Network Working Group P. Nikander
Internet-Draft Ericsson Research Nomadic Lab
Expires: September 2, 2006 J. Laganier
DoCoMo Euro-Labs
F. Dupont
CELAR
March 1, 2006
An IPv6 Prefix for Overlay Routable Cryptographic Hash Identifiers
(ORCHID)
draft-laganier-ipv6-khi-01
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Abstract
This document introduces Overlay Routable Cryptographic Hash
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Identifiers (ORCHID) as a new, experimental class of IPv6-address-
like identifiers. These identifiers are intended to be used as end-
point identifiers at applications and 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 vanilla IPv6 addresses, they are expected to be
routable at an overlay level. Consequently, while they are
considered as non-routable addresses from the IPv6 layer point of
view, all existing IPv6 applications are expected to be able to use
them in a manner compatible with current IPv6 addresses.
This document requests IANA to allocate a temporary prefix out of the
IPv6 addressing space for Overlay Routable Cryptographic Hash
Identifiers.
1. Introduction
This document introduces Overlay Routable Cryptographic Hash
Identifiers (ORCHID), a new class of IP-address-like identifiers.
These identifiers are intended to be globally unique in a statistical
sense (see Section 4), 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 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
(HIT) in the Host Identity Protocol (HIP) [I-D.ietf-hip-base] and
Temporary Mobile Identifiers (TMI) for Mobile IPv6 Privacy Extension
[I-D.dupont-mip6-privacyext].
As these identifiers are expected to be used alongside with IPv6
addresses at both applications and APIs, co-ordination is desired to
make sure that an ORCHID is not inappropriately taken for a vanilla
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 usage at the IP layer.
While being technically possible to use ORCHIDs between consenting
hosts without any co-ordination with the IETF and the IANA, the
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authors would consider such practice potentially dangerous. A
specific danger would be realised 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 balkanisation of the Internet, similar to what has
happened as a result of historical hijackings of non-RFC1918 IPv4
addresses 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 longer 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 experiment
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
experiments with minimum user effort.
For example, there already exists, at the time of this writing, 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, those
implementations are already now 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.
This document argues for no more than allocating an experimental
prefix for such purposes, thereby paving the way for large-scale
experiments with cryptographic identifiers without the dangers caused
by address-space hijacking.
1.2. ORCHID properties
ORCHIDs are designed to have the following properties:
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o Statistically uniqueness; see also Section 4
o Secure binding to the input parameters used in their generation
(i.e., the context identifier and a bitstring.)
o Conformance with the IPv6 global unicast address format as defined
in Section 2.5.4 of [RFC3513].
o Aggregation under a single IPv6 prefix. Note that this is only
needed due to the co-ordination need, as indicated above. Without
such co-ordination need, the ORCHID name space 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 Section 4 for a
discussion of the related API and kernel level implementation issues,
and Section 5 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 (HIT) in the Host Identity
Protocol [I-D.ietf-hip-base] and the Temporary Mobile Identifiers
(TMI) in the Simple Privacy Extension for Mobile IPv6 [I-D.dupont-
mip6-privacyext]. The format is designed to be extensible to allow
other experimental proposals to share the same name space.
1.4. Action plan
This document requests IANA to allocate an experimental prefix out of
the IPv6 addressing space for Overlay Routable Cryptographic Hash
Identifiers.
2. Cryptographic Hash Identifier Construction
An ORCHID is generated using the algorithm below. The algorithm
takes a bitstring and a context identifier as input and produces an
ORCHID as output.
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Input := any bitstring
Hash Input := Context ID | Input
Hash := SHA1( Expand( Hash Input ) )
ORCHID := Prefix | Encode_n( Hash )
where:
| : Denotes concatenation of bitstrings
Input : A bitstring 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.
As a baseline (TO BE DISCUSSED), we propose sharing the
name space introduced for CGA Type Tags; see
http://www.iana.org/assignments/cga-message-types
and RFC 3972.
Expand( ) : An expansion function designed to overcome recent
attacks on SHA-1.
As a baseline (TO BE DISCUSSED), we propose using the
method defined in [I-D.irtf-cfrg-sha1-ime].
Alternatively, it would be possible to use some other
hash function, such as SHA-256, instead of SHA-1.
Encode_n( ): An extraction function which output is obtained by
extracting an <n>-bits-long bitstring from the argument
bitstring.
As a baseline (TO BE DISCUSSED), we propose taking
<n> middlemost bits from the SHA1 output.
Prefix : A constant ( 128 - <n> bits long ) bitstring value,
TBD, assigned by IANA.
To form an ORCHID, two pieces of input data are needed. The first
piece can be any bitstring, but is typically expected to contain a
public cryptographic key and some other data. The second piece is a
context identifier, which is an 128-bits-long datum, allocated as
specified in Section 7. Each specific experiment (such as HIP HITs
or MIP6 TMIs) is expected to allocate their own, specific context
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identifier.
The input bitstring and context identifier are concatenated to form
an input datum, which is then fed to a cryptographic hash function.
