ACE P. van der Stok
Internet-Draft Consultant
Intended status: Standards Track P. Kampanakis
Expires: September 1, 2018 Cisco Systems
S. Kumar
Philips Lighting Research
M. Richardson
M. Furuhed
Nexus Group
S. Raza
February 28, 2018

EST over secure CoAP (EST-coaps)


Enrollment over Secure Transport (EST) [RFC7030] is used as a certificate management protocol over HTTPS.

Low-resource devices often use the lightweight Constrained Application Protocol (CoAP) [RFC7252] for message exchanges. This document defines how to transport EST payloads over secure CoAP (EST-coaps). This allows low-resource constrained devices to re-use existing EST functionality. Example low-resource use cases for EST are: secure bootstrapping and certificate enrollment.

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

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 September 1, 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 ( 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.

Table of Contents

1. Introduction

Enrollment over Secure Transport (EST) [RFC7030] is used for authenticated/authorized endpoint certificate enrollment (and optionally key provisioning) through a Certificate Authority (CA) or Registration Authority (RA). This functionality is also needed for low resource devices.

"Classical" EST uses HTTPS and this specification defines a new transport for EST using CoAP. It also profiles the use of EST to a smaller subset.

IPv6 over Low-power Wireless Personal Area Networks (6LoWPANs) [RFC4944] on IEEE 802.15.4 [ieee802.15.4] wireless networks are becoming common in many industry application domains such as lighting controls. Although IEEE 802.15.4 defines how security can be enabled between nodes within a single mesh network, it does not specify the provisioning and management of the keys. Therefore, securing a 6LoWPAN network with devices from multiple manufacturers with different provisioning techniques is often tedious and time consuming. An example use case is the application of Bootstrapping of Remote Secure Infrastructures (BRSKI) [I-D.ietf-anima-bootstrapping-keyinfra]. The low resource aspects are detailed for 6tisch in [I-D.ietf-6tisch-minimal-security] and [I-D.ietf-6tisch-dtsecurity-secure-join].

Constrained networks use DTLS [RFC6347], CoAP [RFC7252], and UDP instead of TLS [RFC5246], HTTP [RFC7230] and TCP. EST-coaps replaces the invocations of TLS and HTTP by DTLS and CoAP invocations thus enabling EST for CoAP-based low-resource devices.

Because the relatively large EST messages cannot be readily transported over constrained (6LoWPAN, LLN) wireless networks, this document specifies the use of CoAP Block-Wise Transfer ("Block") [RFC7959] to fragment EST messages at the application layer.

1.1. EST operational differences

Only the differences to EST with respect to operational scenarios are described in this section. EST-coaps server differs from EST server as follows:

1.2. 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].

Many of the concepts in this document are taken over from [RFC7030]. Consequently, much text is directly traceable to [RFC7030]. The same document structure is followed to point out the differences and commonalities between EST and EST-coaps.

2. Conformance to RFC7925 profiles

This section shows how EST-coaps fits into the profiles of low-resource devices as described in [RFC7925].

EST-coaps can transport certificates and private keys. Private keys can be transported as response to a request to a server-side key generation as described in section 4.4 of [RFC7030].

The mandatory cipher suite for DTLS is TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 defined in [RFC7251] which is the mandatory-to-implement cipher suite in CoAP. Additionally, the curve secp256r1 MUST be supported [RFC4492]; this curve is equivalent to the NIST P-256 curve. The hash algorithm is SHA-256. DTLS implementations MUST use the Supported Elliptic Curves and Supported Point Formats Extensions [RFC4492]; the uncompressed point format MUST be supported; [RFC6090] can be used as an implementation method.

The EST-coaps client MUST be configured with an explicit TA database or at least an implicit TA database from its manufacturer. The authentication of the EST-coaps server by the EST-coaps client is based on Certificate authentication in the DTLS handshake.

