ACE P. van der Stok
Internet-Draft Consultant
Intended status: Standards Track P. Kampanakis
Expires: August 10, 2019 Cisco Systems
M. Richardson
SSW
S. Raza
RISE SICS
February 6, 2019
EST over secure CoAP (EST-coaps)
draft-ietf-ace-coap-est-08
Abstract
Enrollment over Secure Transport (EST) is used as a certificate
provisioning protocol over HTTPS. Low-resource devices often use the
lightweight Constrained Application Protocol (CoAP) for message
exchanges. This document defines how to transport EST payloads over
secure CoAP (EST-coaps), which allows constrained devices to use
existing EST functionality for provisioning certificates.
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 https://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 August 10, 2019.
Copyright Notice
Copyright (c) 2019 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
(https://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Conformance to RFC7925 profiles . . . . . . . . . . . . . . . 6
5. Protocol Design . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Discovery and URIs . . . . . . . . . . . . . . . . . . . 8
5.2. Mandatory/optional EST Functions . . . . . . . . . . . . 10
5.3. Payload formats . . . . . . . . . . . . . . . . . . . . . 10
5.4. Message Bindings . . . . . . . . . . . . . . . . . . . . 12
5.5. CoAP response codes . . . . . . . . . . . . . . . . . . . 13
5.6. Message fragmentation . . . . . . . . . . . . . . . . . . 13
5.7. Delayed Responses . . . . . . . . . . . . . . . . . . . . 14
5.8. Server-side Key Generation . . . . . . . . . . . . . . . 16
6. DTLS Transport Protocol . . . . . . . . . . . . . . . . . . . 18
7. HTTPS-CoAPS Registrar . . . . . . . . . . . . . . . . . . . . 19
8. Parameters . . . . . . . . . . . . . . . . . . . . . . . . . 21
9. Deployment limitations . . . . . . . . . . . . . . . . . . . 21
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10.1. Content-Format Registry . . . . . . . . . . . . . . . . 22
10.2. Resource Type registry . . . . . . . . . . . . . . . . . 22
11. Security Considerations . . . . . . . . . . . . . . . . . . . 23
11.1. EST server considerations . . . . . . . . . . . . . . . 23
11.2. HTTPS-CoAPS Registrar considerations . . . . . . . . . . 25
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 25
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
14.1. Normative References . . . . . . . . . . . . . . . . . . 26
14.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. EST messages to EST-coaps . . . . . . . . . . . . . 30
A.1. cacerts . . . . . . . . . . . . . . . . . . . . . . . . . 31
A.2. enroll / reenroll . . . . . . . . . . . . . . . . . . . . 33
A.3. serverkeygen . . . . . . . . . . . . . . . . . . . . . . 35
A.4. csrattrs . . . . . . . . . . . . . . . . . . . . . . . . 37
Appendix B. EST-coaps Block message examples . . . . . . . . . . 38
B.1. cacerts . . . . . . . . . . . . . . . . . . . . . . . . . 38
B.2. enroll / reenroll . . . . . . . . . . . . . . . . . . . . 42
Appendix C. Message content breakdown . . . . . . . . . . . . . 43
C.1. cacerts . . . . . . . . . . . . . . . . . . . . . . . . . 43
C.2. enroll / reenroll . . . . . . . . . . . . . . . . . . . . 45
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C.3. serverkeygen . . . . . . . . . . . . . . . . . . . . . . 46
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48
1. Change Log
EDNOTE: Remove this section before publication
-08
added application/pkix-cert Content-Format TBD287.
Stated that well-known/est is compulsory
Use of response codes clarified.
removed bugs: Max-Age and Content-Format Options in Request
Accept Option explained for est/skg and added in enroll example
Persistenc of DTLS connection clarified.
Minor text fixes.
-07:
redone examples from scratch with openssl
Updated authors.
Added CoAP RST as a MAY for an equivalent to an HTTP 204 message.
Added serialization example of the /skg CBOR response.
Added text regarding expired IDevIDs and persistent DTLS
connection that will start using the Explicit TA Database in the
new DTLS connection.
Nits and fixes
Removed CBOR envelop for binary data
Replaced TBD8 with 62.
Added RFC8174 reference and text.
Clarified MTI for server-side key generation and Content-Formats.
Defined the /skg MTI (PKCS#8) and the cases where CMS encryption
will be used.
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Moved Fragmentation section up because it was referenced in
sections above it.
-06:
clarified discovery section, by specifying that no discovery may
be needed for /.well-known/est URI.
added resource type values for IANA
added list of compulsory to implement and optional functions.
Fixed issues pointed out by the idnits tool.
Updated CoAP response codes section with more mappings between EST
HTTP codes and EST-coaps CoAP codes.
Minor updates to the MTI EST Functions section.
Moved Change Log section higher.
-05:
repaired again
TBD8 = 62 removed from C-F registration, to be done in CT draft.
-04:
Updated Delayed response section to reflect short and long delay
options.
-03:
Removed observe and simplified long waits
Repaired Content-Format specification
-02:
Added parameter discussion in section 8
Concluded Content-Format specification using multipart-ct draft
examples updated
-01:
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Editorials done.
Redefinition of proxy to Registrar in Section 7. Explained better
the role of https-coaps Registrar, instead of "proxy"
Provide "observe" Option examples
extended block message example.
inserted new server key generation text in Section 5.8 and
motivated server key generation.
Broke down details for DTLS 1.3
New Media-Type uses CBOR array for multiple Content-Format
payloads
provided new Content-Format tables
new media format for IANA
-00
copied from vanderstok-ace-coap-04
2. Introduction
"Classical" 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). EST transports messages over HTTPS.
This document defines a new transport for EST based on the
Constrained Application Protocol (CoAP) since some Internet of Things
(IoT) devices use CoAP instead of HTTP. Therefore, this
specification utilizes DTLS [RFC6347], CoAP [RFC7252] and UDP instead
of TLS [RFC8446], HTTP [RFC7230] and TCP.
EST responses can be relatively large and for this reason this
specification also uses CoAP Block-Wise Transfer [RFC7959] to offer a
fragmentation mechanism of EST messages at the CoAP layer.
This document also profiles the use of EST to only support
certificate-based client authentication. HTTP Basic or Digest
authentication (as described in Section 3.2.3 of [RFC7030] are not
supported.
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3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Many of the concepts in this document are taken from [RFC7030].
Consequently, much text is directly traceable to [RFC7030].
4. Conformance to RFC7925 profiles
This section describes how EST-coaps fits into the profiles of low-
resource devices described in [RFC7925]. EST-coaps can transport
certificates and private keys. Certificates are responses to
(re-)enrollment requests or requests for a trusted certificate list.
Private keys can be transported as responses to a server-side key
generation request as described in section 4.4 of [RFC7030] and
discussed in Section 5.8 of this document.
As per Sections 3.3 and 4.4 of [RFC7925], the mandatory cipher suite
for DTLS in EST-coaps is TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8
[RFC7251]. Curve secp256r1 MUST be supported [RFC8422]; this curve
is equivalent to the NIST P-256 curve. Crypto agility is important,
and the recommendations in [RFC7925] section 4.4 and any updates to
RFC7925 concerning Curve25519 and other CFRG curves also apply.
DTLS1.2 implementations MUST use the Supported Elliptic Curves and
Supported Point Formats Extensions [RFC8422]. Uncompressed point
format MUST also be supported. [RFC6090] is a summary of the ECC
algorithms. DTLS 1.3 [I-D.ietf-tls-dtls13] implementations differ
from DTLS 1.2 because they do not support point format negotiation in
favor of a single point format for each curve. Thus, support for
DTLS 1.3 does not mandate point formation extensions and negotiation.
The authentication of the EST-coaps server by the EST-coaps client is
based on certificate authentication in the DTLS handshake. The EST-
coaps client MUST be configured with at least an Implicit TA database
from its manufacturer which will enable the authentication of the
server the first time before updating its trust anchor (Explicit TA)
[RFC7030].
The authentication of the EST-coaps client MUST be with a client
certificate in the DTLS handshake. This can either be
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o a previously issued client certificate (e.g., an existing
certificate issued by the EST CA); this could be a common case for
simple re-enrollment of clients.
o a previously installed certificate (e.g., manufacturer IDevID
[ieee802.1ar] or a certificate issued by some other party); the
server is expected to trust that certificate. IDevID's are
expected to have a very long life, as long as the device, but
under some conditions could expire. In that case, the server MAY
want to authenticate a client certificate against its trust store
although the certificate is expired (Section 11).
