CoRE Working Group G. Selander
Internet-Draft J. Mattsson
Intended status: Standards Track F. Palombini
Expires: September 15, 2018 Ericsson AB
L. Seitz
RISE SICS
March 14, 2018
Object Security for Constrained RESTful Environments (OSCORE)
draft-ietf-core-object-security-10
Abstract
This document defines Object Security for Constrained RESTful
Environments (OSCORE), a method for application-layer protection of
the Constrained Application Protocol (CoAP), using CBOR Object
Signing and Encryption (COSE). OSCORE provides end-to-end protection
between endpoints communicating using CoAP or CoAP-mappable HTTP.
OSCORE is designed for constrained nodes and networks supporting a
range of proxy operations, including translation between different
transport protocols.
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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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 15, 2018.
Copyright Notice
Copyright (c) 2018 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
carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. The CoAP Object-Security Option . . . . . . . . . . . . . . . 6
3. The Security Context . . . . . . . . . . . . . . . . . . . . 7
3.1. Security Context Definition . . . . . . . . . . . . . . . 7
3.2. Establishment of Security Context Parameters . . . . . . 9
3.3. Requirements on the Security Context Parameters . . . . . 11
4. Protected Message Fields . . . . . . . . . . . . . . . . . . 12
4.1. CoAP Options . . . . . . . . . . . . . . . . . . . . . . 13
4.2. CoAP Header Fields and Payload . . . . . . . . . . . . . 20
4.3. Signaling Messages . . . . . . . . . . . . . . . . . . . 21
5. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 21
5.1. Kid Context . . . . . . . . . . . . . . . . . . . . . . . 23
5.2. Nonce . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.3. Plaintext . . . . . . . . . . . . . . . . . . . . . . . . 24
5.4. Additional Authenticated Data . . . . . . . . . . . . . . 25
6. OSCORE Header Compression . . . . . . . . . . . . . . . . . . 26
6.1. Encoding of the Object-Security Value . . . . . . . . . . 26
6.2. Encoding of the OSCORE Payload . . . . . . . . . . . . . 27
6.3. Examples of Compressed COSE Objects . . . . . . . . . . . 28
7. Sequence Numbers, Replay, Message Binding, and Freshness . . 29
7.1. Message Binding . . . . . . . . . . . . . . . . . . . . . 29
7.2. AEAD Nonce Uniqueness . . . . . . . . . . . . . . . . . . 29
7.3. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 30
7.4. Replay Protection . . . . . . . . . . . . . . . . . . . . 30
7.5. Losing Part of the Context State . . . . . . . . . . . . 31
8. Processing . . . . . . . . . . . . . . . . . . . . . . . . . 32
8.1. Protecting the Request . . . . . . . . . . . . . . . . . 32
8.2. Verifying the Request . . . . . . . . . . . . . . . . . . 33
8.3. Protecting the Response . . . . . . . . . . . . . . . . . 34
8.4. Verifying the Response . . . . . . . . . . . . . . . . . 35
9. Web Linking . . . . . . . . . . . . . . . . . . . . . . . . . 36
10. Proxy and HTTP Operations . . . . . . . . . . . . . . . . . . 37
10.1. CoAP-to-CoAP Forwarding Proxy . . . . . . . . . . . . . 37
10.2. HTTP Processing . . . . . . . . . . . . . . . . . . . . 37
10.3. HTTP-to-CoAP Translation Proxy . . . . . . . . . . . . . 39
10.4. CoAP-to-HTTP Translation Proxy . . . . . . . . . . . . . 40
11. Security Considerations . . . . . . . . . . . . . . . . . . . 42
11.1. End-to-end protection . . . . . . . . . . . . . . . . . 42
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11.2. Security Context Establishment . . . . . . . . . . . . . 43
11.3. Replay Protection . . . . . . . . . . . . . . . . . . . 43
11.4. Cryptographic Considerations . . . . . . . . . . . . . . 43
11.5. Message Fragmentation . . . . . . . . . . . . . . . . . 44
11.6. Privacy Considerations . . . . . . . . . . . . . . . . . 44
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 45
12.1. COSE Header Parameters Registry . . . . . . . . . . . . 45
12.2. CoAP Option Numbers Registry . . . . . . . . . . . . . . 45
12.3. CoAP Signaling Option Numbers Registry . . . . . . . . . 46
12.4. Header Field Registrations . . . . . . . . . . . . . . . 46
12.5. Media Type Registrations . . . . . . . . . . . . . . . . 46
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 48
13.1. Normative References . . . . . . . . . . . . . . . . . . 48
13.2. Informative References . . . . . . . . . . . . . . . . . 49
Appendix A. Scenario Examples . . . . . . . . . . . . . . . . . 51
A.1. Secure Access to Sensor . . . . . . . . . . . . . . . . . 51
A.2. Secure Subscribe to Sensor . . . . . . . . . . . . . . . 52
Appendix B. Deployment examples . . . . . . . . . . . . . . . . 54
B.1. Master Secret Used Once . . . . . . . . . . . . . . . . . 54
B.2. Master Secret Used Multiple Times . . . . . . . . . . . . 54
B.3. Client Aliveness . . . . . . . . . . . . . . . . . . . . 55
Appendix C. Test Vectors . . . . . . . . . . . . . . . . . . . . 56
C.1. Test Vector 1: Key Derivation with Master Salt . . . . . 56
C.2. Test Vector 2: Key Derivation without Master Salt . . . . 57
C.3. Test Vector 3: OSCORE Request, Client . . . . . . . . . . 58
C.4. Test Vector 4: OSCORE Request, Client . . . . . . . . . . 59
C.5. Test Vector 5: OSCORE Response, Server . . . . . . . . . 60
C.6. Test Vector 6: OSCORE Response with Partial IV, Server . 61
Appendix D. Security properties . . . . . . . . . . . . . . . . 63
Appendix E. CDDL Summary . . . . . . . . . . . . . . . . . . . . 63
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 63
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 64
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] is a web
application protocol, designed for constrained nodes and networks
[RFC7228], and may be mapped from HTTP [RFC8075]. CoAP specifies the
use of proxies for scalability and efficiency and references DTLS
([RFC6347]) for security. CoAP-to-CoAP, HTTP-to-CoAP, and CoAP-to-
HTTP proxies require (D)TLS 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 message payload and metadata in transit
between the endpoints. The proxy can also inject, delete, or reorder
packets since they are no longer protected by (D)TLS.
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This document defines the Object Security for Constrained RESTful
Environments (OSCORE) security protocol, protecting CoAP and CoAP-
mappable HTTP requests and responses end-to-end across intermediary
nodes such as CoAP forward proxies and cross-protocol translators
including HTTP-to-CoAP proxies [RFC8075]. In addition to the core
CoAP features defined in [RFC7252], OSCORE supports Observe
[RFC7641], Blockwise [RFC7959], No-Response [RFC7967], and PATCH and
FETCH [RFC8132]. An analysis of end-to-end security for CoAP
messages through some types of intermediary nodes is performed in
[I-D.hartke-core-e2e-security-reqs]. OSCORE essentially protects the
RESTful interactions; the request method, the requested resource, the
message payload, etc. (see Section 4). OSCORE protects neither the
CoAP Messaging Layer nor the CoAP Token which may change between the
endpoints, and those are therefore processed as defined in [RFC7252].
Additionally, since the message formats for CoAP over unreliable
transport [RFC7252] and for CoAP over reliable transport [RFC8323]
differ only in terms of CoAP Messaging Layer, OSCORE can be applied
to both unreliable and reliable transports (see Figure 1).
+-----------------------------------+
| Application |
+-----------------------------------+
+-----------------------------------+ \
| Requests / Responses / Signaling | |
|-----------------------------------| |
| OSCORE | | CoAP
|-----------------------------------| |
| Messaging Layer / Message Framing | |
+-----------------------------------+ /
+-----------------------------------+
| UDP / TCP / ... |
+-----------------------------------+
Figure 1: Abstract Layering of CoAP with OSCORE
OSCORE works in very constrained nodes and networks, thanks to its
small message size and the restricted code and memory requirements in
addition to what is required by CoAP. Examples of the use of OSCORE
are given in Appendix A. OSCORE does not depend on underlying
layers, and can be used anywhere where CoAP or HTTP can be used,
including non-IP transports (e.g., [I-D.bormann-6lo-coap-802-15-ie]).
OSCORE may be used together with (D)TLS over one or more hops in the
end-to-end path, e.g. with HTTPs in one hop and with plain CoAP in
another hop.
The use of OSCORE does not affect the URI scheme and OSCORE can
therefore be used with any URI scheme defined for CoAP or HTTP. The
application decides the conditions for which OSCORE is required.
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OSCORE uses pre-shared keys which may have been established out-of-
band or with a key establishment protocol (see Section 3.2). The
technical solution builds on CBOR Object Signing and Encryption
(COSE) [RFC8152], providing end-to-end encryption, integrity, replay
protection, and secure binding of response to request. A compressed
version of COSE is used, as specified in Section 6. The use of
OSCORE is signaled with the new Object-Security CoAP option or HTTP
header field, defined in Section 2 and Section 10.3. The solution
transforms a CoAP/HTTP message into an "OSCORE message" before
sending, and vice versa after receiving. The OSCORE message is a
CoAP/HTTP message related to the original message in the following
way: the original CoAP/HTTP message is translated to CoAP (if not
already in CoAP) and protected in a COSE object. The encrypted
message fields of this COSE object are transported in the CoAP
payload/HTTP body of the OSCORE message, and the Object-Security
option/header field is included in the message. A sketch of an
OSCORE message exchange in the case of the original message being
CoAP is provided in Figure 2).
Client Server
| OSCORE request - POST example.com: |
| Header, Token, |
| Options: {Object-Security, ...}, |
| Payload: COSE ciphertext |
+--------------------------------------------->|
| |
|<---------------------------------------------+
| OSCORE response - 2.04 (Changed): |
| Header, Token, |
| Options: {Object-Security, ...}, |
| Payload: COSE ciphertext |
| |
Figure 2: Sketch of CoAP with OSCORE
An implementation supporting this specification MAY implement only
the client part, MAY implement only the server part, or MAY implement
only one of the proxy parts. OSCORE is designed to protect as much
information as possible while still allowing proxy operations
(Section 10). It works with legacy CoAP-to-CoAP forward proxies
[RFC7252], but an OSCORE-aware proxy will be more efficient. HTTP-
to-CoAP proxies [RFC8075] and CoAP-to-HTTP proxies can also be used
with OSCORE, as specified in Section 10.
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1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Readers are expected to be familiar with the terms and concepts
described in CoAP [RFC7252], Observe [RFC7641], Blockwise [RFC7959],
COSE [RFC8152], CBOR [RFC7049], CDDL [I-D.ietf-cbor-cddl] as
summarized in Appendix E, and constrained environments [RFC7228].
The term "hop" is used to denote a particular leg in the end-to-end
path. The concept "hop-by-hop" (as in "hop-by-hop encryption" or
"hop-by-hop fragmentation") opposed to "end-to-end", is used in this
document to indicate that the messages are processed accordingly in
the intermediaries, rather than just forwarded to the next node.
The term "stop processing" is used throughout the document to denote
that the message is not passed up to the CoAP Request/Response layer
(see Figure 1).
The terms Common/Sender/Recipient Context, Master Secret/Salt, Sender
ID/Key, Recipient ID/Key, and Common IV are defined in Section 3.1.
