An IPv6 Prefix for Overlay Routable Cryptographic Hash Identifiers Version 2 (ORCHIDv2)
RFC 7343

Internet Engineering Task Force (IETF)                       J. Laganier
Request for Comments: 7343                       Luminate Wireless, Inc.
Obsoletes: 4843                                                F. Dupont
Category: Standards Track                    Internet Systems Consortium
ISSN: 2070-1721                                           September 2014

                           An IPv6 Prefix for
  Overlay Routable Cryptographic Hash Identifiers Version 2 (ORCHIDv2)

Abstract

   This document specifies an updated Overlay Routable Cryptographic
   Hash Identifiers (ORCHID) format that obsoletes that in RFC 4843.
   These identifiers are intended to be used as endpoint identifiers at
   applications and Application Programming Interfaces (APIs) and not as
   identifiers for network location at the IP layer, i.e., locators.
   They are designed to appear as application-layer entities and at the
   existing IPv6 APIs, but they should not appear in actual IPv6
   headers.  To make them more like regular IPv6 addresses, they are
   expected to be routable at an overlay level.  Consequently, while
   they are considered non-routable addresses from the IPv6-layer
   perspective, all existing IPv6 applications are expected to be able
   to use them in a manner compatible with current IPv6 addresses.

   The Overlay Routable Cryptographic Hash Identifiers originally
   defined in RFC 4843 lacked a mechanism for cryptographic algorithm
   agility.  The updated ORCHID format specified in this document
   removes this limitation by encoding, in the identifier itself, an
   index to the suite of cryptographic algorithms in use.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7343.

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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Rationale and Intent  . . . . . . . . . . . . . . . . . .   3
     1.2.  ORCHID Properties . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Expected Use of ORCHIDs . . . . . . . . . . . . . . . . .   5
     1.4.  Action Plan . . . . . . . . . . . . . . . . . . . . . . .   5
     1.5.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Cryptographic Hash Identifier Construction  . . . . . . . . .   5
   3.  Routing and Forwarding Considerations . . . . . . . . . . . .   7
   4.  Design Choices  . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Collision Considerations . . . . . . . . . . . . . .  13
   Appendix B.  Changes from RFC 4843  . . . . . . . . . . . . . . .  13

1.  Introduction

   This document introduces Overlay Routable Cryptographic Hash
   Identifiers (ORCHID), a new class of identifiers that are like IP
   addresses.  These identifiers are intended to be globally unique in a
   statistical sense (see Appendix A), non-routable at the IP layer, and
   routable at some overlay layer.  The identifiers are securely bound,
   via a secure hash function, to the concatenation of an input
   bitstring and a context tag.  Typically, but not necessarily, the
   input bitstring will include a suitably encoded public cryptographic
   key.

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1.1.  Rationale and Intent

   These identifiers are expected to be used at the existing IPv6
   Application Programming Interfaces (APIs) and application protocols
   between consenting hosts.  They may be defined and used in different
   contexts, suitable for different overlay protocols.  Examples of
   these include Host Identity Tags (HITs) in the Host Identity Protocol
   (HIP) [HIPv2] and Temporary Mobile Identifiers (TMIs) for Mobile IPv6
   Privacy Extension [PRIVACYTEXT].

   As these identifiers are expected to be used along with IPv6
   addresses at both applications and APIs, coordination is desired to
   make sure that an ORCHID is not inappropriately taken for a regular
   IPv6 address and vice versa.  In practice, allocation of a separate
   prefix for ORCHIDs seems to suffice, making them compatible with IPv6
   addresses at the upper layers while simultaneously making it trivial
   to prevent their use at the IP layer.

   While being technically possible to use ORCHIDs between consenting
   hosts without any coordination with the IETF and the IANA, the IETF
   would consider such practice potentially dangerous.  A specific
   danger would be realized if the IETF community later decided to use
   the ORCHID prefix for some different purpose.  In that case, hosts
   using the ORCHID prefix would be, for practical purposes, unable to
   use the prefix for the other new purpose.  That would lead to partial
   balkanization of the Internet, similar to what has happened as a
   result of historical hijackings of IPv4 addresses that are not RFC
   1918 [RFC1918] for private use.