5. Protocol Design
EST-coaps uses CoAP to transfer EST messages, aided by Block-Wise
Transfer [RFC7959] to avoid (excessive) fragmentation of UDP
datagrams. The use of Blocks for the transfer of larger EST messages
is specified in Section 5.6. 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
supported message types in EST-coaps are:
o CA certificate retrieval, needed to receive the complete set of CA
certificates.
o Simple enroll and reenroll, for a CA to sign public client-
identity key.
o Certificate Signing Request (CSR) Attributes messages that informs
the client of the fields to include in generated CSR.
o Server-side key generation messages to provide a private client-
identity key when the client choses so.
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5.1. Discovery and URIs
EST-coaps is targeted for low-resource networks with small packets.
Saving header space is important and short EST-coaps URIs are
specified in this document. These URIs are shorter than the ones in
[RFC7030]. Two example EST-coaps resource path names are:
coaps://est-coaps.example.ietf.org:<port>/.well-known/est/<short-est>
coaps://est-coaps.example.ietf.org:<port>/.well-known/est/
ArbitraryLabel/<short-est>
The short-est strings are defined in Table 1. The ArbitraryLabel
path-segment, if used, SHOULD be of the shortest length possible
(Sections 3.1 and 3.2.2 of [RFC7030]. Arbitrary Labels are usually
defined and used by EST CAs in order to route client requests to the
appropriate certificate profile.
The EST-coaps server URIs, obtained through discovery of the EST-
coaps root resource(s) as shown below, are of the form:
coaps://est-coaps.example.ietf.org:<port>/<root-resource>/<short-est>
coaps://est-coaps.example.ietf.org:<port>/<root-resource>/
ArbitraryLabel/<short-est>
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 |
+------------------+-----------+
Table 1: Table 1: Short EST-coaps URI path
Clients and servers MUST support the short resource URIs. The
corresponding longer URIs from [RFC7030] MAY be supported.
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. The example below shows the
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discovery of the presence and location of EST-coaps resources.
Linefeeds are included only for readability.
REQ: GET /.well-known/core?rt=ace.est*
RES: 2.05 Content
</est>; rt="ace.est",
</est/crts>;rt="ace.est.crts";ct="281 TBD287",
</est/sen>;rt="ace.est.sen";ct="281 TBD287",
</est/sren>;rt="ace.est.sren";ct="281 TBD287",
</est/att>;rt="ace.est.att";ct=285,
</est/skg>;rt="ace.est.skg";ct="62 280 284 281 TBD287"
The first line of the discovery response above MUST be included. The
five consecutive lines after the first MAY be included. The return
of the content types allows the client to choose the most appropriate
one.
Port numbers, not returned in the example, are assumed to be the
default numbers 5683 and 5684 for CoAP and CoAPS respectively
(Sections 12.6 and 12.7 of [RFC7252]). Discoverable port numbers MAY
be returned in the <href> of the payload
[I-D.ietf-core-resource-directory]. An example response payload for
non-default CoAPS server port 61617 follows below. Linefeeds were
included only for readability.
REQ: GET /.well-known/core?rt=ace.est*
RES: 2.05 Content
<coap://[2001:db8:3::123]:61617/est>;rt="ace.est";
anchor="coap://[2001:db8:3::123]:61617",
<coap://[2001:db8:3::123]:61617/est/crts>;rt="ace.est.crts";
ct="281 TBD287";anchor="coap://[2001:db8:3::123]:61617",
<coap://[2001:db8:3::123]:61617/est/sen>;rt="ace.est.sen";
ct="281 TBD287";anchor="coap://[2001:db8:3::123]:61617",
<coap://[2001:db8:3::123]:61617/est/sren>;rt="ace.est.sren";
ct="281 TBD287";anchor="coap://[2001:db8:3::123]:61617",
<coap://[2001:db8:3::123]:61617/est/att>;rt="ace.est.att";
ct="285";anchor="coap://[2001:db8:3::123]:61617",
<coap://[2001:db8:3::123]:61617/est/skg>;rt="ace.est.skg";
ct="62 280 284 281 TBD287";anchor="coap://[2001:db8:3::123]:61617"
The server MUST support the default /.well-known/est server root
resource and port 5684. Resource discovery is necessary when the IP
address of the server is unknown to the client. Resource discovery
SHOULD be employed when non-default URIs (like /est or /est/
ArbitraryLabel) or ports are supported by the server, when the client
is unaware of what EST-coaps resources are available or if the client
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considers sending two Uri-Path Options to convey the resource is
wasteful.
It is up to the implementation to choose its root resource;
throughout this document the example root resource /est is used.
5.2. Mandatory/optional EST Functions
This specification contains a set of required-to-implement functions,
optional functions, and not specified functions. The latter ones are
deemed too expensive for low-resource devices in payload and
calculation times.
Table 2 specifies the mandatory-to-implement or optional
implementation of the est-coaps functions.
+------------------+--------------------------+
| EST Functions | EST-coaps implementation |
+------------------+--------------------------+
| /cacerts | MUST |
| /simpleenroll | MUST |
| /simplereenroll | MUST |
| /fullcmc | Not specified |
| /serverkeygen | OPTIONAL |
| /csrattrs | OPTIONAL |
+------------------+--------------------------+
Table 2: Table 2: List of EST-coaps functions
While [RFC7030] permits a number of these functions to be used
without authentication, this specification requires that the client
MUST be authenticated for all functions.
5.3. Payload formats
The Content-Format (HTTP 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 (section 3.2.2
of [RFC7030]) are in EST-coaps specified by the Content-Format Option
(12) of CoAP. The combination of URI-Path and Content-Format in EST-
coaps MUST map to an allowed combination of URI and Media-Type in
EST. The required Content-Formats for these requests and response
messages are defined in Section 10.1. The CoAP response codes are
defined in Section 5.5.
Content-Format TBD287 can be used in place of 281 to carry a single
certificate instead of a PKCS#7 container in a /crts, /sen, /sren or
/skg response. Content-Format 281 MUST be supported by EST-coaps
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servers. Servers MAY also support Content-Format TBD287. It is up
to the client to support only Content-Format 281, TBD287 or both.
The client is expected to use an COAP Accept Option in the request to
express the preferred response Content-Format. If an Accept Option
is not included in the request, the client is not expressing any
preference and the server SHOULD choose format 281. If the preferred
Content-Format cannot be returned, the server MUST send a 4.06 (Not
Acceptable) response, unless another error code takes precedence for
the response [RFC7252].
Content-Format 286 is used in /sen, /sren and /skg requests and 285
in /att responses.
EST-coaps is designed for low-resource devices and hence does not
need to send Base64-encoded data. Simple binary is more efficient
(30% smaller payload) and well supported by CoAP. Thus, the payload
for a given Media-Type follows the ASN.1 structure of the Media-Type
and is transported in binary format.
*application/multipart-core*
A representation with Content-Format identifier 62 contains a
collection of representations along with their respective Content-
Format. The Content-Format identifies the Media-Type application/
multipart-core specified in [I-D.ietf-core-multipart-ct].
The collection is encoded as a CBOR array [RFC7049] with an even
number of elements. The second, fourth, sixth, etc. element is a
binary string containing a representation. The first, third, fifth,
etc. element is an unsigned integer specifying the Content-Format
identifier of the consecutive representation. For example, a
collection containing two representations in response to a EST-coaps
server-side key generation request, could include a private key in
PKCS#8 [RFC5958] with Content-Format identifier 284 (0x011C) and a
single certificate in a PKCS#7 container with Content-Format
identifier 281 (0x0119). Such a collection would look like
[284,h'0123456789abcdef', 281,h'fedcba9876543210'] in diagnostic CBOR
notation. The serialization of such CBOR content would be
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84 # array(4)
19 011C # unsigned(284)
48 # bytes(8)
0123456789ABCDEF # "\x01#Eg\x89\xAB\xCD\xEF"
19 0119 # unsigned(281)
48 # bytes(8)
FEDCBA9876543210 # "\xFE\xDC\xBA\x98vT2\x10"
Multipart /skg response serialization
When the returned certificate is a single X.509 certificate (not a
PKCS#7 container) the Content-Format identifier is TBD287 (0x011F)
instead of 281. In cases where the private key is encrypted with CMS
(as explained in Section 5.8) the Content-Format identifier is 280
(0x0118) instead of 284. The key and certificate representations are
ASN.1 encoded in binary format. An example is shown in Appendix A.3.
