ACE Working Group G. Selander
Internet-Draft J. Mattsson
Intended status: Standards Track F. Palombini
Expires: October 6, 2016 Ericsson AB
April 04, 2016
Ephemeral Diffie-Hellman Over COSE (EDHOC)
draft-selander-ace-cose-ecdhe-01
Abstract
This document specifies Diffie-Hellman key exchange with ephemeral
keys embedded in messages encoded with the CBOR Encoded Message
Syntax.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 6, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. ECDH Public Keys . . . . . . . . . . . . . . . . . . . . . . 4
2.1. COSE_Key Formatting . . . . . . . . . . . . . . . . . . . 4
2.2. Example: ECDH Public Key . . . . . . . . . . . . . . . . 5
3. Authentication with Pre-Shared Keys . . . . . . . . . . . . . 5
3.1. Message 1 with PSK . . . . . . . . . . . . . . . . . . . 5
3.2. Example: Message 1 with PSK . . . . . . . . . . . . . . . 6
3.3. Message 2 with PSK . . . . . . . . . . . . . . . . . . . 7
3.4. Example: Message 2 with PSK . . . . . . . . . . . . . . . 8
3.5. Key Derivation . . . . . . . . . . . . . . . . . . . . . 9
4. Authentication with Raw Public Keys . . . . . . . . . . . . . 9
4.1. Message 1 with RPK . . . . . . . . . . . . . . . . . . . 10
4.2. Example: Message 1 with RPK . . . . . . . . . . . . . . . 10
4.3. Message 2 with RPK . . . . . . . . . . . . . . . . . . . 11
4.4. Example: Message 2 with RPK . . . . . . . . . . . . . . . 12
4.5. Key Derivation . . . . . . . . . . . . . . . . . . . . . 13
5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Implementing EDHOC with CoAP . . . . . . . . . . . . 17
Appendix B. Integrating EDHOC with ACE . . . . . . . . . . . . . 17
Appendix C. Deriving Security Context for OSCOAP . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
Security at the application layer provides an attractive option for
protecting Internet of Things (IoT) deployments, for example where
transport layer security is not sufficient
[I-D.hartke-core-e2e-security-reqs]. IoT devices may be constrained
in various ways, including memory, storage, processing capacity, and
energy [RFC7228]. A method for protecting individual messages at
application layer, suitable for constrained devices, is provided by
the CBOR Encoded Message Syntax (COSE, [I-D.ietf-cose-msg]).
In order for a communication session to provide forward secrecy, the
communicating parties could run a Diffie-Hellman (DH) key exchange
protocol with ephemeral keys, from which session keys are derived.
This document specifies two instances of DH key exchange using COSE
messages to transport the ephemeral public keys. The DH key exchange
messages are authenticated using pre-established keys, either a
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secret key (Section 3) or raw public keys (Section 4). The keys may
be pre-established to client and server from a trusted third party,
such as an Authorization Server [I-D.ietf-ace-oauth-authz].
Successful verification of the protocol messages, defined in this
document, provides a method for proof-of-possession of the
corresponding secret or private key
[I-D.ietf-oauth-pop-key-distribution].
This document also specifies derivation of traffic keys, from the
shared secret established through the DH key exchange with ephemeral
keys. The key derivation is identical to TLS 1.3
[I-D.ietf-tls-tls13].
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. These
words may also appear in this document in lowercase, absent their
normative meanings.
The key exchange messages are called "message_1" and "message_2", and
the parties exchanging the messages are called "client" and "server",
see Figure 1. The messages are encoded using the CBOR Encoded
Message Syntax (COSE, [I-D.ietf-cose-msg]), and include an ephemeral
public key ("g^x"/"g^y"). The shared secret "g^(xy)" is used to
derive a key called "traffic_secret_0" using the terminology of TLS
1.3 [I-D.ietf-tls-tls13].
client server
| |
| COSE(g^x) |
+---------------------->|
| message_1 |
| |
| COSE(g^y) |
|<----------------------+
| message_2 |
g^(xy) | | g^(xy)
| |
| |
V V
traffic_secret_0 traffic_secret_0
Figure 1: Diffie-Hellman key exchange and key derivation
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Most keys used in this document have an associated identifier. The
identifiers used in the document are placeholders for values of the
identifiers. The following key identifiers/value representations are
used in the draft:
o kid_x and kid_y represent the values of the key identifiers of the
ECDH ephemeral public keys of the client and server, respectively.
* kid_x is a sequence number used for replay protection of
message_1.
