CoRE Working Group G. Selander
Internet-Draft J. Mattsson
Intended status: Standards Track F. Palombini
Expires: November 4, 2017 Ericsson AB
L. Seitz
SICS Swedish ICT
May 03, 2017
Object Security of CoAP (OSCOAP)
draft-ietf-core-object-security-03
Abstract
This document defines Object Security of CoAP (OSCOAP), a method for
application layer protection of the Constrained Application Protocol
(CoAP), using the CBOR Object Signing and Encryption (COSE). OSCOAP
provides end-to-end encryption, integrity and replay protection to
CoAP payload, options, and header fields, as well as a secure message
binding. OSCOAP is designed for constrained nodes and networks and
can be used across intermediaries and over any layer. The use of
OSCOAP is signaled with the CoAP option Object-Security, also defined
in this document.
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
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This Internet-Draft will expire on November 4, 2017.
Copyright Notice
Copyright (c) 2017 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. The Object-Security Option . . . . . . . . . . . . . . . . . 5
3. The Security Context . . . . . . . . . . . . . . . . . . . . 6
3.1. Security Context Definition . . . . . . . . . . . . . . . 6
3.2. Derivation of Security Context Parameters . . . . . . . . 9
3.3. Requirements on the Security Context Parameters . . . . . 10
4. Protected CoAP Message Fields . . . . . . . . . . . . . . . . 11
4.1. CoAP Payload . . . . . . . . . . . . . . . . . . . . . . 12
4.2. CoAP Header . . . . . . . . . . . . . . . . . . . . . . . 12
4.3. CoAP Options . . . . . . . . . . . . . . . . . . . . . . 12
5. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 18
5.1. Plaintext . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2. Additional Authenticated Data . . . . . . . . . . . . . . 19
6. Sequence Numbers, Replay, Message Binding, and Freshness . . 20
6.1. AEAD Nonce Uniqueness . . . . . . . . . . . . . . . . . . 20
6.2. Replay Protection . . . . . . . . . . . . . . . . . . . . 20
6.3. Sequence Number and Replay Window State . . . . . . . . . 21
6.4. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 22
6.5. Delay and Mismatch Attacks . . . . . . . . . . . . . . . 23
7. Processing . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.1. Protecting the Request . . . . . . . . . . . . . . . . . 23
7.2. Verifying the Request . . . . . . . . . . . . . . . . . . 23
7.3. Protecting the Response . . . . . . . . . . . . . . . . . 25
7.4. Verifying the Response . . . . . . . . . . . . . . . . . 25
8. OSCOAP Compression . . . . . . . . . . . . . . . . . . . . . 26
8.1. Encoding of the Object-Security Option . . . . . . . . . 27
8.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 28
9. Web Linking . . . . . . . . . . . . . . . . . . . . . . . . . 29
10. Security Considerations . . . . . . . . . . . . . . . . . . . 29
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 31
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
12.1. CoAP Option Numbers Registry . . . . . . . . . . . . . . 32
12.2. Media Type Registrations . . . . . . . . . . . . . . . . 32
12.3. CoAP Content Format Registration . . . . . . . . . . . . 33
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
14.1. Normative References . . . . . . . . . . . . . . . . . . 34
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14.2. Informative References . . . . . . . . . . . . . . . . . 35
Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 36
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 36
B.1. Secure Access to Sensor . . . . . . . . . . . . . . . . . 36
B.2. Secure Subscribe to Sensor . . . . . . . . . . . . . . . 37
Appendix C. Object Security of Content (OSCON) . . . . . . . . . 39
C.1. Overhead OSCON . . . . . . . . . . . . . . . . . . . . . 40
C.2. MAC Only . . . . . . . . . . . . . . . . . . . . . . . . 41
C.3. Signature Only . . . . . . . . . . . . . . . . . . . . . 41
C.4. Authenticated Encryption with Additional Data (AEAD) . . 42
C.5. Symmetric Encryption with Asymmetric Signature (SEAS) . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43
1. Introduction
The Constrained Application Protocol (CoAP) is a web application
protocol, designed for constrained nodes and networks [RFC7228].
CoAP specifies the use of proxies for scalability and efficiency. At
the same time CoAP [RFC7252] references DTLS [RFC6347] for security.
Proxy operations on CoAP messages require DTLS to be terminated at
the proxy. The proxy therefore not only has access to the data
required for performing the intended proxy functionality, but is also
able to eavesdrop on, or manipulate any part of the CoAP payload and
metadata, in transit between client and server. The proxy can also
inject, delete, or reorder packages since they are no longer
protected by DTLS.
This document defines Object Security of CoAP (OSCOAP), a data object
based security protocol, protecting CoAP message exchanges end-to-
end, across intermediary nodes. An analysis of end-to-end security
for CoAP messages through intermediary nodes is performed in
[I-D.hartke-core-e2e-security-reqs], this specification addresses the
forwarding case. In addition to the core features defined in
[RFC7252], OSCOAP supports Observe [RFC7641] and Blockwise [RFC7959].
OSCOAP is designed for constrained nodes and networks and provides an
in-layer security protocol for CoAP which does not depend on
underlying layers. OSCOAP can be used anywhere that CoAP can be
used, including unreliable transport [RFC7228], reliable transport
[I-D.ietf-core-coap-tcp-tls], and non-IP transport
[I-D.bormann-6lo-coap-802-15-ie]. OSCOAP may also be used to protect
group communication for CoAP [I-D.tiloca-core-multicast-oscoap]. The
use of OSCOAP does not affect the URI scheme and OSCOAP can therefore
be used with any URI scheme defined for CoAP. The application
decides the conditions for which OSCOAP is required.
OSCOAP builds on CBOR Object Signing and Encryption (COSE)
[I-D.ietf-cose-msg], providing end-to-end encryption, integrity,
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replay protection, and secure message binding. A compressed version
of COSE is used, see Section 8. The use of OSCOAP is signaled with
the CoAP option Object-Security, defined in Section 2. OSCOAP
provides protection of CoAP payload, certain options, and header
fields. The solution transforms an unprotected CoAP message into a
protected CoAP message in the following way: the unprotected CoAP
message is protected by including payload (if present), certain
options, and header fields in a COSE object. The message fields that
have been encrypted are removed from the message whereas the Object-
Security option and the compressed COSE object are added, see
Figure 1.
Client Server
| request: |
| GET example.com |
| [Header, Token, Options:{..., |
| Object-Security:COSE object}] |
+---------------------------------------------->|
| response: |
| 2.05 (Content) |
| [Header, Token, Options:{..., |
| Object-Security:-}, Payload:COSE object] |
|<----------------------------------------------+
| |
Figure 1: Sketch of OSCOAP
OSCOAP may be used in extremely constrained settings, where CoAP over
DTLS may be prohibitive e.g. due to large code size. Alternatively,
OSCOAP can be combined with DTLS, thereby enabling end-to-end
security of e.g. CoAP payload and options, in combination with hop-
by-hop protection of the entire CoAP message, during transport
between end-point and intermediary node. Examples of the use of
OSCOAP are given in Appendix B.
The message protection provided by OSCOAP can alternatively be
applied only to the payload of individual messages. We call this
object security of content (OSCON), which is defined in Appendix C.
1.1. 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]. These
words may also appear in this document in lowercase, absent their
normative meanings.
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Readers are expected to be familiar with the terms and concepts
described in CoAP [RFC7252], Observe [RFC7641], Blockwise [RFC7959],
COSE [I-D.ietf-cose-msg], CBOR [RFC7049], CDDL
[I-D.greevenbosch-appsawg-cbor-cddl], and constrained environments
[RFC7228].
The terms Common/Sender/Recipient Context, Master Secret/Salt, Sender
ID/Key/IV, Recepient ID/Key/IV and Context IV are defined in
Section 3.1.
2. The Object-Security Option
The Object-Security option (see Figure 2) indicates that OSCOAP is
used to protect the CoAP message exchange. The Object-Security
option is critical, safe to forward, part of the cache key, not
repeatable, and opaque.
+-----+---+---+---+---+-----------------+--------+--------+
| No. | C | U | N | R | Name | Format | Length |
+-----+---+---+---+---+-----------------+--------+--------|
| TBD | x | | | | Object-Security | opaque | 0- |
+-----+---+---+---+---+-----------------+--------+--------+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
Figure 2: The Object-Security Option
A successful response to a request with the Object-Security option
SHALL contain the Object-Security option. A CoAP endpoint SHOULD NOT
cache a response to a request with an Object-Security option, since
the response is only applicable to the original client's request.
