Internet DRAFT - draft-palombini-core-oscore-edhoc
draft-palombini-core-oscore-edhoc
CoRE Working Group F. Palombini
Internet-Draft Ericsson
Intended status: Standards Track M. Tiloca
Expires: August 23, 2021 R. Hoeglund
RISE AB
S. Hristozov
Fraunhofer AISEC
G. Selander
Ericsson
February 19, 2021
Combining EDHOC and OSCORE
draft-palombini-core-oscore-edhoc-02
Abstract
This document defines an optimization approach for combining the
lightweight authenticated key exchange protocol EDHOC run over CoAP
with the first subsequent OSCORE transaction. This combination
reduces the number of round trips required to set up an OSCORE
Security Context and to complete an OSCORE transaction using that
Security Context.
Status of This Memo
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. EDHOC Option . . . . . . . . . . . . . . . . . . . . . . . . 5
4. EDHOC Combined with OSCORE . . . . . . . . . . . . . . . . . 6
4.1. Client Processing . . . . . . . . . . . . . . . . . . . . 6
4.2. Server Processing . . . . . . . . . . . . . . . . . . . . 7
5. Example of EDHOC + OSCORE Request . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7.1. CoAP Option Numbers Registry . . . . . . . . . . . . . . 10
8. Normative References . . . . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
This document defines an optimization approach to combine the
lightweight authenticated key exchange protocol EDHOC
[I-D.ietf-lake-edhoc], when running over CoAP [RFC7252], with the
first subsequent OSCORE [RFC8613] transaction.
This allows for a minimum number of round trips necessary to setup
the OSCORE Security Context and complete an OSCORE transaction, for
example when an IoT device gets configured in a network for the first
time.
This optimization is desirable, since the number of protocol round
trips impacts the minimum number of flights, which in turn can have a
substantial impact on the latency of conveying the first OSCORE
request, when using certain radio technologies.
Without this optimization, it is not possible, not even in theory, to
achieve the minimum number of flights. This optimization makes it
possible also in practice, since the last message of the EDHOC
protocol can be made relatively small (see Section 1 of
[I-D.ietf-lake-edhoc]), thus allowing additional OSCORE protected
CoAP data within target MTU sizes.
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1.1. Terminology
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.
The reader is expected to be familiar with terms and concepts defined
in CoAP [RFC7252], CBOR [RFC8949], CBOR sequences [RFC8742], OSCORE
[RFC8613] and EDHOC [I-D.ietf-lake-edhoc].
2. Background
EDHOC is a 3-message key exchange protocol. Section 7.2 of
[I-D.ietf-lake-edhoc] specifies how to transport EDHOC over CoAP: the
EDHOC data (referred to as "EDHOC messages") are transported in the
payload of CoAP requests and responses.
This draft deals with the case of the Initiator acting as CoAP Client
and the Responder acting as CoAP Server; instead, the case of the
Initiator acting as CoAP Server cannot be optimized by using this
approach.
That is, the CoAP Client sends a POST request containing EDHOC
message_1 to a reserved resource at the CoAP Server. This triggers
the EDHOC exchange on the CoAP Server, which replies with a 2.04
(Changed) Response containing EDHOC message_2. Finally, the CoAP
Client sends EDHOC message_3, as a CoAP POST request to the same
resource used for EDHOC message_1. The Content-Format of these CoAP
messages may be set to "application/edhoc".
After this exchange takes place, and after successful verifications
specified in the EDHOC protocol, the Client and Server derive the
OSCORE Security Context, as specified in Section 7.2.1 of
[I-D.ietf-lake-edhoc]. Then, they are ready to use OSCORE.
This sequential way of running EDHOC and then OSCORE is specified in
Figure 1. As shown in the figure, this mechanism takes 3 round trips
to complete.
