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.

Copyright Notice

Copyright © 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.

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

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:

• Statistical uniqueness (see also Appendix A).

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

• 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.

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

• 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 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).

 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

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.

4. Design Choices

The design of this namespace faces two competing forces:

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

• 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.

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.

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

• Name: ORCHIDv2

• RFC: RFC 7343

• Allocation Date: 2014-07

• Termination Date: N/A

• Source: True

• Destination: True

• Forwardable: True

• Global: True

• 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.

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.
   
   [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.

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

• Updated HIP references to revised HIP specifications.

• 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.

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

• Removed the discussion on overlay routing.

Authors' Addresses

Julien Laganier
Luminate Wireless, Inc.
Cupertino, CA
USA

   EMail: julien.ietf@gmail.com

Francis Dupont
Internet Systems Consortium

EMail:

          fdupont@isc.org