CoRE Working Group G. Selander
Internet-Draft J. Mattsson
Intended status: Standards Track F. Palombini
Expires: June 22, 2017 Ericsson AB
L. Seitz
SICS Swedish ICT
December 19, 2016
Object Security of CoAP (OSCOAP)
draft-ietf-core-object-security-01
Abstract
This memo defines Object Security of CoAP (OSCOAP), a method for
application layer protection of message exchanges with the
Constrained Application Protocol (CoAP), using the CBOR Object
Signing and Encryption (COSE) format. OSCOAP provides end-to-end
encryption, integrity and replay protection to CoAP payload, options,
and header fields, as well as a secure binding between CoAP request
and response messages. The use of OSCOAP is signaled with the CoAP
option Object-Security, also defined in this memo.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 22, 2017.
Copyright Notice
Copyright (c) 2016 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
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these 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 . . . . . . . . 8
3.2.1. Derivation of Sender Key/IV, Recipient Key/IV . . . . 10
3.2.2. Context Identifier . . . . . . . . . . . . . . . . . 11
3.2.3. Sender ID and Recipient ID . . . . . . . . . . . . . 11
3.2.4. Sequence Numbers and Replay Window . . . . . . . . . 11
4. Protected CoAP Message Fields . . . . . . . . . . . . . . . . 11
4.1. CoAP Payload . . . . . . . . . . . . . . . . . . . . . . 12
4.2. CoAP Header . . . . . . . . . . . . . . . . . . . . . . . 12
4.3. CoAP Options . . . . . . . . . . . . . . . . . . . . . . 13
4.3.1. Class E Options . . . . . . . . . . . . . . . . . . . 15
4.3.2. Class A Options . . . . . . . . . . . . . . . . . . . 17
5. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 17
5.1. Plaintext . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2. Additional Authenticated Data . . . . . . . . . . . . . . 19
6. Protecting CoAP Messages . . . . . . . . . . . . . . . . . . 21
6.1. Replay and Freshness Protection . . . . . . . . . . . . . 21
6.2. Protecting the Request . . . . . . . . . . . . . . . . . 22
6.3. Verifying the Request . . . . . . . . . . . . . . . . . . 23
6.4. Protecting the Response . . . . . . . . . . . . . . . . . 24
6.5. Verifying the Response . . . . . . . . . . . . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . . 26
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 28
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
9.1. CoAP Option Numbers Registry . . . . . . . . . . . . . . 28
9.2. COSE Header Parameters Registry . . . . . . . . . . . . . 29
9.3. Media Type Registrations . . . . . . . . . . . . . . . . 29
9.4. CoAP Content Format Registration . . . . . . . . . . . . 30
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
11.1. Normative References . . . . . . . . . . . . . . . . . . 31
11.2. Informative References . . . . . . . . . . . . . . . . . 32
Appendix A. Overhead . . . . . . . . . . . . . . . . . . . . . . 33
A.1. Length of the Object-Security Option . . . . . . . . . . 33
A.2. Size of the COSE Object . . . . . . . . . . . . . . . . . 33
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A.3. Message Expansion . . . . . . . . . . . . . . . . . . . . 34
A.4. Example . . . . . . . . . . . . . . . . . . . . . . . . . 35
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 36
B.1. Secure Access to Sensor . . . . . . . . . . . . . . . . . 36
B.2. Secure Subscribe to Sensor . . . . . . . . . . . . . . . 38
Appendix C. Object Security of Content (OSCON) . . . . . . . . . 39
C.1. Overhead OSCON . . . . . . . . . . . . . . . . . . . . . 40
C.2. MAC Only . . . . . . . . . . . . . . . . . . . . . . . . 41
C.3. Signature Only . . . . . . . . . . . . . . . . . . . . . 42
C.4. Authenticated Encryption with Additional Data (AEAD) . . 43
C.5. Symmetric Encryption with Asymmetric Signature (SEAS) . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] 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 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 without being
protected or detected by DTLS.
This memo 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.
The solution provides an in-layer security protocol for CoAP which
does not depend on underlying layers and is therefore favorable for
providing security for "CoAP over foo", e.g. CoAP messages passing
over both unreliable and reliable transport
[I-D.ietf-core-coap-tcp-tls], CoAP over IEEE 802.15.4 IE
[I-D.bormann-6lo-coap-802-15-ie].
OSCOAP builds on CBOR Object Signing and Encryption (COSE)
[I-D.ietf-cose-msg], providing end-to-end encryption, integrity, and
replay protection. The use of OSCOAP is signaled with the CoAP
option Object-Security, also defined in this memo. 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
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a COSE object. The message fields that have been encrypted are
removed from the message whereas the Object-Security option and the
COSE object are added. We call the result the "protected" CoAP
message. Thus OSCOAP is a security protocol based on the exchange of
protected CoAP messages (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 provides protection of CoAP payload, certain options, and
header fields, as well as a secure binding between CoAP request and
response messages. OSCOAP provides replay protection, but like DTLS,
OSCOAP only provides relative freshness in the sense that the
sequence numbers allows a recipient to determine the relative order
of messages. 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].
OSCOAP may be used in extremely constrained settings, where DTLS
cannot be supported. Alternatively, OSCOAP can be combined with
DTLS, 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. 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) and it 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
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words may also appear in this document in lowercase, absent their
normative meanings.
Readers are expected to be familiar with the terms and concepts
described in [RFC7252] and [RFC7641]. Readers are also expected to
be familiar with [RFC7049] and understand
[I-D.greevenbosch-appsawg-cbor-cddl]. Terminology for constrained
environments, such as "constrained device", "constrained-node
network", is defined in [RFC7228].
2. The Object-Security Option
The Object-Security option indicates that OSCOAP is used to protect
the CoAP message exchange. The protection is achieved by means of a
COSE object included in the protected CoAP message, as detailed in
Section 5.
The Object-Security option is critical, safe to forward, part of the
cache key, and not repeatable. Figure 2 illustrates the structure of
the Object-Security option.
A CoAP proxy 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 of this is that messages with
the Object-Security option will never generate cache hits. To
further prevent caching, a Max-Age option with value zero SHOULD be
added to the protected CoAP responses.