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CoAP Client CoAP Server
| ------------- EDHOC message_1 ------------> |
| |
| <------------ EDHOC message_2 ------------- |
| |
EDHOC verification |
| |
| ------------- EDHOC message_3 ------------> |
| |
| EDHOC verification
| +
OSCORE Sec Ctx OSCORE Sec Ctx
Derivation Derivation
| |
| ------------- OSCORE Request -------------> |
| |
| <------------ OSCORE Response ------------- |
| |
Figure 1: EDHOC and OSCORE run sequentially
The number of roundtrips can be minimized as follows. Already after
receiving EDHOC message_2 and before sending EDHOC message_3, the
CoAP Client has all the information needed to derive the OSCORE
Security Context.
This means that the Client can potentially send at the same time both
EDHOC message_3 and the subsequent OSCORE Request. On a semantic
level, this approach practically requires to send two separate REST
requests at the same time.
The high level message flow of running EDHOC and OSCORE combined is
shown in Figure 2.
Defining the specific details of how to transport the data and of
their processing order is the goal of this specification, as defined
in Section 4.
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CoAP Client CoAP Server
| ------------- EDHOC message_1 ------------> |
| |
| <------------ EDHOC message_2 ------------- |
| |
EDHOC verification |
+ |
OSCORE Sec Ctx |
Derivation |
| |
| ---- EDHOC message_3 + OSCORE Request ----> |
| |
| EDHOC verification
| +
| OSCORE Sec Ctx
| Derivation
| |
| <------------ OSCORE Response ------------- |
| |
Figure 2: EDHOC and OSCORE combined
3. EDHOC Option
This section defines the EDHOC Option, used in a CoAP request to
signal that the request combines EDHOC message_3 and OSCORE protected
data.
The EDHOC Option has the properties summarized in Figure 3, which
extends Table 4 of [RFC7252]. The option is Critical, Safe-to-
Forward, and part of the Cache-Key. The option MUST occur at most
once and is always empty. If any value is sent, the value is simply
ignored. The option is intended only for CoAP requests and is of
Class U for OSCORE [RFC8613].
+-------+---+---+---+---+-------+--------+--------+---------+
| No. | C | U | N | R | Name | Format | Length | Default |
+-------+---+---+---+---+-------+--------+--------+---------+
| TBD13 | x | | | | EDHOC | Empty | 0 | (none) |
+-------+---+---+---+---+-------+--------+--------+---------+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
Figure 3: The EDHOC Option.
The presence of this option means that the message payload contains
also EDHOC data, that must be extracted and processed as defined in
Section 4.2, before the rest of the message can be processed.
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Figure 4 shows the format of a CoAP message containing both the EDHOC
data and the OSCORE ciphertext, using the newly defined EDHOC option
for signalling.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver| T | TKL | Code | Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Token (if any, TKL bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OSCORE option | EDHOC option | other options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: CoAP message for EDHOC and OSCORE combined - signalled with
the EDHOC Option
4. EDHOC Combined with OSCORE
The approach defined in this specification consists of sending EDHOC
message_3 inside an OSCORE protected CoAP message.
The resulting EDHOC + OSCORE request is in practice the OSCORE
Request from Figure 1, sent to a protected resource and with the
correct CoAP method and options, with the addition that it also
transports EDHOC message_3.
Since EDHOC message_3 may be too large to be included in a CoAP
Option, e.g. if containing a large public key certificate chain, it
has to be transported through the CoAP payload.
The use of this approach is explicitly signalled by including an
EDHOC Option (see Section 3) in the EDHOC + OSCORE request.
4.1. Client Processing
The Client prepares an EDHOC + OSCORE request as follows.
1. Compose EDHOC message_3 as per Section 5.4.2 of
[I-D.ietf-lake-edhoc].
Since the Client is the EDHOC Initiator and the used Correlation
Method is 1 (see Section 3.2.4 of [I-D.ietf-lake-edhoc]), the
EDHOC message_3 always includes the Connection Identifier C_R and
CIPHERTEXT_3. Note that C_R is the OSCORE Sender ID of the
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Client, encoded as a bstr_identifier (see Section 5.1 of
[I-D.ietf-lake-edhoc]).