The authentication of the EST-coaps client is based on client certificate in the DTLS handshake. This can either be

3. Protocol Design and Layering

EST-coaps uses CoAP to transfer EST messages, aided by Block-Wise Transfer [RFC7959] to transport CoAP messages in blocks thus avoiding (excessive) 6LoWPAN fragmentation of UDP datagrams. The use of "Block" for the transfer of larger EST messages is specified in Section 3.4. The Figure 1 below shows the layered EST-coaps architecture.

|    EST request/response messages               |
|    CoAP for message transfer and signaling     |
|    DTLS for transport security                 |
|    UDP for transport                           |

Figure 1: EST-coaps protocol layers

The EST-coaps protocol design follows closely the EST design. The parts supported by EST-coaps are identified by their message types:

3.1. Payload format

The content-format (media type equivalent) of the CoAP message determines which EST message is transported in the CoAP payload. The media types specified in the HTTP Content-Type header (see section 3.2.2 of [RFC7030]) are in EST-coaps specified by the Content-Format Option (12) of CoAP. The combination of URI path-suffix and content-format used for CoAP MUST map to an allowed combination of path-suffix and media type as defined for EST. The required content-formats for these request and response messages are defined in Section 8. The CoAP response codes are defined in Section 3.3.

EST-coaps is designed for use between low-resource devices using CoAP and hence does not need to send base64-encoded data. Simple binary is more efficient (30% less payload compared to base64) and well supported by CoAP. Therefore, the content formats specification in Section 8 requires the use of binary for all EST-coaps Content-Formats.

3.2. Message Bindings

This section describes the general EST CoAP message characteristics.

It is RECOMMENDED to use CoAP CON messages. This recommendation does not influence the communication efficiency because all EST-coaps messages expect a response.

The Ver, TKL, Token, and Message ID values of the CoAP header are not influenced by EST.

CoAP options are used to convey Uri-Host, Uri-Path, Uri-Port, Content-Format and more in CoAP. The CoAP Options are used to communicate the HTTP fields specified in the EST REST messages.

EST URLs are HTTPS based (https://), in CoAP these will be assumed to be transformed to coaps (coaps://)

Appendix A includes some practical examples of EST messages translated to CoAP.

3.3. CoAP response codes

Section 5.9 of [RFC7252] specifies the mapping of HTTP response codes to CoAP response codes. Every time the HTTP response code 200 is specified in [RFC7030] in response to a GET (POST) request, in EST-coaps the equivalent CoAP response code 2.05 (2.01) MUST be used. Response code HTTP 202 in EST is mapped to CoAP _.__. In [I-D.hartke-core-pending] it is specified how multiple concurrently open requests may be handled. All other HTTP 2xx response codes are not used by EST. For the following HTTP 4xx error codes that may occur: 400, 401, 403, 404, 405, 406, 412, 413, 415; the equivalent CoAP response code for EST-coaps is 4.xx. For the HTTP 5xx error codes: 500, 501, 502, 503, 504 the equivalent CoAP response code is 5.xx.

3.4. Message fragmentation

DTLS defines fragmentation only for the handshake part and not for secure data exchange (DTLS records). [RFC6347] states that to avoid using IP fragmentation, which involves error-prone datagram reconstitution, invokers of the DTLS record layer SHOULD size DTLS records so that they fit within any Path MTU estimates obtained from the record layer. In addition, invokers residing on a 6LoWPAN over IEEE 802.15.4 network SHOULD attempt to size CoAP messages such that each DTLS record will fit within one or two IEEE 802.15.4 frames.