+-----+---+---+---+---+-----------------+--------+--------+
| 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
The length of the Object-Security option depends on whether the
unprotected message allows payload, on the set of options that are
included in the unprotected message, the length of the integrity tag,
and the length of the information identifying the security context.
o If the unprotected message allows payload, then the COSE object is
the payload of the protected message (see Section 6.2 and
Section 6.4), and the Object-Security option has length zero. An
endpoint receiving a CoAP message with payload, that also contains
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a non-empty Object-Security option SHALL treat it as malformed and
reject it.
o If the unprotected message does not allow payload, then the COSE
object is the value of the Object-Security option and the length
of the Object-Security option is equal to the size of the 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.
Note that according to [RFC7252], new Methods and Response Codes
should specify if the payload is optional, required or not allowed
(Section 12.1.2) in the message, and in case this is not defined the
sender must not include a payload (Section 5.5). Thus, in this case,
the COSE object MUST be the value of the Object-Security option.
More details about the message overhead caused by the Object-Security
option are given in Appendix A.
3. The Security Context
OSCOAP uses COSE with an Authenticated Encryption with Additional
Data (AEAD) algorithm. The 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. Each security
context is identified by a Context Identifier. A Context Identifier
that is no longer in use can be reassigned to a new security context.
For each endpoint, the security context is composed by 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. Each endpoint
has a unique ID used to derive its Sender Context, this identifier is
called "Sender ID". The Recipient Context is derived with the other
endpoint's ID, which is called "Recipient ID". The Recipient ID is
thus the ID of the endpoint from which a CoAP message is received.
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
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Context Identifiers 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."
.-Cid = Cid1-. .-Cid = Cid1-.
| Common, | | Common, |
| Sender, | | Recipient,|
| Recipient | | Sender |
'------------' '------------'
Client Server
| |
Retrieve context for | request: |
target resource | [Token = Token1, |
Protect request with | Cid = Cid1, ...] |
Sender Context +---------------------->| Retrieve context with
| | Cid = Cid1
| | 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 Context Identifier (Cid). Variable length byte string that
identifies the security context. Its value is immutable once the
security context is established.
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 Base Key (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.
The Sender Context contains the following parameters:
o Sender ID. Variable length byte string identifying the endpoint
itself. Its value is immutable once the security context is
established.
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o Sender Key. Byte string containing the symmetric key to protect
messages to send. Length is determined by Algorithm. Its value
is immutable once the security context is established.
o Sender IV. Byte string containing the fixed context IV
[I-D.ietf-cose-msg]) to protect messages to send. Length is
determined by Algorithm. Its value is immutable once the security
context is established.
o Sender Sequence Number. Non-negative integer enumerating the COSE
objects that the endpoint sends using the context. 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
endpoint messages are received from. Its value is immutable once
the security context is established.
o Recipient Key. Byte string containing the symmetric key to verify
messages received. Length is determined by the Algorithm. Its
value is immutable once the security context is established.
o Recipient IV. Byte string containing the context IV to verify
messages received. Length is determined by Algorithm. Its value
is immutable once the security context is established.
o Recipient Replay Window. The replay protection window for
messages received.
The 3-tuple (Cid, Sender ID, Partial IV) is called Transaction
Identifier (Tid), and SHALL be unique for each Base Key. The Tid is
used as a unique challenge in the COSE object of the protected CoAP
request. The Tid is part of the Additional Authenticated Data (AAD,
see Section 5) of the protected CoAP response message, which is how
responses are bound to requests.
3.2. Derivation of Security Context Parameters
This section describes how to derive the initial parameters in the
security context, given a small set of input parameters. We also
give indications on how applications should select the input
parameters.
The following input parameters SHALL be pre-established:
o Context Identifier (Cid)
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o Base Key (master_secret)
o AEAD Algorithm (Alg)
* Default is AES-CCM-64-64-128 (value 12)
The following input parameters MAY be pre-established:
o Sender ID
* Defaults are 0x00 for the endpoint intially being client, and
0x01 for the endpoint initially being server
o Recipient ID
* Defaults are 0x01 for the endpoint intially being client, and
0x00 for the endpoint initially being server
o Key Derivation Function (KDF)
* Default is HKDF SHA-256
o Replay Window Size
* Default is 64
The endpoints MAY interchange the CoAP client and server roles while
maintaining the same security context. When this happens, the former
server still protects the message to send using the Sender Context,
and verifies the message 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.
The input parameters are included unchanged in the security context.
From the input parameters, the following parameters are derived:
o Sender Key, Sender IV, Sender Sequence Number
o Recipient Key, Recipient IV, Recipient Sequence Number
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.
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3.2.1. Derivation of Sender Key/IV, Recipient Key/IV
Given the input parameters, the client and server can derive all the
other parameters in the security context. The derivation procedure
described here MUST NOT be executed more than once using the same
master_secret and Cid. The same master_secret SHOULD NOT be used with
more than one Cid.
The KDF MUST be one of the HKDF [RFC5869] algorithms defined in COSE.
The KDF HKDF SHA-256 is mandatory to implement. The security context
parameters Sender Key/IV, Recipient Key/IV SHALL be derived using
HKDF, and consists of the composition of the HKDF-Extract and HKDF-
Expand steps ({{RFC5869}):
output parameter = HKDF(master_secret, salt, info, output_length),
where:
o master_secret is defined above
o salt is a string of zeros of the length of the hash function
output in octets
o info is a serialized CBOR array consisting of:
info = [
cid : bstr,
id : bstr,
alg : int,
out_type : tstr,
out_len : uint
]
- id is the Sender ID or Recipient ID
- out_type is "Key" or "IV"
- out_len is the key/IV size of the AEAD algorithm
o output_length is the size of the AEAD key/IV in bytes encoded as
an 8-bit unsigned integer
For example, if the algorithm AES-CCM-64-64-128 (see Section 10.2 in
[I-D.ietf-cose-msg]) is used, output_length for the keys is 128 bits
and output_length for the IVs is 56 bits.
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3.2.2. Context Identifier
As mentioned, Cid is pre-established. How this is done is
application specific, but it is RECOMMENDED that the application uses
64-bits long pseudo-random Cids, in order to have globally unique
Context Identifiers. Cid SHOULD be unique in the sets of all
security contexts used by all the endpoints. If it is not the case,
it is the role of the application to specify how to handle
collisions.