2. Encrypt the original CoAP request as per Section 8.1 of
[RFC8613], using the new OSCORE Security Context established
after receiving EDHOC message_2.
Note that the OSCORE ciphertext is not computed over EDHOC
message_3, which is not protected by OSCORE. That is, the result
of this step is the OSCORE Request as in Figure 1.
3. Build a CBOR sequence [RFC8742] composed of two CBOR byte strings
in the following order.
* The first CBOR byte string is the CIPHERTEXT_3 of the EDHOC
message_3 resulting from step 3.
* The second CBOR byte string has as value the OSCORE ciphertext
of the OSCORE protected CoAP request resulting from step 2.
4. Compose the EDHOC + OSCORE request, as the OSCORE protected CoAP
request resulting from step 2, where the payload is replaced with
the CBOR sequence built at step 3.
5. Signal the usage of this approach within the EDHOC + OSCORE
request, by including the new EDHOC Option defined in Section 3.
4.2. Server Processing
When receiving an EDHOC + OSCORE request, the Server performs the
following steps.
1. Check the presence of the EDHOC option defined in Section 3, to
determine that the received request is an EDHOC + OSCORE request.
If this is the case, the Server continues with the steps defined
below.
2. Extract CIPHERTEXT_3 from the payload of the EDHOC + OSCORE
request, as the first CBOR byte string in the CBOR sequence.
3. Rebuild EDHOC message_3, as a CBOR sequence composed of two CBOR
byte strings in the following order.
* The first CBOR byte string is the 'kid' of the Client
indicated in the OSCORE option of the EDHOC + OSCORE request,
encoded as a bstr_identifier (see Section 5.1 of
[I-D.ietf-lake-edhoc]).
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* The second CBOR byte string is the CIPHERTEXT_3 retrieved at
step 2.
4. Perform the EDHOC processing on the EDHOC message_3 rebuilt at
step 3, including verifications, and the OSCORE Security Context
derivation, as per Section 5.4.3 and Section 7.2.1 of
[I-D.ietf-lake-edhoc], respectively.
5. Extract the OSCORE ciphertext from the payload of the EDHOC +
OSCORE request, as the value of the second CBOR byte string in
the CBOR sequence.
6. Rebuild the OSCORE protected CoAP request as the EDHOC + OSCORE
request, where the payload is replaced with the OSCORE ciphertext
resulting from step 5.
7. Decrypt and verify the OSCORE protected CoAP request resulting
from step 6, as per Section 8.2 of [RFC8613], by using the new
OSCORE Security Context established at step 4.
8. Process the CoAP request resulting from step 7.
If steps 4 (EDHOC processing) and 7 (OSCORE processing) are both
successfully completed, the Server MUST reply with an OSCORE
protected response, in order for the Client to achieve key
confirmation (see Section 5.4.2 of [I-D.ietf-lake-edhoc]). The usage
of EDHOC message_4 as defined in Section 7.1 of [I-D.ietf-lake-edhoc]
is not applicable to the approach defined in this specification.
If step 4 (EDHOC processing) fails, the server discontinues the
protocol as per Section 5.4.3 of [I-D.ietf-lake-edhoc] and sends an
EDHOC error message, formatted as defined in Section 6.1 of
[I-D.ietf-lake-edhoc]. In particular, the CoAP response conveying
the EDHOC error message:
o MUST have Content-Format set to application/edhoc defined in
Section 9.5 of [I-D.ietf-lake-edhoc].
o MUST specify a CoAP error response code, i.e. 4.00 (Bad Request)
in case of client error (e.g. due to a malformed EDHOC message_3),
or 5.00 (Internal Server Error) in case of server error (e.g. due
to failure in deriving EDHOC key material).
If step 4 (EDHOC processing) is successfully completed but step 7
(OSCORE processing) fails, the same OSCORE error handling applies as
defined in Section 8.2 of [RFC8613].