That is not always possible. Even though ECC certificates are small in size, they can vary greatly based on signature algorithms, key sizes, and OID fields used. For 256-bit curves, common ECDSA cert sizes are 500-1000 bytes which could fluctuate further based on the algorithms, OIDs, SANs and cert fields. For 384-bit curves, ECDSA certs increase in size and can sometimes reach 1.5KB. Additionally, there are times when the EST cacerts response from the server can include multiple certs that amount to large payloads. Section 4.6 of CoAP [RFC7252] describes the possible payload sizes: "if nothing is known about the size of the headers, good upper bounds are 1152 bytes for the message size and 1024 bytes for the payload size". Section 4.6 of [RFC7252] also suggests that IPv4 implementations may want to limit themselves to more conservative IPv4 datagram sizes such as 576 bytes. From [RFC0791] follows that the absolute minimum value of the IP MTU for IPv4 is as low as 68 bytes, which would leave only 40 bytes minus security overhead for a UDP payload. Thus, even with ECC certs, EST-coaps messages can still exceed sizes in MTU of 1280 for IPv6 or 60-80 bytes for 6LoWPAN [RFC4919] as explained in section 2 of [RFC7959]. EST-coaps needs to be able to fragment EST messages into multiple DTLS datagrams. Fine-grained fragmentation of EST messages is essential.

To perform fragmentation in CoAP, [RFC7959] specifies the "Block1" option for fragmentation of the request payload and the "Block2" option for fragmentation of the return payload of a CoAP flow.

The BLOCK draft defines SZX in the Block1 and Block2 option fields. These are used to convey the size of the blocks in the requests or responses.

The CoAP client MAY specify the Block1 size and MAY also specify the Block2 size. The CoAP server MAY specify the Block2 size, but not the Block1 size. As explained in Section 1 of [RFC7959]), blockwise transfers SHOULD be used in Confirmable CoAP messages to avoid the exacerbation of lost blocks.

The Size1 response MAY be parsed by the client as a size indication of the Block2 resource in the server response or by the server as a request for a size estimate by the client. Similarly, Size2 option defined in BLOCK should be parsed by the server as an indication of the size of the resource carried in Block1 options and by the client as a maximum size expected in the 4.13 (Request Entity Too Large) response to a request.

Examples of fragmented messages are shown in Appendix C.

3.5. Deployment limits

Although EST-coaps paves the way for the utilization of EST for constrained devices on constrained networks, some devices will not have enough resources to handle the large payloads that come with EST-coaps. The specification of EST-coaps is intended to ensure that EST works for networks of constrained devices that choose to limit their communications stack to UDP/CoAP. It is up to the network designer to decide which devices execute the EST protocol and which not.

4. Discovery and URI

EST-coaps is targeted to low-resource networks with small packets. Saving header space is important and an additional EST-coaps URI is specified that is shorter than the EST URI.

In the context of CoAP, the presence and location of (path to) the management data are discovered by sending a GET request to "/.well-known/core" including a resource type (RT) parameter with the value "ace.est" [RFC6690]. Upon success, the return payload will contain the root resource of the EST resources. It is up to the implementation to choose its root resource; throughout this document the example root resource /est is used. The example below shows the discovery of the presence and location of management data.

  REQ: GET /.well-known/core?rt=ace.est

  RES: 2.05 Content 
</est>; rt="ace.est"

The additional EST-coaps server URIs differ from the EST URI by replacing the scheme https by coaps and by specifying a shorter resource path names:


The CoAP short URI exists next to the URI defined in [RFC7030].


Figure 5 in section 3.2.2 of [RFC7030] enumerates the operations and corresponding paths which are supported by EST. Table 1 provides the mapping from the EST URI path to the shorter EST-coaps URI path.

EST EST-coaps
/cacerts /crts
/simpleenroll /sen
/simplereenroll /sren
/csrattrs /att
/serverkeygen /skg

When discovering the root path for the EST resources, the server MAY return the full resource paths and the used content types. This is useful when multiple content types are specified for EST-coaps server. For example, the following more complete response is possible.

  REQ: GET /.well-known/core?rt=ace.est

  RES: 2.05 Content 
</est>; rt="ace.est"
</est/crts>; rt="ace.est";ct=TBD1
</est/sen>; rt="ace.est";ct=TBD1 TBD4
</est/sren>; rt="ace.est";ct=TBD1 TBD4
</est/att>; rt="ace.est";ct=TBD4
</est/skg>; rt="ace.est";ct=TBD1 TBD4 TBD2

The return of the content-types allows the client to choose the most appropriate one from multiple content types.