2. The CoAP Object-Security Option
The CoAP Object-Security option (see Figure 3, which extends Table 4
of [RFC7252]) indicates that the CoAP message is an OSCORE message
and that it contains a compressed COSE object (see Section 5 and
Section 6). The Object-Security option is critical, safe to forward,
part of the cache key, and not repeatable.
+-----+---+---+---+---+-----------------+--------+--------+---------+
| No. | C | U | N | R | Name | Format | Length | Default |
+-----+---+---+---+---+-----------------+--------+--------+---------+
| TBD | x | | | | Object-Security | (*) | 0-255 | (none) |
+-----+---+---+---+---+-----------------+--------+--------+---------+
C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable
(*) See below.
Figure 3: The Object-Security Option
The Object-Security option includes the OSCORE flag bits (Section 6),
the Sender Sequence Number and the Sender ID when present
(Section 3). The detailed format and length is specified in
Section 6. If the OSCORE flag bits is all zero (0x00) the Option
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value SHALL be empty (Option Length = 0). An endpoint receiving a
CoAP message without payload, that also contains an Object-Security
option SHALL treat it as malformed and reject it.
A successful response to a request with the Object-Security option
SHALL contain the Object-Security option. Whether error responses
contain the Object-Security option depends on the error type (see
Section 8).
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 request (see Section 10.1). As the compressed COSE Object
is included in the cache key, messages with the Object-Security
option will never generate cache hits. For Max-Age processing (see
Section 4.1.3.1).
3. The Security Context
OSCORE requires that client and server establish a shared security
context used to process the COSE objects. OSCORE uses COSE with an
Authenticated Encryption with Additional Data (AEAD, [RFC5116])
algorithm for protecting message data between a client and a server.
In this section, we define the security context and how it is derived
in client and server based on a 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 OSCORE. 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 and
servers 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 4.
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.-------------. .-------------.
| Common, | | Common, |
| Sender, | | Recipient, |
| Recipient | | Sender |
'-------------' '-------------'
Client Server
| |
Retrieve context for | OSCORE request: |
target resource | Token = Token1, |
Protect request with | kid = SID, ... |
Sender Context +---------------------->| Retrieve context with
| | RID = kid
| | Verify request with
| | Recipient Context
| OSCORE response: | Protect response with
| Token = Token1, ... | Sender Context
Retrieve context with |<----------------------+
Token = Token1 | |
Verify request with | |
Recipient Context | |
Figure 4: Retrieval and use of the Security Context
The Common Context contains the following parameters:
o AEAD Algorithm. The COSE AEAD algorithm to use for encryption.
o Key Derivation Function. The HMAC based HKDF [RFC5869] used to
derive Sender Key, Recipient Key, and Common IV.
o Master Secret. Variable length, uniformly random byte string
containing the key used to derive traffic keys and IVs.
o Master Salt. Variable length byte string containing the salt used
to derive traffic keys and IVs.
o Common IV. Byte string derived from Master Secret and Master
Salt. Length is determined by the AEAD Algorithm.
The Sender Context contains the following parameters:
o Sender ID. Byte string used to identify the Sender Context and to
assure unique AEAD nonces. Maximum length is determined by the
AEAD Algorithm.
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 the AEAD Algorithm.
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o Sender Sequence Number. Non-negative integer used by the sender
to protect requests and Observe notifications. Used as 'Partial
IV' [RFC8152] to generate unique nonces for the AEAD. Maximum
value is determined by the AEAD Algorithm.
The Recipient Context contains the following parameters:
o Recipient ID. Byte string used to identify the Recipient Context
and to assure unique AEAD nonces. Maximum length is determined by
the AEAD Algorithm.
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 AEAD Algorithm.
o Replay Window (Server only). The replay window to verify requests
received.
All parameters except Sender Sequence Number and Replay Window are
immutable once the security context is established. An endpoint may
free up memory by not storing the Common IV, Sender Key, and
Recipient Key, deriving them from the Master Key and Master Salt 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.
Endpoints MAY operate as both client and server and use the same
security context for those roles. Independent of being client or
server, the endpoint protects messages to send using its Sender
Context, and verifies messages received using its Recipient Context.
The endpoints MUST NOT change the Sender/Recipient ID when changing
roles. In other words, changing the roles does not change the set of
keys to be used.
3.2. Establishment 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
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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
* Default is AES-CCM-16-64-128 (COSE algorithm encoding: 10)
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
([RFC6347])
All input parameters need to be known to and agreed on by both
endpoints, but the replay window may be different in the two
endpoints. How the input parameters are pre-established, is
application specific. The OSCORE profile of the ACE framework may be
used to establish the necessary input parameters
([I-D.ietf-ace-oscore-profile]), or a key exchange protocol such as
the TLS/DTLS handshake ([I-D.mattsson-ace-tls-oscore]) or EDHOC
([I-D.selander-ace-cose-ecdhe]) providing forward secrecy. Other
examples of deploying OSCORE are given in Appendix B.
3.2.1. Derivation of Sender Key, Recipient Key, and Common 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, Recipient Key, and Common 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)
where:
o salt is the Master Salt as defined above
o IKM is the Master Secret as defined above
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o info is a CBOR array consisting of:
info = [
id : bstr,
alg_aead : int / tstr,
type : tstr,
L : uint
]
where:
o id is the Sender ID or Recipient ID when deriving keys and the
empty string when deriving the Common IV. The encoding is
described in Section 5.
o alg_aead is the AEAD Algorithm, encoded as defined in [RFC8152].
o type is "Key" or "IV". The label is an ASCII string, and does not
include a trailing NUL byte.
o L is the size of the key/IV for the AEAD algorithm used, in bytes.
For example, if the algorithm AES-CCM-16-64-128 (see Section 10.2 in
[RFC8152]) is used, the integer value for alg_aead is 10, the value
for L is 16 for keys and 13 for the Common IV.
3.2.2. Initial Sequence Numbers and Replay Window
The Sender Sequence Number is initialized to 0. The supported types
of replay protection and replay window length is application specific
and depends on how OSCORE is transported, see Section 7.4. The
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 and Master Salt. When a
trusted third party assigns identifiers (e.g., using
[I-D.ietf-ace-oauth-authz]) or by using a protocol that allows the
parties to negotiate locally unique identifiers in each endpoint, the
Sender IDs can be very short. The maximum length of Sender ID in
bytes equals the length of AEAD nonce minus 6. For AES-CCM-16-64-128
the maximum length of Sender ID is 7 bytes. Sender IDs MAY be
uniformly random distributed byte strings if the probability of
collisions is negligible.
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If Sender ID uniqueness cannot be guaranteed by construction, Sender
IDs MUST be long uniformly random distributed byte strings such that
the probability of collisions is negligible.
To simplify retrieval of the right Recipient Context, the Recipient
ID SHOULD be unique in the sets of all Recipient Contexts used by an
endpoint. If an endpoint has the same Recipient ID with different
Recipient Contexts, i.e. the Recipient Contexts are derived from
different keying material, then the endpoint may need to try multiple
times before finding the right security context associated to the
Recipient ID. The Client MAY provide a 'kid context' parameter
(Section 5.1) to help the Server find the right context.
While the triple (Master Secret, Master Salt, Sender ID) MUST be
unique, the same Master Salt MAY be used with several Master Secrets
and the same Master Secret MAY be used with several Master Salts.
4. Protected Message Fields
OSCORE transforms a CoAP message (which may have been generated from
an HTTP message) into an OSCORE message, and vice versa. OSCORE
protects as much of the original message as possible while still
allowing certain proxy operations (see Section 10). This section
defines how OSCORE protects the message fields and transfers them
end-to-end between client and server (in any direction).
The remainder of this section and later sections discuss the behavior
in terms of CoAP messages. If HTTP is used for a particular hop in
the end-to-end path, then this section applies to the conceptual CoAP
message that is mappable to/from the original HTTP message as
discussed in Section 10. That is, an HTTP message is conceptually
transformed to a CoAP message and then to an OSCORE message, and
similarly in the reverse direction. An actual implementation might
translate directly from HTTP to OSCORE without the intervening CoAP
representation.
Protection of Signaling messages (Section 5 of [RFC8323]) is
specified in Section 4.3. The other parts of this section target
Request/Response messages.
Message fields of the CoAP message may be protected end-to-end
between CoAP client and CoAP server in different ways:
o Class E: encrypted and integrity protected,
o Class I: integrity protected only, or
o Class U: unprotected.
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The sending endpoint SHALL transfer Class E message fields in the
ciphertext of the COSE object in the OSCORE message. The sending
endpoint SHALL include Class I message fields in the Additional
Authenticated Data (AAD) of the AEAD algorithm, allowing the
receiving endpoint to detect if the value has changed in transfer.
Class U message fields SHALL NOT be protected in transfer. Class I
and Class U message field values are transferred in the header or
options part of the OSCORE message, which is visible to proxies.
Message fields not visible to proxies, i.e., transported in the
ciphertext of the COSE object, are called "Inner" (Class E). Message
fields transferred in the header or options part of the OSCORE
message, which is visible to proxies, are called "Outer" (Class I or
U). There are currently no Class I options defined.
An OSCORE message may contain both an Inner and an Outer instance of
a certain CoAP message field. Inner message fields are intended for
the receiving endpoint, whereas Outer message fields are used to
support proxy operations. Inner and Outer message fields are
processed independently.
4.1. CoAP Options
A summary of how options are protected is shown in Figure 5. Note
that some options may have both Inner and Outer message fields which
are protected accordingly. The options which require special
processing are labelled with asterisks.
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+-----+-----------------+---+---+
| No. | Name | E | 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 | |
| TBD | Object-Security | | * |
| 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 | * | * |
| 258 | No-Response | * | * |
+-----+-----------------+---+---+
E = Encrypt and Integrity Protect (Inner)
U = Unprotected (Outer)
* = Special
Figure 5: Protection of CoAP Options
Options that are unknown or for which OSCORE processing is not
defined SHALL be processed as class E (and no special processing).
Specifications of new CoAP options SHOULD define how they are
processed with OSCORE. A new COAP option SHOULD be of class E unless
it requires proxy processing.
4.1.1. Inner Options
Inner option message fields (class E) are used to communicate
directly with the other endpoint.
The sending endpoint SHALL write the Inner option message fields
present in the original CoAP message into the plaintext of the COSE
object (Section 5.3), and then remove the Inner option message fields
from the OSCORE message.
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The processing of Inner option message fields by the receiving
endpoint is specified in Section 8.2 and Section 8.4.
4.1.2. Outer Options
Outer option message fields (Class U or I) are used to support proxy
operations.
The sending endpoint SHALL include the Outer option message field
present in the original message in the options part of the OSCORE
message. All Outer option message fields, including Object-Security,
SHALL be encoded as described in Section 3.1 of [RFC7252], where the
delta is the difference to the previously included instance of Outer
option message field.
The processing of Outer options by the receiving endpoint is
specified in Section 8.2 and Section 8.4.
A procedure for integrity-protection-only of Class I option message
fields is specified in Section 5.4. Proxies MUST NOT change the
order of option's occurrences, for options repeatable and of class I.
Note: There are currently no Class I option message fields defined.
4.1.3. Special Options
Some options require special processing, marked with an asterisk '*'
in Figure 5; the processing is specified in this section.
4.1.3.1. Max-Age
An Inner Max-Age message field is used to indicate the maximum time a
response may be cached by the client (as defined in [RFC7252]), end-
to-end from the server to the client, taking into account that the
option is not accessible to proxies. The Inner Max-Age SHALL be
processed by OSCORE as specified in Section 4.1.1.