The Object-Security option is included in the cache key for backward
compatibility with proxies not recognizing the Object-Security
option. The effect is that messages with the Object-Security option
will never generate cache hits. For Max-Age processing, see
Section 4.3.1.1.
The protection is achieved by means of a COSE object (see Section 5),
which is compressed and then included in the protected CoAP message.
The placement of the COSE object depends on whether the method/
response code allows payload (see [RFC7252]):
o If the method/response code allows payload, then the compressed
COSE object Section 8 is the payload of the protected message, and
the Object-Security option has length zero. An endpoint receiving
a CoAP message with payload, that also contains a non-empty
Object-Security option SHALL treat it as malformed and reject it.
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o If the method/response code does not allow payload, then the
compressed COSE object Section 8 is the value of the Object-
Security option and the length of the Object-Security option is
equal to the size of the compressed COSE object. An endpoint
receiving a CoAP message without payload, that also contains an
empty Object-Security option SHALL treat it as malformed and
reject it.
The size of the COSE object depends on whether the method/response
code allows payload, if the message is a request or response, on the
set of options that are included in the unprotected message, the AEAD
algorithm, the length of the information identifying the security
context, and the length of the sequence number.
3. The Security Context
OSCOAP uses COSE with an Authenticated Encryption with Additional
Data (AEAD) algorithm between a CoAP client and a CoAP server. An
implementation supporting this specification MAY only implement the
client part or MAY only implement the server part.
This specification requires that client and server establish a
security context to apply to the COSE objects protecting the CoAP
messages. In this section we define the security context, and also
specify how to derive the initial security contexts in client and
server based on common shared secret and a key derivation function
(KDF).
3.1. Security Context Definition
The security context is the set of information elements necessary to
carry out the cryptographic operations in OSCOAP. For each endpoint,
the security context is composed of a "Common Context", a "Sender
Context", and a "Recipient Context".
The endpoints protect messages to send using the Sender Context and
verify messages received using the Recipient Context, both contexts
being derived from the Common Context and other data. Clients need
to be able to retrieve the correct security context to use.
An endpoint uses its Sender ID (SID) to derive its Sender Context,
and the other endpoint uses the same ID, now called Recipient ID
(RID), to derive its Recipient Context. In communication between two
endpoints, the Sender Context of one endpoint matches the Recipient
Context of the other endpoint, and vice versa. Thus the two security
contexts identified by the same IDs in the two endpoints are not the
same, but they are partly mirrored. Retrieval and use of the
security context are shown in Figure 3.
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.------------. .------------.
| Common, | | Common, |
| Sender, | | Recipient,|
| Recipient | | Sender |
'------------' '------------'
Client Server
| |
Retrieve context for | request: |
target resource | [Token = Token1, |
Protect request with | kid = SID, ...] |
Sender Context +---------------------->| Retrieve context with
| | RID = kid
| | Verify request with
| | Recipient Context
| response: | Protect response with
| [Token = Token1, ...] | Sender Context
Retrieve context with |<----------------------+
Token = Token1 | |
Verify request with | |
Recipient Context | |
Figure 3: Retrieval and use of the Security Context
The Common Context contains the following parameters:
o Algorithm (Alg). Value that identifies the COSE AEAD algorithm to
use for encryption. Its value is immutable once the security
context is established.
o Master Secret. Variable length, uniformly random byte string
containing the key used to derive traffic keys and IVs. Its value
is immutable once the security context is established.
o Master Salt (OPTIONAL). Variable length byte string containing
the salt used to derive traffic keys and IVs. Its value is
immutable once the security context is established.
The Sender Context contains the following parameters:
o Sender ID. Variable length byte string identifying the Sender
Context. Its value is immutable once the security context is
established.
o Sender Key. Byte string containing the symmetric key to protect
messages to send. Derived from Common Context and Sender ID.
Length is determined by Algorithm. Its value is immutable once
the security context is established.
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o Sender IV. Byte string containing the IV to protect messages to
send. Derived from Common Context and Sender ID. Length is
determined by Algorithm. Its value is immutable once the security
context is established.
o Sequence Number. Non-negative integer used to protect requests
and observe responses to send. Used as partial IV
[I-D.ietf-cose-msg] to generate unique nonces for the AEAD.
Maximum value is determined by Algorithm.
The Recipient Context contains the following parameters:
o Recipient ID. Variable length byte string identifying the
Recipient Context. Its value is immutable once the security
context is established.
o Recipient Key. Byte string containing the symmetric key to verify
messages received. Derived from Common Context and Recipient ID.
Length is determined by the Algorithm. Its value is immutable
once the security context is established.
o Recipient IV. Byte string containing the IV to verify messages
received. Derived from Common Context and Recipient ID. Length
is determined by Algorithm. Its value is immutable once the
security context is established.
o Replay Window. The replay window to verify requests and observe
responses received.
When it is understood which context is referred to (Sender Context or
Recipient Context), the term "Context IV" is used to denote the IV
currently used with this context.
An endpoint may free up memory by not storing the Sender Key, Sender
IV, Recipient Key, and Recipient IV, deriving them from the Common
Context when needed. Alternatively, an endpoint may free up memory
by not storing the Master Secret and Master Salt after the other
parameters have been derived.
The endpoints MAY interchange the client and server roles while
maintaining the same security context. When this happens, the former
server still protects messages to send using its Sender Context, and
verifies messages received using its Recipient Context. The same is
also true for the former client. The endpoints MUST NOT change the
Sender/Recipient ID. In other words, changing the roles does not
change the set of keys to be used.
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3.2. Derivation of Security Context Parameters
The parameters in the security context are derived from a small set
of input parameters. The following input parameters SHALL be pre-
established:
o Master Secret
o Sender ID
o Recipient ID
The following input parameters MAY be pre-established. In case any
of these parameters is not pre-established, the default value
indicated below is used:
o AEAD Algorithm (Alg)
* Default is AES-CCM-64-64-128 (COSE abbreviation: 12)
o Master Salt
* Default is the empty string
o Key Derivation Function (KDF)
* Default is HKDF SHA-256
o Replay Window Type and Size
* Default is DTLS-type replay protection with a window size of 32
How the input parameters are pre-established, is application
specific. The EDHOC protocol [I-D.selander-ace-cose-ecdhe] enables
the establishment of input parameters with the property of forward
secrecy and negotiation of KDF and AEAD, it thus provides all
necessary pre-requisite steps for using OSCOAP as defined here.
3.2.1. Derivation of Sender Key/IV, Recipient Key/IV
The KDF MUST be one of the HMAC based HKDF [RFC5869] algorithms
defined in COSE. HKDF SHA-256 is mandatory to implement. The
security context parameters Sender Key/IV and Recipient Key/IV SHALL
be derived from the input parameters using the HKDF, which consists
of the composition of the HKDF-Extract and HKDF-Expand steps
([RFC5869]):
output parameter = HKDF(salt, IKM, info, L)
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where:
o salt is the Master Salt as defined above
o IKM is the Master Secret is defined above
o info is a CBOR array consisting of:
info = [
id : bstr,
alg : int,
type : tstr,
L : int
]
* id is the Sender ID or Recipient ID
* type is "Key" or "IV"
o L is the size of the key/IV for the AEAD algorithm used, in
octets.
For example, if the algorithm AES-CCM-64-64-128 (see Section 10.2 in
[I-D.ietf-cose-msg]) is used, the value for L is 16 for keys and 7
for IVs.
3.2.2. Initial Sequence Numbers and Replay Window
The Sequence Number is initialized to 0. The supported types of
replay protection and replay window length is application specific
and depends on the lower layers. Default is DTLS-type replay
protection with a window size of 32 initiated as described in
Section 4.1.2.6 of [RFC6347].
3.3. Requirements on the Security Context Parameters
As collisions may lead to the loss of both confidentiality and
integrity, Sender ID SHALL be unique in the set of all security
contexts using the same Master Secret. Normally (e.g. when using
EDHOC [I-D.selander-ace-cose-ecdhe]) Sender IDs can be very short.
Note that Sender IDs of different lengths can be used with the same
Master Secret. E.g. the SID with value 0x00 is different from the
SID with the value 0x0000. If Sender ID uniqueness cannot be
guaranteed, random Sender IDs MUST be used. Random Sender IDs MUST
be long enough so that the probability of collisions is negligible.
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To enable retrieval of the right Recipient Context, the Recipient ID
SHOULD be unique in the sets of all Recipient Contexts used by an
endpoint.
