amsuess-t2trg-onion-coap-01.txt

Internet DRAFT - draft-amsuess-t2trg-onion-coap

draft-amsuess-t2trg-onion-coap

Last Version:	draft-amsuess-t2trg-onion-coap-01.txt	Tracker Entry
Date:	`05-Jul-2023`
Disposition:	expired

t2trg C. Amsüss
Internet-Draft
Intended status: Experimental M. Tiloca
Expires: 5 January 2024 R. Höglund
RISE AB
4 July 2023

Using onion routing with CoAP
draft-amsuess-t2trg-onion-coap-01

Abstract

The CoAP protocol was designed with direct connections and proxies in
mind. This document defines mechanisms by which chains of proxies
can be set up. In combination, they enable the operation of hidden
services and client similar to how Tor (The Onion Router) enables it
for TCP based protocols.

Discussion Venues

This note is to be removed before publishing as an RFC.

Discussion of this document takes place on the Thing-to-Thing
Research Group mailing list (t2trg@irtf.org), which is archived at
https://mailarchive.ietf.org/arch/search/?email_list=t2trg.

Source for this draft and an issue tracker can be found at
https://gitlab.com/chrysn/onion-coap.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on 5 January 2024.

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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Components overview . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Client proxy chain . . . . . . . . . . . . . . . . . . . 4
3.1.1. Guidance for setting up proxy chains . . . . . . . . 5
3.2. Server proxy chain . . . . . . . . . . . . . . . . . . . 6
3.3. Proxy discovery . . . . . . . . . . . . . . . . . . . . . 6
3.4. Naming and name resolution . . . . . . . . . . . . . . . 7
3.5. Establishing TLS connections between proxies . . . . . . 7
3.6. Other tricks . . . . . . . . . . . . . . . . . . . . . . 8
3.7. Overhead and optimizations . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Normative References . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 12
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12

1. Introduction

[ See also abstract. ]

1.1. Components overview

This document introduces separate mechanisms that in combination
enable setups similar to how Tor is used for anonymous web access.
Some of the mechanisms need no new protocol components, but merely
describe which existing steps are used to obtain the desired results.

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* A client can use EDHOC to establish a unilaterally authenticated
OSCORE context with proxies (see Section 3.1).

* A server can use EDHOC to establish a unilaterally authenticated
OSCORE context to establish a reverse proxy address (see
Section 3.2).

* A discovery mechanism for proxies (see Section 3.3).

* A naming and discovery mechanism for hidden services (see
Section 3.4).

Note that these mechanisms should be largely independent: A server
that does not intend to hide its position can still advertise a
cryptographic name at its real network coordinates, and thus be
available both to clients that do hide their location (even if their
proxies do not work as “exit nodes” in Tor terminology) and to
clients on a local network.

Figure 1 illustrates an example topology, and Figure 2 illustrates a
cross-section of the OSCORE layers along one path.

P1 P2
^ \ ^ |
| \ / |
Client 1 -> P3 | v P4 / | P5 ---> Server 1
\ | (published as | ^
\ | server addr.) | /
v | | /
P6 v /
P7

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Figure 1: Example topology of an onion style CoAP network showing
two routes in separate graphs. Note that the hop P4-->P2 is
present in both chains, and can be pooled into one TLS connection

Client 1 P3 P6 P1 P4 P2 P7 P5 Server 1
11--- 13--- 16--- ---6 ---4 ---2 TLS connections
18------------------------------------------------------ End-to-end OSCORE
17--------------------------- ------------------------7
14--------------------- ------------------5
12--------------- ------------3
10--------- ------1

Figure 2: Cross section of the Client 1 --> Server 1 connection
of [ TBD reference from label ]. Numbers indicate the sequence
in which EDHOC is performed, and are placed on the side of the
initiator. Not depicted at step 8: Server 1 and/or P4 (TBD)
publishes P4 as the public address of the hidden service; 9:
client obtains the list of available proxies. 15: Client 1 looks
up the introduction point of Server 1 through the proxy chain up
to P1 to discover P4.

2. Conventions and Definitions

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”,
“SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and
“OPTIONAL” in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Mechanisms

3.1. Client proxy chain

A client can pick one or more proxies to hide its position in the
network.