An Outer Max-Age message field is used to avoid unnecessary caching
of OSCORE error responses at OSCORE unaware intermediary nodes. A
server MAY set a Class U Max-Age message field with value zero to
OSCORE error responses, which are described in Section 7.4,
Section 8.2 and Section 8.4. Such message field is then processed
according to Section 4.1.2.
Successful OSCORE responses do not need to include an Outer Max-Age
option since the responses are non-cacheable by construction (see
Section 4.2).
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4.1.3.2. The Block Options
Blockwise [RFC7959] is an optional feature. An implementation MAY
support [RFC7252] and the Object-Security option without supporting
Blockwise. The Block options (Block1, Block2, Size1, Size2), when
Inner message fields, provide secure message fragmentation such that
each fragment can be verified. The Block options, when Outer message
fields, enables hop-by-hop fragmentation of the OSCORE message.
Inner and Outer block processing may have different performance
properties depending on the underlying transport. The end-to-end
integrity of the message can be verified both in case of Inner and
Outer Blockwise provided all blocks are received.
4.1.3.2.1. Inner Block Options
The sending CoAP endpoint MAY fragment a CoAP message as defined in
[RFC7959] before the message is processed by OSCORE. In this case
the Block options SHALL be processed by OSCORE as Inner options
(Section 4.1.1). The receiving CoAP endpoint SHALL process the
OSCORE message according to Section 4.1.1 before processing Blockwise
as defined in [RFC7959].
4.1.3.2.2. Outer Block Options
Proxies MAY fragment an OSCORE message using [RFC7959], by
introducing Block option message fields that are Outer
(Section 4.1.2) and not generated by the sending endpoint. Note that
the Outer Block options are neither encrypted nor integrity
protected. As a consequence, a proxy can maliciously inject block
fragments indefinitely, since the receiving endpoint needs to receive
the last block (see [RFC7959]) to be able to compose the OSCORE
message and verify its integrity. Therefore, applications supporting
OSCORE and [RFC7959] MUST specify a security policy defining a
maximum unfragmented message size (MAX_UNFRAGMENTED_SIZE) considering
the maximum size of message which can be handled by the endpoints.
Messages exceeding this size SHOULD be fragmented by the sending
endpoint using Inner Block options (Section 4.1.3.2.1).
An endpoint receiving an OSCORE message with an Outer Block option
SHALL first process this option according to [RFC7959], until all
blocks of the OSCORE message have been received, or the cumulated
message size of the blocks exceeds MAX_UNFRAGMENTED_SIZE. In the
former case, the processing of the OSCORE message continues as
defined in this document. In the latter case the message SHALL be
discarded.
Because of encryption of Uri-Path and Uri-Query, messages to the same
server may, from the point of view of a proxy, look like they also
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target the same resource. A proxy SHOULD mitigate a potential mix-up
of blocks from concurrent requests to the same server, for example
using the Request-Tag processing specified in Section 3.3.2 of
[I-D.ietf-core-echo-request-tag].
4.1.3.3. Proxy-Uri
Proxy-Uri, when present, is split by OSCORE into class U options and
class E options, which are processed accordingly. When Proxy-Uri is
used in the original CoAP message, Uri-* are not present [RFC7252].
The sending endpoint SHALL first decompose the Proxy-Uri value of the
original CoAP message into the Proxy-Scheme, Uri-Host, Uri-Port, 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 SHALL be protected and
processed as Inner options (Section 4.1.1).
The Proxy-Uri option of the OSCORE message SHALL be set to the
composition of Proxy-Scheme, Uri-Host, and Uri-Port options (if
present) as specified in Section 6.5 of [RFC7252], and processed as
an Outer option of Class U (Section 4.1.2).
Note that replacing the Proxy-Uri value with the Proxy-Scheme and
Uri-* options works by design for all CoAP URIs (see Section 6 of
[RFC7252]). OSCORE-aware HTTP servers should not use the userinfo
component of the HTTP URI (as defined in Section 3.2.1 of [RFC3986]),
so that this type of replacement is possible in the presence of CoAP-
to-HTTP proxies. In future documents specifying cross-protocol
proxying behavior using different URI structures, it is expected that
the authors will create Uri-* options that allow decomposing the
Proxy-Uri, and specify in which OSCORE class they belong.
An example of how Proxy-Uri is processed is given here. Assume that
the original CoAP message contains:
o Proxy-Uri = "coap://example.com/resource?q=1"
During OSCORE processing, Proxy-Uri is split into:
o Proxy-Scheme = "coap"
o Uri-Host = "example.com"
o Uri-Port = "5683"
o Uri-Path = "resource"
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o Uri-Query = "q=1"
Uri-Path and Uri-Query follow the processing defined in
Section 4.1.1, 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 OSCORE message, which has value:
o Proxy-Uri = "coap://example.com"
See Sections 6.1 and 12.6 of [RFC7252] for more information.
4.1.3.4. 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).
In order for an OSCORE-unaware proxy to support forwarding of Observe
messages ([RFC7641]), there SHALL be an Outer Observe option, i.e.,
present in the options part of the OSCORE message. The processing of
the CoAP Code for Observe messages is described in Section 4.2.
To secure the order of notifications, the client SHALL maintain a
Notification Number for each Observation it registers. The
Notification Number is a non-negative integer containing the largest
Partial IV of the successfully received notifications for the
associated Observe registration (see Section 7.4). The Notification
Number is initialized to the Partial IV of the first successfully
received notification response to the registration request. In
contrast to [RFC7641], the received Partial IV MUST always be
compared with the Notification Number, which thus MUST NOT be
forgotten after 128 seconds. The client MAY ignore the Observe
option value.
If the verification fails, the client SHALL stop processing the
response.
The Observe option in the CoAP request 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 OSCORE client
receives a response to an Observe request without an Outer Observe
value, then it MUST verify the response as a non-Observe response.
If the OSCORE client receives a response to a non-Observe request
with an Outer Observe value, it stops processing the message, as
specified in Section 8.4.
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Clients can re-register observations to ensure that the observation
is still active and establish freshness again ([RFC7641]
Section 3.3.1). When an OSCORE observation is refreshed, not only
the ETags, but also the partial IV (and thus the payload and Object-
Security option) change. The server uses the new request's Partial
IV as the 'request_piv' of new responses.
4.1.3.5. No-Response
No-Response is defined in [RFC7967]. Clients using No-Response MUST
set both an Inner (Class E) and an Outer (Class U) No-Response
option, with same value.
The Inner No-Response option is used to communicate to the server the
client's disinterest in certain classes of responses to a particular
request. The Inner No-Response SHALL be processed by OSCORE as
specified in Section 4.1.1.
The Outer No-Response option is used to support proxy functionality,
specifically to avoid error transmissions from proxies to clients,
and to avoid bandwidth reduction to servers by proxies applying
congestion control when not receiving responses. The Outer No-
Response option is processed according to Section 4.1.2.
In particular, step 8 of Section 8.4 is applied to No-Response.
Applications should consider that a proxy may remove the Outer No-
Response option from the request. Applications using No-Response can
specify policies to deal with cases where servers receive an Inner
No-Response option only, which may be the result of the request
having traversed a No-Response unaware proxy, and update the
processing in Section 8.4 accordingly. This avoids unnecessary error
responses to clients and bandwidth reductions to servers, due to No-
Response unaware proxies.
4.1.3.6. Object-Security
The Object-Security option is only defined to be present in OSCORE
messages, as an indication that OSCORE processing have been
performed. The content in the Object-Security option is neither
encrypted nor integrity protected as a whole but some part of the
content of this option is protected (see Section 5.4). "OSCORE
within OSCORE" is not supported: If OSCORE processing detects an
Object-Security option in the original CoAP message, then processing
SHALL be stopped.
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4.2. CoAP Header Fields and Payload
A summary of how the CoAP header fields and payload are protected is
shown in Figure 6, including fields specific to CoAP over UDP and
CoAP over TCP (marked accordingly in the table).
+------------------+---+---+
| Field | E | U |
+------------------+---+---+
| Version (UDP) | | x |
| Type (UDP) | | x |
| Length (TCP) | | x |
| Token Length | | x |
| Code | x | |
| Message ID (UDP) | | x |
| Token | | x |
| Payload | x | |
+------------------+---+---+
E = Encrypt and Integrity Protect (Inner)
U = Unprotected (Outer)
Figure 6: Protection of CoAP Header Fields and Payload
Most CoAP Header fields (i.e. the message fields in the fixed 4-byte
header) are required to be read and/or changed by CoAP proxies and
thus cannot in general be protected end-to-end between the endpoints.
As mentioned in Section 1, OSCORE protects the CoAP Request/Response
layer only, and not the Messaging Layer (Section 2 of [RFC7252]), so
fields such as Type and Message ID are not protected with OSCORE.
The CoAP Header field Code is protected by OSCORE. Code SHALL be
encrypted and integrity protected (Class E) to prevent an
intermediary from eavesdropping or manipulating the Code (e.g.,
changing from GET to DELETE).
The sending endpoint SHALL write the Code of the original CoAP
message into the plaintext of the COSE object (see Section 5.3).
After that, the Outer Code of the OSCORE message SHALL be set to 0.02
(POST) for requests without Observe option, to 0.05 (FETCH) for
requests with Observe option, and to 2.04 (Changed) for responses.
Using FETCH with Observe allows OSCORE to be compliant with the
Observe processing in OSCORE-unaware proxies. The choice of POST and
FETCH ([RFC8132]) allows all OSCORE messages to have payload.
The receiving endpoint SHALL discard the Code in the OSCORE message
and write the Code of the plaintext in the COSE object (Section 5.3)
into the decrypted CoAP message.
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The other currently defined CoAP Header fields are Unprotected (Class
U). The sending endpoint SHALL write all other header fields of the
original message into the header of the OSCORE message. The
receiving endpoint SHALL write the header fields from the received
OSCORE message into the header of the decrypted CoAP message.
The CoAP Payload, if present in the original CoAP message, SHALL be
encrypted and integrity protected and is thus an Inner message field.
The sending endpoint writes the payload of the original CoAP message
into the plaintext (Section 5.3) input to the COSE object. The
receiving endpoint verifies and decrypts the COSE object, and
recreates the payload of the original CoAP message.
4.3. Signaling Messages
Signaling messages (CoAP Code 7.00-7.31) were introduced to exchange
information related to an underlying transport connection in the
specific case of CoAP over reliable transports ([RFC8323]). The use
of OSCORE for protecting Signaling is application dependent.
OSCORE MAY be used to protect Signaling if the endpoints for OSCORE
coincide with the endpoints for the connection. If OSCORE is used to
protect Signaling then:
o Signaling messages SHALL be protected as CoAP Request messages,
except in the case the Signaling message is a response to a
previous Signaling message, in which case it SHALL be protected as
a CoAP Response message. For example, 7.02 (Ping) is protected as
a CoAP Request and 7.03 (Pong) as a CoAP response.
o The Outer Code for Signaling messages SHALL be set to 0.02 (POST),
unless it is a response to a previous Signaling message, in which
case it SHALL be set to 2.04 (Changed).
o All Signaling options, except the Object-Security option, SHALL be
Inner (Class E).
NOTE: Option numbers for Signaling messages are specific to the CoAP
Code (see Section 5.2 of [RFC8323]).
If OSCORE is not used to protect Signaling, Signaling messages SHALL
be unaltered by OSCORE.
5. The COSE Object
This section defines how to use COSE [RFC8152] to wrap and protect
data in the original message. OSCORE uses the untagged COSE_Encrypt0
structure with an Authenticated Encryption with Additional Data
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(AEAD) algorithm. The key lengths, IV length, nonce length, and
maximum Sender Sequence Number are algorithm dependent.