5. DTLS Transport Protocol

EST-coaps depends on a secure transport mechanism over UDP that can secure (confidentiality, authenticity) the CoAP messages exchanged.

DTLS is one such secure protocol. When "TLS" is referred to in the context of EST, it is understood that in EST-coaps, security is provided using DTLS instead. No other changes are necessary (all provisional modes etc. are the same as for TLS).

CoAP was designed to avoid fragmentation. DTLS is used to secure CoAP messages. However, fragmentation is still possible at the DTLS layer during the DTLS handshake when using ECC ciphersuites. If fragmentation is necessary, "DTLS provides a mechanism for fragmenting a handshake message over a number of records, each of which can be transmitted separately, thus avoiding IP fragmentation" [RFC6347].

CoAP and DTLS can provide proof of identity for EST-coaps clients and server with simple PKI messages conformant to section 3.1 of [RFC5272]. EST-coaps supports the certificate types and Trust Anchors (TA) that are specified for EST in section 3 of [RFC7030].

PRF(master_secret, finished_label, Hash(handshake_messages)) 

Channel-binding information for linking proof-of-identity with connection-based proof-of-possession is optional for EST-coaps. When proof-of-possession is desired, a set of actions are required regarding the use of tls-unique, described in section 3.5 in [RFC7030]. The tls-unique information translates to the contents of the first "Finished" message in the TLS handshake between server and client [RFC5929]. The client is then supposed to add this "Finished" message as a ChallengePassword in the attributes section of the PKCS#10 Request Info to prove that the client is indeed in control of the private key at the time of the TLS session when performing a /simpleenroll, for example. In the case of EST-coaps, the same operations can be performed during the DTLS handshake. In the event of handshake message fragmentation, the Hash of the handshake messages used in the MAC calculation of the Finished message [RFC6347].

In a constrained CoAP environment, endpoints can't afford to establish a DTLS connection for every EST transaction. Authenticating and negotiating DTLS keys requires resources on low-end endpoints and consumes valuable bandwidth. The DTLS connection SHOULD remain open for persistent EST connections. For example, an EST cacerts request that is followed by a simpleenroll request can use the same authenticated DTLS connection. Given that after a successful enrollment, it is more likely that a new EST transaction will take place after a significant amount of time, the DTLS connections SHOULD only be kept alive for EST messages that are relatively close to each other.

Support for Observe CoAP options [RFC7641] is out-of-scope for this document. Observe options could be used by the server to notify clients about a change in the cacerts or csr attributes (resources) and might be an area of future work.

6. Proxying

In real-world deployments, the EST server will not always reside within the CoAP boundary. The EST-server can exist outside the constrained network in a non-constrained network that supports TLS/HTTP. In such environments EST-coaps is used by the client within the CoAP boundary and TLS is used to transport the EST messages outside the CoAP boundary. A proxy entity at the edge is required to operate between the CoAP environment and the external HTTP network. The ESTcoaps-to-HTTPS proxy SHOULD terminate EST-coaps downstream and initiate EST connections over TLS upstream.

One possible use-case, shown in one figure below, is expected to be deployed in practice:

                                     Constrained Network
                  .---------.    .----------------------------.
                  |   RA    |    |.--------------------------.|
                  '---------'    ||                          ||
                        |        ||                          ||
.------.  HTTP   .-----------------.  CoAPS   .-----------.  ||
| EST  |<------->|ESTcoaps-to-HTTPS|<-------->| EST Client|  ||
|Server|over TLS |      Proxy      |          '-----------'  ||
'------'         '-----------------'                         ||
                                 ||                          ||

ESTcoaps-to-HTTPS proxy at the CoAP boundary.