Without OSCORE proxies, only one proxy hop can be chosen, because the
CoAP requests contains at most two addresses: The address in the IP
header, and the address in the Uri-Host option. With the mechanisms
introduced in [I-D.tiloca-core-oscore-capable-proxies], CoAP request
can contain a Uri-Host option in each layer of OSCORE, effectively
building a source routing chain.

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To build the chain, the client first chooses its first proxy hop, and
runs EDHOC to establish an OSCORE context. In this process, the
proxy authenticates with its long-term credentials, whereas the
client uses an ephemeral key (a plain CWT Cliams Set, [RFC8392]).
The process can take as little as one round-trip per proxy; when
message 3 of EDHOC is sent along with the OSCORE message (see
[I-D.ietf-core-oscore-edhoc]) that contains the next hop’s message 1,

Once one proxy context is established, EDHOC can be run through that
proxy with the next proxy, until a chain of sufficient length has
been established. Care has to be taken to never use one of the later
proxies with any chain other than the chain through which the
connection was established, for otherwise the client can be
deanonymized mor easily.

When forwarding messages, every forward proxy strips off a layer of
OSCORE from the request, and adds one to the response.

Possible optimizations:

* Can EDHOC be run without transmitting two public keys (G_X and
G_I) for the client? (I.e., Can G_X be re-used as G_I without
harm to EDHOC (likely not), and how would that be communicated?)

* For hops that are only ever used with a single next-hop, as is
typical with all but the first proxy (see guidance below): Can
default values for Proxy-Scheme and Uri-Host be communicated
during EDHOC, values that would later be elided? Otherwise, every
request would contain explicit addresses of the full chain. If
taken to the extreme, this might be setting up a SCHC context that
also compresses parts of the OSCORE option, where the client tells
each proxy what the KID used with the next proxy is, and uses the
same sender sequence number for the hops. (This has own security
considerations; might be necessary to apply offsets, at which
point it gets overly complex).

Effectively, setting a default value for Proxy-Scheme and Uri-Host
makes that (originally forward) proxy a reverse proxy.

3.1.1. Guidance for setting up proxy chains

TBD: This section should contain guidance distilled from Tor
operations. In particular, it might recommend that a client pick one
proxy hop as a long-term first hop, while building the remaining
chain individually for each new origin server.

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Following common tor practice, it is expected that typical chain
lengths are around 3 hops. Note that the amount of processing on the
peer side is independent of the length of the chain chosen by a host.
If a client deems a one-hop setup sufficient and only has resources
for maintaining one extra OSCORE context, it can still use a server
that is hidden behind a 3 long proxy chain.

3.2. Server proxy chain

A server can pick one or more proxies to hide its position in the
network.

Unlike forward proxies, which are configured per request, this
requires a dedicated mechanism.

TBD: This document does not yet specify such a mechanism, but may
draw upon the reverse proxy request of Section 2 of
[I-D.amsuess-core-resource-directory-extensions].

When forwarding messages, every reverse proxy adds a layer of OSCORE
to the request, and removes one from the response.

3.3. Proxy discovery

A mechanism for discovering forward proxies is already described in
[I-D.ietf-core-transport-indication]; discovery of reverse proxies
suitable for servers will depend on the precise mechanism used.

Both proxies’ discovery process may need to be augmented with
metadata that indicates whether the proxy is willing to proxy to
arbitrary locations on the Internet, or merely to hidden peers. That
distinction in forwrd proxies would be similar to how Tor
distinguishes relay and exit nodes. In reverse proxies, there is an
analogous distinction that is not so much based on policy but rather
on the structure of the authority component used by that reverse
proxy: If the name is resolvable on regular CoAP stacks (i.e., DNS
can resolve it to a global IP address), then regular CoAP clients can
use the introduction address as an entry point. It is still up to
the hidden server’s policy to decide whether to allow access that is
merely protected by the chain of hidden proxies, but not end-to-end.
Note that the server will be unable to tell whether the client used
just one layer of EDHOC and OSCORE to reach the introduction point,
or has built its own client chain -- the former is merely a client
proxy chain of zero length.

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Along with discovering the addresses of proxies, a well-maintained
Tor-like network would provide authentication information for them.
This would allow participants to trust that the proxy chain they are
building is not all controlled by a single entity, but have been
around independently for some time.

TBD: Such a service is not specified yet.