The AEAD algorithm AES-CCM-16-64-128 defined in Section 10.2 of
[RFC8152] is mandatory to implement. For AES-CCM-16-64-128 the
length of Sender Key and Recipient Key is 128 bits, the length of
nonce and Common IV is 13 bytes. The maximum Sender Sequence Number
is specified in Section 11.
As specified in [RFC5116], plaintext denotes the data that is to be
encrypted and integrity protected, and Additional Authenticated Data
(AAD) denotes the data that is to be 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 Sender
Sequence Number. All leading zeroes SHALL be removed when
encoding the Partial IV. The value 0 encodes to the byte
string 0x00. This parameter SHALL be present in requests. In
case of Observe (Section 4.1.3.4) the Partial IV SHALL be
present in responses, and otherwise the Partial IV will not
typically be present in responses. (A non-Observe example
where the Partial IV is included in a response is provided in
Section 7.5.2.)
* The 'kid' parameter. The value is set to the Sender ID. This
parameter SHALL be present in requests and will not typically
be present in responses. An example where the Sender ID is
included in a response is the extension of OSCORE to group
communication [I-D.ietf-core-oscore-groupcomm].
* Optionally, a 'kid context' parameter as defined in
Section 5.1. This parameter MAY be present in requests and
SHALL NOT be present in responses.
o The 'ciphertext' field is computed from the secret key (Sender Key
or Recipient Key), AEAD nonce (see Section 5.2), plaintext (see
Section 5.3), and the Additional Authenticated Data (AAD) (see
Section 5.4) following Section 5.2 of [RFC8152].
The encryption process is described in Section 5.3 of [RFC8152].
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5.1. Kid Context
For certain use cases, e.g. deployments where the same kid is used
with multiple contexts, it is necessary or favorable for the sender
to provide an additional identifier of the security material to use,
in order for the receiver to retrieve or establish the correct key.
The kid context parameter is used to provide such additional input.
The kid context and kid are used to determine the security context,
or to establish the necessary input parameters to derive the security
context (see Section 3.2). The application defines how this is done.
The kid context is implicitly integrity protected, as manipulation
that leads to the wrong key (or no key) being retrieved which results
in an error, as described in Section 8.2.
A summary of the COSE header parameter kid context defined above can
be found in Figure 7.
Some examples of relevant uses of kid context are the following:
o If the client has an identifier in some other namespace which can
be used by the server to retrieve or establish the security
context, then that identifier can be used as kid context. The kid
context may be used as Master Salt (Section 3.1) for additional
entropy of the security contexts (see for example Appendix B.2 or
[I-D.ietf-6tisch-minimal-security]).
o In case of a group communication scenario
[I-D.ietf-core-oscore-groupcomm], if the server belongs to
multiple groups, then a group identifier can be used as kid
context to enable the server to find the right security context.
+----------+--------+------------+----------------+-----------------+
| name | label | value type | value registry | description |
+----------+--------+------------+----------------+-----------------+
| kid | kidctx | bstr | | Identifies the |
| context | | | | kid context |
+----------+--------+------------+----------------+-----------------+
Figure 7: Additional common header parameter for the COSE object
5.2. Nonce
The AEAD nonce is constructed in the following way (see Figure 8):
1. left-padding the Partial IV (in network byte order) with zeroes
to exactly 5 bytes,
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2. left-padding the (Sender) ID of the endpoint that generated the
Partial IV (in network byte order) with zeroes to exactly nonce
length - 6 bytes,
3. concatenating the size of the ID (S) with the padded ID and the
padded Partial IV,
4. and then XORing with the Common IV.
Note that in this specification only algorithms that use nonces equal
or greater than 7 bytes are supported. The nonce construction with
S, ID of PIV generator, and Partial IV together with endpoint unique
IDs and encryption keys make it easy to verify that the nonces used
with a specific key will be unique.
When Observe is not used, the request and the response may use the
same nonce. In this way, the Partial IV does not have to be sent in
responses, which reduces the size. For processing instructions see
Section 8.
+---+-----------------------+--+--+--+--+--+
| S | ID of PIV generator | Partial IV |----+
+---+-----------------------+--+--+--+--+--+ |
|
+------------------------------------------+ |
| Common IV |->(XOR)
+------------------------------------------+ |
|
+------------------------------------------+ |
| Nonce |<---+
+------------------------------------------+
Figure 8: AEAD Nonce Formation
5.3. Plaintext
The plaintext is formatted as a CoAP message without Header (see
Figure 9) consisting of:
o the Code of the original CoAP message as defined in Section 3 of
[RFC7252]; and
o all Inner option message fields (see Section 4.1.1) present in the
original CoAP message (see Section 4.1). The options are encoded
as described in Section 3.1 of [RFC7252], where the delta is the
difference to the previously included instance of Class E option;
and
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o the Payload of original 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Class E options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(only if there
is payload)
Figure 9: Plaintext
NOTE: The plaintext contains all CoAP data that needs to be encrypted
end-to-end between the endpoints.
5.4. Additional Authenticated Data
The external_aad SHALL be a CBOR array as defined below:
external_aad = [
oscore_version : uint,
algorithms : [ alg_aead : int / tstr ],
request_kid : bstr,
request_piv : bstr,
options : bstr
]
where:
o oscore_version: contains the OSCORE version number.
Implementations of this specification MUST set this field to 1.
Other values are reserved for future versions.
o alg_aead: contains the AEAD 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_piv: contains the value of the 'Partial IV' in the COSE
object of the request (see Section 5).
o options: contains the Class I options (see Section 4.1.2) present
in the original CoAP message encoded as described in Section 3.1
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of [RFC7252], where the delta is the difference to the previously
included instance of class I option.
NOTE: The format of the external_aad is for simplicity the same for
requests and responses, although some parameters, e.g. request_kid
need not be integrity protected in the requests.
6. OSCORE Header Compression
The Concise Binary Object Representation (CBOR) [RFC7049] combines
very small message sizes with extensibility. The CBOR Object Signing
and Encryption (COSE) [RFC8152] 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 stateless header compression mechanism, simply removing redundant
information from the COSE objects, which significantly reduces the
per-packet overhead. The result of applying this mechanism to a COSE
object is called the "compressed COSE object".
The COSE_Encrypt0 object used in OSCORE is transported in the Object-
Security option and in the Payload. The Payload contains the
Ciphertext and the headers of the COSE object are compactly encoded
as described in the next section.
6.1. Encoding of the Object-Security Value
The value of the Object-Security option SHALL contain the OSCORE flag
bits, the Partial IV parameter, the kid context parameter (length and
value), and the kid parameter as follows:
0 1 2 3 4 5 6 7 <--------- n bytes ------------->
+-+-+-+-+-+-+-+-+---------------------------------
|0 0 0|h|k| n | Partial IV (if any) ...
+-+-+-+-+-+-+-+-+---------------------------------
<- 1 byte -> <------ s bytes ----->
+------------+----------------------+------------------+
| s (if any) | kid context (if any) | kid (if any) ... |
+------------+----------------------+------------------+
Figure 10: Object-Security Value
o The first byte of flag bits encodes the following set of flags and
the length of the Partial IV parameter:
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* The three least significant bits encode the Partial IV length
n. If n = 0 then the Partial IV is not present in the
compressed COSE object. The values n = 6 and n = 7 are
reserved.
* The fourth least significant bit is the kid flag, k: it is set
to 1 if the kid is present in the compressed COSE object.
* The fifth least significant bit is the kid context flag, h: it
is set to 1 if the compressed COSE object contains a kid
context (see Section 5.1).
* The sixth to eighth least significant bits are reserved for
future use. These bits SHALL be set to zero when not in use.
According to this specification, if any of these bits are set
to 1 the message is considered to be malformed and
decompression fails as specified in item 3 of Section 8.2.
o The following n bytes encode the value of the Partial IV, if the
Partial IV is present (n > 0).
o The following 1 byte encode the length of the kid context
(Section 5.1) s, if the kid context flag is set (h = 1).
o The following s bytes encode the kid context, if the kid context
flag is set (h = 1).
o The remaining bytes encode the value of the kid, if the kid is
present (k = 1).
Note that the kid MUST be the last field of the object-security
value, even in case reserved bits are used and additional fields are
added to it.
The length of the Object-Security option thus depends on the presence
and length of Partial IV, kid context, kid, as specified in this
section, and on the presence and length of the other parameters, as
defined in the separate documents.
6.2. Encoding of the OSCORE Payload
The payload of the OSCORE message SHALL encode the ciphertext of the
COSE object.
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6.3. Examples of Compressed COSE Objects
6.3.1. Examples: Requests
1. Request with kid = 0x25 and Partial IV = 0x05
Before compression (24 bytes):
[
h'',
{ 4:h'25', 6:h'05' },
h'aea0155667924dff8a24e4cb35b9'
]
After compression (17 bytes):
Flag byte: 0b00001001 = 0x09
Option Value: 09 05 25 (3 bytes)
Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 (14 bytes)
2. Request with kid = empty string and Partial IV = 0x00
After compression (16 bytes):
Flag byte: 0b00001001 = 0x09
Option Value: 09 00 (2 bytes)
Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 (14 bytes)
3. Request with kid = empty string, Partial IV = 0x05, and kid
context = 0x44616c656b
After compression (22 bytes):
Flag byte: 0b00011001 = 0x19
Option Value: 19 05 05 44 61 6c 65 6b (8 bytes)
Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 (14 bytes)
6.3.2. Example: Response (without Observe)
Before compression (18 bytes):
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[
h'',
{},
h'aea0155667924dff8a24e4cb35b9'
]
After compression (14 bytes):
Flag byte: 0b00000000 = 0x00
Option Value: (0 bytes)
Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 (14 bytes)
6.3.3. Example: Response (with Observe)
Before compression (21 bytes):
[
h'',
{ 6:h'07' },
h'aea0155667924dff8a24e4cb35b9'
]
After compression (16 bytes):
Flag byte: 0b00000001 = 0x01
Option Value: 01 07 (2 bytes)
Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 (14 bytes)
7. Sequence Numbers, Replay, Message Binding, and Freshness
7.1. Message Binding
In order to prevent response delay and mismatch attacks
[I-D.mattsson-core-coap-actuators] from on-path attackers and
compromised proxies, OSCORE binds responses to the requests by
including the kid and Partial IV of the request in the AAD of the
response. The server therefore needs to store the kid and Partial IV
of the request until all responses have been sent.
7.2. 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 Sender
Sequence Number used to protect requests, and - in case of Observe -
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responses. If messages are processed concurrently, the operation of
reading and increasing the Sender Sequence Number MUST be atomic.
The maximum Sender Sequence Number is algorithm dependent (see
Section 11), and no greater than 2^40 - 1. If the Sender Sequence
Number exceeds the maximum, 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.
7.3. Freshness
For requests, OSCORE provides only the guarantee that the request is
not older than the security context. For applications having
stronger demands on request freshness (e.g., control of actuators),
OSCORE needs to be augmented with mechanisms providing freshness, for
example as specified in [I-D.ietf-core-echo-request-tag].
For responses, the message binding guarantees that a response is not
older than its request. For responses without Observe, this gives
strong absolute freshness. For responses with Observe, the absolute
freshness gets weaker with time, and it is RECOMMENDED that the
client regularly re-register the observation.
For requests, and responses with Observe, OSCORE also provides
relative freshness in the sense that the received Partial IV allows a
recipient to determine the relative order of responses.
7.4. Replay Protection
In order to protect from replay of requests, the server's Recipient
Context includes a Replay Window. A server SHALL verify that a
Partial IV received in the COSE object has not been received before.