Table 1 contains the URI mapping between the EST-coaps and EST the proxy SHOULD adhere to. Section 7 of [RFC8075] and Section 3.3 define the mapping between EST-coaps and HTTP response codes, that determines how a proxy translates CoAP response codes from/to HTTP status codes. The mapping from Content-Type to media type is defined in Section 8. The conversion from binary to BSD64 needs to be done in the proxy. Conversion is possible because a TLS link exists between EST-coaps-to-HTTP proxy and EST server and a corresponding DTLS linked exists between EST-coaps-to-HTTP proxy and EST client.

Due to fragmentation of large messages into blocks, an EST-coaps-to-HTTP proxy SHOULD reassemble the BLOCKs before translating the binary content to BSD64, and consecutively relay the message upstream into the HTTP environment.

For the discovery of the EST server by the EST client in the coap environment, the EST-coaps-to-HTTP proxy MUST announce itself according to the rules of Section 4. The available functions of the proxies MUST be announced with as many resource paths. The discovery of EST server in the http environment follow the rules specified in [RFC7030].

[ EDNOTE: PoP will be addressed here. ]

A proxy SHOULD authenticate the client downstream and it should be authenticated by the EST server or CA upstream. The Registration Authority (RA) is necessary to (re-)create the secure connection from DTLS to TLS and vice versa. A trust relationship needs to be pre-established between the proxy and the EST servers to be able to proxy these connections on behalf of various clients.

[EDNOTE: To add more details about trust relations in this section. ]

7. Parameters

[EDNOTE: This section to be populated. It will address transmission parameters described in sections 4.7 and 4.8 of the CoAP draft. EST does not impose any unique parameters that affect the CoAP parameters in Table 2 and 3 in the CoAP draft but the ones in CoAP could be affecting EST. For example, the processing delay of CAs could be less then 2s, but in this case they should send a CoAP ACK every 2s while processing.]

8. IANA Considerations

8.1. Content-Format registry

Additions to the sub-registry "CoAP Content-Formats", within the "CoRE Parameters" registry are needed for the below media types. These can be registered either in the Expert Review range (0-255) or IETF Review range (256-9999).

8.2. Resource Type registry

Additions to the sub-registry "CoAP Resource Type", within the "CoRE Parameters" registry are needed for a new resource type.

9. Security Considerations

9.1. proxy considerations

The proxy proposed in Section 6 must be deployed with great care, and only when the recommended connections are impossible.

[EDNOTE: To add more details about trust relations through proxies in this section. ]

9.2. EST server considerations

The security considerations of section 6 of [RFC7030] are only partially valid for the purposes of this document. As HTTP Basic Authentication is not supported, the considerations expressed for using passwords do not apply.

Given that the client has only limited resources and may not be able to generate sufficiently random keys to encrypt its identity, it is possible that the client uses server generated private/public keys to encrypt its certificate. The transport of these keys is inherently risky. A full probability analysis MUST be done to establish whether server side key generation enhances or decreases the probability of identity stealing.

When a client uses the Implicit TA database for certificate validation, the client cannot verify that the implicit data base can act as an RA. It is RECOMMENDED that such clients include "Linking Identity and POP Information" Section 5 in requests (to prevent such requests from being forwarded to a real EST server by a man in the middle). It is RECOMMENDED that the Implicit Trust Anchor database used for EST server authentication be carefully managed to reduce the chance of a third-party CA with poor certification practices from being trusted. Disabling the Implicit Trust Anchor database after successfully receiving the Distribution of CA certificates response (Section 4.1.3 of [RFC7030]) limits any vulnerability to the first DTLS exchange.

In accordance with [RFC7030], TLS cipher suites that include "_EXPORT_" and "_DES_" in their names MUST NOT be used. More information about recommendations of TLS and DTLS are included in [RFC7525].

As described in CMC, Section 6.7 of [RFC5272], "For keys that can be used as signature keys, signing the certification request with the private key serves as a POP on that key pair". The inclusion of tls-unique in the certification request links the proof-of-possession to the TLS proof-of-identity. This implies but does not prove that the authenticated client currently has access to the private key.

