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Network Working Group                                        R. AtkinsonRequest for Comments: 1825                     Naval Research LaboratoryCategory: Standards Track                                    August 1995Security Architecture for the Internet ProtocolStatus 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.Atkinson                    Standards Track                     [Page 1]

RFC 1825              Security Architecture for IP           August 1995   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.Atkinson                    Standards Track                     [Page 2]

RFC 1825              Security Architecture for IP           August 19951.3 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-endAtkinson                    Standards Track                     [Page 3]

RFC 1825              Security Architecture for IP           August 1995   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.Atkinson                    Standards Track                     [Page 4]

RFC 1825              Security Architecture for IP           August 1995   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].Atkinson                    Standards Track                     [Page 5]

RFC 1825              Security Architecture for IP           August 1995   - 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.Atkinson                    Standards Track                     [Page 6]

RFC 1825              Security Architecture for IP           August 1995   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].Atkinson                    Standards Track                     [Page 7]

RFC 1825              Security Architecture for IP           August 19953. 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).Atkinson                    Standards Track                     [Page 8]

RFC 1825              Security Architecture for IP           August 1995   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.Atkinson                    Standards Track                     [Page 9]

RFC 1825              Security Architecture for IP           August 1995   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].Atkinson                    Standards Track                    [Page 10]

RFC 1825              Security Architecture for IP           August 19953.4 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 inAtkinson                    Standards Track                    [Page 11]

RFC 1825              Security Architecture for IP           August 1995   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 oneAtkinson                    Standards Track                    [Page 12]

RFC 1825              Security Architecture for IP           August 1995   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].Atkinson                    Standards Track                    [Page 13]

RFC 1825              Security Architecture for IP           August 19954.6 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.Atkinson                    Standards Track                    [Page 14]

RFC 1825              Security Architecture for IP           August 19955.1 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.Atkinson                    Standards Track                    [Page 15]

RFC 1825              Security Architecture for IP           August 19955.2 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 haveAtkinson                    Standards Track                    [Page 16]

RFC 1825              Security Architecture for IP           August 1995   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]). SomeAtkinson                    Standards Track                    [Page 17]

RFC 1825              Security Architecture for IP           August 1995   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 willAtkinson                    Standards Track                    [Page 18]

RFC 1825              Security Architecture for IP           August 1995   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,            No. 2, March 1989.Atkinson                    Standards Track                    [Page 19]

RFC 1825              Security Architecture for IP           August 1995   [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.   [BFC93]  A. Ballardie, P. Francis, & J. Crocroft, "Core Based Trees:            An Architecture for Scalable Inter-Domain Multicast Routing",            Proceedings of ACM SIGCOMM 93, ACM Computer Communications            Review, Volume. 23, Number 4, October 1993, pp. 85-95.   [BL73]   Bell, D.E. & LaPadula, L.J., "Secure Computer Systems:            Mathematical Foundations and Model", Technical Report            M74-244, The MITRE Corporation, Bedford, MA, May 1973.   [CB94]   William R. Cheswick & Steven M. Bellovin, Firewalls &            Internet Security, Addison-Wesley, Reading, MA, 1994.   [DIA]    US Defense Intelligence Agency, "Compartmented Mode            Workstation Specification", Technical Report            DDS-2600-6243-87.   [DoD85]  US National Computer Security Center, "Department of Defense            Trusted Computer System Evaluation Criteria", DoD            5200.28-STD, US Department of Defense, Ft. Meade, MD.,            December 1985.   [DoD87]  US National Computer Security Center, "Trusted Network            Interpretation of the Trusted Computer System Evaluation            Criteria", NCSC-TG-005, Version 1, US Department of Defense,            Ft. Meade, MD., 31 July 1987.   [DH76]   W. Diffie & M. Hellman, "New Directions in Cryptography",            IEEE Transactions on Information Theory, Vol. IT-22, No. 6,            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.Atkinson                    Standards Track                    [Page 20]

RFC 1825              Security Architecture for IP           August 1995   [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           Implementation of Network-layer Security Under Unix",           Proceedings of 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.   [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,           USC/Information Sciences Institute, September 1993.   [MS95]  Metzger, P., and W. Simpson, "IP Authentication with Keyed           MD5",RFC 1828, Piermont, Daydreamer, August 1995.   [KMS95] Karn, P., Metzger, P., and W. Simpson, "The ESP DES-CBC           Transform",RFC 1829, Qualcomm, Inc., Piermont, Daydreamer,           August 1995.   [NS78]  R.M. Needham & M.D. Schroeder, "Using Encryption for           Authentication in Large Networks of Computers", Communications           of the ACM, Vol. 21, No. 12, December 1978, pp. 993-999.   [NS81]  R.M. Needham & M.D. Schroeder, "Authentication Revisited",           ACM Operating Systems Review, Vol. 21, No. 1., 1981.   [OTA94] US Congress, Office of Technology Assessment, "Information           Security & Privacy in Network Environments", OTA-TCT-606,           Government Printing Office, Washington, DC, September 1994.   [Sch94] Bruce Schneier, Applied Cryptography,Section 8.6,           John Wiley & Sons, New York, NY, 1994.Atkinson                    Standards Track                    [Page 21]

RFC 1825              Security Architecture for IP           August 1995   [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.milAtkinson                    Standards Track                    [Page 22]