If this verification fails the server SHALL stop processing the
message, and MAY optionally respond with a 4.01 Unauthorized error
message. Also, the server MAY set an Outer Max-Age option with value
zero. The diagnostic payload MAY contain the "Replay protection
failed" string. The size and type of the Replay Window depends on
the use case and the protocol with which the OSCORE message is
transported. In case of reliable and ordered transport from endpoint
to endpoint, e.g. TCP, the server MAY just store the last received
Partial IV and require that newly received Partial IVs equals the
last received Partial IV + 1. However, in case of mixed reliable and
unreliable transports and where messages may be lost, such a replay
mechanism may be too restrictive and the default replay window be
more suitable (see Section 3.2.2).
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Responses to non-Observe requests are protected against replay as
they are cryptographically bound to the request.
In the case of Observe, a client receiving a notification SHALL
verify that the Partial IV of a received notification is greater than
the Notification Number bound to that Observe registration. If the
verification fails, the client SHALL stop processing the response.
If the verification succeeds, the client SHALL overwrite the
corresponding Notification Number with the received Partial IV.
If messages are processed concurrently, the Partial IV needs to be
validated a second time after decryption and before updating the
replay protection data. The operation of validating the Partial IV
and updating the replay protection data MUST be atomic.
7.5. Losing Part of the Context State
To prevent reuse of the AEAD nonce with the same key, or from
accepting replayed messages, an endpoint needs to handle the
situation of losing rapidly changing parts of the context, such as
the request Token, Sender Sequence Number, Replay Window, and
Notification Numbers. These are typically stored in RAM and
therefore lost in the case of an unplanned reboot.
After boot, an endpoint MAY reject to use pre-existing security
contexts, and MAY establish a new security context with each endpoint
it communicates with. However, establishing a fresh security context
may have a non-negligible cost in terms of, e.g., power consumption.
After boot, an endpoint MAY use a partly persistently stored security
context, but then the endpoint MUST NOT reuse a previous Sender
Sequence Number and MUST NOT accept previously accepted messages.
Some ways to achieve this are described in the following sections.
7.5.1. Sequence Number
To prevent reuse of Sender Sequence Numbers, an endpoint MAY perform
the following procedure during normal operations:
o Each time the Sender Sequence Number is evenly divisible by K,
where K is a positive integer, store the Sender Sequence Number in
persistent memory. After boot, the endpoint initiates the Sender
Sequence Number to the value stored in persistent memory + K - 1.
Storing to persistent memory can be costly. The value K gives a
trade-off between the number of storage operations and efficient
use of Sender Sequence Numbers.
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7.5.2. Replay Window
To prevent accepting replay of previously received requests, the
server MAY perform the following procedure after boot:
o For each stored security context, the first time after boot the
server receives an OSCORE request, the server responds with the
Echo option [I-D.ietf-core-echo-request-tag] to get a request with
verifiable freshness. The server MUST use its Partial IV when
generating the AEAD nonce and MUST include the Partial IV in the
response.
If the server using the Echo option can verify a second request as
fresh, then the Partial IV of the second request is set as the lower
limit of the replay window.
7.5.3. Replay Protection of Observe Notifications
To prevent accepting replay of previously received notification
responses, the client MAY perform the following procedure after boot:
o The client rejects notifications bound to the earlier
registration, removes all Notification Numbers and re-registers
using Observe.
8. Processing
This section describes the OSCORE message processing.
8.1. Protecting the Request
Given a CoAP request, the client SHALL perform the following steps to
create an OSCORE request:
1. Retrieve the Sender Context associated with the target resource.
2. Compose the Additional Authenticated Data and the plaintext, as
described in Section 5.4 and Section 5.3.
3. Compute the AEAD nonce from the Sender ID, Common IV, and Partial
IV (Sender Sequence Number in network byte order) as described in
Section 5.2 and (in one atomic operation, see Section 7.2)
increment the Sender Sequence Number by one.
4. Encrypt the COSE object using the Sender Key. Compress the COSE
Object as specified in Section 6.
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5. Format the OSCORE message according to Section 4. The Object-
Security option is added (see Section 4.1.2).
6. Store the association Token - Security Context, in order to be
able to find the Recipient Context from the Token in the
response.
8.2. 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 4.1.3.2).
2. Discard the message Code and all non-special Inner option
message fields (marked with 'x' in column E of Figure 5) present
in the received message. For example, an If-Match Outer option
is discarded, but an Uri-Host Outer option is not discarded.
3. Decompress the COSE Object (Section 6) and retrieve the
Recipient Context associated with the Recipient ID in the 'kid'
parameter. If 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.
If:
* either the decompression or the COSE message fails to decode,
the server MAY respond with a 4.02 Bad Option error message.
The server MAY set an Outer Max-Age option with value zero.
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 MAY respond with a 4.01 Unauthorized error
message. The server MAY set an Outer Max-Age option with
value zero. The diagnostic payload SHOULD contain the string
"Security context not found".
4. Verify the 'Partial IV' parameter using the Replay Window, as
described in Section 7.4.
5. Compose the Additional Authenticated Data, as described in
Section 5.4.
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6. Compute the AEAD nonce from the Recipient ID, Common IV, and the
'Partial IV' parameter, received in the COSE Object.
7. Decrypt the COSE object using the Recipient Key, as per
[RFC8152] Section 5.3. (The decrypt operation includes the
verification of the integrity.)
* If decryption fails, the server MUST stop processing the
request and MAY respond with a 4.00 Bad Request error
message. The server MAY set an Outer Max-Age option with
value zero. The diagnostic payload SHOULD contain the
"Decryption failed" string.
* If decryption succeeds, update the Replay Window, as
described in Section 7.
8. For each decrypted option, check if the option is also present
as an Outer option: if it is, discard the Outer. For example:
the message contains a Max-Age Inner and a Max-Age Outer option.
The Outer Max-Age is discarded.
9. Add decrypted code, options and payload to the decrypted
request. The Object-Security option is removed.
10. The decrypted CoAP request is processed according to [RFC7252]
8.3. Protecting the Response
If a CoAP response is generated in response to an OSCORE request, the
server SHALL perform the following steps to create an OSCORE
response. Note that CoAP error responses derived from CoAP
processing (point 10. in Section 8.2) are protected, as well as
successful CoAP responses, while the OSCORE errors (point 3, 4, and 7
in Section 8.2) do not follow the processing below, but are sent as
simple CoAP responses, without OSCORE processing.
1. Retrieve the Sender Context in the Security Context used to
verify the request.
2. Compose the Additional Authenticated Data and the plaintext, as
described in Section 5.4 and Section 5.3.
3. Compute the AEAD nonce
* If Observe is used, compute the nonce from the Sender ID,
Common IV, and Partial IV (Sender Sequence Number in network
byte order). Then (in one atomic operation, see Section 7.2)
increment the Sender Sequence Number by one.
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* If Observe is not used, either the nonce from the request is
used or a new Partial IV is used (see bullet on 'Partial IV'
in Section 5).
4. Encrypt the COSE object using the Sender Key. Compress the COSE
Object as specified in Section 6. If the AEAD nonce was
constructed from a new Partial IV, this Partial IV MUST be
included in the message. If the AEAD nonce from the request was
used, the Partial IV MUST NOT be included in the message.
5. Format the OSCORE message according to Section 4. The Object-
Security option is added (see Section 4.1.2).
8.4. 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 OSCORE message have been received (see
Section 4.1.3.2).
2. Discard the message Code and all non-special Class E options
from the message. For example, ETag Outer option is discarded,
Max-Age Outer option is not discarded.
3. Retrieve the Recipient Context associated with the Token.
Decompress the COSE Object (Section 6). If either the
decompression or the COSE message fails to decode, then go to
11.
4. For Observe notifications, verify the received 'Partial IV'
parameter against the corresponding Notification Number as
described in Section 7.4. If the client receives a notification
for which no Observe request was sent, then go to 11.
5. Compose the Additional Authenticated Data, as described in
Section 5.4.
6. Compute the AEAD nonce
1. If the Observe option and the Partial IV are not present in
the response, the nonce from the request is used.
2. If the Observe option is present in the response, and the
Partial IV is not present in the response, then go to 11.
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3. If the Partial IV is present in the response, compute the
nonce from the Recipient ID, Common IV, and the 'Partial IV'
parameter, received in the COSE Object.
7. Decrypt the COSE object using the Recipient Key, as per
[RFC8152] Section 5.3. (The decrypt operation includes the
verification of the integrity.)
* If decryption fails, then go to 11.
* If decryption succeeds and Observe is used, update the
corresponding Notification Number, as described in Section 7.
8. For each decrypted option, check if the option is also present
as an Outer option: if it is, discard the Outer. For example:
the message contains a Max-Age Inner and a Max-Age Outer option.
The Outer Max-Age is discarded.
9. Add decrypted code, options and payload to the decrypted
request. The Object-Security option is removed.
10. The decrypted CoAP response is processed according to [RFC7252]
11. (Optional) In case any of the previous erroneous conditions
apply: the client SHALL stop processing the response.
An error condition occurring while processing a response in an
observation does not cancel the observation. A client MUST NOT react
to failure in step 7 by re-registering the observation immediately.
9. Web Linking
The use of OSCORE MAY be indicated by a target attribute "osc" in a
web link [RFC8288] to a resource. This attribute is a hint
indicating that the destination of that link is to be accessed using
OSCORE. Note that this is simply a hint, it does not include any
security context material or any other information required to run
OSCORE.
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.
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10. Proxy and HTTP Operations
RFC 7252 defines operations for a CoAP-to-CoAP proxy (see Section 5.7
of [RFC7252]) and for proxying between CoAP and HTTP (Section 10 of
[RFC7252]). A more detailed description of the HTTP-to-CoAP mapping
is provided by [RFC8075]. This section describes the operations of
OSCORE-aware proxies.
10.1. CoAP-to-CoAP Forwarding Proxy
OSCORE is designed to work with legacy CoAP-to-CoAP forward proxies
[RFC7252], but OSCORE-aware proxies MAY provide certain
simplifications as specified in this section.
Security requirements for forwarding are presented in Section 2.2.1
of [I-D.hartke-core-e2e-security-reqs]. OSCORE complies with the
extended security requirements also addressing Blockwise ([RFC7959])
and CoAP-mappable HTTP. In particular caching is disabled since the
CoAP response is only applicable to the original CoAP request. An
OSCORE-aware proxy SHALL NOT cache a response to a request with an
Object-Security option. As a consequence, the search for cache hits
and CoAP freshness/Max-Age processing can be omitted.
Proxy processing of the (Outer) Proxy-Uri option is as defined in
[RFC7252].
Proxy processing of the (Outer) Block options is as defined in
[RFC7959].
Proxy processing of the (Outer) Observe option is as defined in
[RFC7641]. OSCORE-aware proxies MAY look at the Partial IV value
instead of the Outer Observe option.
10.2. HTTP Processing
OSCORE was initially designed to work between CoAP endpoints only,
but the interest in use cases with one endpoint being an HTTP
endpoint has driven the extension specified here. OSCORE is intended
to be used with at least one endpoint being a CoAP endpoint.
In order to use OSCORE with HTTP, an endpoint needs to be able to map
HTTP messages to CoAP messages (see [RFC8075]), and to apply OSCORE
to CoAP messages (as defined in this document).