Regarding the CSR attributes that the CA may list for inclusion in an enrollment request, an adversary could exclude attributes that a server may want, include attributes that a server may not want, and render meaningless other attributes that a server may want. The CA is expected to be able to enforce policies to recover from improper CSR requests.

Interpreters of ASN.1 structures should be aware of the use of invalid ASN.1 length fields and should take appropriate measures to guard against buffer overflows, stack overruns in particular, and malicious content in general.

10. Acknowledgements

The authors are very grateful to Klaus Hartke for his detailed explanations on the use of Block with DTLS. The authors would like to thank Esko Dijk and Michael Verschoor for the valuable discussions that helped in shaping the solution. They would also like to thank Peter Panburana from Cisco for his feedback on technical details of the solution. Constructive comments were received from Eliot Lear, Jim Schaad, Hannes Tschofenig, and Julien Vermillard.

11. Change Log


12. References

12.1. 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.
[RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S​/​MIME) Version 3.2 Message Specification", RFC 5751, DOI 10.17487/RFC5751, January 2010.
[RFC5967] Turner, S., "The application/pkcs10 Media Type", RFC 5967, DOI 10.17487/RFC5967, August 2010.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, August 2012.
[RFC7030] Pritikin, M., Yee, P. and D. Harkins, "Enrollment over Secure Transport", RFC 7030, DOI 10.17487/RFC7030, October 2013.
[RFC7252] Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014.
[RFC7959] Bormann, C. and Z. Shelby, "Block-Wise Transfers in the Constrained Application Protocol (CoAP)", RFC 7959, DOI 10.17487/RFC7959, August 2016.
[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.

12.2. Informative References

[I-D.hartke-core-pending] Stok, P. and K. Hartke, ""Pending" Responses for the Constrained Application Protocol (CoAP)", Internet-Draft draft-hartke-core-pending-02, February 2018.
[I-D.ietf-6tisch-dtsecurity-secure-join] Richardson, M., "6tisch Secure Join protocol", Internet-Draft draft-ietf-6tisch-dtsecurity-secure-join-01, February 2017.
[I-D.ietf-6tisch-minimal-security] Vucinic, M., Simon, J., Pister, K. and M. Richardson, "Minimal Security Framework for 6TiSCH", Internet-Draft draft-ietf-6tisch-minimal-security-04, October 2017.
[I-D.ietf-anima-bootstrapping-keyinfra] Pritikin, M., Richardson, M., Behringer, M., Bjarnason, S. and K. Watsen, "Bootstrapping Remote Secure Key Infrastructures (BRSKI)", Internet-Draft draft-ietf-anima-bootstrapping-keyinfra-11, February 2018.
[ieee802.15.4] Institute of Electrical and Electronics Engineers, "IEEE Standard 802.15.4-2006", 2006.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, DOI 10.17487/RFC0791, September 1981.
[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.
[RFC4919] Kushalnagar, N., Montenegro, G. and C. Schumacher, "IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and Goals", RFC 4919, DOI 10.17487/RFC4919, August 2007.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J. and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.
[RFC5929] Altman, J., Williams, N. and L. Zhu, "Channel Bindings for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010.
[RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958, DOI 10.17487/RFC5958, August 2010.
[RFC6090] McGrew, D., Igoe, K. and M. Salter, "Fundamental Elliptic Curve Cryptography Algorithms", RFC 6090, DOI 10.17487/RFC6090, February 2011.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014.
[RFC7251] McGrew, D., Bailey, D., Campagna, M. and R. Dugal, "AES-CCM Elliptic Curve Cryptography (ECC) Cipher Suites for TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014.
[RFC7525] Sheffer, Y., Holz, R. and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015.
[RFC7641] Hartke, K., "Observing Resources in the Constrained Application Protocol (CoAP)", RFC 7641, DOI 10.17487/RFC7641, September 2015.
[RFC7925] Tschofenig, H. and T. Fossati, "Transport Layer Security (TLS) / Datagram Transport Layer Security (DTLS) Profiles for the Internet of Things", RFC 7925, DOI 10.17487/RFC7925, July 2016.