The same Master Salt MAY be used with several Master Secrets.
4. Protected CoAP Message Fields
OSCOAP transforms an unprotected CoAP message into a protected CoAP
message, and vice versa. This section defines how the CoAP message
fields are protected. Note that OSCOAP protects messages from the
CoAP Requests/Responses layer only, and not from the Messaging layer
(Section 2 of [RFC7252]): this means that RST and ACK empty messages
are not protected, while ACK with piggybacked responses are protected
using the process defined in this document. All the messages
mentioned in this document refer to CON, NON and non-empty ACK
messages.
OSCOAP protects as much of the unprotected CoAP message as possible,
while still allowing forward proxy operations
[I-D.hartke-core-e2e-security-reqs]. Message fields may either be
o Class E: encrypted and integrity protected,
o Class I: integrity protected only, or
o Class U: unprotected.
This section also outlines how the message fields are transferred, a
detailed description of the processing is provided in Section 7.
Message fields of the unprotected CoAP message are either transferred
in the header/options part of the protected CoAP message, or in the
plaintext of the COSE object. Depending on which, the location of
the message field in the protected CoAP message is called "inner" or
"outer":
o Inner message field: message field included in the plaintext of
the COSE object of the protected CoAP message (see Section 5.1).
The inner message fields are by definition encrypted and integrity
protected by the COSE object (Class E).
o Outer message field: message field included in the header or
options part of the protected CoAP message. The outer message
fields are not encrypted and thus visible to an intermediary, but
may be integrity protected by including the message field values
in the Additional Authenticated Data (AAD) of the COSE object (see
Section 5.2). I.e. outer message fields may be Class I or Class
U.
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Note that, even though the message formats are slightly different,
OSCOAP complies with CoAP over unreliable transport [RFC7252] as well
as CoAP over reliable transport [I-D.ietf-core-coap-tcp-tls].
4.1. CoAP Payload
The CoAP Payload SHALL be encrypted and integrity protected (Class
E), and thus is an inner message field.
The sending endpoint writes the payload of the unprotected CoAP
message into the plaintext of the COSE object.
The receiving endpoint verifies and decrypts the COSE object, and
recreates the payload of the unprotected CoAP message.
4.2. CoAP Header
Many CoAP header fields are required to be read and changed during a
normal message exchange or when traversing a proxy and thus cannot in
general be protected between the endpoints, e.g. CoAP message layer
fields such as Message ID.
The CoAP header field Code MUST be sent in plaintext to support
RESTful processing, but MUST be integrity protected to prevent an
intermediary from changing, e.g. from GET to DELETE (Class I). The
CoAP version number MUST be integrity protected to prevent potential
future version-based attacks (Class I). Note that while the version
number is not sent in each CoAP message over reliable transport
[I-D.ietf-core-coap-tcp-tls], its value is known to client and
server.
The other CoAP header fields SHALL neither be integrity protected nor
encrypted (Class U). All CoAP header fields are thus outer message
fields.
The sending endpoint SHALL copy the header fields from the
unprotected CoAP message to the header of the protected CoAP message.
The receiving endpoint SHALL copy the header fields from the
protected CoAP message to the header of the unprotected CoAP message.
Both sender and receiver include the CoAP version number and header
field Code in the AAD of the COSE object (see Section 5.2).
4.3. CoAP Options
Most options are encrypted and integrity protected (Class E), and
thus inner message fields. But to allow certain proxy operations,
some options have outer values, i.e. are present as options in the
protected CoAP message. Certain options may have both an inner value
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and a potentially different outer value, where the inner value is
intended for the destination endpoint and the outer value is intended
for the proxy.
A summary of how options are protected and processed is shown in
Figure 4. Options within each class are protected and processed in a
similar way, but certain options which require special processing are
indicated by a * in Figure 4 and described in the subsections below.
+----+----------------+---+---+---+
| No.| Name | E | I | U |
+----+----------------+---+---+---+
| 1 | If-Match | x | | |
| 3 | Uri-Host | | | x |
| 4 | ETag | x | | |
| 5 | If-None-Match | x | | |
| 6 | Observe | | * | |
| 7 | Uri-Port | | | x |
| 8 | Location-Path | x | | |
| 11 | Uri-Path | x | | |
| 12 | Content-Format | x | | |
| 14 | Max-Age | * | | |
| 15 | Uri-Query | x | | |
| 17 | Accept | x | | |
| 20 | Location-Query | x | | |
| 23 | Block2 | * | | |
| 27 | Block1 | * | | |
| 28 | Size2 | * | | |
| 35 | Proxy-Uri | | | * |
| 39 | Proxy-Scheme | | | x |
| 60 | Size1 | * | | |
+----+----------------+---+---+---+
E=Encrypt and Integrity Protect, I=Integrity Protect only,
U=Unprotected, *=Special
Figure 4: Protection of CoAP Options
Unless specified otherwise, CoAP options not listed in Figure 4 SHALL
be encrypted and integrity protected and processed as class E
options.
Specifications of new CoAP options SHOULD define how they are
processed with OSCOAP. New COAP options SHOULD be of class E and
SHOULD NOT have outer values unless a forwarding proxy needs to read
that option value. If a certain option has both inner and outer
values, the two values SHOULD NOT be the same.
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4.3.1. Class E Options
For options in class E (see Figure 4) the option value in the
unprotected CoAP message, if present, SHALL be encrypted and
integrity protected between the endpoints. Hence the actions
resulting from the use of such options is analogous to communicating
in a protected manner directly with the endpoint. For example, a
client using an If-Match option will not be served by a proxy.
The sending endpoint SHALL write the class E option from the
unprotected CoAP message into the plaintext of the COSE object.
Except for the special options described in the subsections, the
sending endpoint SHALL NOT use the outer options of class E.
However, note that an intermediary may, legitimately or not, add,
change or remove the value of an outer option.
Except for the Block options Section 4.3.1.2, the receiving endpoint
SHALL discard any outer options of class E from the protected CoAP
message and SHALL write the Class E options present in the plaintext
of the COSE object into the unprotected CoAP message.
4.3.1.1. Max-Age
An inner Max-Age option, like other class E options, is used as
defined in [RFC7252] taking into account that it is not accessible to
proxies.
Since OSCOAP binds CoAP responses to requests, a cached response
would not be possible to use for any other request. To avoid
unnecessary caching, a server MAY add an outer Max-Age option with
value zero to protected CoAP responses (see Section 5.6.1 of
[RFC7252]). The outer Max-Age option is not integrity protected.
4.3.1.2. The Block Options
Blockwise [RFC7959] is an optional feature. An implementation MAY
comply with [RFC7252] and the Object-Security option without
implementing [RFC7959].
The Block options (Block1, Block2, Size1 and Size2) MAY be either
only inner options, only outer options or both inner and outer
options. The inner and outer options are processed independently.
The inner block options are used for endpoint-to-endpoint secure
fragmentation of payload into blocks and protection of information
about the fragmentation (block number, block size, last block). In
this case, the CoAP client fragments the CoAP message as defined in
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[RFC7959] before the message is processed by OSCOAP. The CoAP server
first processes the OSCOAP message before processing blockwise as
defined in [RFC7959].
There SHALL be a security policy defining a maximum unfragmented
message size for inner Block options such that messages exceeding
this size SHALL be fragmented by the sending endpoint.
Additionally, a proxy may arbitrarily do block fragmentation on any
CoAP message, in particular an OSCOAP message, as defined in
[RFC7959] and thereby add outer Block options to a block and send on
the next hop. The outer block options are thus neither encrypted nor
integrity protected.
An endpoint receiving a message with an outer Block option SHALL
first process this option according to [RFC7959], until all blocks of
the protected CoAP message has been received, or the cumulated
message size of the exceeds the maximum unfragmented message size.
In the latter case the message SHALL be discarded. In the former
case, the processing of the protected CoAP message continues as
defined in this document.
If the unprotected CoAP message in turn contains Block options, the
receiving endpoint processes this according to [RFC7959].
TODO: Update processing to support multiple concurrently proceeding
requests
4.3.2. Class I Options
A Class I option is an outer option and hence visible in the options
part of the protected CoAP message. Except for special options
described in the subsections, for options in Class I (see Figure 4)
the option value SHALL be integrity protected between the endpoints,
see (Section 5.2). Unless otherwise specified, the sending endpoint
SHALL encode the Class I options in the protected CoAP message as
described in Section 4.3.4.