5.4. Message Bindings
The general EST-coaps message characteristics are:
o All EST-coaps messages expect a response from the server, thus the
client MUST send the requests over confirmable CON CoAP messages.
o The Ver, TKL, Token, and Message ID values of the CoAP header are
not affected by EST.
o The CoAP Options used are Uri-Host, Uri-Path, Uri-Port, Content-
Format, Accept and Location-Path. These CoAP Options are used to
communicate the HTTP fields specified in the EST REST messages.
The Uri-Host and Uri-Port Options are optional. They are usually
omitted as the DTLS destination and port are sufficient. Explicit
Uri-Host and Uri-Port Options are typically used when an endpoint
hosts multiple virtual servers and uses the Options to route the
requests accordingly. Alternatively, if a UDP port to a server is
blocked, someone could send the DTLS packets to a known open port
on the server and use the Uri-Port to convey the intended port he
is attempting to reach.
o EST URLs are HTTPS based (https://), in CoAP these are assumed to
be translated to CoAPS (coaps://)
Table 1 provides the mapping from the EST URI path to the EST-coaps
URI path. Appendix A includes some practical examples of EST
messages translated to CoAP.
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5.5. CoAP response codes
Section 5.9 of [RFC7252] and Section 7 of [RFC8075] specify 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 request (/cacerts, /csrattrs), in EST-coaps the equivalent CoAP
response code 2.05 or 2.03 MUST be used. Similarly, 2.01, 2.02 or
2.04 MUST be used in response to EST POST requests (/simpleenroll,
/simplereenroll, /serverkeygen).
Response code HTTP 202 Retry-After that existed in EST has no
equivalent in CoAP. Retry-After is used in EST for delayed server
responses. Section 5.7 specifies how EST-coaps handles delayed
messages.
EST makes use of HTTP 204 and 404 responses when a resource is not
available for the client. The equivalent CoAP codes to use in an
EST-coaps responses are 2.04 and 4.04. Additionally, EST's HTTP 401
error translates to 4.01 in EST-coaps. Other EST HTTP error messages
are 400, 423 and 503. Their equivalent CoAP errors are 4.00, 4.03
and 5.03 respectively. In case a CoAP Option is unrecognized and
critical, the server is expected to return a 4.02 (Bad Option).
Moreover, if the Content-Format requested in the client Accept
Option, is not supported the server MUST return a 4.06 (Not
Acceptable), unless another error code takes precedence for the
response.
5.6. Message fragmentation
DTLS defines fragmentation only for the handshake 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 [ieee802.15.4] 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 in EST-coaps. Even though ECC
certificates are small in size, they can vary greatly based on
signature algorithms, key sizes, and Object Identifier (OID) fields
used. For 256-bit curves, common ECDSA cert sizes are 500-1000 bytes
which could fluctuate further based on the algorithms, OIDs, Subject
Alternative Names (SAN) and cert fields. For 384-bit curves, ECDSA
certificates increase in size and can sometimes reach 1.5KB.
Additionally, there are times when the EST cacerts response from the
server can include multiple certificates that amount to large
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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. Even with ECC,
EST-coaps messages can still exceed MTU sizes on the Internet or
6LoWPAN [RFC4919] (Section 2 of [RFC7959]). EST-coaps needs to be
able to fragment messages into multiple DTLS datagrams.
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. As explained
in Section 1 of [RFC7959], block-wise transfers should be used in
Confirmable CoAP messages to avoid the exacerbation of lost blocks.
The EST-coaps client and server MUST support Block2. Block1 MUST be
supported for EST-coaps enrollment requests that exceed the Path MTU.
[RFC7959] also defines Size1 and Size2 Options to provide size
information about the resource representation in a request and
response. EST-client and server MAY support Size1 and Size2 Options.
Examples of fragmented EST-coaps messages are shown in Appendix B.
5.7. Delayed Responses
Server responses can sometimes be delayed. According to section
5.2.2 of [RFC7252], a slow server can acknowledge the request and
respond later with the requested resource representation. In
particular, a slow server can respond to an EST-coaps enrollment
request with an empty ACK with code 0.00, before sending the
certificate to the server after a short delay. If the certificate
response is large, the server will need more than one Block2 blocks
to transfer it. This situation is shown in Figure 2. The client
sends an enrollment request that uses N1+1 Block1 blocks. The server
uses an empty 0.00 ACK to announce the delayed response which is
provided later with 2.04 messages containing N2+1 Block2 Options.
The first 2.04 is a confirmable message that is acknowledged by the
client with an ACK. Onwards, having received the first 256 bytes in
the first Block2 block, the client asks for a block reduction to 128
bytes in a confirmable enrollment request Uri-Path and acknowledges
the Block2 blocks sent up to that point.
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POST [2001:db8::2:1]:61616/est/sen (CON)(1:0/1/256) {CSR req} -->
<-- (ACK) (1:0/1/256) (2.31 Continue)
POST [2001:db8::2:1]:61616/est/sen (CON)(1:1/1/256) {CSR req} -->
<-- (ACK) (1:1/1/256) (2.31 Continue)
.
.
.
POST [2001:db8::2:1]:61616/est/sen (CON)(1:N1/0/256){CSR req} -->
<-- (0.00 empty ACK)
|
...... short delay before certificate is ready ......
|
<-- (CON) (1:N1/0/256)(2:0/1/256)(2.04 Changed) {Cert resp}
(ACK) -->
POST [2001:db8::2:1]:61616/est/sen (CON)(2:1/0/128) -->
<-- (ACK) (2:1/1/128) (2.04 Changed) {Cert resp}
.
.
.
POST [2001:db8::2:1]:61616/est/sen (CON)(2:N2/0/128) -->
<-- (ACK) (2:N2/0/128) (2.04 Changed) {Cert resp}
Figure 2: EST-COAP enrollment with short wait
If the server is very slow (i.e. minutes) in providing the response
(i.e. when a manual intervention is needed), the server SHOULD
respond with an ACK containing response code 5.03 (Service
unavailable) and a Max-Age Option to indicate the time the client
SHOULD wait to request the content later. After a delay of Max-Age,
the client SHOULD resend the identical CSR to the server. As long as
the server responds with response code 5.03 (Service Unavailable)
with a Max-Age Option, the client SHOULD keep resending the
enrollment request until the server responds with the certificate or
the client abandons for other reasons.
To demonstrate this scenario, Figure 3 shows a client sending an
enrollment request that uses N1+1 Block1 blocks to send the CSR to
the server. The server needs N2+1 Block2 blocks to respond, but also
needs to take a long delay (minutes) to provide the response.
Consequently, the server uses a 5.03 ACK response with a Max-Age
Option. The client waits for a period of Max-Age as many times as he
receives the same 5.03 response and retransmits the enrollment
request until he receives a certificate in a fragmented 2.01
response. Note that the server asks for a decrease in the block size
when acknowledging the first Block2.
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POST [2001:db8::2:1]:61616/est/sen (CON)(1:0/1/256) {CSR req} -->
<-- (ACK) (1:0/1/256) (2.31 Continue)
POST [2001:db8::2:1]:61616/est/sen (CON)(1:1/1/256) {CSR req} -->
<-- (ACK) (1:1/1/256) (2.31 Continue)
.
.
POST [2001:db8::2:1]:61616/est/sen (CON)(1:N1/0/256){CSR req} -->
<-- (ACK) (1:N1/0/256) (2:0/0/128)(5.03 Service Unavailable)
(Max-Age)
|
|
Client tries one or more times after Max-Age with identical payload
|
|
POST [2001:db8::2:1]:61616/est/sen (CON)(1:N1/0/256){CSR req} -->
<-- (ACK) (1:N1/0/256) (2:0/1/128) (2.01 Created){Cert resp}
POST [2001:db8::2:1]:61616/est/sen (CON)(2:1/0/128) -->
<-- (ACK) (2:1/1/128) (2.01 Created) {Cert resp}
.
.
.
POST [2001:db8::2:1]:61616/est/sen (CON)(2:N2/0/128) -->
<-- (ACK) (2:N2/0/128) (2.01 Created) {Cert resp}
Figure 3: EST-COAP enrollment with long wait
5.8. Server-side Key Generation
Constrained devices sometimes do not have the necessary hardware to
generate statistically random numbers for private keys and DTLS
ephemeral keys. Past experience has also shown that low-resource
endpoints sometimes generate numbers which could allow someone to
decrypt the communication or guess the private key and impersonate as
the device [PsQs] [RSAorig]. Additionally, random number key
generation is costly, thus energy draining. Even though the random
numbers that constitute the identity/cert do not get generated often,
an endpoint may not want to spend time and energy generating
keypairs, and just ask for one from the server.