3.4. Naming and name resolution

The mechanisms discussed in [I-D.amsuess-t2trg-rdlink] can be used by
hidden services to come up with names for their services. (That
document will need to be updated to use mechanisms from
[I-D.ietf-core-transport-indication]). Unlike described there, they
would not enter their network address into the distributed directory,
but the address of their most remote reverse proxy (the introduction
point).

Along with the service’s public key (that is announced as part of the
name), the published record may also include the public key of the
introduction point, as that will allow the client to establish an
extra layer with the introduction point. TBD: The benefits of this
layer are yet unclear, as is whether this needs to be a layer of
OSCORE, or whether the mechanisms of [I-D.selander-lake-authz] could
be used to roll that into the end-to-end establishment. (Likely, an
extra layer is needed, for even if that mechanism is used for some
kind of encrypted origin indication, the later OSOCRE phase will need
some origin indication at the introduction point to distinguish
multiple hidden services behind the introduction point in such a way
that an observer of the introduction’s ingress point can not tell
which services are being used).

3.5. Establishing TLS connections between proxies

Proxy-to-proxy requests, which are the majority of transmitted
request, are transmitted between unconstrained devices across the
Internet. As such, protecting those connections with an extra layer
of TLS (as specified in [RFC8323]) is desirable, because

* it profits from TCP's flow control,

* it hides more request metadata (even though none of the metadata
visible at this point should be linkable to the server or the
client, with the exception of message length), and

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* it allows requests to be mixed across MTU and thus to hide their
length and number (provided there is sufficient traffic on the
link to ensure that multiple messages are processed within one
Nagle interval).

[ TBD: Explore whether coercing traffic through specific pairs of
nodes instead of random node pairings would make sense. If it is
dangerous, maybe servers might pair up on their own to ensure that it
is hard to monitor their ingress and egress traffic for correlation.
]

A challenge in establishing TLS connections on that link is that
proxies are advertised with EDHOC credentials in the network’s
discovery area. The tools of [I-D.tschofenig-tls-cwt] may help
bridging that gap.

3.6. Other tricks

TBD. Current ideas:

* For increased privacy, it may make sense to spool requests and
responses in proxies for "as long as practical". Those setting up
the proxy may indicate that in the security context. While it
increases the proxy's memory requirements a lot to keep several
seconds of traffic around, it is not expected that these proxies
will be operating at network capacity limits.

* Add chatter between proxies. With the stark contrast between
constrained device bandwidths and Internet bandwidths, this can be
tolerable.

* Access point assistance: While all of this is aimed at constrained
devices as defined in [RFC7228], devices of Class 1 may not be
capable of using the full proxy discovery service or setting up
security contexts with all the hops in the chain. Instead, they
may only provide end-to-end encryption, and use a service provided
by a local node (e.g. the border router in a 6LoWPAN network
[RFC8138]) to set up the hops. Such a setup can also be used to
defer policy decisions to the network, which may decide to
advertise its own address as an introduction point, or use a
suitable length of reverse proxies.

* The introduction point would be the only suitable location to
place a caching proxy when [I-D.amsuess-core-cachable-oscore] is
used. As the server is be aware that cacheable OSCORE is used, it
can select a reverse proxy that was advertised with caching
capabilities (with that metadatum still TBD).

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3.7. Overhead and optimizations

TBD. Main points:

* Establishing a default Uri-Host likely gives most savings.

* For intermediate hops, using a shorter AEAD tag might be an
option.

4. Security Considerations

* When using proxy chains, only contact a proxy through the one
chain it is set up with, and only accept messages into a context
if they were transported in the hop they are expected to be
received from.

It is of utmost importance to not have observably different
behavior between messages with an unknown context and messages
whose context is known but not expected at this point. For
example, if an attacker controls a server’s introduction point and
intends to deanonymize clients, it may attempt to send responses
directly to the suspected address of the client.

In implementations, this can be mitigated by first looking up the
list of contexts depending on the outer layer, and then looking up
inside that list whether the security context is known and the
message expected.

* What are the effects of sequence numbers on correlation? Is it
good or bad to use the same sequence number for all hops in a
chain?

5. IANA Considerations

TBD.

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

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6.2. Informative References

[I-D.amsuess-core-cachable-oscore]
Amsüss, C. and M. Tiloca, "Cacheable OSCORE", Work in
Progress, Internet-Draft, draft-amsuess-core-cachable-
oscore-06, 11 January 2023,
<https://datatracker.ietf.org/doc/html/draft-amsuess-core-
cachable-oscore-06>.