   The whole need for the proposed allocation grows from the desire to
   be able to use ORCHIDs with existing applications and APIs.  This
   desire leads to the potential conflict, mentioned above.  Resolving
   the conflict requires the proposed allocation.

   One can argue that the desire to use these kinds of identifiers via
   existing APIs is architecturally wrong, and there is some truth in
   that argument.  Indeed, it would be more desirable to introduce a new
   API and update all applications to use identifiers, rather than
   locators, via that new API.  That is exactly what we expect to happen
   in the long run.

   However, given the current state of the Internet, we do not consider
   it viable to introduce any changes that, at once, require
   applications to be rewritten and host stacks to be updated.  Rather
   than that, we believe in piece-wise architectural changes that
   require only one of the existing assets to be touched.  ORCHIDs are
   designed to address this situation: to allow people to implement with
   protocol stack extensions, such as secure overlay routing, HIP, or

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   Mobile IP privacy extensions, without requiring them to update their
   applications.  The goal is to facilitate large-scale deployments with
   minimum user effort.

   For example, at the time of this writing, there already exist HIP
   implementations that run fully in user space, using the operating
   system to divert a certain part of the IPv6 address space to a user-
   level daemon for HIP processing.  In practical terms, these
   implementations are already using a certain IPv6 prefix for
   differentiating HIP identifiers from IPv6 addresses, allowing them
   both to be used by the existing applications via the existing APIs.

   The Overlay Routable Cryptographic Hash Identifiers originally
   defined in [RFC4843] lacked a mechanism for cryptographic algorithm
   agility.  The updated ORCHID format specified in this document
   removes this limitation by encoding, in the identifier itself, an
   index to the suite of cryptographic algorithms in use.

   Because the updated ORCHIDv2 format is not backward compatible, IANA
   has allocated a new 28-bit prefix out of the IANA IPv6 Special
   Purpose Address Block, namely 2001:0000::/23, as per [RFC6890].  The
   prefix that was temporarily allocated for the experimental ORCHID was
   returned to IANA in March 2014 [RFC4843].

1.2.  ORCHID Properties

   ORCHIDs are designed to have the following properties:

   o  Statistical uniqueness (see also Appendix A).

   o  Secure binding to the input parameters used in their generation
      (i.e., the Context Identifier and a bitstring).

   o  Aggregation under a single IPv6 prefix.  Note that this is only
      needed due to the coordination need as indicated above.  Without
      such coordination need, the ORCHID namespace could potentially be
      completely flat.

   o  Non-routability at the IP layer, by design.

   o  Routability at some overlay layer, making them, from an
      application point of view, semantically similar to IPv6 addresses.

   As mentioned above, ORCHIDs are intended to be generated and used in
   different contexts, as suitable for different mechanisms and
   protocols.  The Context Identifier is meant to be used to
   differentiate between the different contexts; see Appendix A for a

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   discussion of the related API issues implementation issues and
   Section 4 for the design choices explaining why the Context
   Identifiers are used.

1.3.  Expected Use of ORCHIDs

   Examples of identifiers and protocols that are expected to adopt the
   ORCHID format include Host Identity Tags (HITs) in the Host Identity
   Protocol [HIPv2] and the Temporary Mobile Identifiers (TMIs) in the
   Simple Privacy Extension for Mobile IPv6 [PRIVACYTEXT].  The format
   is designed to be extensible to allow other experimental proposals to
   share the same namespace.

1.4.  Action Plan

   This document requests IANA to allocate a prefix out of the IPv6
   addressing space for Overlay Routable Cryptographic Hash Identifiers.

1.5.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Cryptographic Hash Identifier Construction

   An ORCHID is generated using the ORCHID Generation Algorithm (OGA).
   The algorithm takes a bitstring and a Context Identifier as input and
   produces an ORCHID as output.  The hash function used in the ORCHID
   Generation Algorithm is defined for each OGA identifier by the
   specification for the respective usage context (e.g., HIPv2).