Appendix A. EST messages to EST-coaps

This section takes all examples from Appendix A of [RFC7030], changes the payload from Base64 to binary and replaces the http headers by their CoAP equivalents.

The corresponding CoAP headers are only shown in Appendix A.1. Creating CoAP headers are assumed to be generally known.

[EDNOTE: The payloads of the examples need to be re-generated with appropriate tools and example certificates.]

A.1. cacerts

In EST-coaps, a coaps cacerts IPv4 message can be:

GET coaps://[]/est/crts

The corresponding CoAP header fields are shown below. The use of block and DTLS are worked out in Appendix C.

  Ver = 1
  T = 0 (CON)
  Code = 0x01 (0.01 is GET)
   Option1 (Uri-Host)
     Option Delta = 0x3  (option nr = 3)
     Option Length = 0x9
     Option Value =
   Option2 (Uri-Port)
     Option Delta = 0x4  (option nr = 4+3=7)
     Option Length = 0x4
     Option Value = 8085
   Option3 (Uri-Path)
     Option Delta = 0x4   (option nr = 7+4= 11)
     Option Length = 0x9
     Option Value = /est/crts
  Payload = [Empty]

A 2.05 Content response with a cert in EST-coaps will then be:

2.05 Content (Content-Format: application/pkcs7-mime)

with CoAP fields

  Ver = 1
  T = 2 (ACK)
  Code = 0x45 (2.05 Content)
    Option1 (Content-Format)
      Option Delta = 0xC  (option nr = 12)
      Option Length = 0x2
      Option Value = TBD1 (defined in this document) 
  Payload =

A.2. csrattrs

In the following valid /csrattrs exchange, the EST-coaps client authenticates itself with a certificate issued by the connected CA.

The initial DTLS handshake is identical to the enrollment example. The IPv6 CoAP GET request looks like:

GET coaps://[2001:db8::2:1]:61616/est/att

A 2.05 Content response contains attributes which are relevant for the authenticated client. In this example, the EST-coaps server two attributes that the client can ignore when they are unknown to him.:

A.3. enroll / reenroll

[EDNOTE: We might need a new Option for the Retry-After response message. We might need a new Option for the WWW-Authenticate response.]

During the Enroll/Reenroll exchange, the EST-coaps client uses a CSR (PKCS#10) request in the POST request payload.

After verification of the certificate by the server, a 2.05 Content response with the issued certificate will be returned.

POST [2001:db8::2:1]:61616/est/sen
(Content-Format: application/pkcs10)

2.05 Content (Content-Format: application/pkcs7-mime)

[EDNOTE: If POP is used, make sure tls-unique in the CSR is a valid HMAC output. ]

A.4. serverkeygen

During this valid /serverkeygen exchange, the EST-coaps client authenticates itself using the certificate provided by the connected CA.

[EDNOTE: the client incudes a CSR with a public key that the server should ignore, so we need a content-format here. ]

[EDNote: If POP is used, make sure tls-unique in the CSR is a valid HMAC output. ]

The initial DTLS handshake is identical to the enrollment example. The CoAP GET request looks like:

POST coaps://[]/est/skg

2.05 Content (Content-Format: application/pkcs8)

Without the DecryptKeyIdentifier attribute, the response has no additional encryption beyond DTLS. [EDNOTE: Add comment about deriving symmetric keys by using the TLS KEM draft. ]

The response contains first a preamble that can be ignored. The EST-coaps server can use the preamble to include additional explanations, like ownership or support information

Appendix B. Encoding for server side key generation

Sever side key generation for CoAP can be implemented efficiently using multipart encoding

[EDNOTE: text to be written.]

Appendix C. EST-coaps Block message examples

This section provides a detailed example of the messages using DTLS and BLOCK option Block2. The minimum PMTU is 1280 bytes, which is the example value assumed for the DTLS datagram size. The example block length is taken as 64 which gives an SZX value of 2.

