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.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . 9
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Collision Considerations . . . . . . . . . . . . . . 12
Appendix B. Changes from RFC 4843 . . . . . . . . . . . . . . . 12
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:
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
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 pre-image 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:
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.
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
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.
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
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.
Authors' Addresses
Julien Laganier
Luminate Wireless, Inc.
Cupertino, CA
USA
EMail: julien.ietf@gmail.com
Francis Dupont
Internet Systems Consortium
EMail: fdupont@isc.org