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 Input      :=  any bitstring
 OGA ID     :=  4-bit Orchid Generation Algorithm identifier
 Hash Input :=  Context ID | Input
 Hash       :=  Hash_function( Hash Input )
 ORCHID     :=  Prefix | OGA ID | Encode_96( Hash )

 where:

 |               : Denotes concatenation of bitstrings

 Input           : A bitstring that is unique or statistically unique
                   within a given context.  The bitstring is intended
                   to be associated with the to-be-created ORCHID in
                   the given context.

 Context ID      : A randomly generated value defining the expected
                   usage context for the particular ORCHID and the
                   hash function to be used for generation of ORCHIDs
                   in this context.  These values are allocated out of
                   the namespace introduced for Cryptographically
                   Generated Addresses (CGA) Type Tags (see RFC 3972 and
                   http://www.iana.org/assignments/cga-message-types).

 OGA ID          : A 4-bit-long identifier for the Hash_function
                   in use within the specific usage context.

 Hash_function   : The one-way hash function (i.e., hash function
                   with preimage resistance and second-preimage
                   resistance) to be used as identified by the
                   value for the OGA ID according document
                   defining the context usage identified by the
                   Context ID.  For example, version 2 of the
                   HIP specification defines truncated SHA1 [RFC3174] as
                   the hash function to be used to generate
                   ORCHIDv2 in the HIPv2 protocol when the
                   OGA ID is 3 [HIPv2].

 Encode_96( )    : An extraction function in which output is obtained
                   by extracting the middle 96-bit-long bitstring
                   from the argument bitstring.

 Prefix          : A constant 28-bit-long bitstring value
                   (2001:20::/28).

   To form an ORCHID, two pieces of input data are needed.  The first
   piece can be any bitstring, but it is typically expected to contain a
   public cryptographic key and some other data.  The second piece is a

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   Context Identifier, which is a 128-bit-long datum, allocated as
   specified in Section 6.  Each specific ORCHIDv2 application (such as
   HIP HITs or MIP6 TMIs) is expected to allocate their own, specific
   Context Identifier.

   The input bitstring and Context Identifier are concatenated to form
   an input datum, which is then fed to the cryptographic hash function
   to be used for the value of the OGA identifier according to the
   document defining the context usage identified by the Context ID.
   The result of the hash function is processed by an encoding function,
   resulting in a 96-bit-long value.  This value is prepended with the
   concatenation of the 28-bit ORCHID prefix and the 4-bit OGA ID.  The
   result is the ORCHID, a 128-bit-long bitstring that can be used at
   the IPv6 APIs in hosts participating to the particular experiment.

   The ORCHID prefix is allocated under the IPv6 global unicast address
   block.  Hence, ORCHIDs are indistinguishable from IPv6 global unicast
   addresses.  However, it should be noted that ORCHIDs do not conform
   with the IPv6 global unicast address format defined in Section 2.5.4
   of [RFC4291] since they do not have a 64-bit Interface ID formatted
   as described in Section 2.5.1. of [RFC4291].

3.  Routing and Forwarding Considerations

   ORCHIDs are designed to serve as location-independent endpoint
   identifiers rather than IP-layer locators.  Therefore, routers MAY be
   configured not to forward any packets containing an ORCHID as a
   source or a destination address.  If the destination address is an
   ORCHID but the source address is a valid unicast source address,
   routers MAY be configured to generate an ICMP Destination
   Unreachable, Administratively Prohibited message.

   ORCHIDs are not designed for use in IPv6 routing protocols, since
   such routing protocols are based on the architectural definition of
   IPv6 addresses.  Future non-IPv6 routing systems, such as overlay
   routing systems, may be designed based on ORCHIDs.  Any such ORCHID-
   based routing system is out of scope of this document.

   Router software MUST NOT include any special handling code for
   ORCHIDs.  In other words, the non-routability property of ORCHIDs, if
   implemented, is to be implemented via configuration rather than by
   hardwired software code, e.g., the ORCHID prefix can be blocked by a
   simple configuration rule such as an Access Control List entry.

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4.  Design Choices

   The design of this namespace faces two competing forces:

   o  As many bits as possible should be preserved for the hash result.

   o  It should be possible to share the namespace between multiple
      mechanisms.