* kid_y is used to identify a resulting traffic key, as a means
for the server to ensure that different clients establishing
traffic keys using this method have different identifiers.
o kid_0 represents the value of the key identifier of the pre-shared
key between client and server (Section 3).
o kid_c and kid_s represent the values of the key identifiers of the
static public keys of the client and server, respectively
(Section 4).
* kid_c and kid_s are used to identify client and server,
respectively.
+------------+-----+-----------------------------------------+
| Key | Key | Use |
| Identifier | | |
+------------+-----+-----------------------------------------+
| kid_x | g^x | ECDH ephemeral public key of the client |
| kid_y | g^y | ECDH ephemeral public key of the server |
| kid_0 | PSK | Pre-shared key (Section 3) |
| kid_c | PKc | Static client public key (Section 4) |
| kid_s | PKs | Static server public key (Section 4) |
+------------+-----+-----------------------------------------+
Figure 2: Notation of keys and key identifiers.
2. ECDH Public Keys
This section defines the formatting of the ephemeral public keys g^x
and g^y.
2.1. COSE_Key Formatting
The ECDH ephemeral public key SHALL be formatted as a COSE_Key with
the following fields and values:
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o kty: The value SHALL be 2 (Elliptic Curve Keys)
o kid:
o crv: The value 1 SHALL be supported by the server (NIST P-256
a.k.a. secp256r1 [RFC4492])
o x:
o y: The value SHOULD be boolean.
[TODO: Consider replacing P-256 with Curve25519]
2.2. Example: ECDH Public Key
An example of COSE_Key structure, representing an ECDH public key, is
given in Figure 3, using CBOR's diagnostic notation. In this
example, the ephemeral key is identified by a 4 bytes 'kid'.
/ ephemeral / -1:{
/ kty / 1:2,
/ kid / 2:h'78f67901',
/ crv / -1:1,
/ x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590b
bfbf054e1c7b4d91d6280',
/ y / -3:true
}
Figure 3: Example of an ECDH public key formatted as a COSE_Key
The equivalent CBOR encoding is: h'a120a50102024478f67901200121582098
f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91d628022f5',
which has a size of 50 bytes.
3. Authentication with Pre-Shared Keys
This section defines the DH key exchange protocol messages, when the
exchange is authenticated with Message Authentication Codes (MACs)
calculated with a pre-shared key.
The client and server are assumed to have a pre-shared key, PSK, the
value of its identifier is represented by kid_0.
3.1. Message 1 with PSK
message_1 contains the client's ephemeral public key, g^x, and a MAC
over g^x, calculated with the pre-shared key.
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Before sending message_1, the client SHALL generate a fresh ephemeral
ECDH key pair. The ephemeral public key, g^x, SHALL be formatted as
in Section 2. The 'kid' value kid_x of the ephemeral public key g^x
SHALL be a sequence number increased by 1 for each message_1
associated to kid_0. Note that each ephemeral ECDH key pair SHALL
NOT be re-used with any other node than the one it was initially
generated for.
The key identifier kid_0 SHALL be unique for client and server.
message_1 SHALL have the COSE_Mac0_Tagged structure
[I-D.ietf-cose-msg] with the following fields and values:
o Header
* Protected
+ Alg: 4 (HMAC 256/64)
+ Kid: kid_0
* Unprotected: Empty, except for the case specified in Appendix B
o Payload: g^x (with 'kid' = kid_x, which is the sequence number)
o Tag: As in section 6.3 of [I-D.ietf-cose-msg]
[TODO: Error handling]
3.2. Example: Message 1 with PSK
An example of COSE encoding for message_1 is given in Figure 4 using
CBOR's diagnostic notation. In this example, kid_0, the identifier
of PSK is 4 bytes, and kid_x is one byte.
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996(
[
/ protected / h'a201040444e19648b5' / {
/ alg / 1:4, / HMAC 256//64 /
/ kid / 4:h'e19648b5' / kid_0
} / ,
/ unprotected / {},
/ payload / h'a120a50102024103200121582098f50a4ff6c05861c8860d13
a638ea56c3f5ad7590bbfbf054e1c7b4d91d628022f5' / COSE_Key g^x / {
/ ephemeral / -1:{
/ kty / 1:2,
/ kid / 2:h'03', / kid_x
/ crv / -1:1,
/ x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfb
f054e1c7b4d91d6280',
/ y / -3:true
}
} / ,
/ tag / h'e77fe81c66c3b5c0'
]
)
Figure 4: Example of message_1 authenticated with PSK
The equivalent CBOR encoding is: h'd903e48449a201040444e19648b5a0582c
a120a50102024103200121582098f50a4ff6c05861c8860d13a638ea56c3f5ad7590b
bfbf054e1c7b4d91d628022f548e77fe81c66c3b5c0', which has a size of 73
bytes.
3.3. Message 2 with PSK
message_2 contains the server's ephemeral public key, g^y, and a MAC
over g^y and message_1, calculated with the pre-shared key.
Before sending message_2, the server SHALL verify message_1 using the
pre-shared key, PSK, and that the kid_x is greater than previously
verified message_1 associated to kid_0. The server SHALL generate a
fresh ephemeral ECDH key pair. The ephemeral public key, g^y, SHALL
be formatted as in Section 2, its identifier (kid_y) SHALL be unique
among key identifiers used for traffic keys by the server.
At reception, the client SHALL verify message_2 using the pre-shared
key PSK and the sent message_1.
message_2 SHALL have the COSE_Mac0_Tagged structure
[I-D.ietf-cose-msg] with the following fields and values:
o Header
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* Protected
+ Alg: 4 (HMAC 256/64)
+ Kid: kid_0
* Unprotected: empty
o Payload: g^y (with 'kid' = kid_y)
o external_aad: message_1
o Tag: as in [I-D.ietf-cose-msg], including the external_aad in the
MAC_structure.
[TODO: Error handling]
3.4. Example: Message 2 with PSK
An example of COSE encoding for message_2 is given in Figure 5 using
CBOR's diagnostic notation. In this example, kid_0, the identifier
of PSK, and kid_y, the identifier of the server's ephemeral public
key, is 4 bytes.
996(
[
/ protected / h'a201040444e19648b5' / {
/ alg / 1:4, / HMAC 256//64 /
/ kid / 4:h'e19648b5' / kid_0
} / ,
/ unprotected / {},
/ payload / h'a120a5010202442edb61f92001215820acbee6672a28340a
ffce41c721901ebd7868231bd1d86e41888a07822214050022f5'
/ COSE_Key g^y / {
/ ephemeral / -1:{
/ kty / 1:2,
/ kid / 2:h'2edb61f9', / kid_y
/ crv / -1:1,
/ x / -2:h'acbee6672a28340affce41c721901ebd7868231bd1d
86e41888a078222140500',
/ y / -3:true
}
} / ,
/ tag / h'6113268ad246f2c9'
]
)
Figure 5: Example of message_2 authenticated with PSK
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The equivalent CBOR encoding is: h'd903e48449a201040444e19648b5a05832
a120a4010202481e6f0c642001215820acbee6672a28340affce41c721901ebd78682
31bd1d86e41888a07822214050022f5486113268ad246f2c9', which has a size
of 76 bytes.
3.5. Key Derivation
The client and server SHALL derive "traffic_secret_0" from the
information available through the key exchange, as described in this
section. The key derivation is identical to Section 7 of
[I-D.ietf-tls-tls13], using the PSK + ECDHE operational mode and HKDF
[RFC5869] with SHA-256:
o The Static Secret (SS) SHALL be the pre-shared key
o The Ephemeral Secret (ES) SHALL be the ECDH shared secret,
generated from the ephemeral keys, as specified in section 7.3.3.
of [I-D.ietf-tls-tls13]
o The generic string "TLS 1.3, " in HkdfLabel (Section 7.1) SHALL be
replaced by "EDHOC, "
o The handshake_hash is replaced by the exchange_hash = SHA-
256(message_1 + message_2), where '+' denotes concatenation of
octet strings
The procedure for deriving "traffic_secret_0" in Section 7 in
[I-D.ietf-tls-tls13] SHALL be followed. The "traffic_secret_0" SHALL
be identified by the identifier of the server's ephemeral public key
(kid_y).
Appendix C provides an example of how to derive a security context
from "traffic_secret_0".
4. Authentication with Raw Public Keys
This section defines the DH key exchange protocol messages, when the
exchange is authenticated with signatures calculated with Pre-
Established Raw Public Keys (RPK).
o The client's static public key, denoted PKc, is pre-established to
the server, and SHALL be uniquely identified at the server by
kid_c.
o The server's static public key, denoted PKs, is pre-established to
the client, and SHALL be uniquely identified at the client by
kid_s.
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4.1. Message 1 with RPK
message_1 contains the client's ephemeral public key, g^x, and a
signature over g^x, computed with the client's static private key.
Before sending message_1, the client SHALL generate a fresh ephemeral
ECDH key pair. The client's ephemeral public key, g^x, SHALL be
formatted as in Section 2. The 'kid' value kid_x of the ephemeral
public key g^x SHALL be a sequence number increased by 1 for each
message_1 associated to kid_c. Note that each ephemeral ECDH key
pair SHALL NOT be re-used with any other node than the one it was
initially generated for.
message_1 SHALL have the COSE_Sign_Tagged structure
[I-D.ietf-cose-msg], with the following fields and values:
o Header
* Protected: empty
* Unprotected: empty, except in the specified in Appendix B
o Payload: g^x (with 'kid' = kid_x, which is the sequence number)
o Signatures
* Protected
+ Alg: -7 (ECDSA 256)
+ Kid: kid_c
* Unprotected: empty
* Signature: as in {{I-D.ietf-cose-msg}
o external_aad: kid_s
[TODO: Error handling]
4.2. Example: Message 1 with RPK
An example of COSE encoding for message_1 is given in Figure 6, using
CBOR's diagnostic notation. In this example, the size of the
identifiers of the static public keys kid_c and kid_s are 4 bytes,
and the identifier of the client's ephemeral key, kid_x is 1 byte.
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991(
[
/ protected / h'',
/ unprotected / {},
/ payload / h'a120a50102024112200121582098f50a4ff6c05861c8860d13
a638ea56c3f5ad7590bbfbf054e1c7b4d91d628022f5' / COSE_Key g^x / {
/ ephemeral / -1:{
/ kty / 1:2,
/ kid / 2:h'12', / kid_x
/ crv / -1:1,
/ x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfb
f054e1c7b4d91d6280',
/ y / -3:true
}
} / ,
/ signatures / [
[
/ protected / h'a201260444c150d41c' / {
/ alg / 1:-7, / ECDSA 256 /
/ kid / 4:h'c150d41c', / kid_c /
} / ,
/ unprotected / {},
/ signature / h'eae868ecc1276883766c5dc5ba5b8dca25dab3c2e56a
51ce5705b793914348e14eea4aee6e0c9f09db4ef3ddeca8f3506cd1a98a8fb64327
be470355c9657ce0'
]
]
]
)
Figure 6: Example of message_1 authenticated by the client
The equivalent CBOR encoding is: d903df8440a0582fa120a501020241122001
21582098f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91d628
022f5818349a201260444c150d41ca05840eae868ecc1276883766c5dc5ba5b8dca25
dab3c2e56a51ce5705b793914348e14eea4aee6e0c9f09db4ef3ddeca8f3506cd1a98
a8fb64327be470355c9657ce0, which has a size of 134 bytes.
4.3. Message 2 with RPK
message_2 contains the server's ephemeral public key, g^y, and a
signature over g^y and message_1, computed with the server's static
private key.
Before sending message_2, the server SHALL verify message_1, and that
the kid_x is greater than previously verified with public key
corresponding to kid = kid_c. The server SHALL generate a fresh
ephemeral ECDH key pair, formatted as in Section 2, the value of the
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key identifier (kid_y) SHALL be unique among key identifiers used for
traffic keys by the server.
At reception, the client SHALL verify message 2 using the pre-shared
key PSK and the sent message_1.
message_2 SHALL have the COSE_Sign_Tagged structure
[I-D.ietf-cose-msg] with the following fields and values:
o Header
* Protected: empty
* Unprotected: empty
o Payload: g^y (with 'kid' = kid_y)
o Signatures
* Protected
+ Alg: -7 (ECDSA 256)
+ Kid: kid_s
* Unprotected: empty
* Signature: as in {{I-D.ietf-cose-msg}
o external_aad: message_1
[TODO: Error handling]
4.4. Example: Message 2 with RPK
An example of COSE encoding for Message 2 is given in Figure 7, using
CBOR's diagnostic notation. In this example, the size of the
identifiers of the public keys: kid_x, kid_y, and kid_s are 4 bytes.
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991(
[
/ protected / h'',
/ unprotected / {},
/ payload / h'a120a5010202442edb61f9200121582098f50a4ff6c0
5861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91d628022f5'
/ COSE_Key g^y / {
/ ephemeral / -1:{
/ kty / 1:2,
/ kid / 2:h'2edb61f9', / kid_y
/ crv / -1:1,
/ x / -2:h'acbee6672a28340affce41c721901ebd7868231bd1d
86e41888a078222140500',
/ y / -3:true
}
} / ,
/ signatures / [
[
/ protected / h'a2012604447a2af164' / {
/ alg / 1:-7, / ECDSA 256 /
/ kid / 4:h'7a2af164', / kid_s /
} / ,
/ unprotected / {},
/ signature / h'2374e27a3d9eeb4f66c5dc5ba5b8dca25dab3c2e56a5
51ce5705b793914348e14eea4aee6e0c9f09db4ef3ddeca8f3506cd1a98a8fb64327
be470355c9657ce0'
]
]
]
)
Figure 7: Example of message_2 authenticated by server
The equivalent CBOR encoding is: h'd903df8440a05832a120a5010202442edb
61f9200121582098f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b
4d91d628022f5818349a2012604447a2af164a058402374e27a3d9eeb4f66c5dc5ba5
b8dca25dab3c2e56a551ce5705b793914348e14eea4aee6e0c9f09db4ef3ddeca8f35
06cd1a98a8fb64327be470355c9657ce0', which has a size of 137 bytes.
4.5. Key Derivation
The client and server SHALL derive "traffic_secret_0" from the
information available through the key exchange, as described in this
section. The key derivation is identical to Section 7 of
[I-D.ietf-tls-tls13], using the ECDHE operational mode and HKDF
[RFC5869] with SHA-256:
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o The Static Secret (SS) and the Ephemeral Secret (ES) SHALL be the
ECDH shared secret, generated from the ephemeral keys, as
specified in section 7.3.3. of [I-D.ietf-tls-tls13]
o The generic string "TLS 1.3, " in HkdfLabel (Section 7.1) SHALL be
replaced by "EDHOC, "
o The handshake_hash is replaced by the exchange_hash = SHA-
256(message_1 + message_2), where '+' denotes concatenation of
octet strings
The procedure for deriving "traffic_secret_0" in Section 7 in
[I-D.ietf-tls-tls13] SHALL be followed. The "traffic_secret_0" SHALL
be identified with the value of the 'kid' field of the server's
ephemeral public key (kid_y).
Appendix C provides an example of how to derive a security context
from "traffic_secret_0".
5. Security Considerations
After the key derivation is completed, the intermediate computed
values should be securely deleted, along with any ephemeral ECDH
secrets.
The choice of key length used in the different algorithms needs to be
harmonized, so that right security level is maintained throughout the
calculations.
The identifier of the ephemeral key of the client is used for replay
protection or client requests.
The verification of client and server identity is important, the
static public key identifiers serve the role to identify the entity
holding the private key. In case of pre-shared key, the key
identifier serves the role to identify the "other" party.
With the current protocol, key confirmation of the Diffie-Hellman
shared secret/traffic keys is performed when the keys are
successfully used. The addition of key confirmation to the protocol
is for further study.
A two-pass authenticated key exchange protocol can at most provide a
weak form of forward secrecy in the following sense; when agents'
long-term keys are compromised, the secrecy of previously established
session-keys is guaranteed, but only for sessions in which the
adversary did not actively interfere.
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The algorithm for generating the shared secret has important security
consequences. In this part we follow closely TLS 1.3 and may inherit
any vulnerabilities in that construction.
[TODO: Expand on the security considerations in a future version of
the draft]
6. Privacy Considerations
[TODO: ]
7. IANA Considerations
8. Acknowledgments
The authors are grateful to Ilari Liusvaara for pointing out
vulnerabilities in version -00. The authors wish to thank Ludwig
Seitz for timely review and helpful comments.
9. References
9.1. Normative References
[I-D.ietf-cose-msg]
Schaad, J., "CBOR Encoded Message Syntax", draft-ietf-
cose-msg-11 (work in progress), March 2016.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-12 (work in progress),
March 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
9.2. Informative References
[I-D.hartke-core-e2e-security-reqs]
Selander, G., Palombini, F., Hartke, K., and L. Seitz,
"Requirements for CoAP End-To-End Security", draft-hartke-
core-e2e-security-reqs-00 (work in progress), March 2016.
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[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authorization for the Internet of Things
using OAuth 2.0", draft-ietf-ace-oauth-authz-01 (work in
progress), February 2016.
[I-D.ietf-oauth-pop-key-distribution]
Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
"OAuth 2.0 Proof-of-Possession: Authorization Server to
Client Key Distribution", draft-ietf-oauth-pop-key-
distribution-02 (work in progress), October 2015.
[I-D.selander-ace-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security of CoAP (OSCOAP)", draft-selander-ace-
object-security-04 (work in progress), March 2016.