4.3.2.1. Observe
Observe [RFC7641] is an optional feature. An implementation MAY
support [RFC7252] and the Object-Security option without supporting
[RFC7641]. The Observe option as used here targets the requirements
on forwarding of [I-D.hartke-core-e2e-security-reqs]
(Section 2.2.1.2).
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In order for a proxy to support forwarding of Observe messages, there
must be an Observe option present in options part of the protected
CoAP message ([RFC7641]), so Observe must have an outer value:
o The Observe option of the unprotected CoAP request SHALL be
encoded in the protected CoAP request as described in
Section 4.3.4.
To secure the order of the notifications, responses with the Observe
option SHALL be integrity protected in the following way:
o The Observe option SHALL be included in the external_aad of the
response (see Section 5.2), with value set to the 3 least
significant bytes of the Sequence Number of the response.
The Observe option in the CoAP request SHALL NOT be integrity
protected, since it may be legitimately removed by a proxy.
If the Observe option is removed from a CoAP request by a proxy, then
the server can still verify the request (as a non-Observe request),
and produce a non-Observe response. If the OSCOAP client receives a
response to an Observe request without an outer Observe value, then
it MUST verify the response as a non-Observe response, i.e. not
include the Sequence Number of the response in the external_aad.
4.3.3. Class U Options
Options in Class U have outer values and are used to support forward
proxy operations. Unless otherwise specified, the sending endpoint
SHALL encode the Class U options in the options part of the protected
CoAP message as described in Section 4.3.4.
4.3.3.1. Uri-Host, Uri-Port, and Proxy-Scheme
The sending endpoint SHALL copy Uri-Host, Uri-Port, and Proxy-Scheme
from the unprotected CoAP message to the options part of the
protected CoAP message. When Uri-Host, Uri-Port, or Proxy-Scheme
options are present, Proxy-Uri is not used [RFC7252].
4.3.3.2. Proxy-Uri
Proxy-Uri, when present, is split by OSCOAP into class U options and
class E options, which are processed accordingly. When Proxy-Uri is
used in the unprotected CoAP message, Uri-* are not present
[RFC7252].
The sending endpoint SHALL first decompose the Proxy-Uri value of the
unprotected CoAP message into the Proxy-Scheme, Uri-Host, Uri-Port,
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Uri-Path and Uri-Query options (if present) according to section 6.4
of [RFC7252].
Uri-Path and Uri-Query are class E options and MUST be protected and
processed as if obtained from the unprotected CoAP message, see
Section 4.3.1.
The value of the Proxy-Uri option of the protected CoAP message MUST
be replaced with Proxy-Scheme, Uri-Host and Uri-Port options (if
present) composed according to section 6.5 of [RFC7252] and MUST be
processed as a class U option, see Section 4.3.3.
An example of how Proxy-Uri is processed is given here. Assume that
the unprotected CoAP message contains:
o Proxy-Uri = "coap://example.com/resource?q=1"
During OSCOAP processing, Proxy-Uri is split into:
o Proxy-Scheme = "coap"
o Uri-Host = "example.com"
o Uri-Port = "5863"
o Uri-Path = "resource"
o Uri-Query = "q=1"
Uri-Path and Uri-Query follow the processing defined in
Section 4.3.1, and are thus encrypted and transported in the COSE
object. The remaining options are composed into the Proxy-Uri
included in the options part of the protected CoAP message, which has
value:
o Proxy-Uri = "coap://example.com"
4.3.4. Outer Options in the Protected CoAP Message
All options with outer values present in the protected CoAP message,
including the Object-Security option, SHALL be encoded as described
in Section 3.1 of [RFC7252], where the delta is the difference to the
previously included outer option value.
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5. The COSE Object
This section defines how to use COSE [I-D.ietf-cose-msg] to wrap and
protect data in the unprotected CoAP message. OSCOAP uses the
untagged COSE_Encrypt0 structure with an Authenticated Encryption
with Additional Data (AEAD) algorithm. The key lengths, IV lengths,
and maximum sequence number are algorithm dependent.
The AEAD algorithm AES-CCM-64-64-128 defined in Section 10.2 of
[I-D.ietf-cose-msg] is mandatory to implement. For AES-CCM-64-64-128
the length of Sender Key and Recipient Key is 128 bits, the length of
nonce, Sender IV, and Recipient IV is 7 bytes. The maximum Sequence
Number is specified in Section 10.
The nonce is constructed as described in Section 3.1 of
[I-D.ietf-cose-msg], i.e. by padding the partial IV (Sequence Number
in network byte order) with zeroes and XORing it with the Context IV
(Sender IV or Recipient IV), with the following addition: The most
significant bit in the first byte of the Context IV SHALL be flipped
for responses, in case there is a unique response (not Observe). In
this way, the same sequence number can be reused for requests and
corresponding responses, which reduces the size of the responses in
the most common case. For detailed processing instructions, see
Section 7.
We denote by Plaintext the data that is encrypted and integrity
protected, and by Additional Authenticated Data (AAD) the data that
is integrity protected only.
The COSE Object SHALL be a COSE_Encrypt0 object with fields defined
as follows
o The "protected" field is empty.
o The "unprotected" field includes:
* The "Partial IV" parameter. The value is set to the Sequence
Number. The Partial IV SHALL be of minimum length needed to
encode the sequence number. This parameter SHALL be present in
requests. In case of Observe (Section 4.3.2.1) the Partial IV
SHALL be present in the response, and otherwise the Partial IV
SHALL NOT be present in the response.
* The "kid" parameter. The value is set to the Sender ID (see
Section 3). This parameter SHALL be present in requests and
SHALL NOT be present in responses.
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o The "ciphertext" field is computed from the Plaintext (see
Section 5.1) and the Additional Authenticated Data (AAD) (see
Section 5.2) following Section 5.2 of [I-D.ietf-cose-msg].
The encryption process is described in Section 5.3 of
[I-D.ietf-cose-msg].
5.1. Plaintext
The Plaintext is formatted as a CoAP message without Header (see
Figure 5) consisting of:
o all Class E option values Section 4.3.1 present in the unprotected
CoAP message (see Section 4.3). The options are encoded as
described in Section 3.1 of [RFC7252], where the delta is the
difference to the previously included Class E option; and
o the Payload of unprotected CoAP message, if present, and in that
case prefixed by the one-byte Payload Marker (0xFF).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Class E options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(only if there
is payload)
Figure 5: Plaintext
5.2. Additional Authenticated Data
The external_aad SHALL be a CBOR array as defined below:
external_aad = [
ver : uint,
code : uint,
options : bstr,
alg : int,
request_kid : bstr,
request_seq : bstr
]
where:
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o ver: contains the CoAP version number, as defined in Section 3 of
[RFC7252].
o code: contains is the CoAP Code of the unprotected CoAP message,
as defined in Section 3 of [RFC7252].
o options: contains the Class I options Section 4.3.2 present in the
unprotected CoAP message encoded as described in Section 3.1 of
[RFC7252], where the delta is the difference to the previously
included class I option
o alg: contains the Algorithm from the security context used for the
exchange (see Section 3.1).
o request_kid: contains the value of the 'kid' in the COSE object of
the request (see Section 5).
o request_seq: contains the value of the 'Partial IV' in the COSE
object of the request (see Section 5).
6. Sequence Numbers, Replay, Message Binding, and Freshness
Sequence numbers and replay window are initialized as defined in
Section 3.2.2.
6.1. AEAD Nonce Uniqueness
An AEAD nonce MUST NOT be used more than once per AEAD key. In order
to assure unique nonces, each Sender Context contains a Sequence
Number used to protect requests, and - in case of Observe -
responses. The maximum sequence number is algorithm dependent, see
Section 10. If the Sequence Number exceeds the maximum sequence
number, the endpoint MUST NOT process any more messages with the
given Sender Context. The endpoint SHOULD acquire a new security
context (and consequently inform the other endpoint) before this
happens. The latter is out of scope of this document.
6.2. Replay Protection
In order to protect from replay of messages, each Recipient Context
contains a Replay Window used to verify request, and - in case of
Observe - responses. A receiving endpoint SHALL verify that a
Sequence Number (Partial IV) received in the COSE object has not been
received before in the Recipient Context. For requests, if this
verification fails and the message received is a CON message, the
server SHALL respond with a 4.00 Bad Request error message. The
diagnostic payload MAY contain the "Replay protection failed" string.
For responses, if this verification fails and the message received is
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a CON message, the client SHALL respond with an empty ACK and stop
processing the response.