The result of the hash function is processed by an encoding function,
resulting in an n-bits-long value. This value is prepended with the
ORCHID prefix. The result is the ORCHID, an 128-bits-long bitstring
that can be used at the IPv6 APIs in hosts participating to the
particular experiment.
3. Routing Considerations
ORCHIDs are designed to serve as location independent end-point-
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 a
ORCHID but the source address is a valid unicast source address,
routers MAY be configured to generate an ICMP Destination
Unreachable, Administratively Prohibited message.
Due to the experimental nature of ORCHIDs, router software MUST NOT
include any special handling code for ORCHIDs. In other words, the
non-routability property of ORCHIDs, if implemented, MUST be
implemented via configuration and NOT by hard-wired software code.
At this time, it is RECOMMENDED that the default router configuration
does not handle ORCHIDs in any special way. In other words, there is
no need to touch existing or new routers due to this experiment. If
such reason should later appear, for example, due to a faulty
implementation leaking ORCHIDs to the IP layer, the prefix can be and
should be blocked by a simple configuration rule.
3.1. Overlay Routing
As mentioned multiple times, ORCHIDs are designed to be non-routable
at the IP layer. However, there are multiple ongoing research
efforts for creating various overlay routing and resolution
mechanisms for flat identifiers. For example, the Host Identity
Indirection Infrastructure (Hi3) [hi3] proposal outlines a way for
using a Distributed Hash Table to forward HIP packets based on the
Host Identity Tag.
What is common to the various research proposals is that they create
a new kind of resolution or routing infrastructure on the top of the
existing Internet routing structure. In practical terms, they allow
delivery of packets based on flat, non-routable identifiers,
utilising information stored in a distributed data base. Usually the
database used is based on Distributed Hash Tables. This effectively
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creates a new routing network on the top of the existing IP-based
routing network, capable of routing packets that are not addressed by
IP addresses but some other kind of identifiers.
Typical benefits from overlay routing include location independence,
more scalable multi-cast, any-cast, and multi-homing support than in
IP, and better DoS resistance than in the vanilla Internet. The main
drawback is typically an order of magnitude slower performance,
caused by an easily largish number of extra look-up or forwarding
steps needed. Consequently, in most practical cases the overlay
routing system is used only during initial protocol state set-up (cf.
TCP handshake), after which the communicating end-points exchange
packets directly with IP, bypassing the overlay network.
The net result of the typical overlay routing approaches is a
communication service whose basic functionality is comparable to that
of provided by classical IP but that provides considerably better
resilience that vanilla IP in dynamic networking environments. Some
experiments also introduce additional functionality, such as enhanced
security or ability to effectively route through several IP
addressing domains.
The authors expect ORCHIDs to become fully routable, via one or more
overlay systems, before the end of the experiment.
4. Collision Considerations
As noted above, the aim is that ORCHIDs are 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 5.
Consider next the very rare case that some ORCHID happens to refer to
two different entities at the same time at two different locations in
the Internet. Even in that case the probability of this fact
becoming visible (and therefore a matter of consideration) at any
single location in the Internet is negligible. For the vast majority
of cases the two simultaneous uses of the ORCHID will never cross
each other. However, while rare such collisions are still possible.
This section gives reasonable guidelines on how to mitigate the
consequences in the case such a collision happens.
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
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the ORCHID name space. However, the context ID is not present in the
ORCHID itself, but 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.
Now, 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 in order to determine
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-set-up 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-set-up protocol.
Even though ORCHID collisions are expected to be extremely rare, two
kinds of collisions may still happen. First, it is possible that two
different input bitstrings within the same context may map to the
same ORCHID. In that case, the state-set-up mechanism is expected to
resolve the conflict, for example, by indicating to the peer that the
ORCHID in question is already in use.
A second type of collision may happen if two input bitstrings, used
in different usage contexts, map to the same ORCHID. In this case
the main confusion is about which context to use. In order to
prevent these types of collisions, it is RECOMMENDED that
implementations that simultaneously support multiple different
contexts maintain a node-wide unified database of known ORCHIDs, and
indicate a conflict if any of the mechanisms attempt to register a
ORCHID that is already in use. For example, if a given ORCHID is
already being used as a HIT in HIP, it cannot simultaneously be used
as a TMI in Mobile IP. Instead, if Mobile IP attempts to use the
ORCHID, it will be notified (by the kernel) that the ORCHID in
question is already in use.
5. Design Choices
The design of this name space faces two competing forces:
As many bits as possible should be preserved for the hash result.
It should be possible to share the name space between multiple
mechanisms.
The desire to have a long hash result requires the prefix to be as
short as possible, and to use few (if any) bits for additional
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encoding. The present design takes this desire to the maxim: all the
bits beyond the prefix are used as hash output. This leaves no bits
in the ORCHID itself available for identifying the context.
Additionally, due to security considerations, the present design
REQUIRES that the hash function used in constructing ORCHIDs be
constant; see Section 6.
The authors explicitly considered including a hash extension
mechanism, similar to the one in CGA [RFC3972], but decided to leave
it out. There were two reasons: desire for simplicity, and the
somewhat unclear IPR situation around the hash extension mechanism.
If there is a future revision of this document, we strongly advise
the future authors to reconsider the decision.