If the application has total control of both clients and servers,
shorter unique Cids MAY be used. Note that Cids of different lengths
can be used by different clients and that e.g. a Cid with the value
0x00 is different from the Cid with the value 0x0000.
In the same phase during which the Cid is established in the
endpoint, the application informs the endpoint what resources can be
accessed using the corresponding security contexts. Resources that
are accessed with OSCOAP are called "protected" resources. The set
of resources that can be accessed using a certain security context is
decided by the application (resource, host, etc.). The client SHALL
save the association resource-Cid, in order to be able to retrieve
the correct security context to access a protected resource. The
server SHALL save the association resource-Cid, in order to determine
whether a particular resource may be accessed using a certain Cid.
3.2.3. Sender ID and Recipient ID
The Sender ID and Recipient ID SHALL be unique in the set of all
endpoints using the same security context. Collisions may lead to
the loss of both confidentiality and integrity. If random IDs are
used, they MUST be long enough so that the probability of collisions
is negligible.
3.2.4. Sequence Numbers and Replay Window
The Sender Sequence Number is initialized to 0. The Recipient Replay
Window is initiated as described in Section 4.1.2.6 of [RFC6347].
4. Protected CoAP Message Fields
OSCOAP transforms an unprotected CoAP message into a protected CoAP
message, and vice versa. This section defines how the unprotected
CoAP message fields are protected. OSCOAP protects as much of the
unprotected CoAP message as possible, while still allowing forward
proxy operations [I-D.hartke-core-e2e-security-reqs].
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This section also outlines how the message fields are processed and
transferred, a detailed description is provided in Section 6.
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 "outer" or
"inner":
o Inner message field = message field included in the plaintext of
the COSE object of the protected CoAP message (see Section 5.1)
o Outer message field = message field included in the header or
options part of the protected CoAP message
The inner message fields are encrypted and integrity protected by the
COSE object. The outer message fields are sent in plain text but may
be integrity protected by including the message field values in the
AAD of the COSE object (see Section 5.2).
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, 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 (see Section 6.2 and
Section 6.4).
The receiving endpoint verifies and decrypts the COSE object, and
recreates the payload of the unprotected CoAP message (see
Section 6.3 and Section 6.5).
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 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. The CoAP
version number SHALL be integrity protected to prevent potential
future version-based attacks. Note that while the version number is
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not sent in each CoAP message over reliable transport
[I-D.ietf-core-coap-tcp-tls], its value is known to client and
server.
Other CoAP header fields SHALL neither be integrity protected nor
encrypted. The CoAP header fields are thus outer message fields.
The sending endpoint SHALL copy the header fields from the
unprotected CoAP message to the protected CoAP message. The
receiving endpoint SHALL copy the header fields from the protected
CoAP message to the unprotected CoAP message. Both sender and
receiver inserts the CoAP version number and header field Code in the
AAD of the COSE object (see section Section 5.2).
4.3. CoAP Options
As with the message fields described in the previous sections, CoAP
options may be encrypted and integrity protected, integrity protected
only, or neither encrypted nor integrity protected.
Most options are encrypted and integrity protected (see Figure 4),
and thus inner message fields. But to allow certain proxy
operations, some options have outer values and require special
processing. Indeed, certain options may or must have both an inner
value 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.
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+----+---+---+---+---+----------------+--------+--------+---+---+
| No.| C | U | N | R | Name | Format | Length | E | A |
+----+---+---+---+---+----------------+--------+--------+---+---+
| 1 | x | | | x | If-Match | opaque | 0-8 | x | |
| 3 | x | x | - | | Uri-Host | string | 1-255 | | x |
| 4 | | | | x | ETag | opaque | 1-8 | x | |
| 5 | x | | | | If-None-Match | empty | 0 | x | |
| 6 | | x | - | | Observe | uint | 0-3 | * | |
| 7 | x | x | - | | Uri-Port | uint | 0-2 | | x |
| 8 | | | | x | Location-Path | string | 0-255 | x | |
| 11 | x | x | - | x | Uri-Path | string | 0-255 | x | |
| 12 | | | | | Content-Format | uint | 0-2 | x | |
| 14 | | x | - | | Max-Age | uint | 0-4 | * | |
| 15 | x | x | - | x | Uri-Query | string | 0-255 | x | |
| 17 | x | | | | Accept | uint | 0-2 | x | |
| 20 | | | | x | Location-Query | string | 0-255 | x | |
| 23 | x | x | - | - | Block2 | uint | 0-3 | * | |
| 27 | x | x | - | - | Block1 | uint | 0-3 | * | |
| 28 | | | x | | Size2 | unit | 0-4 | * | |
| 35 | x | x | - | | Proxy-Uri | string | 1-1034 | | * |
| 39 | x | x | - | | Proxy-Scheme | string | 1-255 | | x |
| 60 | | | x | | Size1 | uint | 0-4 | * | |
+----+---+---+---+---+----------------+--------+--------+---+---+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable,
E=Encrypt and Integrity Protect, A=Integrity Protect, *=Special
Figure 4: Protection of CoAP Options
A summary of how options are protected and processed is shown in
Figure 4. The CoAP options are partitioned into two classes:
o E - options which are encrypted and integrity protected, and
o A - options which are only integrity protected.
Options within each class are protected and processed in a similar
way, but certain options which require special processing as
described in the subsections and indicated by a '*' in Figure 4.
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 specify how they are
processed with OSCOAP. New COAP options SHOULD be of class E and
SHOULD NOT have outer options unless a forwarding proxy needs to read
an option value. If a certain option is both inner and outer, the
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two values SHOULD NOT be the same, unless a proxy is required by
specification to be able to read the end-to-end value.
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, and thus is not visible to
or possible to change by intermediary nodes. Hence the actions
resulting from the use of such options is analogous to communicating
in a protected manner with the endpoint. For example, a client using
an ETag 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 (see
Section 6.2 and Section 6.4).
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, legimitimately or not, add,
change or remove the value of an outer option.
Execept for the Block options Section 4.3.1.3, the receiving endpoint
SHALL discard any outer options of class E from the protected CoAP
message and SHALL replace it with the value from the COSE object when
present (see Section 6.3 and Section 6.5).