The size and type of the Replay Window depends on the use case and
lower protocol layers. In case of reliable and ordered transport
from endpoint to endpoint, the recipient MAY just store the last
received sequence number and require that newly received Sequence
Numbers equals the last received Sequence Number + 1.
6.3. Sequence Number and Replay Window State
To prevent reuse of the Nonce/Sequence Number with the same key, or
from accepting replayed messages, a node needs to handle the
situation of suddenly losing sequence number and replay window state
in RAM, e.g. as a result of a reboot.
After boot, a node MAY reject to use existing security contexts from
before it booted and MAY establish a new security context with each
party it communicates, e.g. using EDHOC
[I-D.selander-ace-cose-ecdhe]. However, establishing a fresh
security context may have a non-negligible cost in terms of e.g.
power consumption.
If a stored security context is to be used after reboot, then the
node MUST NOT reuse a previous Sequence Number and MUST NOT accept
previously accepted messages.
6.3.1. The Basic Case
To prevent reuse of Sequence Number, the node MAY perform the
following procedure during normal operations:
o Before sending a message, the client stores in persistent memory a
sequence number associated to the stored security context higher
than any sequence number which has been or are being sent using
this security context. After boot, the client does not use any
lower sequence number in a request than what was persistently
stored with that security context.
* Storing to persistent memory can be costly. Instead of storing
a sequence number for each request, the client may store Seq +
K to persistent memory every K requests, where Seq is the
current sequence number and K > 1. This is a trade-off between
the number of storage operations and efficient use of sequence
numbers.
To prevent accepting replay of previously received messages, the node
MAY perform the following procedure:
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o After boot, before verifying a message using a security context
stored before boot, the server synchronizes the replay window so
that no old messages are being accepted. The server uses the
Repeat option [I-D.mattsson-core-coap-actuators] for synchronizing
the replay window: For each stored security context, the first
time after boot the server receives an OSCOAP request, it
generates a pseudo-random nonce and responds with the Repeat
option set to the nonce as described in
[I-D.mattsson-core-coap-actuators]. If the server receives a
repeated OSCOAP request containing the Repeat option and the same
nonce, and if the server can verify the request, then the sequence
number obtained in the repeated message is set as the lower limit
of the replay window.
6.3.2. The Observe Case
To prevent reuse of Sequence Number in case of Observe, the node MAY
perform the following procedure during normal operations:
o Before sending a notification, the server stores in persistent
memory a sequence number associated to the stored security context
higher than any sequence number for which a notification has been
or are being sent using this security context. After boot, the
server does not use any lower sequence number in an Observe
response than what was persistently stored with that security
context.
* Storing to persistent memory can be costly. Instead of storing
a sequence number for each notification, the server may store
Seq + K to persistent memory every K requests, where Seq is the
current sequence number and K > 1. This is a trade-off between
the number of storage operations and efficient use of sequence
numbers.
Note that a client MAY continue an ongoing observation after reboot
using a stored security context. With Observe, the client can only
verify the order of the notifications, as they may be delayed. If
the client wants to synchronize with a server resource it MAY restart
an observation.
6.4. Freshness
For responses without Observe, OSCOAP provides absolute freshness.
For requests, and responses with Observe, OSCOAP provides relative
freshness in the sense that the sequence numbers allows a recipient
to determine the relative order of messages.
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For applications having stronger demands on freshness (e.g. control
of actuators), OSCOAP needs to be augmented with mechanisms providing
absolute freshness [I-D.mattsson-core-coap-actuators].
6.5. Delay and Mismatch Attacks
In order to prevent response delay and mismatch attacks
[I-D.mattsson-core-coap-actuators] from on-path attackers and
compromised proxies, OSCOAP binds responses to the request by
including the request's ID (Sender ID or Recipient ID) and sequence
number in the AAD of the response. The server therefore needs to
store the request's ID (Sender ID or Recipient ID) and sequence
number until all responses have been sent.
7. Processing
7.1. Protecting the Request
Given an unprotected request, the client SHALL perform the following
steps to create a protected request:
1. Retrieve the Sender Context associated with the target resource.
2. Compose the Additional Authenticated Data, as described in
Section 5.
3. Compose the AEAD nonce by XORing the Context IV (Sender IV) with
the partial IV (Sequence Number in network byte order).
4. Encrypt the COSE object using the Sender Key. Compress the COSE
Object as specified in Section 8.
5. Format the protected CoAP message according to Section 4. The
Object-Security option is added, see Section 4.3.4.
6. Store the association Token - Security Context. The client SHALL
be able to find the Recipient Context from the Token in the
response.
7. Increment the Sequence Number by one.
7.2. Verifying the Request
A server receiving a request containing the Object-Security option
SHALL perform the following steps:
1. Process outer Block options according to [RFC7959], until all
blocks of the request have been received, see Section 4.3.1.2.
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2. Decompress the COSE Object (Section 8) and retrieve the Recipient
Context associated with the Recipient ID in the 'kid' parameter.
If the request is a CON message, and:
* either the decompression or the COSE message fails to decode,
the server SHALL respond with a 4.02 Bad Option error message.
The diagnostic payload SHOULD contain the string "Failed to
decode COSE".
* the server fails to retrieve a Recipient Context with
Recipient ID corresponding to the 'kid' parameter received,
the server SHALL respond with a 4.01 Unauthorized error
message. The diagnostic payload MAY contain the string
"Security context not found".
If the request is a NON message and either the decompression or the
COSE message fails to decode, or the server fails to retrieve a
Recipient Context with Recipient ID corresponding to the 'kid'
parameter received, then the server SHALL stop processing the
request.
1. Verify the Sequence Number in the 'Partial IV' parameter, as
described in Section 6.
2. Compose the Additional Authenticated Data, as described in
Section 5.
3. Compose the AEAD nonce by XORing the Context IV (Recipient IV)
with the padded 'Partial IV' parameter, received in the COSE
Object.
4. Decrypt the COSE object using the Recipient Key.
* If decryption fails, the server MUST stop processing the
request and, if the request is a CON message, the server MUST
respond with a 4.00 Bad Request error message. The diagnostic
payload MAY contain the "Decryption failed" string.
* If decryption succeeds, update the Recipient Replay Window, as
described in Section 6.
5. Add decrypted options and payload to the unprotected request,
processing the E options as described in (Section 4). The
Object-Security option is removed.
6. The unprotected CoAP request is processed according to [RFC7252]
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7.3. Protecting the Response
Given an unprotected response, the server SHALL perform the following
steps to create a protected response:
1. Retrieve the Sender Context in the Security Context used to
verify the request.
2. Compose the Additional Authenticated Data, as described in
Section 5.
3. Compose the AEAD nonce
* If Observe is not used, compose the AEAD nonce by XORing the
Context IV (Sender IV with the most significant bit in the
first byte flipped) with the padded Partial IV parameter from
the request.
* If Observe is used, compose the AEAD nonce by XORing the
Context IV (Sender IV) with the Partial IV of the response
(Sequence Number in network byte order).
4. Encrypt the COSE object using the Sender Key. Compress the COSE
Object as specified in Section 8.
5. Format the protected CoAP message according to Section 4. The
Object-Security option is added, see Section 4.3.4.
6. If Observe is used, increment the Sequence Number by one.
7.4. Verifying the Response
A client receiving a response containing the Object-Security option
SHALL perform the following steps:
1. Process outer Block options according to [RFC7959], until all
blocks of the protected CoAP message have been received, see
Section 4.3.1.2.
2. Retrieve the Recipient Context associated with the Token.
Decompress the COSE Object (Section 8). If the response is a CON
message and either the decompression or the COSE message fails to
decode, then the client SHALL send an empty ACK back and stop
processing the response. If the response is a NON message and
any of the previous conditions appear, then the client SHALL
simply stop processing the response.
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1. For Observe notifications, verify the Sequence Number in the
'Partial IV' parameter as described in Section 6.
2. Compose the Additional Authenticated Data, as described in
Section 5.
3. Compose the AEAD nonce
* If the Observe option is not present in the response, compose
the AEAD nonce by XORing the Context IV (Recipient IV with the
the most significant bit in the first byte flipped) with the
padded Partial IV parameter from the request.
* If the Observe option is present in the response, compose the
AEAD nonce by XORing the Context IV (Recipient IV) with the
padded Partial IV parameter from the response.
4. Decrypt the COSE object using the Recipient Key.
* If decryption fails, the client MUST stop processing the
response and, if the request is a CON message, the client MUST
respond with an empty ACK back.