[I-D.amsuess-core-resource-directory-extensions]
Amsüss, C., "CoRE Resource Directory Extensions", Work in
Progress, Internet-Draft, draft-amsuess-core-resource-
directory-extensions-08, 13 March 2023,
<https://datatracker.ietf.org/doc/html/draft-amsuess-core-
resource-directory-extensions-08>.

[I-D.amsuess-t2trg-rdlink]
Amsüss, C., "rdlink: Robust distributed links to
constrained devices", Work in Progress, Internet-Draft,
draft-amsuess-t2trg-rdlink-01, 23 September 2019,
<https://datatracker.ietf.org/doc/html/draft-amsuess-
t2trg-rdlink-01>.

[I-D.ietf-core-oscore-edhoc]
Palombini, F., Tiloca, M., Höglund, R., Hristozov, S., and
G. Selander, "Using EDHOC with CoAP and OSCORE", Work in
Progress, Internet-Draft, draft-ietf-core-oscore-edhoc-07,
13 March 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-oscore-edhoc-07>.

[I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
and J. Park, "Group Object Security for Constrained
RESTful Environments (Group OSCORE)", Work in Progress,
Internet-Draft, draft-ietf-core-oscore-groupcomm-18, 22
June 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-core-oscore-groupcomm-18>.

[I-D.ietf-core-transport-indication]
Amsüss, C., "CoAP Protocol Indication", Work in Progress,
Internet-Draft, draft-ietf-core-transport-indication-02,
13 March 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-transport-indication-02>.

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[I-D.ietf-lake-edhoc]
Selander, G., Mattsson, J. P., and F. Palombini,
"Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
Progress, Internet-Draft, draft-ietf-lake-edhoc-19, 3
February 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-lake-edhoc-19>.

[I-D.selander-lake-authz]
Selander, G., Mattsson, J. P., Vučinić, M., Richardson,
M., and A. Schellenbaum, "Lightweight Authorization for
EDHOC", Work in Progress, Internet-Draft, draft-selander-
lake-authz-02, 21 April 2023,
<https://datatracker.ietf.org/doc/html/draft-selander-
lake-authz-02>.

[I-D.tiloca-core-oscore-capable-proxies]
Tiloca, M. and R. Höglund, "OSCORE-capable Proxies", Work
in Progress, Internet-Draft, draft-tiloca-core-oscore-
capable-proxies-06, 5 April 2023,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
oscore-capable-proxies-06>.

[I-D.tiloca-core-oscore-capable-proxies-06]
Tiloca, M. and R. Höglund, "OSCORE-capable Proxies", Work
in Progress, Internet-Draft, draft-tiloca-core-oscore-
capable-proxies-06, 5 April 2023,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
oscore-capable-proxies-06>.

[I-D.tschofenig-tls-cwt]
Tschofenig, H. and M. Brossard, "Using CBOR Web Tokens
(CWTs) in Transport Layer Security (TLS) and Datagram
Transport Layer Security (DTLS)", Work in Progress,
Internet-Draft, draft-tschofenig-tls-cwt-02, 13 July 2020,
<https://datatracker.ietf.org/doc/html/draft-tschofenig-
tls-cwt-02>.

[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/rfc/rfc7228>.

[RFC8138] Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
"IPv6 over Low-Power Wireless Personal Area Network
(6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
April 2017, <https://www.rfc-editor.org/rfc/rfc8138>.

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[RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
RFC 8323, DOI 10.17487/RFC8323, February 2018,
<https://www.rfc-editor.org/rfc/rfc8323>.

[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/rfc/rfc8392>.

Appendix A. Change log

Since -00:

* Shaped into separate sections on the bits and pieces involved.

* Added illustrations.

* Moved all points from the previous notes in with the new text.

Since [I-D.tiloca-core-oscore-capable-proxies-06]:

* The main body of the text was moved here and will be absent from
the -07 version of that document.

* An abstract was added.

Acknowledgments

TBD.

Authors' Addresses

Christian Amsüss
Austria
Email: christian@amsuess.com

Marco Tiloca
RISE AB
Isafjordsgatan 22
SE-16440 Kista
Sweden
Email: marco.tiloca@ri.se

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Rikard Höglund
RISE AB
Isafjordsgatan 22
SE-16440 Kista
Sweden
Email: rikard.hoglund@ri.se

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