In these scenarios, server-side key generation can be used. The
client asks for the server or proxy to generate the private key and
the certificate which are transferred back to the client in the
server-side key generation response. In all respects, the server
SHOULD treat the CSR as it would treat any enroll or re-enroll CSR;
the only distinction here is that the server MUST ignore the public
key values and signature in the CSR. These are included in the
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request only to allow re-use of existing codebases for generating and
parsing such requests.
The client /skg request needs to communicate to the server the
Content-Format of the application/multipart-core elements. The key
Content-Format requested by the client is depicted in the PKCS#10
request. If the request contains SMIMECapabilities the client is
expecting Content-Format 280. Otherwise he expects a PKCS#8 key in
Content-Format 284. The client expresses the preferred certificate
Content-Format in his /skg request by using an Accept Option. The
Accept Option is 281 when preferring a certificate in a PKCS#7
container or TBD287 when preferring a single X.509 certificate.
[RFC7030] provides two methods, symmetric and asymmetric, to
optionally encrypt the generated key. The methods are signaled by
the client by using the relevant attributes (SMIMECapabilities and
DecryptKeyIdentifier or AsymmetricDecryptKeyIdentifier) in the CSR
request. The symmetric key or the asymmetric keypair establishment
method is out of scope of the specification.
The EST-coaps server-side key generation response is returned with
Content-Format application/multipart-core
[I-D.ietf-core-multipart-ct] containing a CBOR array with four items
Section 5.3. The certificate part exactly matches the response from
an enrollment response. The private key can be in unprotected PKCS#8
[RFC5958] format (Content-Format 284) or protected inside of CMS
SignedData (Content-Format 280). The SignedData is signed by the
party that generated the private key, which may be the EST server or
the EST CA. The SignedData is further protected by placing it inside
of a CMS EnvelopedData as explained in Section 4.4.2 of [RFC7030].
In summary, the symmetrically encrypted key is included in the
encryptedKey attribute in a KEKRecipientInfo structure. In the case
where the asymmetric encryption key is suitable for transport key
operations the generated private key is encrypted with a symmetric
key which is encrypted by the client defined (in the CSR) asymmetric
public key and is carried in an encryptedKey attribute in a
KeyTransRecipientInfo structure. Finally, if the asymmetric
encryption key is suitable for key agreement, the generated private
key is encrypted with a symmetric key which is encrypted by the
client defined (in the CSR) asymmetric public key and is carried in
an recipientEncryptedKeys attribute in a KeyAgreeRecipientInfo.
[RFC7030] recommends the use of additional encryption of the returned
private key. For the context of this specification, clients and
servers that choose to support server-side key generation MUST
support unprotected (PKCS#8) private keys (Content-Format 284).
Symmetric or asymmetric encryption of the private key (CMS
EnvelopedData, Content-Format 280) SHOULD be supported for
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deployments where end-to-end encryption needs to be provided between
the client and a server. Such cases could include architectures
where an entity between the client and the CA terminates the DTLS
connection (Registrar in Figure 4).
6. DTLS Transport Protocol
EST-coaps depends on a secure transport mechanism over UDP that
secures the exchanged CoAP messages. DTLS is one such secure
protocol. EST depended in TLS. No other changes are necessary
regarding the secure transport of EST messages.
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 several records, each of which
can be transmitted separately, thus avoiding IP fragmentation"
[RFC6347].
The DTLS handshake is authenticated by using certificates. EST-coaps
supports the certificate types and Trust Anchors (TA) that are
specified for EST in Section 3 of [RFC7030].
CoAP and DTLS can provide proof-of-identity for EST-coaps clients and
servers with simple PKI messages as described in Section 3.1 of
[RFC5272]. Moreover, 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 consists of the contents of
the first "Finished" message in the (D)TLS handshake between server
and client [RFC5929]. The client adds the "Finished" message as a
ChallengePassword in the attributes section of the PKCS#10 Request
[RFC5967] to prove that the client is indeed in control of the
private key at the time of the (D)TLS session establishment.
In the case of EST-coaps, the same operations can be performed during
the DTLS handshake. For DTLS 1.2, in the event of handshake message
fragmentation, the Hash of the handshake messages used in the MAC
calculation of the Finished message MUST be computed as if each
handshake message had been sent as a single fragment (Section 4.2.6
of [RFC6347]). The Finished message is calculated as shown in
Section 7.4.9 of [RFC5246]. Similarly, for DTLS 1.3, the Finished
message MUST be computed as if each handshake message had been sent
as a single fragment (Section 5.8 of [I-D.ietf-tls-dtls13]) following
the algorithm described in 4.4.4 of [RFC8446].
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In a constrained CoAP environment, endpoints can't always 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. To alleviate this
situation, an EST-coaps DTLS connection MAY remain open for
sequential EST transactions. For example, an EST csrattrs request
that is followed by a simpleenroll request can use the same
authenticated DTLS connection. However, when a cacerts request is
included in the set of sequential EST transactions, some additional
security considerations apply regarding the use of the Implicit and
Explicit TA database as explained in Section 11.1.
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. In some cases, like NAT
rebinding, keeping the state of a connection is not possible when
devices sleep for extended periods of time. In such occasions,
[I-D.ietf-tls-dtls-connection-id] negotiates a connection ID that can
eliminate the need for new handshake and its additional cost.
7. HTTPS-CoAPS Registrar
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 which case it will support TLS/HTTP instead of
CoAPS. 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 Registrar at the edge is required to
operate between the CoAP environment and the external HTTP network as
shown in Figure 4.
Constrained Network
.------. .----------------------------.
| CA | |.--------------------------.|
'------' || ||
| || ||
.------. HTTP .-----------------. CoAPS .-----------. ||
| EST |<------->|EST-coaps-to-HTTPS|<------->| EST Client| ||
|Server|over TLS | Registrar | '-----------' ||
'------' '-----------------' ||
|| ||
|'--------------------------'|
'----------------------------'
Figure 4: EST-coaps-to-HTTPS Registrar at the CoAP boundary.
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The EST-coaps-to-HTTPS Registrar MUST terminate EST-coaps downstream
and initiate EST connections over TLS upstream. The Registrar MUST
authenticate and OPTIONALLY authorize the clients and it MUST be
authenticated by the EST server or CA. The trust relationship
between the Registrar and the EST server SHOULD be pre-established
for the Registrar to proxy these connections on behalf of various
clients.
When enforcing Proof-of-Possession (POP) linking, the DTLS tls-unique
value of the (D)TLS session is used to prove that the private key
corresponding to the public key is in the possession of and was used
to establish the connection by the client as explained in Section 6).
The POP linking information is lost between the EST-coaps client and
the EST server when a Registrar is present. The EST server becomes
aware of the presence of a Registrar from its TLS client certificate
that includes id-kp-cmcRA [RFC6402] extended key usage extension
(EKU). As explained in Section 3.7 of [RFC7030], the EST server
SHOULD apply an authorization policy consistent with a Registrar
client. For example, it could be configured to accept POP linking
information that does not match the current TLS session because the
authenticated EST client Registrar has verified this information when
acting as an EST server.
For some use cases, clients that leverage server-side key generation
might prefer for the enrolled keys to be generated by the Registrar
if the CA does not support server-side key generation. In these
cases, the Registrar MUST support random number generation using
proper entropy. Such Registrar is responsible for generating a new
CSR signed by a new key which will be returned to the client along
with the certificate from the CA.
Table 1 contains the URI mappings between EST-coaps and EST that the
Registrar MUST adhere to. Section 5.5 of this specification and
Section 7 of [RFC8075] define the mappings between EST-coaps and HTTP
response codes, that determine how the Registrar MUST translate CoAP
response codes from/to HTTP status codes. The mapping from CoAP
Content-Format to HTTP Media-Type is defined in Section 10.1.
Additionally, a conversion from CBOR major type 2 to Base64 encoding
MUST take place at the Registrar when server-side key generation is
supported. If CMS end-to-end encryption is employed for the private
key, the encrypted CMS EnvelopedData blob MUST be converted to binary
in CBOR type 2 downstream to the client.