4.3.1.1. Max-Age
An inner Max-Age option 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. Therefore, there
SHOULD be an outer Max-Age option with value zero to prevent caching
of responses (see Section 5.6.1 of [RFC7252]).
The outer Max-Age option SHALL NOT be encrypted and SHALL NOT be
integrity protected.
4.3.1.2. Observe
The Observe option as used here targets the requirements on
forwarding of [I-D.hartke-core-e2e-security-reqs] (Section 2.2.1.2).
An inner Observe option is used between endpoints. In order for a
proxy to support forwarding of notifications, there SHALL be an outer
Observe option. To simplify the processing in the server, the outer
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option SHOULD have the same value as the inner Observe option. The
outer Observe option MAY have different values than the inner, but
the order of the different values is SHALL be the same as for the
inner Observe option.
The outer Observe option SHALL neither be encrypted nor integrity
protected.
4.3.1.3. The Block Options
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, last block, etc.).
Additionally, a proxy may arbitrarily do fragmentation operations on
the protected CoAP message, adding outer block options that are not
intended to be verified by any endpoint or proxy.
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.
In addition to the processing defined for the inner Block options
inherent to class E options, the AEAD Tag from each block SHALL be
included in the calculation of the Tag for the next block (see
Section 5.2), so that each block in the order being sent can be
verified as it arrives.
The protected CoAP message may be fragmented by the sending endpoint
or proxy as defined in [RFC7959], in which case the outer Block
options are being used. The outer Block options SHALL neither be
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 (see Section 6.3 and Section 6.5).
If the unprotected CoAP message contains Block options, the receiving
endpoint processes this according to {{RFC7959}.
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4.3.2. Class A Options
Options in this class are used to support forward proxy operations.
Class A options SHALL only have outer values and SHALL NOT be
encrypted. In order for the destination endpoint to verify the Uri,
class A options SHALL be integrity protected.
Uri-Host, Uri-Port, Proxy-Scheme and Proxy-Uri are class A options.
When Uri-Host, Uri-Port, Proxy-Scheme options are present, Proxy-Uri
is not used [RFC7252]. Proxy-Uri is processed like the other class A
options after a pre-processing step (see Section 4.3.2.1.
Except for Proxi-Uri, the sending endpoint SHALL copy the class A
option from the unprotected CoAP message to the protected CoAP
message. The class A options are inserted in the AAD of the COSE
object (see unencrypted-Uri Section 5.2).
4.3.2.1. Proxy-Uri
Proxy-Uri, when present, is split by OSCOAP into class A options and
privacy sensitive 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 unencrypted-Uri (Section 5.2) and
Uri-Path/Query options according to section 6.4 of [RFC7252].
Uri-Path and Uri-Query are class E options and SHALL 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 SHALL
be replaced with unencrypted-Uri and SHALL be protected and processed
as a class A option, see Section 4.3.2.
5. The COSE Object
This section defines how to use the COSE format [I-D.ietf-cose-msg]
to wrap and protect data in the unprotected CoAP message. OSCOAP
uses the COSE_Encrypt0 structure with an Authenticated Encryption
with Additional Data (AEAD) algorithm.
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 SHALL be 128 bits, the
length of nonce, Sender IV, and Recipient IV SHALL be 7 bytes, and
the maximum Sequence Number SHALL be 2^56-1. The nonce is
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constructed as described in Section 3.1 of [I-D.ietf-cose-msg], i.e.
by padding the Partial IV (Sequence Number) with zeroes and XORing it
with the context IV (Sender IV or Recipient IV).
Since OSCOAP only makes use of a single COSE structure, there is no
need to explicitly specify the structure, and OSCOAP uses the
untagged version of the COSE_Encrypt0 structure (Section 2. of
[I-D.ietf-cose-msg]). If the COSE object has a different structure,
the recipient MUST reject the message, treating it as malformed.
OSCOAP introduces a new COSE Header Parameter, the Sender Identifier:
sid: This parameter is used to identify the sender of the message.
Applications MUST NOT assume that 'sid' values are unique. This
is not a security critical field. For this reason, it can be
placed in the unprotected headers bucket.
+------+-------+------------+----------------+-------------------+
| name | label | value type | value registry | description |
+------+-------+------------+----------------+-------------------+
| sid | TBD | bstr | | Sender Identifier |
+------+-------+------------+----------------+-------------------+
Table 1: Additional COSE Header Parameter
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, in the COSE object.
The fields of COSE_Encrypt0 structure are defined as follows (see
example in Appendix C.4).
o The "Headers" field is formed by:
* The "protected" field, which SHALL include:
+ The "Partial IV" parameter. The value is set to the Sender
Sequence Number. The Partial IV is a byte string (type:
bstr), and SHOULD be of minimum length needed to encode the
sequence number.
+ The "kid" parameter. The value is set to the Context
Identifier (see Section 3). This parameter is optional if
the message is a CoAP response.
+ Optionally, the parameter called "sid", defined below. The
value is set to the Sender ID (see Section 3). Note that
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since this parameter is sent in clear, privacy issues SHOULD
be considered by the application defining the Sender ID.
* The "unprotected" field, which SHALL be empty.
o The "ciphertext" field is computed from the Plaintext (see
Section 5.1) and the Additional Authenticated Data (AAD) (see
Section 5.2) and encoded as a byte string (type: bstr), following
Section 5.2 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 CoAP Options present in the unprotected message which are
encrypted (see Section 4), in the order as given by the Option
number (each Option with Option Header including delta to previous
included encrypted option); and
o the CoAP Payload, 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options to Encrypt (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 Additional Authenticated Data ("Enc_structure") as described is
Section 5.3 of [I-D.ietf-cose-msg] includes:
o the "context" parameter, which has value "Encrypted"
o the "protected" parameter, which includes the "protected" part of
the "Headers" field;
o the "external_aad" is a serialized CBOR array Figure 6 where the
exact content is different in requests (external_aad_req) and
repsonses (external_aad_resp). It contains:
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* ver: uint, contains the CoAP version number, as defined in
Section 3 of [RFC7252]
* code: uint, contains is the CoAP Code of the unprotected CoAP
message, as defined in Section 3 of [RFC7252].
