as is described in the "IP Key Management
   Requirements" section below.

4.5 Multicast Key Distribution

   Multicast key distribution is an active research area in the
   published literature as of this writing.  For multicast groups having
   relatively few members, manual key distribution or multiple use of
   existing unicast key distribution algorithms such as modified
   Diffie-Hellman appears feasible.  For very large groups, new scalable
   techniques will be needed.  The use of Core-Based Trees (CBT) to
   provide session key management as well as multicast routing might be
   an approach used in the future [BFC93].

4.6 IP Key Management Requirements

   This section defines key management requirements for all IPv6
   implementations and for those IPv4 implementations that implement the
   IP Authentication Header, the IP Encapsulating Security Payload, or
   both.  It applies equally to the IP Authentication Header and the IP
   Encapsulating Security Payload.

   All such implementations MUST support manual configuration of
   Security Associations.

   All such implementations SHOULD support an Internet standard Security
   Association establishment protocol (e.g., IKMP, Photuris) once such a
   protocol is published as an Internet standards-track RFC.

   Implementations MAY also support other methods of configuring
   Security Associations.

   Given two endpoints, it MUST be possible to have more than one
   concurrent Security Association for communications between them.
   Implementations on multi-user hosts SHOULD support user granularity
   (i.e., "user-oriented") Security Associations.

   All such implementations MUST permit the configuration of host-
   oriented keying.

   A device that encrypts or authenticates IP packets originated other
   systems, for example a dedicated IP encryptor or an encrypting
   gateway, cannot generally provide user-oriented keying for traffic
   originating on other systems.  Such systems MAY additionally
   implement support for user-oriented keying for traffic originating on
   other systems.

   The method by which keys are configured on a particular system is
   implementation-defined.  A flat file containing security association
   identifiers and the security parameters, including the key(s), is an
   example of one possible method for manual key distribution.  An IP
   system MUST take reasonable steps to protect the keys and other
   security association information from unauthorised examination or
   modification because all of the security lies in the keys.

5. USAGE

   This section describes the possible use of the security mechanisms
   provided by IP in several different environments and applications in
   order to give the implementer and user a better idea of how these
   mechanisms can be used to reduce security risks.

5.1 USE WITH FIREWALLS

   Firewalls are not uncommon in the current Internet [CB94].  While
   many dislike their presence because they restrict connectivity, they
   are unlikely to disappear in the near future.  Both of these IP
   mechanisms can be used to increase the security provided by
   firewalls.

   Firewalls used with IP often need to be able to parse the headers and
   options to determine the transport protocol (e.g., UDP or TCP) in use
   and the port number for that protocol.  Firewalls can be used with
   the Authentication Header regardless of whether that firewall is
   party to the appropriate Security Assocation, but a firewall that is
   not party to the applicable Security Association will not normally be
   able to decrypt an encrypted upper-layer protocol to view the
   protocol or port number needed to perform per-packet filtering OR to
   verify that the data (e.g., source, destination, transport protocol,
   port number) being used for access control decisions is correct and
   authentic.  Hence, authentication might be performed not only within
   an organisation or campus but also end to end with remote systems
   across the Internet.  This use of the Authentication Header with IP
   provides much more assurance that the data being used for access
   control decisions is authentic.

   Organisations with two or more sites that are interconnected using
   commercial IP service might wish to use a selectively encrypting
   firewall.  If an encrypting firewall were placed between each site of
   a company and the commercial IP service provider, the firewall could
   provide an encrypted IP tunnel among all the company's sites.  It
   could also encrypt traffic between the company and its suppliers,
   customers, and other affiliates.  Traffic with the Network
   Information Center, with public Internet archives, or some other
   organisations might not be encrypted because of the unavailability of
   a standard key management protocol or as a deliberate choice to
   facilitate better communications, improved network performance, and
   increased connectivity.  Such a practice could easily protect the
   company's sensitive traffic from eavesdropping and modification.

   Some organisations (e.g., governments) might wish to use a fully
   encrypting firewall to provide a protected virtual network over
   commercial IP service.  The difference between that and a bulk IP
   encryption device is that a fully encrypting firewall would provide
   filtering of the decrypted traffic as well as providing encryption of
   IP packets.