* If decryption succeeds and Observe is used, update the
Recipient Replay Window, as described in Section 6.
5. Add decrypted options or payload to the unprotected response
overwriting any outer E options (see Section 4). The Object-
Security option is removed.
* If Observe is used, replace the Observe value with the 3 least
significant bytes in the sequence number.
6. The unprotected CoAP response is processed according to [RFC7252]
8. OSCOAP Compression
The Concise Binary Object Representation (CBOR) [RFC7049] combines
very small message sizes with extensibility. The CBOR Object Signing
and Encryption (COSE) [I-D.ietf-cose-msg] uses CBOR to create compact
encoding of signed and encrypted data. COSE is however constructed
to support a large number of different stateless use cases, and is
not fully optimized for use as a stateful security protocol, leading
to a larger than necessary message expansion. In this section we
define a simple stateless compression mechanism for OSCOAP, which
significantly reduces the per-packet overhead.
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8.1. Encoding of the Object-Security Option
The value of the Object-Security option SHALL be encoded as follows:
o The first byte MUST encode a set of flags and the length of the
Partial IV parameter.
* The three least significant bits encode the Partial IV size.
If their value is 0, the Partial IV is not present in the
compressed message.
* The fourth least significant bit is set to 1 if the kid is
present in the compressed message.
* The fifth-eighth least significant bits (= most significant
half-byte) are reserved and SHALL be set to zero when not in
use.
o The following n bytes (n being the value of the Partial IV size in
the first byte) encode the value of the Partial IV, if the Partial
IV is present (size not 0).
o The following byte encodes the size of the kid parameter, if the
kid is present (flag bit set to 1)
o The following m bytes (m given by the previous byte) encode the
value of the kid, if the kid is present (flag bit set to 1)
o The remainining bytes encode the ciphertext.
The presence of Partial IV and kid in requests and responses is
specified in Section 5, and summarized in Figure 6.
7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+ k: kid flag bit
|0 0 0 0|k|pivsz| pivsz: Partial IV size (3 bits)
+-+-+-+-+-+-+-+-+
+-------+---------+------------+-----------+
| | Request | Resp with- | Resp with |
| | | out observe| observe |
+-------+---------+------------+-----------+
| k | 1 | 0 | 0 |
| pivsz | > 0 | 0 | > 0 |
+-------+---------+------------+-----------+
Figure 6: Flag byte for OSCOAP compression
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8.2. Examples
This section provides examples of COSE Objects before and after
OSCOAP compression.
8.2.1. Example: Request
Before compression:
[
h'',
{ 4:h'25', 6:h'05' },
h'aea0155667924dff8a24e4cb35b9'
]
0x83 40 a2 04 41 25 06 41 05 4e ae a0 15 56 67 92
4d ff 8a 24 e4 cb 35 b9 (24 bytes)
After compression:
First byte: 0b00001001 = 0x09
0x09 05 01 25 ae a0 15 56 67 92 4d ff 8a 24 e4 cb
35 b9 (18 bytes)
8.2.2. Example: Response (without Observe)
Before compression:
[
h'',
{},
h'aea0155667924dff8a24e4cb35b9'
]
0x83 40 a0 4e ae a0 15 56 67 92 4d ff 8a 24 e4 cb
35 b9 (18 bytes)
After compression:
First byte: 0b00000000 = 0x00
0x00 ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9
(15 bytes)
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8.2.3. Example: Response (with Observe)
Before compression:
[
h'',
{ 6:h'07' },
h'aea0155667924dff8a24e4cb35b9'
]
0x83 40 a1 06 41 07 4e ae a0 15 56 67 92 4d ff
8a 24 e4 cb 35 b9 (21 bytes)
After compression:
First byte: 0b00000001 = 0x01
0x01 07 ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9
(16 bytes)
9. Web Linking
The use of OSCOAP MAY be indicated by a target attribute "osc" in a
web link [RFC5988] to a CoAP resource. This attribute is a hint
indicating that the destination of that link is to be accessed using
OSCOAP. Note that this is simply a hint, it does not include any
security context material or any other information required to run
OSCOAP.
A value MUST NOT be given for the "osc" attribute; any present value
MUST be ignored by parsers. The "osc" attribute MUST NOT appear more
than once in a given link-value; occurrences after the first MUST be
ignored by parsers.
10. Security Considerations
In scenarios with intermediary nodes such as proxies or brokers,
transport layer security such as DTLS only protects data hop-by-hop.
As a consequence the intermediary nodes can read and modify
information. The trust model where all intermediate nodes are
considered trustworthy is problematic, not only from a privacy
perspective, but also from a security perspective, as the
intermediaries are free to delete resources on sensors and falsify
commands to actuators (such as "unlock door", "start fire alarm",
"raise bridge"). Even in the rare cases, where all the owners of the
intermediary nodes are fully trusted, attacks and data breaches make
such an architecture brittle.
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DTLS protects hop-by-hop the entire CoAP message, including header,
options, and payload. OSCOAP protects end-to-end the payload, and
all information in the options and header, that is not required for
forwarding (see Section 4). DTLS and OSCOAP can be combined, thereby
enabling end-to-end security of CoAP payload, in combination with
hop-by-hop protection of the entire CoAP message, during transport
between end-point and intermediary node.
The CoAP message layer, however, cannot be protected end-to-end
through intermediary devices since the parameters Type and Message
ID, as well as Token and Token Length may be changed by a proxy.
Moreover, messages that are not possible to verify should for
security reasons not always be acknowledged but in some cases be
silently dropped. This would not comply with CoAP message layer, but
does not have an impact on the application layer security solution,
since message layer is excluded from that.
The use of COSE to protect CoAP messages as specified in this
document requires an established security context. The method to
establish the security context described in Section 3.2 is based on a
common shared secret material in client and server, which may be
obtained e.g. by using EDHOC [I-D.selander-ace-cose-ecdhe] or the ACE
framework [I-D.ietf-ace-oauth-authz]. An OSCOAP profile of ACE is
described in [I-D.seitz-ace-oscoap-profile].
The mandatory-to-implement AEAD algorithm AES-CCM-64-64-128 is
selected for broad applicability in terms of message size (2^64
blocks) and maximum number of messages (2^56). Compatibility with
CCM* is achieved by using the algorithm AES-CCM-16-64-128
[I-D.ietf-cose-msg].
Most AEAD algorithms require a unique nonce for each message, for
which the sequence numbers in the COSE message field "Partial IV" is
used. If the recipient accepts any sequence number larger than the
one previously received, then the problem of sequence number
synchronization is avoided. With reliable transport it may be
defined that only messages with sequence number which are equal to
previous sequence number + 1 are accepted. The alternatives to
sequence numbers have their issues: very constrained devices may not
be able to support accurate time, or to generate and store large
numbers of random nonces. The requirement to change key at counter
wrap is a complication, but it also forces the user of this
specification to think about implementing key renewal.
The maximum sequence number to guarantee nonce uniqueness
(Section 6.1) is algorithm dependent. Using AES_CCM, with the
maximum sequence number SHALL be 2^(min(nonce length in bits, 56) -
1) - 1. The "-1" in the exponent stems from the same partial IV and
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flipped bit of IV (Section 5) is used in request and response. The
compression algorithm (Section 8) assumes that the partial IV is 56
bits or less (which is the reason for min(,) in the exponent).
The inner block options enable the sender to split large messages
into protected blocks such that the receiving node can verify blocks
before having received the complete message. The outer block options
allow for arbitrary proxy fragmentation operations that cannot be
verified by the endpoints, but can by policy be restricted in size
since the encrypted options allow for secure fragmentation of very
large messages. A maximum message size (above which the sending
endpoint fragments the message and the receiving endpoint discards
the message, if complying to the policy) may be obtained as part of
normal resource discovery.
Applications need to use a padding scheme if the content of a message
can be determined solely from the length of the payload. As an
example, the strings "YES" and "NO" even if encrypted can be
distinguished from each other as there is no padding supplied by the
current set of encryption algorithms. Some information can be
determined even from looking at boundary conditions. An example of
this would be returning an integer between 0 and 100 where lengths of
1, 2 and 3 will provide information about where in the range things
are. Three different methods to deal with this are: 1) ensure that
all messages are the same length. For example using 0 and 1 instead
of 'yes' and 'no'. 2) Use a character which is not part of the
responses to pad to a fixed length. For example, pad with a space to
three characters. 3) Use the PKCS #7 style padding scheme where m
bytes are appended each having the value of m. For example,
appending a 0 to "YES" and two 1's to "NO". This style of padding
means that all values need to be padded.