For this purpose, this specification defines a new HTTP header field
named CoAP-Object-Security, see Section 12.4. The CoAP-Object-
Security header field is only used in POST requests and 200 (OK)
responses. All field semantics is given within the CoAP-Object-
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Security header field. The header field is neither appropriate to
list in the Connection header field (see Section 6.1 of [RFC7230]),
nor in a Vary response header field (see Section 7.1.4 of [RFC7231]),
nor allowed in trailers (see Section 4.1 of [RFC7230]).
Intermediaries are not allowed to insert, delete, or modify the
field's value. The header field is not preserved across redirects.
A sending endpoint uses [RFC8075] to translate an HTTP message into a
CoAP message. It then protects the message with OSCORE processing,
and add the Object-Security option (as defined in this document).
Then, the endpoint maps the resulting CoAP message to an HTTP message
that includes the HTTP header field CoAP-Object-Security, whose value
is:
o "" if the CoAP Object-Security option is empty, or
o the value of the CoAP Object-Security option (Section 6.1) in
base64url encoding (Section 5 of [RFC4648]) without padding (see
[RFC7515] Appendix C for implementation notes for this encoding).
Note that the value of the HTTP body is the CoAP payload, i.e. the
OSCORE payload (Section 6.2).
The HTTP header field Content-Type is set to 'application/oscore'
(see Section 12.5).
The resulting message is an OSCORE message that uses HTTP.
A receiving endpoint uses [RFC8075] to translate an HTTP message into
a CoAP message, with the following addition. The HTTP message
includes the CoAP-Object-Security header field, which is mapped to
the CoAP Object-Security option in the following way. The CoAP
Object-Security option value is:
o empty if the value of the HTTP CoAP-Object-Security header field
is ""
o the value of the HTTP CoAP-Object-Security header field decoded
from base64url (Section 5 of [RFC4648]) without padding (see
[RFC7515] Appendix C for implementation notes for this decoding).
Note that the value of the CoAP payload is the HTTP body, i.e. the
OSCORE payload (Section 6.2).
The resulting message is an OSCORE message that uses CoAP.
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The endpoint can then verify the message according to the OSCORE
processing and get a verified CoAP message. It can then translate
the verified CoAP message into a verified HTTP message.
10.3. HTTP-to-CoAP Translation Proxy
Section 10.2 of [RFC7252] and [RFC8075] specify the behavior of an
HTTP-to-CoAP proxy. As requested in Section 1 of [RFC8075], this
section describes the HTTP mapping for the OSCORE protocol extension
of CoAP.
The presence of the Object-Security option, both in requests and
responses, is expressed in an HTTP header field named CoAP-Object-
Security in the mapped request or response. The value of the field
is:
o "" if the CoAP Object-Security option is empty, or
o the value of the CoAP Object-Security option (Section 6.1) in
base64url encoding (Section 5 of [RFC4648]) without padding (see
[RFC7515] Appendix C for implementation notes for this encoding).
The header field Content-Type 'application/oscore' (see Section 12.5)
is used for OSCORE messages transported in HTTP. The CoAP Content-
Format option is omitted for OSCORE messages transported in CoAP.
The value of the body is the OSCORE payload (Section 6.2).
Example:
Mapping and notation here is based on "Simple Form" (Section 5.4.1.1
of [RFC8075]).
[HTTP request -- Before client object security processing]
GET http://proxy.url/hc/?target_uri=coap://server.url/orders
HTTP/1.1
[HTTP request -- HTTP Client to Proxy]
POST http://proxy.url/hc/?target_uri=coap://server.url/ HTTP/1.1
Content-Type: application/oscore
CoAP-Object-Security: CSU
Body: 09 07 01 13 61 f7 0f d2 97 b1 [binary]
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[CoAP request -- Proxy to CoAP Server]
POST coap://server.url/
Object-Security: 09 25
Payload: 09 07 01 13 61 f7 0f d2 97 b1 [binary]
[CoAP request -- After server object security processing]
GET coap://server.url/orders
[CoAP response -- Before server object security processing]
2.05 Content
Content-Format: 0
Payload: Exterminate! Exterminate!
[CoAP response -- CoAP Server to Proxy]
2.04 Changed
Object-Security: [empty]
Payload: 00 31 d1 fc f6 70 fb 0c 1d d5 ... [binary]
[HTTP response -- Proxy to HTTP Client]
HTTP/1.1 200 OK
Content-Type: application/oscore
CoAP-Object-Security: ""
Body: 00 31 d1 fc f6 70 fb 0c 1d d5 ... [binary]
[HTTP response -- After client object security processing]
HTTP/1.1 200 OK
Content-Type: text/plain
Body: Exterminate! Exterminate!
Note that the HTTP Status Code 200 in the next-to-last message is the
mapping of CoAP Code 2.04 (Changed), whereas the HTTP Status Code 200
in the last message is the mapping of the CoAP Code 2.05 (Content),
which was encrypted within the compressed COSE object carried in the
Body of the HTTP response.
10.4. CoAP-to-HTTP Translation Proxy
Section 10.1 of [RFC7252] describes the behavior of a CoAP-to-HTTP
proxy. RFC 8075 [RFC8075] does not cover this direction in any more
detail and so an example instantiation of Section 10.1 of [RFC7252]
is used below.
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Example:
[CoAP request -- Before client object security processing]
GET coap://proxy.url/
Proxy-Uri=http://server.url/orders
[CoAP request -- CoAP Client to Proxy]
POST coap://proxy.url/
Proxy-Uri=http://server.url/
Object-Security: 09 25
Payload: 09 07 01 13 61 f7 0f d2 97 b1 [binary]
[HTTP request -- Proxy to HTTP Server]
POST http://server.url/ HTTP/1.1
Content-Type: application/oscore
CoAP-Object-Security: CSU
Body: 09 07 01 13 61 f7 0f d2 97 b1 [binary]
[HTTP request -- After server object security processing]
GET http://server.url/orders HTTP/1.1
[HTTP response -- Before server object security processing]
HTTP/1.1 200 OK
Content-Type: text/plain
Body: Exterminate! Exterminate!
[HTTP response -- HTTP Server to Proxy]
HTTP/1.1 200 OK
Content-Type: application/oscore
CoAP-Object-Security: ""
Body: 00 31 d1 fc f6 70 fb 0c 1d d5 ... [binary]
[CoAP response - Proxy to CoAP Client]
2.04 Changed
Object-Security: [empty]
Payload: 00 31 d1 fc f6 70 fb 0c 1d d5 ... [binary]
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[CoAP response -- After client object security processing]
2.05 Content
Content-Format: 0
Payload: Exterminate! Exterminate!
Note that the HTTP Code 2.04 (Changed) in the next-to-last message is
the mapping of HTTP Status Code 200, whereas the CoAP Code 2.05
(Content) in the last message is the value that was encrypted within
the compressed COSE object carried in the Body of the HTTP response.
11. Security Considerations
11.1. End-to-end protection
In scenarios with intermediary nodes such as proxies or gateways,
transport layer security such as (D)TLS only protects data hop-by-
hop. As a consequence, the intermediary nodes can read and modify
information. The trust model where all intermediary 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.
(D)TLS protects hop-by-hop the entire message. OSCORE protects end-
to-end all information that is not required for proxy operations (see
Section 4). (D)TLS and OSCORE can be combined, thereby enabling end-
to-end security of the message payload, in combination with hop-by-
hop protection of the entire message, during transport between end-
point and intermediary node. The CoAP messaging layer, including
header fields such as Type and Message ID, as well as CoAP message
fields Token and Token Length may be changed by a proxy and thus
cannot be protected end-to-end. Error messages occurring during CoAP
processing are protected end-to-end. Error messages occurring during
OSCORE processing are not always possible to protect, e.g. if the
receiving endpoint cannot locate the right security context. It may
still be favorable to send an unprotected error message, e.g. to
prevent extensive retransmissions, so unprotected error messages are
allowed as specified. Similar to error messages, signaling messages
are not always possible to protect as they may be intended for an
intermediary. Hop-by-hop protection of signaling messages can be
achieved with (D)TLS. Applications using unprotected error and
signaling messages need to consider the threat that these messages
may be spoofed.
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11.2. Security Context Establishment
The use of COSE to protect 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 the ACE framework [I-D.ietf-ace-oauth-authz]. An
OSCORE profile of ACE is described in [I-D.ietf-ace-oscore-profile].
11.3. Replay Protection
Most AEAD algorithms require a unique nonce for each message, for
which the sender 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.
11.4. Cryptographic Considerations
The maximum sender sequence number is dependent on the AEAD
algorithm. The maximum sender sequence number SHALL be 2^40 - 1, or
any algorithm specific lower limit, after which a new security
context must be generated. The mechanism to build the nonce
(Section 5.2) assumes that the nonce is at least 56 bit-long, and the
Partial IV is at most 40 bit-long. The mandatory-to-implement AEAD
algorithm AES-CCM-16-64-128 is selected for compatibility with CCM*.
The security level of a system with m Masters Keys of length k used
together with Master Salts with entropy n is k + n - log2(m).
Similarly, the security level of a system with m AEAD keys of length
k used together with AEAD nonces of length n is k + n - log2(m).
Security level here means that an attacker can recover one of the m
keys with complexity 2^(k + n) / m. Protection against such attacks
can be provided by increasing the size of the keys or the entropy of
the Master Salt. The complexity of recovering a specific key is
still 2^k (assuming the Master Salt/AEAD nonce is public). The
Master Secret, Sender Key, and Recipient Key MUST be secret, the rest
of the parameters MAY be public. The Master Secret MUST be uniformly
random.
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11.5. Message Fragmentation
The Inner Block options enable the sender to split large messages
into OSCORE-protected blocks such that the receiving endpoint 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 Inner Block 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.
11.6. Privacy Considerations
Privacy threats executed through intermediary nodes are considerably
reduced by means of OSCORE. End-to-end integrity protection and
encryption of the message payload and all options that are not used
for proxy operations, provide mitigation against attacks on sensor
and actuator communication, which may have a direct impact on the
personal sphere.
The unprotected options (Figure 5) 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. OSCORE does
not provide protection for HTTP header fields which are not CoAP-
mappable.
Unprotected error messages reveal information about the security
state in the communication between the endpoints. Unprotected
signalling messages reveal information about the reliable transport
used on a leg of the path. Using the mechanisms described in
Section 7.5 may reveal when a device goes through a reboot. This can
be mitigated by the device storing the precise state of sender
sequence number and replay window on a clean shutdown.
The length of message fields can reveal information about the
message. Applications may use a padding scheme to protect against
traffic analysis. 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
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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. Similar
arguments apply to other message fields such as resource names.
12. IANA Considerations
Note to RFC Editor: Please replace all occurrences of "[[this
document]]" with the RFC number of this specification.
Note to IANA: Please note all occurrences of "TBD" in this
specification should be assigned the same number.
12.1. COSE Header Parameters Registry
The 'kid context' parameter is added to the "COSE Header Parameters
Registry":
o Name: kid context
o Label: TBD1 (Integer value between 1 and 255)
o Value Type: bstr
o Value Registry:
o Description: kid context
o Reference: Section 5.1 of this document
12.2. CoAP Option Numbers Registry
The Object-Security option is added to the CoAP Option Numbers
registry:
+--------+-----------------+-------------------+
| Number | Name | Reference |
+--------+-----------------+-------------------+
| TBD | Object-Security | [[this document]] |
+--------+-----------------+-------------------+
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12.3. CoAP Signaling Option Numbers Registry
The Object-Security option is added to the CoAP Signaling Option
Numbers registry:
+------------+--------+---------------------+-------------------+
| Applies to | Number | Name | Reference |
+------------+--------+---------------------+-------------------+
| 7.xx | TBD | Object-Security | [[this document]] |
+------------+--------+---------------------+-------------------+
12.4. Header Field Registrations
The HTTP header field CoAP-Object-Security is added to the Message
Headers registry:
+----------------------+----------+----------+-------------------+
| Header Field Name | Protocol | Status | Reference |
+----------------------+----------+----------+-------------------+
| CoAP-Object-Security | http | standard | [[this document]] |
+----------------------+----------+----------+-------------------+
12.5. Media Type Registrations
This section registers the 'application/oscore' media type in the
"Media Types" registry.