   The desire to have a long hash result requires that the prefix be as
   short as possible and use few (if any) bits for additional encoding.
   The present design takes this desire to the maximum: all the bits
   beyond the prefix and the ORCHID Generation Algorithm Identifier are
   used as hash output.  This leaves no bits in the ORCHID itself
   available for identifying the context; however, the 4 bits used to
   encode the ORCHID Generation Algorithm Identifier provides
   cryptographic agility with respect to the hash function in use for a
   given context (see Section 5).

   The desire to allow multiple mechanisms to share the namespace has
   been resolved by including the Context Identifier in the hash
   function input.  While this does not allow the mechanism to be
   directly inferred from an ORCHID, it allows one to verify that a
   given input bitstring and ORCHID belong to a given context, with high
   probability (but also see Section 5).

5.  Security Considerations

   ORCHIDs are designed to be securely bound to the Context ID and the
   bitstring used as the input parameters during their generation.  To
   provide this property, the ORCHID Generation Algorithm relies on the
   second-preimage resistance (a.k.a. one-way) property of the hash
   function used in the generation [RFC4270].  To have this property and
   to avoid collisions, it is important that the allocated prefix is as
   short as possible, leaving as many bits as possible for the hash
   output.

   For a given Context ID, all mechanisms using ORCHIDs MUST use exactly
   the same mechanism for generating an ORCHID from the input bitstring.
   Allowing different mechanisms, without explicitly encoding the
   mechanism in the Context ID or the ORCHID itself, would allow
   so-called bidding-down attacks.  That is, if multiple different hash
   functions were allowed to construct ORCHIDs valid for the same
   Context ID, and if one of the hash functions became insecure, that
   would allow attacks against even those ORCHIDs valid for the same
   Context ID that had been constructed using the other, still secure
   hash functions.

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   An identifier for the hash function to be used for the ORCHID
   generation is encoded in the ORCHID itself, while the semantic for
   the values taken by this identifier are defined separately for each
   Context ID.  Therefore, the present design allows the use of
   different hash functions per given Context ID for constructing
   ORCHIDs from input bitstrings.  The intent is that the protocol or
   application using an ORCHIDv2 allocates a Context ID for that use and
   defines, within the scope of that Context ID, the registry for the
   ORCHID Generation Algorithm (OGA) ID.  The rationale for this is to
   allow different applications to use different hash functions that
   best satisfy their specific requirements, such that the relatively
   small OGA ID namespace (4 bits wide, i.e., 16 different values) does
   not get exhausted too quickly.  If more secure hash functions are
   later needed, newer values for the ORCHID Generation Algorithm can be
   defined for the given Context ID.

   In order to preserve a low enough probability of collisions (see
   Appendix A), each method MUST utilize a mechanism that makes sure
   that the distinct input bitstrings are either unique or statistically
   unique within that context.  There are several possible methods to
   ensure this; for example, one can include into the input bitstring a
   globally maintained counter value, a pseudorandom number of
   sufficient entropy (minimum 96 bits), or a randomly generated public
   cryptographic key.  The Context ID makes sure that input bitstrings
   from different contexts never overlap.  These together make sure that
   the probability of collisions is determined only by the probability
   of natural collisions in the hash space and is not increased by a
   possibility of colliding input bitstrings.

   The generation of an ORCHIDv2 identifier from an input bitstring
   involves truncation of a hash output to construct a fixed-size
   identifier in a fashion similar to the scheme specified in "Naming
   Things with Hashes" [RFC6920].  Accordingly, the Security
   Considerations of [RFC6920] pertaining to truncation of the hash
   output during identifier generation are also applicable to ORCHIDv2
   generation.

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6.  IANA Considerations

   Because the updated ORCHIDv2 format is not backward compatible with
   the earlier one, IANA has allocated a new 28-bit prefix out of the
   IANA IPv6 Special Purpose Address Block, namely 2001:0000::/23, as
   per [RFC6890].  The prefix that was temporarily allocated for the
   experimental ORCHID was returned to IANA in March 2014 [RFC4843].
   The registry information for the allocation is as follows:

   o  Address Block: 2001:20::/28

   o  Name: ORCHIDv2

   o  RFC: RFC 7343

   o  Allocation Date: 2014-07

   o  Termination Date: N/A

   o  Source: True

   o  Destination: True

   o  Forwardable: True

   o  Global: True

   o  Reserved-by-Protocol: False

   The Context Identifier (or Context ID) is a randomly generated value
   defining the usage context of an ORCHID and the hash function to be
   used for generation of ORCHIDs in this context.  This document
   defines no specific value.  The Context ID shares the namespace
   introduced for CGA Type Tags.  Hence, defining new values follows the
   rules of Section 8 of [RFC3972], i.e., First Come, First Served.
   However, no IANA actions are required.