* alg: int, contains the Algorithm from the security context used
for the exchange (see Section 3.1);
* unencrypted-uri: tstr with tag URI, contains the part of the
URI which is not encrypted, and is composed of the request
scheme (Proxy-Scheme if present), Uri-Host and Uri-Port (if
present) options according to the method described in
Section 6.5 of [RFC7252], if the message is a CoAP request;
* cid : bstr, contains the cid for the request (which is same as
the cid for the response).
* id : bstr, is the identifier for the endpoint sending the
request and verifying the response; which means that for the
endpoint sending the response, the id has value Recipient ID,
while for the endpoint receiving the response, id has the value
Sender ID.
* seq : bstr, is the value of the "Partial IV" in the COSE object
of the request (see Section 5).
* tag-previous-block: bstr, contains the AEAD Tag of the message
containing the previous block in the sequence, as enumerated by
Block1 in the case of a request and Block2 in the case of a
response, if the message is fragmented using a block option
[RFC7959].
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external_aad = external_aad_req / external_aad_resp
external_aad_req = [
ver : uint,
code : uint,
alg : int,
unencrypted-uri : uri,
? tag-previous-block : bstr
]
external_aad_resp = [
ver : uint,
code : uint,
alg : int,
cid : bstr,
id : bstr,
seq : bstr,
? tag-previous-block : bstr
]
Figure 6: External AAD (external_aad)
The encryption process is described in Section 5.3 of
[I-D.ietf-cose-msg].
6. Protecting CoAP Messages
6.1. Replay and Freshness Protection
In order to protect from replay of messages and verify freshness, a
CoAP endpoint SHALL maintain a Sender Sequence Number and a Recipient
Replay Window in the security context. An endpoint uses the Sender
Sequence Number to protect messages to send and the Recipient Replay
Window to verify received messages, as described in Section 3.
A receiving endpoint SHALL verify that the Sequence Number (Partial
IV) received in the COSE object has not been received before in the
security context identified by the Cid. The size of the Replay Window
depends on the use case and lower protocol layers. In case of
reliable and ordered transport, the recipient MAY just store the last
received sequence number and require that newly received Sequence
Numbers equals the last received Recipient Sequence Number + 1.
The receiving endpoint SHALL reject messages with a sequence number
greater than the maximum value of the Partial IV. This maximum value
is algorithm specific, for example for AES-CCM-64-64-128 it is
2^56-1.
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OSCOAP responses are verified to match a prior request, by including
the unique transaction identifier (Tid as defined in Section 3) of
the request in the Additional Authenticated Data of the response
message. In case of CoAP observe, each notification MUST be verified
using the Tid of the observe registration, so the Tid of the
registration needs to be cached by the observer until the observation
ends.
If a CoAP server receives a request with the Object-Security option,
then the server SHALL include the Tid of the request in the AAD of
the response, as described in Section 6.4.
If the CoAP client receives a response with the Object-Security
option, then the client SHALL verify the integrity of the response,
using the Tid of its own associated request in the AAD, as described
in Section 6.5.
6.2. Protecting the Request
Given an unprotected CoAP request, including header, options and
payload, the client SHALL perform the following steps to create a
protected CoAP request using a security context associated with the
target resource (see Section 3.2.2).
When using Uri-Host or Proxy-Uri in the construction of the request,
the <host> value MUST be a reg-name ([RFC3986]), and not an IP-
literal or IPv4address, for canonicalization of the destination
address.
1. Compute the COSE object as specified in Section 5
* the AEAD nonce is created by XORing the Sender IV (context IV)
with the Sender Sequence Number (partial IV).
* If the block option is used, the AAD includes the AEAD Tag
from the previous block sent (from the second block and
following) Section 5.2. This means that the endpoint MUST
store the Tag of each last-sent block to compute the
following.
* Note that the 'sid' field containing the Sender ID is included
in the COSE object (Section 5) if the application needs it.
2. Format the protected CoAP message as an ordinary CoAP message,
with the following Header, Options, and Payload, based on the
unprotected CoAP message:
* The CoAP header is the same as the unprotected CoAP message.
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* If present, the CoAP option Proxy-Uri is decomposed as
described in Section 4.3.2.1.
* The CoAP options which are of class E (Section 4) are removed.
The Object-Security option is added.
* If the message type of the unprotected CoAP message does not
allow Payload, then the value of the Object-Security option is
the COSE object. If the message type of the unprotected CoAP
message allows Payload, then the Object-Security option is
empty and the Payload of the protected CoAP message is the
COSE object.
3. Store the association Token - Cid. The Client SHALL be able to
find the correct security context used to protect the request and
verify the response with use of the Token of the message
exchange.
4. Increment the Sender Sequence Number by one. If the Sender
Sequence Number exceeds the maximum number for the AEAD
algorithm, the client MUST NOT process any more requests with the
given security context. The client SHOULD acquire a new security
context (and consequently inform the server about it) before this
happens. The latter is out of scope of this memo.
6.3. Verifying the Request
A CoAP server receiving an unprotected CoAP request to access a
protected resource (as defined Section 3.2.2) SHALL reject the
message with error code 4.01 (Unauthorized).
A CoAP server receiving a message containing the Object-Security
option and a outer Block option SHALL first process this option
according to [RFC7959], until all blocks of the protected CoAP
message has been received, see Section 4.3.1.3.
A CoAP server receiving a message containing the Object-Security
option SHALL perform the following steps, using the security context
identified by the Context Identifier in the "kid" parameter in the
received COSE object:
1. Verify the Sequence Number in the Partial IV parameter, as
described in Section 6.1. If it cannot be verified that the
Sequence Number has not been received before, the server MUST
stop processing the request.
2. Recreate the Additional Authenticated Data, as described in
Section 5.
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* If the block option is used, the AAD includes the AEAD Tag
from the previous block received (from the second block and
following) Section 5.2. This means that the endpoint MUST
store the Tag of each last-received block to compute the
following.
* Note that the server's <host> value MUST be a reg-name
([RFC3986]), and not an IP-literal or IPv4address.
3. Compose the AEAD nonce by XORing the Recipient IV (context IV)
with the padded Partial IV parameter, received in the COSE
Object.
4. Retrieve the Recipient Key.
5. Verify and decrypt the message. If the verification fails, the
server MUST stop processing the request.