These media types are used to indicate that the content is an OSCORE
message.
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Type name: application
Subtype name: oscore
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See the Security Considerations section
of [[This document]].
Interoperability considerations: N/A
Published specification: [[This document]]
Applications that use this media type: IoT applications sending
security content over HTTP(S) transports.
Fragment identifier considerations: N/A
Additional information:
* Deprecated alias names for this type: N/A
* 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
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13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://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, <https://www.rfc-editor.org/info/rfc7049>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014, <https://www.rfc-
editor.org/info/rfc7231>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014, <https://www.rfc-
editor.org/info/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015, <https://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, <https://www.rfc-
editor.org/info/rfc7959>.
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[RFC8075] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
E. Dijk, "Guidelines for Mapping Implementations: HTTP to
the Constrained Application Protocol (CoAP)", RFC 8075,
DOI 10.17487/RFC8075, February 2017, <https://www.rfc-
editor.org/info/rfc8075>.
[RFC8132] van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
FETCH Methods for the Constrained Application Protocol
(CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
<https://www.rfc-editor.org/info/rfc8132>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017, <https://www.rfc-
editor.org/info/rfc8288>.
[RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
RFC 8323, DOI 10.17487/RFC8323, February 2018,
<https://www.rfc-editor.org/info/rfc8323>.
13.2. 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.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-03 (work in progress), July 2017.
[I-D.ietf-6tisch-minimal-security]
Vucinic, M., Simon, J., Pister, K., and M. Richardson,
"Minimal Security Framework for 6TiSCH", draft-ietf-
6tisch-minimal-security-05 (work in progress), March 2018.
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[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-10 (work in progress), February 2018.
[I-D.ietf-ace-oscore-profile]
Seitz, L., Palombini, F., and M. Gunnarsson, "OSCORE
profile of the Authentication and Authorization for
Constrained Environments Framework", draft-ietf-ace-
oscore-profile-00 (work in progress), December 2017.
[I-D.ietf-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR data structures", draft-ietf-cbor-cddl-02
(work in progress), February 2018.
[I-D.ietf-core-echo-request-tag]
Amsuess, C., Mattsson, J., and G. Selander, "Echo and
Request-Tag", draft-ietf-core-echo-request-tag-00 (work in
progress), October 2017.
[I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., and J. Park,
"Secure group communication for CoAP", draft-ietf-core-
oscore-groupcomm-01 (work in progress), March 2018.
[I-D.mattsson-ace-tls-oscore]
Mattsson, J., "Using Transport Layer Security (TLS) to
Secure OSCORE", draft-mattsson-ace-tls-oscore-00 (work in
progress), October 2017.
[I-D.mattsson-core-coap-actuators]
Mattsson, J., Fornehed, J., Selander, G., Palombini, F.,
and C. Amsuess, "Controlling Actuators with CoAP", draft-
mattsson-core-coap-actuators-04 (work in progress), March
2018.
[I-D.selander-ace-cose-ecdhe]
Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace-
cose-ecdhe-07 (work in progress), July 2017.
[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,
<https://www.rfc-editor.org/info/rfc3986>.
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[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010, <https://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, <https://www.rfc-
editor.org/info/rfc7228>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/info/rfc7515>.
[RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for
No Server Response", RFC 7967, DOI 10.17487/RFC7967,
August 2016, <https://www.rfc-editor.org/info/rfc7967>.
Appendix A. Scenario Examples
This section gives examples of OSCORE, targeting scenarios in
Section 2.2.1.1 of [I-D.hartke-core-e2e-security-reqs]. 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 (compressed) COSE message format.
A.1. Secure Access to Sensor
This example illustrates a client requesting the alarm status from a
server.
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Client Proxy Server
| | |
+------>| | Code: 0.02 (POST)
| POST | | Token: 0x8c
| | | Object-Security: [kid:5f,Partial IV:42]
| | | Payload: {Code:0.01,
| | | Uri-Path:"alarm_status"}
| | |
| +------>| Code: 0.02 (POST)
| | POST | Token: 0x7b
| | | Object-Security: [kid:5f,Partial IV:42]
| | | Payload: {Code:0.01,
| | | Uri-Path:"alarm_status"}
| | |
| |<------+ Code: 2.04 (Changed)
| | 2.04 | Token: 0x7b
| | | Object-Security: -
| | | Payload: {Code:2.05, "OFF"}
| | |
|<------+ | Code: 2.04 (Changed)
| 2.04 | | Token: 0x8c
| | | Object-Security: -
| | | Payload: {Code:2.05, "OFF"}
| | |
Figure 11: Secure Access to Sensor. Square brackets [ ... ] indicate
content of compressed COSE object. Curly brackets { ... } indicate
encrypted data.
The request/response Codes are encrypted by OSCORE and only dummy
Codes (POST/Changed) are visible in the header of the OSCORE message.
The option Uri-Path ("alarm_status") and payload ("OFF") are
encrypted.
The COSE header of the request contains an identifier (5f),
indicating which security context was used to protect the message and
a Partial IV (42).
The server verifies the request as specified in Section 8.2. The
client verifies the response as specified in Section 8.4.
A.2. Secure Subscribe to Sensor
This example illustrates a client requesting subscription to a blood
sugar measurement resource (GET /glucose), first receiving the value
220 mg/dl and then a second value 180 mg/dl.
Client Proxy Server
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| | |
+------>| | Code: 0.05 (FETCH)
| FETCH | | Token: 0x83
| | | Observe: 0
| | | Object-Security: [kid:ca,Partial IV:15]
| | | Payload: {Code:0.01,
| | | Uri-Path:"glucose"}
| | |
| +------>| Code: 0.05 (FETCH)
| | FETCH | Token: 0xbe
| | | Observe: 0
| | | Object-Security: [kid:ca,Partial IV:15]
| | | Payload: {Code:0.01,
| | | Uri-Path:"glucose"}
| | |
| |<------+ Code: 2.04 (Changed)
| | 2.04 | Token: 0xbe
| | | Observe: 7
| | | Object-Security: [Partial IV:32]
| | | Payload: {Code:2.05,
| | | Content-Format:0, "220"}
| | |
|<------+ | Code: 2.04 (Changed)
| 2.04 | | Token: 0x83
| | | Observe: 7
| | | Object-Security: [Partial IV:32]
| | | Payload: {Code:2.05,
| | | Content-Format:0, "220"}
... ... ...
| | |
| |<------+ Code: 2.04 (Changed)
| | 2.04 | Token: 0xbe
| | | Observe: 8
| | | Object-Security: [Partial IV:36]
| | | Payload: {Code:2.05,
| | | Content-Format:0, "180"}
| | |
|<------+ | Code: 2.04 (Changed)
| 2.04 | | Token: 0x83
| | | Observe: 8
| | | Object-Security: [Partial IV:36]
| | | Payload: {Code:2.05,
| | | Content-Format:0, "180"}
| | |
Figure 12: Secure Subscribe to Sensor. Square brackets [ ... ]
indicate content of compressed COSE object header. Curly brackets {
... } indicate encrypted data.
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The request/response Codes are encrypted by OSCORE and only dummy
Codes (FETCH/Changed) are visible in the header of the OSCORE
message. The options Content-Format (0) and the payload ("220" and
"180"), are encrypted.
The COSE header of the request contains an identifier (ca),
indicating the security context used to protect the message and a
Partial IV (15). The COSE headers of the responses contains Partial
IVs (32 and 36).
The server verifies that the Partial IV has not been received before.
The client verifies that the responses are bound to the request and
that the Partial IVs are greater than any Partial IV previously
received in a response bound to the request.
Appendix B. Deployment examples
OSCORE may be deployed in a variety of settings, a few examples are
given in this section.
B.1. Master Secret Used Once
For settings where the Master Secret is only used during deployment,
the uniqueness of AEAD nonce may be assured by persistent storage of
the security context as described in this specification (see
Section 7.5). For many IoT deployments, a 128 bit uniformly random
Master Key is sufficient for encrypting all data exchanged with the
IoT device throughout its lifetime.
B.2. Master Secret Used Multiple Times
In cases where the Master Secret needs to be used to derive multiple
security contexts, e.g. due to recommissioning or where the security
context is not persistently stored, a stochastically unique Master
Salt prevents the reuse of AEAD nonce and key. The Master Salt may
be transported between client and server in the kid context parameter
(see Section 5.1) of the request.
In this section we give an example of a procedure which may be
implemented in client and server to establish the OSCORE security
context based on pre-established input parameters (see Section 3.2)
except for the Master Salt which is transported in kid context.
1. In order to establish a security context with a server for the
first time, or a new security context replacing an old security
context, the client generates a (pseudo-)random uniformly
distributed 64-bit Master Salt and derives the security context
as specified in Section 3.2. The client protects a request with
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the new Sender Context and sends the message with kid context set
to the Master Salt.
2. The server, receiving an OSCORE request with a non-empty kid
context derives the new security context using the received kid
context as Master Salt. The server processes the request as
specified in this document using the new Recipient Context. If
the processing of the request completes without error, the server
responds with an Echo option as specified in
[I-D.ietf-core-echo-request-tag]. The response is protected with
the new Sender Context.
3. The client, receiving a response with an Echo option to a request
which used a new security context, verifies the response using
the new Recipient Context, and if valid repeats the request with
the Echo option (see [I-D.ietf-core-echo-request-tag]) using the
new Sender Context. Subsequent message exchanges (unless
superseded) are processed using the new security context without
including the Master Salt in the kid context.
4. The server, receiving a request with a kid context and a valid
Echo option (see [I-D.ietf-core-echo-request-tag]), repeats the
processing described in step 2. If it completes without error,
then the new security context is established, and the request is
valid. If the server already had an old security context with
this client that is now replaced by the new security context.
If the server receives a request without kid context from a client
with which no security context is established, then the server
responds with a 4.01 Unauthorized error message with diagnostic
payload containing the string "Security context not found". This
could be the result of the server having lost its security context or
that a new security context has not been successfully established,
which may be a trigger for the client to run this procedure.
B.3. Client Aliveness
The use of a single OSCORE request and response enables the client to
verify that the server's identity and aliveness through actual
communications. While a verified OSCORE request enables the server
to verify the identity of the entity who generated the message, it
does not verify that the client is currently involved in the
communication, since the message may be a delayed delivery of a
previously generated request which now reaches the server. To verify
the aliveness of the client the server may initiate an OSCORE
protected message exchange with the client, e.g. by switching the
roles of client and server as described in Section 3.1, or by using
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the Echo option in the response to a request from the client
[I-D.ietf-core-echo-request-tag].
Appendix C. Test Vectors
This appendix includes the test vectors for different examples of
CoAP messages using OSCORE.
C.1. Test Vector 1: Key Derivation with Master Salt
Given a set of inputs, OSCORE defines how to set up the Security
Context in both the client and the server. The default values are
used for AEAD Algorithm and KDF.