Due to fragmentation of large messages into blocks, an EST-coaps-to-
HTTP Registrar MUST reassemble the BLOCKs before translating the
binary content to Base64, and consecutively relay the message
upstream.
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For the discovery of the EST server by the EST client in the CoAP
environment, the EST-coaps-to-HTTP Registrar MUST announce itself
according to the rules in Section 5.1. The available actions of the
Registrars MUST be announced with as many resource paths necessary.
8. Parameters
This section addresses transmission parameters described in sections
4.7 and 4.8 of [RFC7252]. EST does not impose any unique values on
the CoAP parameters in [RFC7252], but the EST parameter values need
to be tuned to the CoAP parameter values.
It is RECOMMENDED, based on experiments, to follow the default CoAP
configuration parameters ([RFC7252]). However, depending on the
implementation scenario, retransmissions and timeouts can also occur
on other networking layers, governed by other configuration
parameters. A change in a server parameter MUST ensure the adjusted
value is also available to all the endpoints with which these
adjusted values are to be used to communicate.
Some further comments about some specific parameters, mainly from
Table 2 in [RFC7252]:
o NSTART: Limit the number of simultaneous outstanding interactions
that a client maintains to a given server. EST-coaps clients
SHOULD use 1, which is the default. A EST-coaps client is not
expected to interact with more than one servers at the same time.
o DEFAULT_LEISURE: This setting is only relevant in multicast
scenarios, outside the scope of EST-coaps.
o PROBING_RATE: A parameter which specifies the rate of re-sending
non-confirmable messages. The EST messages are defined to be sent
as CoAP confirmable messages, hence this setting is not
applicable.
Finally, the Table 3 parameters in [RFC7252] are mainly derived from
Table 2. Directly changing parameters on one table would affect
parameters on the other.
9. Deployment limitations
Although EST-coaps paves the way for the utilization of EST by
constrained devices in constrained networks, some classes of devices
[RFC7228] 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/DTLS/CoAP. It is
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up to the network designer to decide which devices execute the EST
protocol and which do not.
10. IANA Considerations
10.1. Content-Format Registry
Additions to the sub-registry "CoAP Content-Formats", within the
"CoRE Parameters" registry [COREparams] are specified in Table 3.
These have been registered provisionally in the Expert Review range
(0-255).
+------------------------------+-------+----------------------------+
| HTTP Media-Type | ID | Reference |
+------------------------------+-------+----------------------------+
| application/pkcs7-mime; | 280 | [RFC7030] [I-D.ietf-lamps- |
| smime-type=server-generated- | | rfc5751-bis] |
| key | | |
| application/pkcs7-mime; | 281 | [I-D.ietf-lamps-rfc5751-bi |
| smime-type=certs-only | | s] |
| application/pkcs7-mime; | 282 | [RFC5273] [I-D.ietf-lamps- |
| smime-type=CMC-request | | rfc5751-bis] |
| application/pkcs7-mime; | 283 | [RFC5273] [I-D.ietf-lamps- |
| smime-type=CMC-response | | rfc5751-bis] |
| application/pkcs8 | 284 | [RFC5958] [I-D.ietf-lamps- |
| | | rfc5751-bis] |
| application/csrattrs | 285 | [RFC7030] [RFC7231] |
| application/pkcs10 | 286 | [RFC5967] [I-D.ietf-lamps- |
| | | rfc5751-bis] |
| application/pkix-cert | TBD28 | [RFC2585] |
| | 7 | |
+------------------------------+-------+----------------------------+
Table 3: Table 3: New CoAP Content-Formats
The Content-Formats 281 to 286 have been the subject of an earlier
temporary allocation. It is suggested that 287 is allocated to
TBD287.
10.2. Resource Type registry
This memo registers a new Resource Type (rt=) Link Target Attributes
in the "Resource Type (rt=) Link Target Attribute Values" subregistry
under the "Constrained RESTful Environments (CoRE) Parameters"
registry.
o rt="ace.est". This EST resource is used to query and return the
supported EST resources of a CoAP server.
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o rt="ace.est.crts". This resource depicts the support of EST get
cacerts.
o rt="ace.est.sen". This resource depicts the support of EST simple
enroll.
o rt="ace.est.sren". This resource depicts the support of EST
simple reenroll.
o rt="ace.est.att". This resource depicts the support of EST CSR
attributes.
o rt="ace.est.skg". This resource depicts the support of EST
server-side key generation.
11. Security Considerations
11.1. 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.
The transport of these keys is inherently risky. Analysis SHOULD be
done to establish whether server-side key generation enhances or
decreases the probability of identity stealing.
It is also RECOMMENDED that the Implicit Trust Anchor database used
for EST server authentication is carefully managed to reduce the
chance of a third-party CA with poor certification practices
jeopardizing authentication. Disabling the Implicit Trust Anchor
database after successfully receiving the Distribution of CA
certificates response (Section 4.1.3 of [RFC7030]) limits any risk to
the first DTLS exchange. Alternatively, in a case where a /sen
request immediately follows a /crt, a client MAY choose to keep the
connection authenticated by the Implicit TA open for efficiency
reasons (Section 6). A client that pipelines EST-coaps /crt request
with other requests in the same DTLS connection SHOULD revalidate the
server certificate chain against the updated Explicit TA from the
/crt response before proceeding with the subsequent requests. If the
server certificate chain does not authenticate against the database,
the client SHOULD close the connection without completing the rest of
the requests. The updated Explicit TA MUST continue to be used in
new DTLS connections.
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In cases where the IDevID used to authenticate the client is expired
the server MAY still authenticate the client because IDevIDs are
expected to live as long as the device itself (Section 4). In such
occasions, checking the certificate revocation status or authorizing
the client using another method is important for the server to ensure
that the client is to be trusted.
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 certificate request links the proof-of-possession to
the TLS proof-of-identity. This implies but does not prove that only
the authenticated client currently has access to the private key.
What's more, POP linking uses tls-unique as it is defined in
[RFC5929]. The 3SHAKE attack [tripleshake] poses a risk by allowing
a man-in-the-middle to leverage session resumption and renegotiation
to inject himself between a client and server even when channel
binding is in use. The attack was possible because of certain (D)TLS
implementation imperfections. In the context of this specification,
an attacker could invalidate the purpose of the POP linking
ChallengePassword in the client request by resuming an EST-coaps
connection. Even though the practical risk of such an attack to EST-
coaps is not devastating, we would rather use a more secure channel
binding mechanism. Such a mechanism could include an updated tls-
unique value generation like the tls-unique-prf defined in
[I-D.josefsson-sasl-tls-cb] by using a TLS exporter [RFC5705] in TLS
1.2 or TLS 1.3's updated exporter (Section 7.5 of [RFC8446]). Such
mechanism has not been standardized yet. Adopting in this document a
channel binding value generated from an exporter would break
backwards compatibility. Thus, in this specification we still depend
in the tls-unique mechanism defined in [RFC5929], especially since
the practicality of such an attack would not expose any messages
exchanged with EST-coaps.
Regarding the Certificate Signing Request (CSR), a 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.
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11.2. HTTPS-CoAPS Registrar considerations
The Registrar proposed in Section 7 must be deployed with care, and
only when the recommended connections are impossible. When POP
linking is used the Registrar terminating the TLS connection
establishes a new one with the upstream CA. Thus, it is impossible
for POP linking to be enforced end-to-end for the EST transaction.
The EST server could be configured to accept POP linking information
that does not match the current TLS session because the authenticated
EST Registrar client has verified this information when acting as an
EST server.
The introduction of an EST-coaps-to-HTTP Registrar assumes the client
can trust the registrar using its implicit or explicit TA database.
It also assumes the Registrar has a trust relationship with the
upstream EST server in order to act on behalf of the clients. When a
client uses the Implicit TA database for certificate validation, he
SHOULD confirm if the server is acting as an RA by the presence of
the id-kp-cmcRA [RFC6402] EKU in the server certificate. If the
server certificate does not include the EKU, it is RECOMMENDED that
the client includes "Linking Identity and POP Information"
(Section 6) in requests.
In a server-side key generation case, if no end-to-end encryption is
used, the Registrar may be able see the private key as it acts as a
man-in-the-middle. Thus, the client puts its trust on the Registrar
not exposing the private key.