6. If the message verifies, update the Recipient Replay Window, as
described in Section 6.1.
7. Restore the unprotected request by adding any decrypted options
or payload from the plaintext. Any outer E options (Section 4)
are overwritten. The Object-Security option is removed.
6.4. Protecting the Response
A server receiving a valid request with a protected CoAP message
(i.e. containing an Object-Security option) SHALL respond with a
protected CoAP message.
Given an unprotected CoAP response, including header, options, and
payload, the server SHALL perform the following steps to create a
protected CoAP response, using the security context identified by the
Context Identifier of the received request:
1. Compute the COSE object as specified in Section Section 5
* The AEAD nonce is created by XORing the Sender IV (context IV)
and the padded Sender Sequence Number.
* If the block option [RFC7959] is used, the AAD includes the
AEAD Tag from the previous block sent (from the second block
and following) Section 5.2. This means that the endpoint MUST
store the Tag of each last-sent block to compute the
following. Note that this applies even for random access of
blocks, i.e. when blocks are not requested in the order of
their relative number (NUM).
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2. Format the protected CoAP message as an ordinary CoAP message,
with the following Header, Options, and Payload based on the
unprotected CoAP message:
* The CoAP header is the same as the unprotected CoAP message.
* The CoAP options which are of class E are removed, except any
special option (labelled '*') that is present which has its
outer value (Section 4). The Object-Security option is added.
* If the message type of the unprotected CoAP message does not
allow Payload, then the value of the Object-Security option is
the COSE object. If the message type of the unprotected CoAP
message allows Payload, then the Object-Security option is
empty and the Payload of the protected CoAP message is the
COSE object.
3. Increment the Sender Sequence Number by one. If the Sender
Sequence Number exceeds the maximum number for the AEAD
algorithm, the server MUST NOT process any more responses with
the given security context. The server SHOULD acquire a new
security context (and consequently inform the client about it)
before this happens. The latter is out of scope of this memo.
Note the differences between generating a protected request, and a
protected response, for example whether "kid" is present in the
header, or whether Destination URI or Tid is present in the AAD, of
the COSE object.
6.5. Verifying the Response
A CoAP client receiving a message containing the Object-Security
option SHALL perform the following steps, using the security context
identified by the Token of the received response:
1. If the message contain an outer Block option the client SHALL
process this option according to [RFC7959], until all blocks of
the protected CoAP message has been received, see
Section 4.3.1.3.
2. Verify the Sequence Number in the Partial IV parameter as
described in Section 6.1. If it cannot be verified that the
Sequence Number has not been received before, the client MUST
stop processing the response.
3. Recreate the Additional Authenticated Data as described in
Section 5.
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* If the block option is used, the AAD includes the AEAD Tag
from the previous block received (from the second block and
following) Section 5.2. This means that the endpoint MUST
store the Tag of each last-received block to compute the
following.
4. Compose the AEAD nonce by XORing the Recipient IV (context IV)
with the Partial IV parameter, received in the COSE Object.
5. Retrieve the Recipient Key.
6. Verify and decrypt the message. If the verification fails, the
client MUST stop processing the response.
7. If the message verifies, update the Recipient Replay Window, as
described in Section 6.1.
8. Restore the unprotected response by adding any decrypted options
or payload from the plaintext. Any class E options (Section 4)
are overwritten. The Object-Security option is removed.
7. 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.
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
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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 and key derivation function in client
and server. EDHOC [I-D.selander-ace-cose-ecdhe] describes an
augmented Diffie-Hellman key exchange to produce forward secret
keying material and agree on crypto algorithms necessary for OSCOAP,
authenticated with pre-established credentials. These pre-
established credentials may, in turn, be provisioned using a trusted
third party such as described in the OAuth-based 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 no. messages (2^56-1). 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 encrypted 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. In order to protect
from attacks replacing blocks from a different message with the same
block number between same endpoints and same resource at roughly the
same time, the AEAD Tag from the message containing one block is
included in the external_aad of the message containing the next
block.
The unencrypted 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
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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.
8. 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.
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.
9. IANA Considerations
Note to RFC Editor: Please replace all occurrences of "[[this
document]]" with the RFC number of this specification.
9.1. CoAP Option Numbers Registry
The Object-Security option is added to the CoAP Option Numbers
registry:
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+--------+-----------------+-------------------+
| Number | Name | Reference |
+--------+-----------------+-------------------+
| TBD | Object-Security | [[this document]] |
+--------+-----------------+-------------------+
9.2. COSE Header Parameters Registry
The "sid" parameter is added to the COSE Header Parameter Registry:
+------+-------+------------+----------------+-------------------+
| name | label | value type | value registry | description |
+------+-------+------------+----------------+-------------------+
| sid | TBD | bstr | | Sender Identifier |
+------+-------+------------+----------------+-------------------+
9.3. 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]]
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:
iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: N/A
Author: Goeran Selander, goran.selander@ericsson.com
Change Controller: IESG
Provisional registration? No
9.4. CoAP Content Format Registration
The "application/oscon" content format is added to the CoAP Content
Format registry:
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+-------------------+----------+----+-------------------+
| Media type | Encoding | ID | Reference |
+-------------------+----------+----+-------------------+
| application/oscon | - | 70 | [[this document]] |
+-------------------+----------+----+-------------------+
10. Acknowledgments
The following individuals provided input to this document: Carsten
Bormann, Joakim Brorsson, Martin Gunnarsson, Klaus Hartke, Jim
Schaad, Marco Tiloca, and Malisa Vucinic.
Ludwig Seitz and Goeran Selander worked on this document as part of
the CelticPlus project CyberWI, with funding from Vinnova.
11. References
11.1. 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>.
[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>.
[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>.
[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>.
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[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[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>.
11.2. Informative References
[I-D.selander-ace-cose-ecdhe]
Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace-
cose-ecdhe-04 (work in progress), October 2016.
[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-01 (work in progress), July 2016.
[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.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.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-04 (work in progress), October 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.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-05 (work in progress),
October 2016.
[I-D.greevenbosch-appsawg-cbor-cddl]
Vigano, C. and H. Birkholz, "CBOR data definition language
(CDDL): a notational convention to express CBOR data
structures", draft-greevenbosch-appsawg-cbor-cddl-09 (work
in progress), September 2016.