C.1.1. Client
Inputs:
o Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)
o Master Salt: 0x9e7ca92223786340 (8 bytes)
o Sender ID: 0x (0 byte)
o Recipient ID: 0x01 (1 byte)
From the previous parameters,
o info (for Sender Key): 0x84400A634b657910 (8 bytes)
o info (for Recipient Key): 0x8441010A634b657910 (9 bytes)
o info (for Common IV): 0x84400a6249560d (7 bytes)
Outputs:
o Sender Key: 0x7230aab3b549d94c9224aacc744e93ab (16 bytes)
o Recipient Key: 0xe534a26a64aa3982e988e31f1e401e65 (16 bytes)
o Common IV: 0x01727733ab49ead385b18f7d91 (13 bytes)
C.1.2. Server
Inputs:
o Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)
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o Master Salt: 0x9e7ca92223786340 (64 bytes)
o Sender ID: 0x01 (1 byte)
o Recipient ID: 0x (0 byte)
From the previous parameters,
o info (for Sender Key): 0x8441010A634b657910 (9 bytes)
o info (for Recipient Key): 0x84400A634b657910 (8 bytes)
o info (for Common IV): 0x84400a6249560d (7 bytes)
Outputs:
o Sender Key: 0xe534a26a64aa3982e988e31f1e401e65 (16 bytes)
o Recipient Key: 0x7230aab3b549d94c9224aacc744e93ab (16 bytes)
o Common IV: 0x01727733ab49ead385b18f7d91 (13 bytes)
C.2. Test Vector 2: Key Derivation without Master Salt
Given a set of inputs, OSCORE defines how to set up the Security
Context in both the client and the server. The default values are
used for AEAD Algorithm, KDF, and Master Salt.
C.2.1. Client
Inputs:
o Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)
o Sender ID: 0x00 (1 byte)
o Recipient ID: 0x01 (1 byte)
From the previous parameters,
o info (for Sender Key): 0x8441000A634b657910 (9 bytes)
o info (for Recipient Key): 0x8441010A634b657910 (9 bytes)
o info (for Common IV): 0x84400a6249560d (7 bytes)
Outputs:
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o Sender Key: 0xf8f3b887436285ed5a66f6026ac2cdc1 (16 bytes)
o Recipient Key: 0xd904cb101f7341c3f4c56c300fa69941 (16 bytes)
o Common IV: 0xd1a1949aa253278f34c528d2cc (13 bytes)
C.2.2. Server
Inputs:
o Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)
o Sender ID: 0x01 (1 byte)
o Recipient ID: 0x00 (1 byte)
From the previous parameters,
o info (for Sender Key): 0x8441010A634b657910 (9 bytes)
o info (for Recipient Key): 0x8441000A634b657910 (9 bytes)
o info (for Common IV): 0x84400a6249560d (7 bytes)
Outputs:
o Sender Key: 0xd904cb101f7341c3f4c56c300fa69941 (16 bytes)
o Recipient Key: 0xf8f3b887436285ed5a66f6026ac2cdc1 (16 bytes)
o Common IV: 0xd1a1949aa253278f34c528d2cc (13 bytes)
C.3. Test Vector 3: OSCORE Request, Client
This section contains a test vector for a OSCORE protected CoAP GET
request using the security context derived in Appendix C.1. The
unprotected request only contains the Uri-Path option.
Unprotected CoAP request:
0x440149c60000f2a7396c6f63616c686f737483747631 (22 bytes)
Common Context:
o AEAD Algorithm: 10 (AES-CCM-16-64-128)
o Key Derivation Function: HKDF SHA-256
o Common IV: 0xd1a1949aa253278f34c528d2cc (13 bytes)
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Sender Context:
o Sender ID: 0x00 (1 byte)
o Sender Key: 0xf8f3b887436285ed5a66f6026ac2cdc1 (16 bytes)
o Sender Sequence Number: 20
The following COSE and cryptographic parameters are derived:
o Partial IV: 0x14 (1 byte)
o kid: 0x00 (1 byte)
o external_aad: 0x8501810a4100411440 (9 bytes)
o AAD: 0x8368456e63727970743040498501810a4100411440 (21 bytes)
o plaintext: 0x01b3747631 (5 bytes)
o encryption key: 0xf8f3b887436285ed5a66f6026ac2cdc1 (16 bytes)
o nonce: 0xd0a1949aa253278f34c528d2d8 (13 bytes)
From the previous parameter, the following is derived:
o Object-Security value: 0x091400 (3 bytes)
o ciphertext: 0x55b3710d47c611cd3924838a44 (13 bytes)
From there:
o Protected CoAP request (OSCORE message): 0x44026dd30000acc5396c6f6
3616c686f7374d305091400ff55b3710d47c611cd3924838a44 (37 bytes)
C.4. Test Vector 4: OSCORE Request, Client
This section contains a test vector for a OSCORE protected CoAP GET
request using the security context derived in Appendix C.2. The
unprotected request only contains the Uri-Path option.
Unprotected CoAP request:
0x440149c60000f2a7396c6f63616c686f737483747631 (22 bytes)
Common Context:
o AEAD Algorithm: 10 (AES-CCM-16-64-128)
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o Key Derivation Function: HKDF SHA-256
o Common IV: 0x01727733ab49ead385b18f7d91 (13 bytes)
Sender Context:
o Sender ID: 0x (0 bytes)
o Sender Key: 0x7230aab3b549d94c9224aacc744e93ab (16 bytes)
o Sender Sequence Number: 20
The following COSE and cryptographic parameters are derived:
o Partial IV: 0x14 (1 byte)
o kid: 0x (0 byte)
o external_aad: 0x8501810a40411440 (8 bytes)
o AAD: 0x8368456e63727970743040488501810a40411440 (20 bytes)
o plaintext: 0x01b3747631 (5 bytes)
o encryption key: 0x7230aab3b549d94c9224aacc744e93ab (16 bytes)
o nonce: 0x01727733ab49ead385b18f7d85 (13 bytes)
From the previous parameter, the following is derived:
o Object-Security value: 0x0914 (2 bytes)
o ciphertext: 0x6be9214aad448260ff1be1f594 (13 bytes)
From there:
o Protected CoAP request (OSCORE message): 0x44023bfc000066ef396c6f6
3616c686f7374d2050914ff6be9214aad448260ff1be1f594 (36 bytes)
C.5. Test Vector 5: OSCORE Response, Server
This section contains a test vector for a OSCORE protected 2.05
Content response to the request in Appendix C.3. The unprotected
response has payload "Hello World!" and no options. The protected
response does not contain a kid nor a Partial IV.
Unprotected CoAP response:
0x644549c60000f2a7ff48656c6c6f20576f726c6421 (21 bytes)
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Common Context:
o AEAD Algorithm: 10 (AES-CCM-16-64-128)
o Key Derivation Function: HKDF SHA-256
o Common IV: 0xd1a1949aa253278f34c528d2cc (13 bytes)
Sender Context:
o Sender ID: 0x01 (1 byte)
o Sender Key: 0xd904cb101f7341c3f4c56c300fa69941 (16 bytes)
o Sender Sequence Number: 0
The following COSE and cryptographic parameters are derived:
o external_aad: 0x8501810a4100411440 (9 bytes)
o AAD: 0x8368456e63727970743040498501810a4100411440 (21 bytes)
o plaintext: 0x45ff48656c6c6f20576f726c6421 (14 bytes)
o encryption key: 0xd904cb101f7341c3f4c56c300fa69941 (16 bytes)
o nonce: 0xd0a1949aa253278f34c528d2d8 (13 bytes)
From the previous parameter, the following is derived:
o Object-Security value: 0x (0 bytes)
o ciphertext: e4e8c28c41c8f31ca56eec24f6c71d94eacbcdffdc6d (22
bytes)
From there:
o Protected CoAP response (OSCORE message): 0x64446dd30000acc5d008ff
e4e8c28c41c8f31ca56eec24f6c71d94eacbcdffdc6d (33 bytes)
C.6. Test Vector 6: OSCORE Response with Partial IV, Server
This section contains a test vector for a OSCORE protected 2.05
Content response to the request in Appendix C.3. The unprotected
response has payload "Hello World!" and no options. The protected
response does not contain a kid, but contains a Partial IV.
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Unprotected CoAP response:
0x644549c60000f2a7ff48656c6c6f20576f726c6421 (21 bytes)
Common Context:
o AEAD Algorithm: 10 (AES-CCM-16-64-128)
o Key Derivation Function: HKDF SHA-256
o Common IV: 0xd1a1949aa253278f34c528d2cc (13 bytes)
Sender Context:
o Sender ID: 0x01 (1 byte)
o Sender Key: 0xd904cb101f7341c3f4c56c300fa69941 (16 bytes)
o Sender Sequence Number: 0
The following COSE and cryptographic parameters are derived:
o Partial IV: 0x00 (1 byte)
o external_aad: 0x8501810a4100411440 (9 bytes)
o AAD: 0x8368456e63727970743040498501810a4100411440 (21 bytes)
o plaintext: 0x45ff48656c6c6f20576f726c6421 (14 bytes)
o encryption key: 0xd904cb101f7341c3f4c56c300fa69941 (16 bytes)
o nonce: 0xd0a1949aa253278e34c528d2cc (13 bytes)
From the previous parameter, the following is derived:
o Object-Security value: 0x0100 (2 bytes)
o ciphertext: 0xa7e3ca27f221f453c0ba68c350bf652ea096b328a1bf (22
bytes)
From there:
o Protected CoAP response (OSCORE message): 0x64442b130000b29ed20801
00ffa7e3ca27f221f453c0ba68c350bf652ea096b328a1bf (35 bytes)
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Appendix D. Security properties
This appendix discusses security properties of OSCORE.
TODO
Appendix E. CDDL Summary
Data structure definitions in the present specification employ the
CDDL language for conciseness and precision. CDDL is defined in
[I-D.ietf-cbor-cddl], which at the time of writing this appendix is
in the process of completion. As the document is not yet available
for a normative reference, the present appendix defines the small
subset of CDDL that is being used in the present specification.
Within the subset being used here, a CDDL rule is of the form "name =
type", where "name" is the name given to the "type". A "type" can be
one of:
o a reference to another named type, by giving its name. The
predefined named types used in the present specification are:
"uint", an unsigned integer (as represented in CBOR by major type
0); "int", an unsigned or negative integer (as represented in CBOR
by major type 0 or 1); "bstr", a byte string (as represented in
CBOR by major type 2); "tstr", a text string (as represented in
CBOR by major type 3);
o a choice between two types, by giving both types separated by a
"/";
o an array type (as represented in CBOR by major type 4), where the
sequence of elements of the array is described by giving a
sequence of entries separated by commas ",", and this sequence is
enclosed by square brackets "[" and "]". Arrays described by an
array description contain elements that correspond one-to-one to
the sequence of entries given. Each entry of an array description
is of the form "name : type", where "name" is the name given to
the entry and "type" is the type of the array element
corresponding to this entry.
Acknowledgments
The following individuals provided input to this document: Christian
Amsuess, Tobias Andersson, Carsten Bormann, Joakim Brorsson, Esko
Dijk, Thomas Fossati, Martin Gunnarsson, Klaus Hartke, Jim Schaad,
Peter van der Stok, Dave Thaler, Marco Tiloca, and Malisa Vucinic.
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Ludwig Seitz and Goeran Selander worked on this document as part of
the CelticPlus project CyberWI, with funding from Vinnova.
Authors' Addresses
Goeran Selander
Ericsson AB
Email: goran.selander@ericsson.com
John Mattsson
Ericsson AB
Email: john.mattsson@ericsson.com
Francesca Palombini
Ericsson AB
Email: francesca.palombini@ericsson.com
Ludwig Seitz
RISE SICS
Email: ludwig.seitz@ri.se
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