Clients that leverage server-side key generation without end-to-end
encryption of the private key (Section 5.8) have no knowledge if the
Registrar will be generating the private key and enrolling the
certificates with the CA or if the CA will be responsible for
generating the key. In such cases, the existence of a Registrar
requires the client to put its trust on the registrar doing the right
thing if it is generating the private key.
12. Contributors
Martin Furuhed contributed to the EST-coaps specification by
providing feedback based on the Nexus EST over CoAPS server
implementation that started in 2015. Sandeep Kumar kick-started this
specification and was instrumental in drawing attention to the
importance of the subject.
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13. Acknowledgements
The authors are very grateful to Klaus Hartke for his detailed
explanations on the use of Block with DTLS and his support for the
Content-Format specification. 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 for his feedback on technical details of the solution.
Constructive comments were received from Benjamin Kaduk, Eliot Lear,
Jim Schaad, Hannes Tschofenig, Julien Vermillard, John Manuel, Oliver
Pfaff and Pete Beal.
Interop tests were done by Oliver Pfaff, Thomas Werner, Oskar
Camezind, Bjorn Elmers and Joel Hoglund.
Robert Moskowitz provided code to create the examples.
14. References
14.1. Normative References
[I-D.ietf-core-multipart-ct]
Fossati, T., Hartke, K., and C. Bormann, "Multipart
Content-Format for CoAP", draft-ietf-core-multipart-ct-02
(work in progress), August 2018.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-30 (work in progress),
November 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP",
RFC 2585, DOI 10.17487/RFC2585, May 1999,
<https://www.rfc-editor.org/info/rfc2585>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
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[RFC5967] Turner, S., "The application/pkcs10 Media Type", RFC 5967,
DOI 10.17487/RFC5967, August 2010,
<https://www.rfc-editor.org/info/rfc5967>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
"Enrollment over Secure Transport", RFC 7030,
DOI 10.17487/RFC7030, October 2013,
<https://www.rfc-editor.org/info/rfc7030>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
[RFC8075] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
E. Dijk, "Guidelines for Mapping Implementations: HTTP to
the Constrained Application Protocol (CoAP)", RFC 8075,
DOI 10.17487/RFC8075, February 2017,
<https://www.rfc-editor.org/info/rfc8075>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
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14.2. Informative References
[COREparams]
IANA, "Constrained RESTful Environments (CoRE)
Parameters", <https://www.iana.org/assignments/core-
parameters/core-parameters.xhtml>.
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
Amsuess, "CoRE Resource Directory", draft-ietf-core-
resource-directory-19 (work in progress), January 2019.
[I-D.ietf-lamps-rfc5751-bis]
Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", draft-ietf-lamps-rfc5751-bis-12
(work in progress), September 2018.
[I-D.ietf-tls-dtls-connection-id]
Rescorla, E., Tschofenig, H., Fossati, T., and T. Gondrom,
"Connection Identifiers for DTLS 1.2", draft-ietf-tls-
dtls-connection-id-02 (work in progress), October 2018.
[I-D.josefsson-sasl-tls-cb]
Josefsson, S., "Channel Bindings for TLS based on the
PRF", draft-josefsson-sasl-tls-cb-03 (work in progress),
March 2015.
[I-D.moskowitz-ecdsa-pki]
Moskowitz, R., Birkholz, H., Xia, L., and M. Richardson,
"Guide for building an ECC pki", draft-moskowitz-ecdsa-
pki-04 (work in progress), September 2018.
[ieee802.15.4]
Institute of Electrical and Electronics Engineers, "IEEE
Standard 802.15.4-2006", 2006.
[ieee802.1ar]
Institute of Electrical and Electronics Engineers, "IEEE
802.1AR Secure Device Identifier", December 2009.
[PsQs] Nadia Heninger, Zakir Durumeric, Eric Wustrow, J. Alex
Halderman, "Mining Your Ps and Qs: Detection of Widespread
Weak Keys in Network Devices", USENIX Security Symposium
2012 ISBN 978-931971-95-9, August 2012.
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[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,
<https://www.rfc-editor.org/info/rfc4919>.
[RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,
<https://www.rfc-editor.org/info/rfc5272>.
[RFC5273] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC): Transport Protocols", RFC 5273,
DOI 10.17487/RFC5273, June 2008,
<https://www.rfc-editor.org/info/rfc5273>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
<https://www.rfc-editor.org/info/rfc5929>.
[RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958,
DOI 10.17487/RFC5958, August 2010,
<https://www.rfc-editor.org/info/rfc5958>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090,
DOI 10.17487/RFC6090, February 2011,
<https://www.rfc-editor.org/info/rfc6090>.
[RFC6402] Schaad, J., "Certificate Management over CMS (CMC)
Updates", RFC 6402, DOI 10.17487/RFC6402, November 2011,
<https://www.rfc-editor.org/info/rfc6402>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
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[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[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,
<https://www.rfc-editor.org/info/rfc7251>.
[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, <https://www.rfc-editor.org/info/rfc7525>.
[RFC7925] Tschofenig, H., Ed. 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,
<https://www.rfc-editor.org/info/rfc7925>.
[RFC8422] Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
Curve Cryptography (ECC) Cipher Suites for Transport Layer
Security (TLS) Versions 1.2 and Earlier", RFC 8422,
DOI 10.17487/RFC8422, August 2018,
<https://www.rfc-editor.org/info/rfc8422>.
[RSAorig] Petr Svenda, Matus Nemec, Peter Sekan, Rudolf Kvasnovsky,
David Formanek, David Komarek, Vashek Matyas, "The
Million-Key Question - Investigating the Origins of RSA
Public Keys", USENIX Security Symposium 2016 ISBN
978-1-931971-32-4, August 2016.
[tripleshake]
Karthikeyan Bhargavan, Antoine Delignat-Lavaud, Cedric
Fournet, Alfredo Pironti, Pierre-Yves Strub, "Triple
Handshakes and Cookie Cutters: Breaking and Fixing
Authentication over TLS", IEEE Security and Privacy ISBN
978-1-4799-4686-0, May 2014.
Appendix A. EST messages to EST-coaps
This section shows similar examples to the ones presented in
Appendix A of [RFC7030]. The payloads in the examples are the hex
encoded binary, generated with 'xxd -p', of the PKI certificates
created following [I-D.moskowitz-ecdsa-pki]. The payloads are shown
unencrypted. In practice the message content would be binary
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formatted and transferred over an encrypted DTLS tunnel. The
hexadecimal representations in the examples below would NOT be
transported in hex, but in binary. Hex is used for visualization
purposes because a binary representation cannot be rendered well in
text.
The certificate responses included in the examples contain Content-
Format 281 (application/pkcs7). If the client had requested Content-
Format TBD287 (application/pkix-cert) with an Accept Option, the
server would respond a single DER binary certificate.
These examples assume that the resource discovery, returned a short
base path of "/est".
The corresponding CoAP headers are only shown in Appendix A.1.
Creating CoAP headers is assumed to be generally understood.
The message content breakdown is presented in Appendix C.
A.1. cacerts
In EST-coaps, a cacerts message can be:
GET coaps://est-coaps.example.ietf.org:9085/est/crts
(Accept: 281)
The corresponding CoAP header fields are shown below. The use of
block and DTLS are worked out in Appendix B.
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Ver = 1
T = 0 (CON)
Code = 0x01 (0.01 is GET)
Token = 0x9a (client generated)
Options
Option (Uri-Host) [optional]
Option Delta = 0x3 (option# 3)
Option Length = 0x9
Option Value = est-coaps.example.ietf.org
Option (Uri-Port) [optional]
Option Delta = 0x4 (option# 3+4=7)
Option Length = 0x4
Option Value = 9085
Option (Uri-Path)
Option Delta = 0x4 (option# 7+4=11)
Option Length = 0x5
Option Value = "est"
Option (Uri-Path)
Option Delta = 0x0 (option# 11+0=11)
Option Length = 0x6
Option Value = "crts"
Option (Accept)
Option Delta = 0x6 (option# 11+6=17)
Option Length = 0x2
Option Value = 281
Payload = [Empty]
The Uri-Host and Uri-Port Options are optional. They are usually
omitted as the DTLS destination and port are sufficient. Explicit
Uri-Host and Uri-Port Options are typically used when an endpoint
hosts multiple virtual servers and uses the Options to route the
requests accordingly. Alternatively, if a UDP port to a server is
blocked, someone could send the DTLS packets to a known open port on
the server and use the Uri-Port to convey the intended port he is
attempting to reach.