11. Privacy Considerations
Privacy threats executed through intermediate nodes are considerably
reduced by means of OSCOAP. End-to-end integrity protection and
encryption of CoAP payload and all options that are not used for
forwarding, provide mitigation against attacks on sensor and actuator
communication, which may have a direct impact on the personal sphere.
The unprotected options (Figure 4) may reveal privacy sensitive
information. In particular Uri-Host SHOULD NOT contain privacy
sensitive information.
CoAP headers sent in plaintext allow for example matching of CON and
ACK (CoAP Message Identifier), matching of request and responses
(Token) and traffic analysis.
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12. IANA Considerations
Note to RFC Editor: Please replace all occurrences of "[[this
document]]" with the RFC number of this specification.
12.1. CoAP Option Numbers Registry
The Object-Security option is added to the CoAP Option Numbers
registry:
+--------+-----------------+-------------------+
| Number | Name | Reference |
+--------+-----------------+-------------------+
| TBD | Object-Security | [[this document]] |
+--------+-----------------+-------------------+
12.2. Media Type Registrations
The "application/oscon" media type is added to the Media Types
registry:
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Type name: application
Subtype name: oscon
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See Appendix C of this document.
Interoperability considerations: N/A
Published specification: [[this document]] (this document)
Applications that use this media type: To be identified
Fragment identifier considerations: N/A
Additional information:
* Magic number(s): N/A
* File extension(s): N/A
* Macintosh file type code(s): N/A
Person & email address to contact for further information:
Goeran Selander <goran.selander@ericsson.com>
Intended usage: COMMON
Restrictions on usage: N/A
Author: Goeran Selander, goran.selander@ericsson.com
12.3. CoAP Content Format Registration
The "application/oscon" content format is added to the CoAP Content
Format registry:
+-------------------+----------+----+-------------------+
| Media type | Encoding | ID | Reference |
+-------------------+----------+----+-------------------+
| application/oscon | - | 70 | [[this document]] |
+-------------------+----------+----+-------------------+
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13. Acknowledgments
The following individuals provided input to this document: Christian
Amsuess, Carsten Bormann, Joakim Brorsson, Martin Gunnarsson, Klaus
Hartke, Jim Schaad, Marco Tiloca, and Malisa Vučinić.
Ludwig Seitz and Goeran Selander worked on this document as part of
the CelticPlus project CyberWI, with funding from Vinnova.
14. References
14.1. Normative References
[I-D.ietf-cose-msg]
Schaad, J., "CBOR Object Signing and Encryption (COSE)",
draft-ietf-cose-msg-24 (work in progress), November 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<http://www.rfc-editor.org/info/rfc5869>.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
<http://www.rfc-editor.org/info/rfc5988>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <http://www.rfc-editor.org/info/rfc7049>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<http://www.rfc-editor.org/info/rfc7641>.
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[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<http://www.rfc-editor.org/info/rfc7959>.
14.2. Informative References
[I-D.bormann-6lo-coap-802-15-ie]
Bormann, C., "Constrained Application Protocol (CoAP) over
IEEE 802.15.4 Information Element for IETF", draft-
bormann-6lo-coap-802-15-ie-00 (work in progress), April
2016.
[I-D.greevenbosch-appsawg-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "CBOR data
definition language (CDDL): a notational convention to
express CBOR data structures", draft-greevenbosch-appsawg-
cbor-cddl-10 (work in progress), March 2017.
[I-D.hartke-core-e2e-security-reqs]
Selander, G., Palombini, F., and K. Hartke, "Requirements
for CoAP End-To-End Security", draft-hartke-core-e2e-
security-reqs-02 (work in progress), January 2017.
[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE)", draft-ietf-ace-oauth-
authz-06 (work in progress), March 2017.
[I-D.ietf-core-coap-tcp-tls]
Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
draft-ietf-core-coap-tcp-tls-08 (work in progress), April
2017.
[I-D.mattsson-core-coap-actuators]
Mattsson, J., Fornehed, J., Selander, G., and F.
Palombini, "Controlling Actuators with CoAP", draft-
mattsson-core-coap-actuators-02 (work in progress),
November 2016.
[I-D.seitz-ace-oscoap-profile]
Seitz, L. and F. Palombini, "OSCOAP profile of ACE",
draft-seitz-ace-oscoap-profile-01 (work in progress),
October 2016.
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[I-D.selander-ace-cose-ecdhe]
Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace-
cose-ecdhe-06 (work in progress), April 2017.
[I-D.tiloca-core-multicast-oscoap]
Tiloca, M., Selander, G., and F. Palombini, "Secure group
communication for CoAP", draft-tiloca-core-multicast-
oscoap-01 (work in progress), March 2017.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<http://www.rfc-editor.org/info/rfc7228>.
Appendix A. Test Vectors
TODO: This section needs to be updated.
Appendix B. Examples
This section gives examples of OSCOAP. The message exchanges are
made, based on the assumption that there is a security context
established between client and server. For simplicity, these
examples only indicate the content of the messages without going into
detail of the COSE message format.
B.1. Secure Access to Sensor
This example targets the scenario in Section 3.1 of
[I-D.hartke-core-e2e-security-reqs] and illustrates a client
requesting the alarm status from a server.
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Client Proxy Server
| | |
+----->| | Code: 0.01 (GET)
| GET | | Token: 0x8c
| | | Object-Security: [kid:5f, seq:42,
| | | {Uri-Path:"alarm_status"}]
| | | Payload: -
| | |
| +----->| Code: 0.01 (GET)
| | GET | Token: 0x7b
| | | Object-Security: [kid:5f, seq:42,
| | | {Uri-Path:"alarm_status"}]
| | | Payload: -
| | |
| |<-----+ Code: 2.05 (Content)
| | 2.05 | Token: 0x7b
| | | Object-Security: -
| | | Payload: [{"OFF"}]
| | |
|<-----+ | Code: 2.05 (Content)
| 2.05 | | Token: 0x8c
| | | Object-Security: -
| | | Payload: [{"OFF"}]
| | |
Figure 7: Secure Access to Sensor. Square brackets [ ... ] indicate
a COSE object. Curly brackets { ... } indicate encrypted data.
Since the method (GET) doesn't allow payload, the Object-Security
option carries the COSE object as its value. Since the response code
(Content) allows payload, the COSE object is carried as the CoAP
payload.
The COSE header of the request contains an identifier (5f),
indicating which security context was used to protect the message and
a sequence number (42). The option Uri-Path ("alarm_status") and
payload ("OFF") are encrypted.
The server verifies that the sequence number has not been received
before. The client verifies that the response is bound to the
request.
B.2. Secure Subscribe to Sensor
This example targets the scenario in Section 3.2 of
[I-D.hartke-core-e2e-security-reqs] and illustrates a client
requesting subscription to a blood sugar measurement resource (GET
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/glucose), first receiving the value 220 mg/dl and then a second
value 180 mg/dl.
Client Proxy Server
| | |
+----->| | Code: 0.01 (GET)
| GET | | Token: 0x83
| | | Observe: 0
| | | Object-Security: [kid:ca, seq:15,
| | | {Uri-Path:"glucose"}]
| | | Payload: -
| | |
| +----->| Code: 0.01 (GET)
| | GET | Token: 0xbe
| | | Observe: 0
| | | Object-Security: [kid:ca, seq:15,
| | | {Uri-Path:"glucose"}]
| | | Payload: -
| | |
| |<-----+ Code: 2.05 (Content)
| | 2.05 | Token: 0xbe
| | | Observe: 000032
| | | Object-Security: -
| | | Payload: [seq:32, {Content-Format:0, "220"}]
| | |
|<-----+ | Code: 2.05 (Content)
| 2.05 | | Token: 0x83
| | | Observe: 000032
| | | Object-Security: -
| | | Payload: [seq:32, {Content-Format:0, "220"}]
... ... ...
| | |
| |<-----+ Code: 2.05 (Content)
| | 2.05 | Token: 0xbe
| | | Observe: 000036
| | | Object-Security: -
| | | Payload: [seq:36, {Content-Format:0, "180"}]
| | |
|<-----+ | Code: 2.05 (Content)
| 2.05 | | Token: 0x83
| | | Observe: 000036
| | | Object-Security: -
| | | Payload: [seq:36, {Content-Format:0, "180"}]
| | |
Figure 8: Secure Subscribe to Sensor. Square brackets [ ... ]
indicate a COSE object. Curly brackets { ... } indicate encrypted
data.