The following is an example of a valid /cacerts exchange over DTLS. The content length of the cacerts response in appendix A.1 of [RFC7030] is 4246 bytes using base64. This leads to a length of 2509 bytes in binary. The CoAP message adds around 10 bytes, the DTLS record 29 bytes. To avoid IP fragmentation, the CoAP block option is used and an MTU of 127 is assumed to stay within one IEEE 802.15.4 packet. To stay below the MTU of 127, the payload is split in 39 packets with a payload of 64 bytes each, followed by a packet of 13 bytes. The client sends an IPv6 packet containing the UDP datagram with the DTLS record that encapsulates the CoAP Request 40 times. The server returns an IPv6 packet containing the UDP datagram with the DTLS record that encapsulates the CoAP response. The CoAP request-response exchange with block option is shown below. Block option is shown in a decomposed way indicating the kind of Block option (2 in this case because used in the response) followed by a colon, and then the block number (NUM), the more bit (M = 0 means last block), and block size exponent (2**(SZX+4)) separated by slashes. The Length 64 is used with SZX= 2 to avoid IP fragmentation. The CoAP Request is sent with confirmable (CON) option and the content format of the Response is /application/cacerts.

GET []/est/crts     -->
              <--   (2:0/1/39) 2.05 Content
    GET URI (2:1/1/39)                           -->
              <--   (2:1/1/39) 2.05 Content
     GET URI (2:65/1/39)                         -->
              <--   (2:65/0/39) 2.05 Content

For further detailing the CoAP headers of the first two blocks are written out.

The header of the first GET looks like:

  Ver = 1
  T = 0 (CON)
  Code = 0x01 (0.1 GET) 
   Option1 (Uri-Host)
     Option Delta = 0x3  (option nr = 3)
     Option Length = 0x9
     Option Value =
   Option2 (Uri-Port)
     Option Delta = 0x4   (option nr = 3+4=7)
     Option Length = 0x4
     Option Value = 8085
   Option3 (Uri-Path)
     Option Delta = 0x4    (option nr = 7+4=11)
     Option Length = 0x9
     Option Value = /est/crts
  Payload = [Empty]

The header of the first response looks like:

  Ver = 1
  T = 2 (ACK)
  Code = 0x45 (2.05 Content.) 
    Option1 (Content-Format)
      Option Delta = 0xC  (option 12)
      Option Length = 0x2
      Option Value = TBD1                  
    Option2 (Block2)
      Option Delta = 0xB  (option 23 = 12 + 11)
      Option Length = 0x1
      Option Value = 0x0A (block number = 0, M=1, SZX=2)
  Payload = 

The second Block2:

  Ver = 1
  T = 2 (means ACK)
  Code = 0x45 (2.05 Content.)
    Option1 (Content-Format)
      Option Delta = 0xC   (option 12)
      Option Length = 0x2 
      Option Value = TBD1
    Option2 (Block2)
      Option Delta = 0xB  (option 23 = 12 + 11)
      Option Length = 0x1
      Option Value = 0x1A (block number = 1, M=1, SZX=2)
  Payload = 

The 40th and final Block2:

  Ver = 1
  T = 2 (means ACK)
  Code = 0x21
    Option1 (Content-Format)
      Option Delta = 0xC   (option 12)
      Option Length = 0x2
      Option Value = TBD1 
    Option2 (Block2)
      Option Delta = 0xB  (option 23 = 12 + 11)
      Option Length = 0x2
      Option Value = 0x272 (block number = 39, M=0, SZX=2)
  Payload = 73a30d0c006343116f58403100

Authors' Addresses

Peter van der Stok Consultant EMail:
Panos Kampanakis Cisco Systems EMail:
Sandeep S. Kumar Philips Lighting Research High Tech Campus 7 Eindhoven, 5656 AE NL EMail:
Michael C. Richardson Sandelman Software Works EMail: URI:
Martin Furuhed Nexus Group EMail:
Shahid Raza RISE SICS Isafjordsgatan 22 Kista, Stockholm 16440 SE EMail: