ACE Working Group G. Selander
Internet-Draft J. Mattsson
Intended status: Standards Track F. Palombini
Expires: September 22, 2016 Ericsson AB
March 21, 2016

Ephemeral Diffie-Hellman Over COSE (EDHOC)
draft-selander-ace-cose-ecdhe-00

Abstract

This document specifies the 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.

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Table of Contents

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 secret key (Section 3) or public keys (Section 4). The pre-established keys may be transferred 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) and a Message Authentication Code (MAC). 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, MAC)     |  
             +---------------------->|
             |      message_1        |
             |                       |                
             |    COSE(g^y, MAC)     |
             |<----------------------+
             |      message_2        |
     g^(xy)  |                       |  g^(xy)           
       |                                  |                                             
       |                                  |          
       V                                  V      
traffic_secret_0                   traffic_secret_0
                                                     

Figure 1: Diffie-Hellman key exchange and key derivation

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:

+------------+-----+-----------------------------------------------+
|    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    | g^c | ECDH static public key of the client (Sec. 4) |
|   kid_s    | g^s | ECDH static public key of the server (Sec. 4) |
+------------+-----+-----------------------------------------------+

Figure 2: Notation of keys and key identifiers.

The server ephemeral key identifier key_y is also used to identify the resulting traffic key security context, which means that the server can ensure that different clients establishing traffic keys using this method have different context 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:

  • kty: The value SHALL be 2 (Elliptic Curve Keys)
  • kid:
  • crv: The value 1 SHALL be supported by the server (NIST P-256 a.k.a. secp256r1 [RFC4492])
  • x:
  • 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 pre-shared 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’a120a50102024478f67901200121582098f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91d628022f5’, which has a size of 50 bytes.

3. Authentication with Pre-Shared Keys

This section defines the DH key exchange protocol messages, when the MAC is 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.

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, with the ‘kid’ field omitted.

message_1 SHALL have the COSE_Mac0_Tagged structure [I-D.ietf-cose-msg] with the following fields and values:

  • Header
    • Protected
      • Alg: 4 (HMAC 256/64)
      • Kid: kid_0
    • Unprotected: Empty, except for the case specified in Appendix B
  • Payload: g^x (‘kid’ field omitted)
  • 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.

   996(
     [
       / protected / h'a201040444e19648b5' / {
           / alg / 1:4, / HMAC 256//64 /
           / kid / 4:h'e19648b5' / kid_0
         } / ,
       / unprotected / {},
       / payload / h'a120a40102200121582098f50a4ff6c05861c8860d13a638
       ea56c3f5ad7590bbfbf054e1c7b4d91d628022f5' / COSE_Key g^x / {
          / ephemeral / -1:{
            / kty / 1:2,
            / 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’d903e48449a201040444e19648b5a0582ca120a40102200121582098f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91d628022f548e77fe81c66c3b5c0’, which has a size of 70 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 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.

message_2 SHALL have the COSE_Mac0_Tagged structure [I-D.ietf-cose-msg] with the following fields and values:

  • Header
    • Protected
      • Alg: 4 (HMAC 256/64)
      • Kid: kid_0
    • Unprotected: empty
  • Payload: g^y
  • external_aad: message_1
  • 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

The equivalent CBOR encoding is: h’d903e48449a201040444e19648b5a05832a120a4010202481e6f0c642001215820acbee6672a28340affce41c721901ebd7868231bd1d86e41888a07822214050022f5486113268ad246f2c9’, 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:

  • The Static Secret (SS) SHALL be the pre-shared key
  • 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]
  • The generic string “TLS 1.3, “ in HkdfLabel (Section 7.1) SHALL be replaced by “EDHOC, “
  • 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”.

TODO: Align key derivation with that used with ECDH-SS (Section 4).

4. Authentication with Static ECDH Keys

This section defines the DH key exchange protocol messages, when the MAC is calculated with a key derived from static ECDH keys.

The client and the server are assumed to have static ECDH keys of a common curve. Curve P-256 SHALL be implemented by the server.

  • The client’s static public key is denoted g^c, and identified by kid_c
  • The server’s static public key is denoted g^s, and identified by kid_s

4.1. Message 1 with ECDH-SS

message_1 contains the client’s ephemeral public key, g^x, and a MAC over g^x, computed with a key derived from the shared secret g^(cs), calculated from the client’s and server’s static public keys.

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, and identified by kid_x.

message_1 SHALL have the COSE_Mac_Tagged structure [I-D.ietf-cose-msg], with the following fields and values:

  • Header
    • Protected
      • Alg: 4 (HMAC 256/64)
    • Unprotected: empty, except in the specified in Appendix B
  • Payload: g^x
  • Tag: as in [I-D.ietf-cose-msg]
  • Recipients
    • COSE_recipient
      • Header
        • Protected
          • Alg: -27 (ECDH-SS + HKDF-256)
        • Unprotected
          • Static Kid: kid_s
          • Kid: kid_c
          • U Nonce: pseudo-random octet string
      • Ciphertext: nil

TODO: Error handling

4.2. Example: Message 1 with ECDH-SS

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 ECDH public keys: kid_x (the client’s ephemeral), kid_c (the client’s static), and kid_s (the server’s static) are 4 bytes, while the length of U Nonce is 32 bytes.

   994(
     [
       / protected / h'a10104' / {
           / alg / 1:4 / HMAC 256//64 /
         } / ,
       / unprotected / {},
       / payload / h'a120a50102024478f67901200121582098f50a4ff6c05861
       c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91d628022f5' 
       / COSE_Key g^x / {
          / ephemeral / -1:{
            / kty / 1:2,
            / kid / 2: h'78f67901', / kid_x
            / crv / -1:1,
            / x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfb
            f054e1c7b4d91d6280',
            / y / -3:true
          } 
        } / ,        
       / tag / h'9287cb4ead0c171d',
       / recipients / [
         [
           / protected / h'a101381a' / {
               \ alg \ 1:-27 \ ECDH-SS + HKDF-256 \
             } / ,
           / unprotected / {
             / static kid / -3:h'c150d41c', / kid_s /
             / kid / 4:h'f6b70552', / kid_c /
             / U nonce / -22:h'4d8553e7e74f3c6a3a9dd3ef286a8195cbf8a2
             3d19558ccfec7d34b824f42d91'
           },
           / ciphertext / h''
         ]
       ]
     ]
   )

Figure 6: Example of message_1 authenticated with static ECDH keys

The equivalent CBOR encoding is: h’d903e28543a10104a05832a120a50102024478f67901200121582098f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91d628022f5489287cb4ead0c171d818344a101381aa32244c150d41c0444f6b705523558204d8553e7e74f3c6a3a9dd3ef286a8195cbf8a23d19558ccfec7d34b824f42d9140’, which has a size of 126 bytes.

4.3. Message 2 with ECDH-SS

message_2 contains the server’s ephemeral public key, g^y, and a MAC over g^y, computed with a key derived from the shared secret g^(xs), calculated from the client’s ephemeral public key (kid_x) and the server’s static key (kid_s).

Before sending message_2, the server SHALL verify message_1. The server SHALL generate a fresh ephemeral ECDH key pair, formatted as in Section 2, the value of the key identifier (kid_y) SHALL be unique among key identifiers used for traffic keys by the server.

message_2 SHALL have the COSE_Mac_Tagged structure [I-D.ietf-cose-msg] with the following fields and values:

  • Header
    • Protected
      • Alg: 4 (HMAC 256/64)
    • Unprotected: empty
  • Payload: g^y
  • Tag: as in [I-D.ietf-cose-msg].
  • Recipients
    • COSE_recipient
      • Header
        • Protected
          • Alg: -27 (ECDH-SS + HKDF-256)
        • Unprotected
          • Static Kid: kid_x
          • Kid: kid_s
          • U Nonce: pseudo-random octet string
      • Ciphertext: nil

TODO: Error handling

4.4. Example: Message 2 with ECDH-SS

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 ECDH public keys: kid_x (the client’s ephemeral), kid_y (the server’s ephemeral), and kid_s (the server’s static) are 4 bytes, while the length of U Nonce is 32 bytes.

   994(
     [
       / protected / h'a10104' / {
           / alg / 1:4 / HMAC 256//64 /
         } / ,
       / 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'2cc75952a7c6dc7f',
       / recipients / [
         [
           / protected / h'a101381a' / {
               \ alg \ 1:-27 \ ECDH-SS + HKDF-256 \
             } / ,
           / unprotected / {
             / static kid / -3:h'78f67901', / kid_x /
             / kid / 4:h'c150d41c', / kid_s /
             / U nonce / -22:h'66aabbadf938799613ccbf8a7da0a15f13be5b
             43d300aa51fceabc07a731232a'
           },
           / ciphertext / h''
         ]
       ]
     ]
   )

Figure 7: Example of message_2 authenticated with static ECDH keys

The equivalent CBOR encoding is: h’d903e28543a10104a05832a120a5010202442edb61f92001215820acbee6672a28340affce41c721901ebd7868231bd1d86e41888a07822214050022f5482cc75952a7c6dc7f818344a101381aa3224478f679010444c150d41c35582066aabbadf938799613ccbf8a7da0a15f13be5b43d300aa51fceabc07a731232a40’, which has a size of 126 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:

  • 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]
  • The generic string “TLS 1.3, “ in HkdfLabel (Section 7.1) SHALL be replaced by “EDHOC, “
  • 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”.

TODO: Align key derivation with that used with ECDH-SS.

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, e.g. the size of PSK and the length of the Client/Server Write Key.

message_1 may be replayed and cause unnecessary resource consumption for the server. A limited mitigation can be provided by caching (the hash of) the received ephemeral keys, and compare the ephemeral keys of a new request with this cache.

With the current protocol, key confirmation of the Diffie-Hellman shared secret/traffic keys is performed when the keys are successfully used. One extension of the protocol is to add key confirmation by the server, so that a client detecting a failed key confirmation can initiate a new key exchange. This may be accomplished by including a counter-MAC in the second message of the key exchange, where the key used in the MAC is derived from the traffic keys. Since the calculation of the traffic keys include the hash of the key exchange messages, the counter-MAC must be excluded from the exchange_hash.

6. Privacy Considerations

TBD

7. IANA Considerations

8. Acknowledgments

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", Internet-Draft draft-ietf-cose-msg-10, February 2016.
[I-D.ietf-tls-tls13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Internet-Draft draft-ietf-tls-tls13-11, December 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.

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", Internet-Draft draft-hartke-core-e2e-security-reqs-00, March 2016.
[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", Internet-Draft draft-ietf-ace-oauth-authz-01, 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", Internet-Draft draft-ietf-oauth-pop-key-distribution-02, October 2015.
[I-D.selander-ace-object-security] Selander, G., Mattsson, J., Palombini, F. and L. Seitz, "Object Security of CoAP (OSCOAP)", Internet-Draft draft-selander-ace-object-security-03, October 2015.
[I-D.wahlstroem-ace-cbor-web-token] Wahlstroem, E., Jones, M. and H. Tschofenig, "CBOR Web Token (CWT)", Internet-Draft draft-wahlstroem-ace-cbor-web-token-00, 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.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, May 2010.
[RFC7228] Bormann, C., Ersue, M. and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, DOI 10.17487/RFC7228, May 2014.
[RFC7252] Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014.
[RFC7519] Jones, M., Bradley, J. and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015.

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:

  • The request method is POST
  • Content-Format is “application/cose+cbor”
  • The Uri-Path is “edhoc”
  • 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:

  • The response Code is 2.04 (Changed)
  • 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 static 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.

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.

Since the protocol in Section 4 requires static ECDH keys from the same curve, information about the curve to use must be available to the client before making the request to the AS. There are different candidate sources for this information, for example: the server, a resource directory or the AS itself. As an example of the latter, the AS could, for example, reject a token request for a server with a public key in the wrong curve and provide information about the right curve in the response. The client could then generate a new static ECDH key pair in the right curve, include the public key in a new request to the AS, for inclusion in the access token delivered to the server.

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:

  • Secret = traffic_secret_0
  • phase = “application data key expansion”
  • purpose = “client write key” / “server write key” / “client write IV” / “server write IV”
  • handshake_context = message_1 + message_2, the concatenation of the exchanged messages
  • key_length for key and IV is algorithm specific.

The first three bullets are identical to TLS 1.3.

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