A 2.05 Content response with a cert in EST-coaps will then be
2.05 Content (Content-Format: 281)
{payload with certificate in binary format}
with CoAP fields
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Ver = 1
T = 2 (ACK)
Code = 0x45 (2.05 Content)
Token = 0x9a (copied from request by server)
Options
Option (Content-Format)
Option Delta = 0xC (option# 12)
Option Length = 0x2
Option Value = 281
[ The hexadecimal representation below would NOT be transported
in hex, but in binary. Hex is used because a binary representation
cannot be rendered well in text. ]
Payload =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The breakdown of the payload is shown in Appendix C.1.
A.2. enroll / reenroll
During the (re-)enroll exchange the EST-coaps client uses a CSR
(Content-Format 286) request in the POST request payload. The Accept
option tells the server that the client is expecting Content-Format
281 (PKCS#7) in the response. As shown in Appendix C.2, the CSR
contains a ChallengePassword which is used for POP linking
(Section 6).
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POST [2001:db8::2:1]:61616/est/sen
(Token: 0x45)
(Accept: 281)
(Content-Format: 286)
[ The hexadecimal representation below would NOT be transported
in hex, but in binary. Hex is used because a binary representation
cannot be rendered well in text. ]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After verification of the CSR by the server, a 2.01 Content response
with the issued certificate will be returned to the client.
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2.01 Created
(Token: 0x45)
(Content-Format: 281)
[ The hexadecimal representation below would NOT be transported
in hex, but in binary. Hex is used because a binary representation
cannot be rendered well in text. ]
3082026e06092a864886f70d010702a082025f3082025b0201013100300b
06092a864886f70d010701a08202413082023d308201e2a0030201020208
7e7661d7b54e4632300a06082a8648ce3d040302305d310b300906035504
0613025553310b300906035504080c02434131143012060355040a0c0b45
78616d706c6520496e6331163014060355040b0c0d636572746966696361
74696f6e3113301106035504030c0a3830322e3141522043413020170d31
39303133313131323931365a180f39393939313233313233353935395a30
5c310b3009060355040613025553310b300906035504080c024341310b30
0906035504070c024c4131143012060355040a0c0b6578616d706c652049
6e63310c300a060355040b0c03496f54310f300d06035504051306577431
3233343059301306072a8648ce3d020106082a8648ce3d03010703420004
c8b421f11c25e47e3ac57123bf2d9fdc494f028bc351cc80c03f150bf50c
ff958d75419d81a6a245dffae790be95cf75f602f9152618f816a2b23b56
38e59fd9a3818a30818730090603551d1304023000301d0603551d0e0416
041496600d8716bf7fd0e752d0ac760777ad665d02a0301f0603551d2304
183016801468d16551f951bfc82a431d0d9f08bc2d205b1160300e060355
1d0f0101ff0404030205a0302a0603551d1104233021a01f06082b060105
05070804a013301106092b06010401b43b0a01040401020304300a06082a
8648ce3d0403020349003046022100c0d81996d2507d693f3c48eaa5ee94
91bda6db214099d98117c63b361374cd86022100a774989f4c321a5cf25d
832a4d336a08ad67df20f1506421188a0ade6d349236a1003100
The breakdown of the request and response is shown in Appendix C.2.
As described in Section 5.7, if the server is not able to provide a
response immediately, it sends an empty ACK with response code 5.03
(Service Unavailable) and the Max-Age Option. See Figure 3 for an
example exchange.
A.3. serverkeygen
In a serverkeygen exchange the CoAP POST request looks like
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POST coaps://192.0.2.1:8085/est/skg
(Token: 0xa5)
(Accept: 281)
(Content-Format: 286)
[ The hexadecimal representation below would NOT be transported
in hex, but in binary. Hex is used because a binary representation
cannot be rendered well in text. ]
3081cf3078020100301631143012060355040a0c0b736b67206578616d70
6c653059301306072a8648ce3d020106082a8648ce3d030107034200041b
b8c1117896f98e4506c03d70efbe820d8e38ea97e9d65d52c8460c5852c5
1dd89a61370a2843760fc859799d78cd33f3c1846e304f1717f8123f1a28
4cc99fa000300a06082a8648ce3d04030203470030440220387cd4e9cf62
8d4af77f92ebed4890d9d141dca86cd2757dd14cbd59cdf6961802202f24
5e828c77754378b66660a4977f113cacdaa0cc7bad7d1474a7fd155d090d
The response would follow [I-D.ietf-core-multipart-ct] and could look
like
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2.01 Content
(Token: 0xa5)
(Content-Format: 62)
[ The hexadecimal representations below would NOT be transported
in hex, but in binary. Hex is used because a binary representation
cannot be rendered well in text. ]
84 # array(4)
19 011C # unsigned(284)
58 8A # bytes(138)
308187020100301306072a8648ce3d020106082a8648ce3d030107046d30
6b02010104200b9a67785b65e07360b6d28cfc1d3f3925c0755799deeca7
45372b01697bd8a6a144034200041bb8c1117896f98e4506c03d70efbe82
0d8e38ea97e9d65d52c8460c5852c51dd89a61370a2843760fc859799d78
cd33f3c1846e304f1717f8123f1a284cc99f
19 0119 # unsigned(281)
59 01D3 # bytes(467)
308201cf06092a864886f70d010702a08201c0308201bc0201013100300b
06092a864886f70d010701a08201a23082019e30820143a0030201020208
126de8571518524b300a06082a8648ce3d04030230163114301206035504
0a0c0b736b67206578616d706c65301e170d313930313039303835373038
5a170d3339303130343038353730385a301631143012060355040a0c0b73
6b67206578616d706c653059301306072a8648ce3d020106082a8648ce3d
030107034200041bb8c1117896f98e4506c03d70efbe820d8e38ea97e9d6
5d52c8460c5852c51dd89a61370a2843760fc859799d78cd33f3c1846e30
4f1717f8123f1a284cc99fa37b307930090603551d1304023000302c0609
6086480186f842010d041f161d4f70656e53534c2047656e657261746564
204365727469666963617465301d0603551d0e04160414494be598dc8dbc
0dbc071c486b777460e5cce621301f0603551d23041830168014494be598
dc8dbc0dbc071c486b777460e5cce621300a06082a8648ce3d0403020349
003046022100a4b167d0f9add9202810e6bf6a290b8cfdfc9b9c9fea2cc1
c8fc3a464f79f2c202210081d31ba142751a7b4a34fd1a01fcfb08716b9e
b53bdaadc9ae60b08f52429c0fa1003100
The private key in the response above is without CMS EnvelopedData
and has no additional encryption beyond DTLS (Section 5.8).
The breakdown of the request and response is shown in Appendix C.3
A.4. csrattrs
Below is a csrattrs exchange
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REQ:
GET coaps://[2001:db8::2:1]:61616/est/att
RES:
2.05 Content
(Content-Format: 285)
[ The hexadecimal representation below would NOT be transported
in hex, but in binary. Hex is used because a binary representation
cannot be rendered well in text. ]
307c06072b06010101011630220603883701311b131950617273652053455
420617320322e3939392e31206461746106092a864886f70d010907302c06
0388370231250603883703060388370413195061727365205345542061732
0322e3939392e32206461746106092b240303020801010b06096086480165
03040202
A 2.05 Content response should contain attributes which are relevant
for the authenticated client. This example is copied from section
A.2 in [RFC7030], where the base64 representation is replaced with a
hexadecimal representation of the equivalent binary format. The EST-
coaps server returns attributes that the client can ignore if they
are unknown to him.
Appendix B. EST-coaps Block message examples
Two examples are presented in this section:
1. a cacerts exchange shows the use of Block2 and the block headers
2. an enroll exchange shows the Block1 and Block2 size negotiation
for request and response payloads.
The payloads are shown unencrypted. In practice the message contents
would be binary formatted and transferred over an encrypted DTLS
tunnel. The corresponding CoAP headers are only shown in
Appendix B.1. Creating CoAP headers are assumed to be generally
known.
B.1. cacerts
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 cacerts exchange over DTLS. The
content length of the cacerts response in appendix A.1 of [RFC7030]
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contains 639 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 9 packets with a payload of 64 bytes each, followed by a last
tenth packet of 63 bytes. The client sends an IPv6 packet containing
the UDP datagram with the DTLS record that encapsulates the CoAP
request 10 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 (block-
option:NUM/M/size) indicating the kind of Block Option (2 in this
case) followed by a colon, and then the block number (NUM), the more
bit (M = 0 in Block2 response means it is last block), and block size
with exponent (2**(SZX+4)) separated by slashes. The Length 64 is
used with SZX=2 to avoid IP fragmentation. The CoAP Request is sent
confirmable (CON) and the Content-Format of the response, even though
not shown, is 281 (application/pkcs7-mime; smime-type=certs-only).