5.2 USE WITH IP MULTICAST

   In the past several years, the Multicast Backbone (MBONE) has grown
   rapidly.  IETF meetings and other conferences are now regularly
   multicast with real-time audio, video, and whiteboards.  Many people
   are now using teleconferencing applications based on IP Multicast in
   the Internet or in private internal networks.  Others are using IP
   multicasting to support distributed simulation or other applications.
   Hence it is important that the security mechanisms in IP be suitable
   for use in an environment where multicast is the general case.

   The Security Parameters Indexes (SPIs) used in the IP security
   mechanisms are receiver-oriented, making them well suited for use in
   IP multicast [Atk95a, Atk95b].  Unfortunately, most currently
   published multicast key distribution protocols do not scale well.
   However, there is active research in this area.  As an interim step,
   a multicast group could repeatedly use a secure unicast key
   distribution protocol to distribute the key to all members or the
   group could pre-arrange keys using manual key distribution.

5.3 USE TO PROVIDE QOS PROTECTION

   The recent IAB Security Workshop identified Quality of Service
   protection as an area of significant interest [BCCH].  The two IP
   security mechanisms are intended to provide good support for real-
   time services as well as multicasting.  This section describes one
   possible approach to providing such protection.

   The Authentication Header might be used, with appropriate key
   management, to provide authentication of packets.  This
   authentication is potentially important in packet classification
   within routers.  The IPv6 Flow Identifier might act as a Low-Level
   Identifier (LLID).  Used together, packet classification within
   routers becomes straightforward if the router is provided with the
   appropriate keying material.  For performance reasons the routers
   might authenticate only every Nth packet rather than every packet,
   but this is still a significant improvement over capabilities in the
   current Internet.  Quality of service provisioning is likely to also
   use the Flow ID in conjunction with a resource reservation protocol,
   such as RSVP [ZDESZ93].  Thus, the authenticated packet
   classification can be used to help ensure that each packet receives
   appropriate handling inside routers.

5.4 USE IN COMPARTMENTED OR MULTI-LEVEL NETWORKS

   A multi-level secure (MLS) network is one where a single network is
   used to communicate data at different sensitivity levels (e.g.,
   Unclassified and Secret) [DoD85] [DoD87].  Many governments have

   significant interest in MLS networking [DIA].  The IP security
   mechanisms have been designed to support MLS networking.  MLS
   networking requires the use of strong Mandatory Access Controls
   (MAC), which ordinary users are incapable of controlling or violating
   [BL73].  This section pertains only to the use of these IP security
   mechanisms in MLS environments.

   The Authentication Header can be used to provide strong
   authentication among hosts in a single-level network.  The
   Authentication Header can also be used to provide strong assurance
   for both mandatory access control decisions in multi-level networks
   and discretionary access control decisions in all kinds of networks.
   If explicit IP sensitivity labels (e.g., IPSO) [Ken91] are used and
   confidentiality is not considered necessary within the particular
   operational environment, the Authentication Header is used to provide
   authentication for the entire packet, including cryptographic binding
   of the sensitivity level to the IP header and user data.  This is a
   significant improvement over labeled IPv4 networks where the label is
   trusted even though it is not trustworthy because there is no
   authentication or cryptographic binding of the label to the IP header
   and user data.  IPv6 will normally use implicit sensitivity labels
   that are part of the Security Association but not transmitted with
   each packet instead of using explicit sensitivity labels.  All
   explicit IP sensitivity labels MUST be authenticated using either
   ESP, AH, or both.

   The Encapsulating Security Payload can be combined with appropriate
   key policies to provide full multi-level secure networking.  In this
   case each key must be used only at a single sensitivity level and
   compartment.  For example, Key "A" might be used only for sensitive
   Unclassified packets, while Key "B" is used only for Secret/No-
   compartments traffic, and Key "C" is used only for Secret/No-Foreign
   traffic.  The sensitivity level of the protected traffic MUST NOT
   dominate the sensitivity level of the Security Association used with
   that traffic.  The sensitivity level of the Security Association MUST
   NOT dominate the sensitivity level of the key that belongs to that
   Security Association.  The sensitivity level of the key SHOULD be the
   same as the sensitivity level of the Security Association.  The
   Authentication Header can also have different keys for the same
   reasons, with the choice of key depending in part on the sensitivity
   level of the packet.