[I-D.wahlstroem-ace-cbor-web-token]
Wahlstroem, E., Jones, M., and H. Tschofenig, "CBOR Web
Token (CWT)", draft-wahlstroem-ace-cbor-web-token-00 (work
in progress), December 2015.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, DOI
10.17487/RFC4492, May 2006,
<http://www.rfc-editor.org/info/rfc4492>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/
RFC5869, May 2010,
<http://www.rfc-editor.org/info/rfc5869>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228, DOI 10.17487/
RFC7228, May 2014,
<http://www.rfc-editor.org/info/rfc7228>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, DOI 10.17487/
RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<http://www.rfc-editor.org/info/rfc7519>.
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Appendix A. Implementing EDHOC with CoAP
The DH key exchange specified in this document can be implemented as
a CoAP [RFC7252] message exchange. A strawman is sketched here.
The client makes the following request:
o The request method is POST
o Content-Format is "application/cose+cbor"
o The Uri-Path is "edhoc"
o The Payload is message_1
The server performs the verifications of the COSE object as specified
in [I-D.ietf-cose-msg]. If successful, then the server provides the
following response:
o The response Code is 2.04 (Changed)
o The Payload is message_2
Appendix B. Integrating EDHOC with ACE
A pre-requisite for using the DH key exchange protocols in Section 3
and Section 4 of this document is that some keys are pre-established
in client and server. The ACE framework [I-D.ietf-ace-oauth-authz]
specifies how an authorization server (AS) supports the establishment
of keys in client and (resource) server, either a shared secret key
or each others' public keys, which is exactly what is required in
Section 3 and Section 4, respectively.
The ACE protocol specifies a client making a 'token request' to the
AS to retrieve an access token (JWT [RFC7519], or CWT
[I-D.wahlstroem-ace-cbor-web-token]) containing authorization
information about the client regarding a certain resource on a
certain server. The client can then transfer the access token to the
server in the CoAP payload of the following request:
POST /authz-info
The access token may also contain a shared secret key or the public
key of the client, for use by the server.
In case of symmetric keys, the AS generates this key and protects it
for the client and server, after which the protocol in Section 3 can
start.
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In case of asymmetric keys, the ACE framework allows the client to
include its public key in the 'token request', which results in the
key being included in the access token reaching the server. The
server's public key can be assumed to be known to the AS, which can
therefore provide also this information to the client in the response
to the token request.
The transfer of the access token as defined in
[I-D.ietf-ace-oauth-authz] can be combined with the execution of
EDHOC, for example, by including the access token in the Unprotected
of Header of message_1. A dedicated resource could be defined for
this combined message exchange, for example:
POST /authz-info-edhoc
The strawman in Appendix A applies also to this case.
Appendix C. Deriving Security Context for OSCOAP
In this section we show how to establish security context for OSCOAP
[I-D.selander-ace-object-security], using the method specified in
this document.
We assume that "traffic_secret_0" has been established, e.g. as
described in Appendix B using a DH key exchange specified in this
document. OSCOAP requires traffic keying material Client/Server
Write Key/IV to be established at client and server, see section 3 of
[I-D.selander-ace-object-security]. The computation of keying
material mimics the traffic key calculation of Section 7.3 in TLS 1.3
[I-D.ietf-tls-tls13] using HKDF with SHA-256 and the following
parameters:
o Secret = traffic_secret_0
o phase = "application data key expansion"
o purpose = "client write key" / "server write key" / "client write
IV" / "server write IV"
o handshake_context = message_1 + message_2, the concatenation of
the exchanged messages
o key_length for key and IV is algorithm specific.
The first three bullets are identical to TLS 1.3.
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With the mandatory OSCOAP algorithm AES-CCM-64-64-128 (see
Section 10.2 in [I-D.ietf-cose-msg]), key_length for the keys is 128
bits and key_length for the static IVs is 56 bits.
The Context Identifier (Cid) is set to the key identifier of
traffic_secret_0 (i.e. kid_y, using the terminology of Section 3 and
Section 4).
Authors' Addresses
Goeran Selander
Ericsson AB
Farogatan 6
Kista SE-16480 Stockholm
Sweden
Email: goran.selander@ericsson.com
John Mattsson
Ericsson AB
Farogatan 6
Kista SE-16480 Stockholm
Sweden
Email: john.mattsson@ericsson.com
Francesca Palombini
Ericsson AB
Farogatan 6
Kista SE-16480 Stockholm
Sweden
Email: francesca.palombini@ericsson.com
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