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Since the method (GET) doesn't allow payload, the Object-Security
option carries the COSE object as its value. Since the response code
(Content) allows payload, the COSE object is carried as the CoAP
payload.
The COSE header of the request contains an identifier (ca),
indicating the security context used to protect the message and a
Sequence Number (15). The COSE header of the responses contains
sequence numbers (32 and 36). The options Content-Format (0) and the
payload ("220" and "180"), are encrypted. The Observe option is
integrity protected. The shown Observe values (000032 and 000036)
are the ones that the client will see after OSCOAP processing.
The server verifies that the sequence number has not been received
before. The client verifies that the sequence number has not been
received before and that the responses are bound to the request.
Appendix C. Object Security of Content (OSCON)
TODO: This section needs to be updated.
OSCOAP protects message exchanges end-to-end between a certain client
and a certain server, targeting the security requirements for forward
proxy of [I-D.hartke-core-e2e-security-reqs]. In contrast, many use
cases require one and the same message to be protected for, and
verified by, multiple endpoints, see caching proxy section of
[I-D.hartke-core-e2e-security-reqs]. Those security requirements can
be addressed by protecting essentially the payload/content of
individual messages using the COSE format ([I-D.ietf-cose-msg]),
rather than the entire request/response message exchange. This is
referred to as Object Security of Content (OSCON).
OSCON transforms an unprotected CoAP message into a protected CoAP
message in the following way: the payload of the unprotected CoAP
message is wrapped by a COSE object, which replaces the payload of
the unprotected CoAP message. We call the result the "protected"
CoAP message.
The unprotected payload shall be the plaintext/payload of the COSE
object. The 'protected' field of the COSE object 'Headers' shall
include the context identifier, both for requests and responses. If
the unprotected CoAP message includes a Content-Format option, then
the COSE object shall include a protected 'content type' field, whose
value is set to the unprotected message Content-Format value. The
Content-Format option of the protected CoAP message shall be replaced
with "application/oscon" (Section 12)
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The COSE object shall be protected (encrypted) and verified
(decrypted) as described in ([I-D.ietf-cose-msg]).
Most AEAD algorithms require a unique nonce for each message.
Sequence numbers for partial IV as specified for OSCOAP may be used
for replay protection as described in Section 6. The use of time
stamps in the COSE header parameter 'operation time'
[I-D.ietf-cose-msg] for freshness may be used.
OSCON shall not be used in cases where CoAP header fields (such as
Code or Version) or CoAP options need to be integrity protected or
encrypted. OSCON shall not be used in cases which require a secure
binding between request and response.
The scenarios in Sections 3.3 - 3.5 of
[I-D.hartke-core-e2e-security-reqs] assume multiple recipients for a
particular content. In this case the use of symmetric keys does not
provide data origin authentication. Therefore the COSE object should
in general be protected with a digital signature.
C.1. Overhead OSCON
In general there are four different kinds of modes that need to be
supported: message authentication code, digital signature,
authenticated encryption, and symmetric encryption + digital
signature. The use of digital signature is necessary for
applications with many legitimate recipients of a given message, and
where data origin authentication is required.
To distinguish between these different cases, the tagged structures
of COSE are used (see Section 2 of [I-D.ietf-cose-msg]).
The sizes of COSE messages for selected algorithms are detailed in
this section.
The size of the header is shown separately from the size of the MAC/
signature. A 4-byte Context Identifier and a 1-byte Sequence Number
are used throughout all examples, with these values:
o Cid: 0xa1534e3c
o Seq: 0xa3
For each scheme, we indicate the fixed length of these two parameters
("Cid+Seq" column) and of the Tag ("MAC"/"SIG"/"TAG"). The "Message
OH" column shows the total expansions of the CoAP message size, while
the "COSE OH" column is calculated from the previous columns.
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Overhead incurring from CBOR encoding is also included in the COSE
overhead count.
To make it easier to read, COSE objects are represented using CBOR's
diagnostic notation rather than a binary dump.
C.2. MAC Only
This example is based on HMAC-SHA256, with truncation to 8 bytes
(HMAC 256/64).
Since the key is implicitly known by the recipient, the
COSE_Mac0_Tagged structure is used (Section 6.2 of
[I-D.ietf-cose-msg]).
The object in COSE encoding gives:
996( # COSE_Mac0_Tagged
[
h'a20444a1534e3c0641a3', # protected:
{04:h'a1534e3c',
06:h'a3'}
{}, # unprotected
h'', # payload
MAC # truncated 8-byte MAC
]
)
This COSE object encodes to a total size of 26 bytes.
Figure 9 summarizes these results.
+------------------+-----+-----+---------+------------+
| Structure | Tid | MAC | COSE OH | Message OH |
+------------------+-----+-----+---------+------------+
| COSE_Mac0_Tagged | 5 B | 8 B | 13 B | 26 B |
+------------------+-----+-----+---------+------------+
Figure 9: Message overhead for a 5-byte Tid using HMAC 256/64
C.3. Signature Only
This example is based on ECDSA, with a signature of 64 bytes.
Since only one signature is used, the COSE_Sign1_Tagged structure is
used (Section 4.2 of [I-D.ietf-cose-msg]).
The object in COSE encoding gives:
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997( # COSE_Sign1_Tagged
[
h'a20444a1534e3c0641a3', # protected:
{04:h'a1534e3c',
06:h'a3'}
{}, # unprotected
h'', # payload
SIG # 64-byte signature
]
)
This COSE object encodes to a total size of 83 bytes.
Figure 10 summarizes these results.
+-------------------+-----+------+---------+------------+
| Structure | Tid | SIG | COSE OH | Message OH |
+-------------------+-----+------+---------+------------+
| COSE_Sign1_Tagged | 5 B | 64 B | 14 B | 83 bytes |
+-------------------+-----+------+---------+------------+
Figure 10: Message overhead for a 5-byte Tid using 64 byte ECDSA
signature.
C.4. Authenticated Encryption with Additional Data (AEAD)
This example is based on AES-CCM with the Tag truncated to 8 bytes.
Since the key is implicitly known by the recipient, the
COSE_Encrypt0_Tagged structure is used (Section 5.2 of
[I-D.ietf-cose-msg]).
The object in COSE encoding gives:
993( # COSE_Encrypt0_Tagged
[
h'a20444a1534e3c0641a3', # protected:
{04:h'a1534e3c',
06:h'a3'}
{}, # unprotected
ciphertext # ciphertext including truncated 8-byte TAG
]
)
This COSE object encodes to a total size of 25 bytes.
Figure 11 summarizes these results.
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+----------------------+-----+-----+---------+------------+
| Structure | Tid | TAG | COSE OH | Message OH |
+----------------------+-----+-----+---------+------------+
| COSE_Encrypt0_Tagged | 5 B | 8 B | 12 B | 25 bytes |
+----------------------+-----+-----+---------+------------+
Figure 11: Message overhead for a 5-byte Tid using AES_128_CCM_8.
C.5. Symmetric Encryption with Asymmetric Signature (SEAS)
This example is based on AES-CCM and ECDSA with 64 bytes signature.
The same assumption on the security context as in Appendix C.4. COSE
defines the field 'counter signature w/o headers' that is used here
to sign a COSE_Encrypt0_Tagged message (see Section 3 of
[I-D.ietf-cose-msg]).
The object in COSE encoding gives:
993( # COSE_Encrypt0_Tagged
[
h'a20444a1534e3c0641a3', # protected:
{04:h'a1534e3c',
06:h'a3'}
{9:SIG}, # unprotected:
09: 64 bytes signature
ciphertext # ciphertext including truncated 8-byte TAG
]
)
This COSE object encodes to a total size of 92 bytes.
Figure 12 summarizes these results.
+----------------------+-----+-----+------+---------+------------+
| Structure | Tid | TAG | SIG | COSE OH | Message OH |
+----------------------+-----+-----+------+---------+------------+
| COSE_Encrypt0_Tagged | 5 B | 8 B | 64 B | 15 B | 92 B |
+----------------------+-----+-----+------+---------+------------+
Figure 12: Message overhead for a 5-byte Tid using AES-CCM
countersigned with ECDSA.
Authors' Addresses
Goeran Selander
Ericsson AB
Email: goran.selander@ericsson.com
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John Mattsson
Ericsson AB
Email: john.mattsson@ericsson.com
Francesca Palombini
Ericsson AB
Email: francesca.palombini@ericsson.com
Ludwig Seitz
SICS Swedish ICT
Email: ludwig@sics.se
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