The desire to allow multiple mechanism to share the name space 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 a ORCHID, it allows one to verify that a given
input bitstring and ORCHID belong to a given context, with high
probability; but see also Section 6.
6. Security Considerations
ORCHIDs are designed to be securely bound to the context identifier
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.
All mechanism using ORCHIDs MUST use exactly the same mechanism for
generating a ORCHID from the input bitstring. Allowing different
mechanisms, without explicitly encoding the mechanism in the ORCHID
itself, would allow so called bidding down attacks. That is, if
multiple different hash functions were allowed in constructing
ORCHIDs in a given shared name space, and if one of the hash
functions became insecure, that would allow attacks against even
those ORCHIDs that had been constructed using the other, still secure
hash functions.
Due to the desire to keep the hash output value as long as possible,
the present design allows only one method for constructing ORCHIDs
from input bitstrings. If other methods (perhaps using more secure
hash functions) are later needed, they MUST use a different prefix.
Consequently, the suggested method to react to the hash result
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becoming too short, due to increased computational power or to the
used hash function becoming insecure due to advances in cryptology,
is to allocate a new prefix and cease to use the present one.
As of today, SHA-1 applied in conjunction with a proper expansion
function of the hash input is considered as satisfying the second-
preimage resistance requirement [I-D.irtf-cfrg-sha1-ime]. Hash
output of at least 100 bits, but preferably up to 120 bits, is
considered to have a low enough probability of collisions.
In order to preserve a low enough probability of collisions (see
Section 4), 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 that; for example, one can include into the input bitstring a
globally maintained counter value, a pseudo-random number of
sufficient entropy (minimum 120 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 bit strings.
7. IANA Considerations
IANA is requested to allocate a temporary non-routable prefix from
the IPv6 address space. As per Sections 2.5.1 and 2.5.4 of
[RFC3513], the prefix must be allocated from the 0000::/3 block,
since ORCHIDs do not have a 64-bit interface identifier part. The
allocation will require updating
http://www.iana.org/assignments/ipv6-address-space
As a baseline (TO BE DISCUSSED), we propose an 8-bit prefix to be
allocated from the 1000::/4 block. During the discussions related to
this draft, it was suggested that other identifier spaces may be
later allocated from this block. However, this document does not
define such a policy or allocations.
The Context Identifier (or Context ID) is a randomly generated value
defining the usage context of a ORCHID. This document defines no
specific value.
As a baseline (TO BE DISCUSSED), we propose sharing the name space
introduced for CGA Type Tags. Hence, defining new values would
follow the rules of Section 8 of [RFC3972], i.e., on a First Come
First Served basis. The policy will require updating the policy for
http://www.iana.org/assignments/cga-message-types
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8. Acknowledgments
Julien Laganier is partly funded by Ambient Networks, a research
project supported by the European Commission under its Sixth
Framework Program.
Special thanks to Geoff Huston for his sharp but constructive critic
during the development of this memo. Tom Henderson helped to clarify
a number of issues.
9. Version history
9.1. -00 to -01
The name Keyed Hash Identifier (KHI) was replaced with Overlay
Routable Cryptographic Hash Identifier (ORCHID). However, the draft
name was not changed.
More text added to explain the rationale behind the proposed
allocation.
Text changed to emphasise that while ORCHIDs are expected to be non-
routable at the IP-layer, they are expected to become fully routable
and/or resolvable at some upper, overlay layer, thereby making their
basic semantics fully compatible with IPv6 addresses.
Removed the proposed expiration date. If such an expiration date is
needed, it can be added later during the discussions.
10. References
10.1. Normative references
[I-D.irtf-cfrg-sha1-ime]
Blumenthal, U., Jutla, C., and A. Patthak, "SHA1-IME: A
SHA-1 Variant with Provably Good Message Expansion Code",
November 2005.
[RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
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10.2. Informative references
[I-D.dupont-mip6-privacyext]
Dupont, F., "A Simple Privacy Extension for Mobile IPv6",
draft-dupont-mip6-privacyext-03 (work in progress),
January 2006.
[I-D.ietf-hip-base]
Moskowitz, R., "Host Identity Protocol",
draft-ietf-hip-base-00 (work in progress), June 2004.
[RFC3587] Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global
Unicast Address Format", RFC 3587, August 2003.
[RFC4270] Hoffman, P. and B. Schneier, "Attacks on Cryptographic
Hashes in Internet Protocols", RFC 4270, November 2005.
[hi3] Nikander, P., Arkko, J., and B. Ohlman, "Host Identity
Indirection Infrastructure (Hi3)", Nov 2004.
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Authors' Addresses
Pekka Nikander
Ericsson Research Nomadic Lab
JORVAS FI-02420
FINLAND
Phone: +358 9 299 1
Email: pekka.nikander@nomadiclab.com
Julien Laganier
DoCoMo Communications Laboratories Europe GmbH
Landsberger Strasse 312
Munich 80687
Germany
Phone: +49 89 56824 231
Email: julien.ietf@laposte.net
URI: http://www.docomolab-euro.com/
Francis Dupont
CELAR
Email: Francis.Dupont@point6.net
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