[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>.
[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. Overhead
OSCOAP transforms an unprotected CoAP message to a protected CoAP
message, and the protected CoAP message is larger than the
unprotected CoAP message. This appendix illustrates the message
expansion.
A.1. Length of the Object-Security Option
The protected CoAP message contains the COSE object. The COSE object
is included in the payload if the message type of the unprotected
CoAP message allows payload or else in the Object-Security option.
In the former case the Object-Security option is empty. So the
length of the Object-Security option is either zero or the size of
the COSE object, depending on whether the CoAP message allows payload
or not.
Length of Object-Security option = { 0, size of COSE Object }
A.2. Size of the COSE Object
The size of the COSE object is the sum of the sizes of
o the Header parameters,
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o the Cipher Text (excluding the Tag),
o the Tag, and
o data incurred by the COSE format itself (including CBOR encoding).
Let's analyze the contributions one at a time:
o The header parameters of the COSE object are the Context
Identifier (Cid) and the Sequence Number (Seq) (also part of the
Transaction Identifier (Tid)) if the message is a request, and Seq
only if the message is a response (see Section 5).
* The size of Cid is recommended to be 64 bits, but may be
shorter, as discussed in Section 3.2.2
* The size of Seq is variable, and increases with the number of
messages exchanged.
* As the AEAD nonce is generated from the padded Sequence Number
and a previously agreed upon context IV it is not required to
send the whole nonce in the message.
o The Cipher Text, excluding the Tag, is the encryption of the
payload and the encrypted options Section 4, which are present in
the unprotected CoAP message.
o The size of the Tag depends on the Algorithm. For example, for
the algorithm AES-CCM-64-64-128, the Tag is 8 bytes.
o The overhead from the COSE format itself depends on the sizes of
the previous fields, and is of the order of 10 bytes.
A.3. Message Expansion
The message expansion is not the size of the COSE object. The
ciphertext in the COSE object is encrypted payload and options of the
unprotected CoAP message - the plaintext of which is removed from the
protected CoAP message. Since the size of the ciphertext is the same
as the corresponding plaintext, there is no message expansion due to
encryption; payload and options are just represented in a different
way in the protected CoAP message:
o The encrypted payload is in the payload of the protected CoAP
message
o The encrypted options are in the Object-Security option or within
the payload.
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Therefore the OSCOAP message expansion is due to Cid (if present),
Seq, Tag, and COSE overhead:
Message Overhead = Cid + Seq + Tag + COSE Overhead
Figure 7: OSCOAP message expansion
A.4. Example
This section gives an example of message expansion in a request with
OSCOAP.
In this example we assume an 8-byte Cid.
o Cid: 0xa1534e3c9cecad84
In the example the sequence number is 225, requiring 1 byte to
encode. (The size of Seq could be larger depending on how many
messages that has been sent as is discussed in Appendix A.2.)
o Seq: 225
The example is based on AES-CCM-64-64-128.
o Tag is 8 bytes
The COSE object is represented in Figure 8 using CBOR's diagnostic
notation.
[
h'a20448a1534e3c9cecad840641e2', / protected:
{04:h'a1534e3c9cecad84',
06:h'e2'} /
{}, / unprotected: - /
Ciph + Tag / ciphertext + 8 byte
authentication tag /
]
Figure 8: Example of message expansion
Note that the encrypted CoAP options and payload are omitted since we
target the message expansion (see Appendix A.3). Therefore the size
of the COSE Cipher Text equals the size of the Tag, which is 8 bytes.
The COSE object encodes to a total size of 26 bytes, which is the
message expansion in this example. The COSE overhead in this example
is 26 - (8 + 1 + 8) = 9 bytes, according to the formula in Figure 7.
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Note that in this example two bytes in the COSE overhead are used to
encode the length of Cid and the length of Seq.
Figure 9 summarizes these results.
+---------+---------+---------+----------+------------+
| Cid | Seq | Tag | COSE OH | Message OH |
+---------+---------+---------+----------+------------+
| 8 bytes | 1 byte | 8 bytes | 9 bytes | 22 bytes |
+---------+---------+---------+----------+------------+
Figure 9: Message overhead for a 8-byte Cid, 1-byte Seq and 8-byte
Tag.
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
Here is an example targeting the scenario in the Section 2.2.1. -
Forwarding of [I-D.hartke-core-e2e-security-reqs]. The example
illustrates a client requesting the alarm status from a server. In
the request, CoAP option Uri-Path is encrypted and integrity
protected, and the CoAP header fields Code and Version are integrity
protected (see Section 4). In the response, the CoAP Payload is
encrypted and integrity protected, and the CoAP header fields Code
and Version are integrity protected.
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Client Proxy Server
| | |
+----->| | Code: 0.01 (GET)
| GET | | Token: 0x8c
| | | Object-Security: [cid:5fdc, seq:42,
| | | {Uri-Path:"alarm_status"},
| | | <Tag>]
| | | Payload: -
| | |
| +----->| Code: 0.01 (GET)
| | GET | Token: 0x7b
| | | Object-Security: [cid:5fdc, seq:42,
| | | {Uri-Path:"alarm_status"},
| | | <Tag>]
| | | Payload: -
| | |
| |<-----+ Code: 2.05 (Content)
| | 2.05 | Token: 0x7b
| | | Max-Age: 0
| | | Object-Security: -
| | | Payload: [seq:56, {"OFF"}, <Tag>]
| | |
|<-----+ | Code: 2.05 (Content)
| 2.05 | | Token: 0x8c
| | | Max-Age: 0
| | | Object-Security: -
| | | Payload: [seq:56, {"OFF"}, <Tag>]
| | |
Figure 10: Indication of CoAP GET protected with OSCOAP. The
brackets [ ... ] indicate a COSE object. The brackets { ... }
indicate encrypted data.
Since the unprotected request message (GET) has no payload, the
Object-Security option carries the COSE object as its value. Since
the unprotected response message (Content) has payload ("OFF"), the
COSE object (indicated with [ ... ]) is carried as the CoAP payload.
The COSE header of the request contains a Context Identifier
(cid:5fdc), indicating which security context was used to protect the
message and a Sequence Number (seq:42).