   Encryption is very useful and desirable even when all of the hosts
   are within a protected environment.  The Internet-standard encryption
   algorithm could be used, in conjunction with appropriate key
   management, to provide strong Discretionary Access Controls (DAC) in
   conjunction with either implicit sensitivity labels or explicit
   sensitivity labels (such as IPSO provides for IPv4 [Ken91]). Some

   environments might consider the Internet-standard encryption
   algorithm sufficiently strong to provide Mandatory Access Controls
   (MAC).  Full encryption SHOULD be used for all communications between
   multi-level computers or compartmented mode workstations even when
   the computing environment is considered to be protected.

6. SECURITY CONSIDERATIONS

   This entire memo discusses the Security Architecture for the Internet
   Protocol.  It is not a general security architecture for the
   Internet, but is instead focused on the IP layer.

   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].

   If more than one sender uses shares a Security Association with a
   destination, then the receiving system can only authenticate that the
   packet was sent from one of those systems and cannot authenticate
   which of those systems sent it.  Similarly, if the receiving system
   does not check that the Security Association used for a packet is
   valid for the claimed Source Address of the packet, then the
   receiving system cannot authenticate whether the packet's claimed
   Source Address is valid.  For example, if senders "A" and "B" each
   have their own unique Security Association with destination "D" and
   "B" uses its valid Security Association with D but forges a Source
   Address of "A", then "D" will be fooled into believing the packet
   came from "A" unless "D" verifies that the claimed Source Address is
   party to the Security Association that was used.

   Users need to understand that the quality of the security provided by
   the mechanisms provided by these two IP security mechanisms depends
   completely on the strength of the implemented cryptographic
   algorithms, the strength of the key being used, the correct
   implementation of the cryptographic algorithms, the security of the
   key management protocol, and the correct implementation of IP and the
   several security mechanisms in all of the participating systems.  The
   security of the implementation is in part related to the security of
   the operating system which embodies the security implementations.
   For example, if the operating system does not keep the private
   cryptologic keys (that is, all symmetric keys and the private
   asymmetric keys) confidential, then traffic using those keys will not
   be secure.  If any of these is incorrect or insufficiently secure,
   little or no real security will be provided to the user.  Because
   different users on the same system might not trust each other, each
   user or each session should usually be keyed separately.  This will

   also tend to increase the work required to cryptanalyse the traffic
   since not all traffic will use the same key.

   Certain security properties (e.g., traffic analysis protection) are
   not provided by any of these mechanisms.  One possible approach to
   traffic analysis protection is appropriate use of link encryption
   [VK83].  Users must carefully consider which security properties they
   require and take active steps to ensure that their needs are met by
   these or other mechanisms.

   Certain applications (e.g., electronic mail) probably need to have
   application-specific security mechanisms.  Application-specific
   security mechanisms are out of the scope of this document.  Users
   interested in electronic mail security should consult the RFCs
   describing the Internet's Privacy-Enhanced Mail system.  Users
   concerned about other application-specific mechanisms should consult
   the online RFCs to see if suitable Internet Standard mechanisms
   exist.

ACKNOWLEDGEMENTS

   Many of the concepts here are derived from or were influenced by the
   US Government's SDNS security protocol specifications, the ISO/IEC's
   NLSP specification, or from the proposed swIPe security protocol
   [SDNS, ISO, IB93, IBK93].  The work done for SNMP Security and SNMPv2
   Security influenced the choice of default cryptological algorithms
   and modes [GM93].  Steve Bellovin, Steve Deering, Richard Hale,
   George Kamis, Phil Karn, Frank Kastenholz, Perry Metzger, Dave
   Mihelcic, Hilarie Orman and Bill Simpson provided careful critiques
   of early versions of this document.

REFERENCES

   [Atk95a] Atkinson, R., "IP Authentication Header", RFC 1826, NRL,
            August 1995.