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7.  Contributors

   Pekka Nikander (pekka.nikander@nomadiclab.com) co-authored an
   earlier, experimental version of this specification [RFC4843].

8.  Acknowledgments

   Special thanks to Geoff Huston for his sharp but constructive
   critique during the development of this memo.  Tom Henderson helped
   to clarify a number of issues.  This document has also been improved
   by reviews, comments, and discussions originating from the IPv6,
   Internet Area, and IETF communities.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005.

9.2.  Informative References

   [HIPv2]    Moskowitz, R., Heer, T., Jokela, P., and T. Henderson,
              "Host Identity Protocol Version 2 (HIPv2)", Work in
              Progress, July 2014.

   [PRIVACYTEXT]
              Dupont, F., "A Simple Privacy Extension for Mobile IPv6",
              Work in Progress, July 2006.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets", BCP
              5, RFC 1918, February 1996.

   [RFC3174]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, September 2001.

   [RFC4270]  Hoffman, P. and B. Schneier, "Attacks on Cryptographic
              Hashes in Internet Protocols", RFC 4270, November 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

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   [RFC4843]  Nikander, P., Laganier, J., and F. Dupont, "An IPv6 Prefix
              for Overlay Routable Cryptographic Hash Identifiers
              (ORCHID)", RFC 4843, April 2007.

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153, RFC
              6890, April 2013.

   [RFC6920]  Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
              Keranen, A., and P. Hallam-Baker, "Naming Things with
              Hashes", RFC 6920, April 2013.

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Appendix A.  Collision Considerations

   As noted earlier, the aim is that so long as keys are not reused,
   ORCHIDs be globally unique in a statistical sense.  That is, given
   the ORCHID referring to a given entity, the probability of the same
   ORCHID being used to refer to another entity elsewhere in the
   Internet must be sufficiently low so that it can be ignored for most
   practical purposes.  We believe that the presented design meets this
   goal (see Section 4).

   As mentioned above, ORCHIDs are expected to be used at the legacy
   IPv6 APIs between consenting hosts.  The Context ID is intended to
   differentiate between the various experiments, or contexts, sharing
   the ORCHID namespace.  However, the Context ID is not present in the
   ORCHID itself but is only in front of the input bitstring as an input
   to the hash function.  While this may lead to certain implementation-
   related complications, we believe that the trade-off of allowing the
   hash result part of an ORCHID being longer more than pays off the
   cost.

   Because ORCHIDs are not routable at the IP layer, in order to send
   packets using ORCHIDs at the API level, the sending host must have
   additional overlay state within the stack to determine which
   parameters (e.g., what locators) to use in the outgoing packet.  An
   underlying assumption here, and a matter of fact in the proposals
   that the authors are aware of, is that there is an overlay protocol
   for setting up and maintaining this additional state.  It is assumed
   that the state-setup protocol carries the input bitstring and that
   the resulting ORCHID-related state in the stack can be associated
   back with the appropriate context and state-setup protocol.

Appendix B.  Changes from RFC 4843

   o  Updated HIP references to revised HIP specifications.

   o  The Overlay Routable Cryptographic Hash Identifiers originally
      defined in [RFC4843] lacked a mechanism for cryptographic
      algorithm agility.  The updated ORCHID format specified in this
      document removes this limitation by encoding, in the identifier
      itself, an index to the suite of cryptographic algorithms in use.

   o  Moved the "Collision Considerations" section into an appendix and
      removed unnecessary discussions.

   o  Removed the discussion on overlay routing.

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Authors' Addresses

   Julien Laganier
   Luminate Wireless, Inc.
   Cupertino, CA
   USA

   EMail: julien.ietf@gmail.com

   Francis Dupont
   Internet Systems Consortium

   EMail: fdupont@isc.org

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