The option Uri-Path (alarm_status) and payload ("OFF") are formatted
as indicated in Section 5, and encrypted in the COSE Cipher Text
(indicated with { ... }).
The server verifies that the Sequence Number has not been received
before (see Section 6.1). The client verifies that the Sequence
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Number has not been received before and that the response message is
generated as a response to the sent request message (see
Section 6.1).
B.2. Secure Subscribe to Sensor
Here is an example targeting the scenario in the Forwarding with
observe case of [I-D.hartke-core-e2e-security-reqs]. The example
illustrates a client requesting subscription to a blood sugar
measurement resource (GET /glucose), and first receiving the value
220 mg/dl, and then a second reading with value 180 mg/dl. The CoAP
options Observe, Uri-Path, Content-Format, and Payload are encrypted
and integrity protected, and the CoAP header field Code is integrity
protected (see Section 4).
Client Proxy Server
| | |
+----->| | Code: 0.01 (GET)
| GET | | Token: 0x83
| | | Observe: 0
| | | Object-Security: [cid:ca, seq:15b7, {Observe:0,
| | | Uri-Path:"glucose"}, <Tag>]
| | | Payload: -
| | |
| +----->| Code: 0.01 (GET)
| | GET | Token: 0xbe
| | | Observe: 0
| | | Object-Security: [cid:ca, seq:15b7, {Observe:0,
| | | Uri-Path:"glucose"}, <Tag>]
| | | Payload: -
| | |
| |<-----+ Code: 2.05 (Content)
| | 2.05 | Token: 0xbe
| | | Max-Age: 0
| | | Observe: 1
| | | Object-Security: -
| | | Payload: [seq:32c2, {Observe:1,
| | | Content-Format:0, "220"}, <Tag>]
| | |
|<-----+ | Code: 2.05 (Content)
| 2.05 | | Token: 0x83
| | | Max-Age: 0
| | | Observe: 1
| | | Object-Security: -
| | | Payload: [seq:32c2, {Observe:1,
| | | Content-Format:0, "220"}, <Tag>]
... ... ...
| | |
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| |<-----+ Code: 2.05 (Content)
| | 2.05 | Token: 0xbe
| | | Max-Age: 0
| | | Observe: 2
| | | Object-Security: -
| | | Payload: [seq:32c6, {Observe:2,
| | | Content-Format:0, "180"}, <Tag>]
| | |
|<-----+ | Code: 2.05 (Content)
| 2.05 | | Token: 0x83
| | | Max-Age: 0
| | | Observe: 2
| | | Object-Security: -
| | | Payload: [seq:32c6, {Observe:2,
| | | Content-Format:0, "180"}, <Tag>]
| | |
Figure 11: Indication of CoAP GET protected with OSCOAP. The
brackets [ ... ] indicates COSE object. The bracket { ... }
indicates encrypted data.
Since the unprotected request message (GET) allows no payload, the
COSE object (indicated with [ ... ]) is carried in the Object-
Security option value. Since the unprotected response message
(Content) has payload, the Object-Security option is empty, and the
COSE object is carried as the payload.
The COSE header of the request contains a Context Identifier
(cid:ca), indicating which security context was used to protect the
message and a Sequence Number (seq:15b7).
The options Observe, Content-Format and the payload are formatted as
indicated in Section 5, and encrypted in the COSE ciphertext
(indicated with { ... }).
The server verifies that the Sequence Number has not been received
before (see Section 6.1). The client verifies that the Sequence
Number has not been received before and that the response message is
generated as a response to the subscribe request.
Appendix C. Object Security of Content (OSCON)
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
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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 9)
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 6.1. 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.
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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
following the formula in Figure 7.
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
]
)
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This COSE object encodes to a total size of 26 bytes.
Figure 12 summarizes these results.
+------------------+-----+-----+---------+------------+
| Structure | Tid | MAC | COSE OH | Message OH |
+------------------+-----+-----+---------+------------+
| COSE_Mac0_Tagged | 5 B | 8 B | 13 B | 26 B |
+------------------+-----+-----+---------+------------+
Figure 12: 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:
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 13 summarizes these results.
+-------------------+-----+------+---------+------------+
| Structure | Tid | SIG | COSE OH | Message OH |
+-------------------+-----+------+---------+------------+
| COSE_Sign1_Tagged | 5 B | 64 B | 14 B | 83 bytes |
+-------------------+-----+------+---------+------------+
Figure 13: Message overhead for a 5-byte Tid using 64 byte ECDSA
signature.
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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
TAG # ciphertext + truncated 8-byte TAG
]
)
This COSE object encodes to a total size of 25 bytes.
Figure 14 summarizes these results.
+----------------------+-----+-----+---------+------------+
| Structure | Tid | TAG | COSE OH | Message OH |
+----------------------+-----+-----+---------+------------+
| COSE_Encrypt0_Tagged | 5 B | 8 B | 12 B | 25 bytes |
+----------------------+-----+-----+---------+------------+
Figure 14: 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:
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993( # COSE_Encrypt0_Tagged
[
h'a20444a1534e3c0641a3', # protected:
{04:h'a1534e3c',
06:h'a3'}
{9:SIG}, # unprotected:
09: 64 bytes signature
TAG # ciphertext + truncated 8-byte TAG
]
)
This COSE object encodes to a total size of 92 bytes.
Figure 15 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 15: Message overhead for a 5-byte Tid using AES-CCM
countersigned with ECDSA.
Authors' Addresses
Goeran Selander
Ericsson AB
Farogatan 6
Kista SE-16480 Stockholm
Sweden
Email: goran.selander@ericsson.com
John Mattsson
Ericsson AB
Farogatan 6
Kista SE-16480 Stockholm
Sweden
Email: john.mattsson@ericsson.com
Selander, et al. Expires June 22, 2017 [Page 44]
Internet-Draft Object Security of CoAP (OSCOAP) December 2016
Francesca Palombini
Ericsson AB
Farogatan 6
Kista SE-16480 Stockholm
Sweden
Email: francesca.palombini@ericsson.com
Ludwig Seitz
SICS Swedish ICT
Scheelevagen 17
Lund 22370
Sweden
Email: ludwig@sics.se
Selander, et al. Expires June 22, 2017 [Page 45]