   [Atk95b] Atkinson, R., "IP Encapsulating Security Payload", RFC 1827,
            NRL, August 1995.

   [BCCH94] Braden, R., Clark, D., Crocker, S., and C. Huitema, "Report
            of IAB Workshop on Security in the Internet Architecture",
            RFC 1636, USC/Information Sciences Institute, MIT, Trusted
            Information Systems, INRIA, June 1994.

   [Bel89]  Steven M. Bellovin, "Security Problems in the TCP/IP
            Protocol Suite", ACM Computer Communications Review, Vol. 19,
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   [Bel95]  Steven M. Bellovin, Presentation at IP Security Working
            Group Meeting, Proceedings of the 32nd Internet Engineering
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            Danvers, MA.

   [BFC93]  A. Ballardie, P. Francis, & J. Crocroft, "Core Based Trees:
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            Review, Volume. 23, Number 4, October 1993, pp. 85-95.

   [BL73]   Bell, D.E. & LaPadula, L.J., "Secure Computer Systems:
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            M74-244, The MITRE Corporation, Bedford, MA, May 1973.

   [CB94]   William R. Cheswick & Steven M. Bellovin, Firewalls &
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   [DIA]    US Defense Intelligence Agency, "Compartmented Mode
            Workstation Specification", Technical Report
            DDS-2600-6243-87.

   [DoD85]  US National Computer Security Center, "Department of Defense
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            5200.28-STD, US Department of Defense, Ft. Meade, MD.,
            December 1985.

   [DoD87]  US National Computer Security Center, "Trusted Network
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            Ft. Meade, MD., 31 July 1987.

   [DH76]   W. Diffie & M. Hellman, "New Directions in Cryptography",
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            November 1976, pp. 644-654.

   [EK94]   D. Eastlake III & C. Kaufman, "Domain Name System Protocol
            Security Extensions", Work in Progress.

   [GM93]   Galvin J., and K. McCloghrie, "Security Protocols for
            version 2 of the Simple Network Management Protocol
            (SNMPv2)", RFC 1446, Trusted Information Systems, Hughes LAN
            Systems, April 1993.

   [HA94]   Haller, N., and R. Atkinson, "On Internet Authentication",
            RFC 1704, Bell Communications Research, NRL, October 1994.

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

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

   [IB93]  John Ioannidis and Matt Blaze, "Architecture and
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           October 1993.

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

   [Ken91] Kent, S., "US DoD Security Options for the Internet Protocol",
           RFC 1108, BBN Communications, November 1991.

   [Ken93] Kent, S., "Privacy Enhancement for Internet Electronic Mail:
           Part II: Certificate-Based Key Management", RFC 1422,
           BBN Communications, February 1993.

   [KB93]  Kohl, J., and B. Neuman, "The Kerberos Network Authentication
           Service (V5)", RFC 1510, Digital Equipment Corporation,
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   [MS95]  Metzger, P., and W. Simpson, "IP Authentication with Keyed
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   [KMS95] Karn, P., Metzger, P., and W. Simpson, "The ESP DES-CBC
           Transform", RFC 1829, Qualcomm, Inc., Piermont, Daydreamer,
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   [NS81]  R.M. Needham & M.D. Schroeder, "Authentication Revisited",
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   [OTA94] US Congress, Office of Technology Assessment, "Information
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           Government Printing Office, Washington, DC, September 1994.

   [Sch94] Bruce Schneier, Applied Cryptography, Section 8.6,
           John Wiley & Sons, New York, NY, 1994.

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

   [VK83]  V.L. Voydock & S.T. Kent, "Security Mechanisms in High-level
           Networks", ACM Computing Surveys, Vol. 15, No. 2, June 1983.

   [ZDESZ93] Zhang, L., Deering, S., Estrin, D., Shenker, S., and
             D. Zappala, "RSVP: A New Resource ReSerVation Protocol",
             IEEE Network magazine, September 1993.

DISCLAIMER

   The views expressed in this note 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
   design.

AUTHOR'S ADDRESS

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

   Phone:  (202) 767-2389
   Fax:    (202) 404-8590
   EMail: atkinson@itd.nrl.navy.mil