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5. Example of EDHOC + OSCORE Request
An example based on the OSCORE test vector from Appendix C.4 of
[RFC8613] and the EDHOC test vector from Appendix B.2 of
[I-D.ietf-lake-edhoc] is given in Figure 5. In particular, the
example assumes that:
o The used OSCORE Partial IV is 0, consistently with the first
request protected with the new OSCORE Security Context.
o The OSCORE Sender ID of the Client is 0x20. This corresponds to
the EDHOC Connection Identifier C_R, which is encoded as the
bstr_identifier 0x08 in EDHOC message_3.
o The EDHOC option is registered with CoAP option number 13.
o OSCORE option value: 0x090020 (3 bytes)
o EDHOC option value: - (0 bytes)
o C_R: 0x20 (1 byte)
o CIPHERTEXT_3: 0x5253c3991999a5ffb86921e99b607c067770e0
(19 bytes)
o EDHOC message_3: 0x08 5253c3991999a5ffb86921e99b607c067770e0
(20 bytes)
o OSCORE ciphertext: 0x612f1092f1776f1c1668b3825e (13 bytes)
From there:
o Protected CoAP request (OSCORE message):
0x44025d1f ; CoAP 4-byte header
00003974 ; Token
39 6c6f63616c686f7374 ; Uri-Host Option: "localhost"
63 090020 ; OSCORE Option
40 ; EDHOC Option
ff 5253c3991999a5ffb86921e99b607c067770e0
4d612f1092f1776f1c1668b3825e
(57 bytes)
Figure 5: Example of CoAP message with EDHOC and OSCORE combined
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6. Security Considerations
The same security considerations from OSCORE [RFC8613] and EDHOC
[I-D.ietf-lake-edhoc] hold for this document.
TODO (more considerations)
7. IANA Considerations
This document has the following actions for IANA.
7.1. CoAP Option Numbers Registry
IANA is asked to enter the following option numbers to the "CoAP
Option Numbers" registry defined in [RFC7252] within the "CoRE
Parameters" registry.
[
The CoAP option numbers 13 and 21 are both consistent with the
properties of the EDHOC Option defined in Section 3, and they both
allow the EDHOC Option to always result in an overall size of 1 byte.
This is because:
o The EDHOC option is always empty, i.e. with zero-length value; and
o Since the OSCORE option with option number 9 is always present in
the CoAP request, the EDHOC option would be encoded with a maximum
delta of 4 or 12, depending on its option number being 13 or 21.
At the time of writing, the CoAP option numbers 13 and 21 are both
unassigned in the "CoAP Option Numbers" registry, as first available
and consistent option numbers for the EDHOC option.
]
+--------+-------+-------------------+
| Number | Name | Reference |
+--------+-------+-------------------+
| TBD13 | EDHOC | [[this document]] |
+--------+-------+-------------------+
8. Normative References
[I-D.ietf-lake-edhoc]
Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
Diffie-Hellman Over COSE (EDHOC)", draft-ietf-lake-
edhoc-03 (work in progress), December 2020.
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[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/info/rfc2119>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[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/info/rfc8174>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
[RFC8742] Bormann, C., "Concise Binary Object Representation (CBOR)
Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
<https://www.rfc-editor.org/info/rfc8742>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
Acknowledgments
The authors sincerely thank Christian Amsuess, Klaus Hartke, Jim
Schaad and Malisa Vucinic for their feedback and comments in the
discussion leading up to this draft.
The work on this document has been partly supported by VINNOVA and
the Celtic-Next project CRITISEC; and by the H2020 project SIFIS-Home
(Grant agreement 952652).
Authors' Addresses
Francesca Palombini
Ericsson
Email: francesca.palombini@ericsson.com
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Marco Tiloca
RISE AB
Isafjordsgatan 22
Kista SE-16440 Stockholm
Sweden
Email: marco.tiloca@ri.se
Rikard Hoeglund
RISE AB
Isafjordsgatan 22
Kista SE-16440 Stockholm
Sweden
Email: rikard.hoglund@ri.se
Stefan Hristozov
Fraunhofer AISEC
Email: stefan.hristozov@aisec.fraunhofer.de
Goeran Selander
Ericsson
Email: goran.selander@ericsson.com
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