The transfer of the 10 blocks with partially filled block NUM=9 is
shown below
GET coaps://est-coaps.example.ietf.org:9085/est/crts (2:0/0/64) -->
<-- (2:0/1/64) 2.05 Content
GET coaps://est-coaps.example.ietf.org:9085/est/crts (2:1/0/64) -->
<-- (2:1/1/64) 2.05 Content
|
|
|
GET coaps://est-coaps.example.ietf.org:9085/est/crts (2:9/0/64) -->
<-- (2:9/0/64) 2.05 Content
The header of the GET request looks like
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Ver = 1
T = 0 (CON)
Code = 0x01 (0.1 GET)
Token = 0x9a (client generated)
Options
Option (Uri-Host) [optional]
Option Delta = 0x3 (option# 3)
Option Length = 0x9
Option Value = est-coaps.example.ietf.org
Option (Uri-Port) [optional]
Option Delta = 0x4 (option# 3+4=7)
Option Length = 0x4
Option Value = 9085
Option (Uri-Path)
Option Delta = 0x4 (option# 7+4=11)
Option Length = 0x5
Option Value = "est"
Option (Uri-Path)Uri-Path)
Option Delta = 0x0 (option# 11+0=11)
Option Length = 0x6
Option Value = "crts"
Option (Accept)
Option Delta = 0x6 (option# 11+6=17)
Option Length = 0x2
Option Value = 281
Payload = [Empty]
The Uri-Host and Uri-Port Options are optional. They are usually
omitted as the DTLS destination and port are sufficient. Explicit
Uri-Host and Uri-Port Options are typically used when an endpoint
hosts multiple virtual servers and uses the Options to route the
requests accordingly. Alternatively, if a UDP port to a server is
blocked, someone could send the DTLS packets to a known open port on
the server and use the Uri-Port to convey the intended port he is
attempting to reach.
For further detailing the CoAP headers, the first two and the last
blocks are written out below. The header of the first Block2
response looks like
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Ver = 1
T = 2 (ACK)
Code = 0x45 (2.05 Content)
Token = 0x9a (copied from request by server)
Options
Option
Option Delta = 0xC (option# 12 Content-Format)
Option Length = 0x2
Option Value = 281
Option
Option Delta = 0xB (option# 12+11=23 Block2)
Option Length = 0x1
Option Value = 0x0A (block#=0, M=1, SZX=2)
[ The hexadecimal representation below would NOT be transported
in hex, but in binary. Hex is used because a binary representation
cannot be rendered well in text. ]
Payload =
3082027b06092a864886f70d010702a082026c308202680201013100300b
06092a864886f70d010701a082024e3082024a308201f0a0030201020209
009189bc
The second Block2:
Ver = 1
T = 2 (means ACK)
Code = 0x45 (2.05 Content)
Token = 0x9a (copied from request by server)
Options
Option
Option Delta = 0xC (option# 12 Content-Format)
Option Length = 0x2
Option Value = 281
Option
Option Delta = 0xB (option 12+11=23 Block2)
Option Length = 0x1
Option Value = 0x1A (block#=1, M=1, SZX=2)
[ The hexadecimal representation below would NOT be transported
in hex, but in binary. Hex is used because a binary representation
cannot be rendered well in text. ]
Payload =
df9c99244b300a06082a8648ce3d0403023067310b300906035504061302
5553310b300906035504080c024341310b300906035504070c024c413114
30120603
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The 10th and final Block2:
Ver = 1
T = 2 (means ACK)
Code = 0x45 (2.05 Content)
Token = 0x9a (copied from request by server)
Options
Option
Option Delta = 0xC (option# 12 Content-Format)
Option Length = 0x2
Option Value = 281
Option
Option Delta = 0xB (option# 12+11=23 Block2 )
Option Length = 0x2
Option Value = 0x92 (block#=9, M=0, SZX=2)
[ The hexadecimal representation below would NOT be transported
in hex, but in binary. Hex is used because a binary representation
cannot be rendered well in text. ]
Payload =
2ec0b4af52d46f3b7ecc9687ddf267bcec368f7b7f1353272f022047a28a
e5c7306163b3c3834bab3c103f743070594c089aaa0ac870cd13b902caa1
003100
B.2. enroll / reenroll
In this example the requested Block2 size of 256 bytes, required by
the client, is transferred to the server in the very first request
message. The block size 256=(2**(SZX+4)) which gives SZX=4. The
notation for block numbering is the same as in Appendix B.1. It is
assumed that CSR takes N1+1 blocks and the cert response takes N2+1
blocks. The header fields and the payload are omitted for brevity.
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POST [2001:db8::2:1]:61616/est/sen (CON)(1:0/1/256) {CSR req} -->
<-- (ACK) (1:0/1/256) (2.31 Continue)
POST [2001:db8::2:1]:61616/est/sen (CON)(1:1/1/256) {CSR req} -->
<-- (ACK) (1:1/1/256) (2.31 Continue)
.
.
.
POST [2001:db8::2:1]:61616/est/sen (CON)(1:N1/0/256){CSR req} -->
<-- (ACK) (1:N1/0/256)(2:0/1/256)(2.04 Changed){Cert resp}
POST [2001:db8::2:1]:61616/est/sen (CON)(2:1/0/256) -->
<-- (ACK) (2:1/1/256)(2.04 Changed) {Cert resp}
.
.
.
POST [2001:db8::2:1]:61616/est/sen (CON)(2:N2/0/256) -->
<-- (ACK) (2:N2/0/256) (2.04 Changed) {Cert resp}
Figure 5: EST-COAP enrollment with multiple blocks
N1+1 blocks have been transferred from client to the server and N2+1
blocks have been transferred from server to client.
Appendix C. Message content breakdown
This appendix presents the breakdown of the hexadecimal dumps of the
binary payloads shown in Appendix A.
C.1. cacerts
Breakdown of cacerts response containing one root CA certificate.
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Certificate:
Data:
Version: 3 (0x2)
Serial Number:
91:89:bc:df:9c:99:24:4b
Signature Algorithm: ecdsa-with-SHA256
Issuer: C=US, ST=CA, L=LA, O=Example Inc,
OU=certification, CN=Root CA
Validity
Not Before: Jan 7 10:40:41 2019 GMT
Not After : Jan 2 10:40:41 2039 GMT
Subject: C=US, ST=CA, L=LA, O=Example Inc,
OU=certification, CN=Root CA
Subject Public Key Info:
Public Key Algorithm: id-ecPublicKey
Public-Key: (256 bit)
pub:
04:81:49:94:08:2b:6e:81:85:f3:df:53:f5:e0:be:
e6:98:97:33:35:20:00:23:dd:f7:8c:d1:7a:44:3f:
fd:8d:dd:40:90:87:69:c5:56:52:ac:2c:cb:75:c4:
a5:0a:7c:7d:db:7c:22:da:e6:c8:5c:ca:53:82:09:
fd:bb:f1:04:c9
ASN1 OID: prime256v1
NIST CURVE: P-256
X509v3 extensions:
X509v3 Subject Key Identifier:
24:95:E8:16:EF:6F:FC:AA:F3:56:CE:4A:DF:FE:33:CF:49:2A:BB:A8
X509v3 Authority Key Identifier:
keyid:
24:95:E8:16:EF:6F:FC:AA:F3:56:CE:4A:DF:FE:33:CF:49:2A:BB:A8
X509v3 Basic Constraints: critical
CA:TRUE
X509v3 Key Usage: critical
Certificate Sign, CRL Sign
X509v3 Subject Alternative Name:
email:certify@example.com
Signature Algorithm: ecdsa-with-SHA256
30:45:02:21:00:da:e3:7c:96:f1:54:c3:2e:c0:b4:af:52:d4:
6f:3b:7e:cc:96:87:dd:f2:67:bc:ec:36:8f:7b:7f:13:53:27:
2f:02:20:47:a2:8a:e5:c7:30:61:63:b3:c3:83:4b:ab:3c:10:
3f:74:30:70:59:4c:08:9a:aa:0a:c8:70:cd:13:b9:02:ca
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C.2. enroll / reenroll
The breakdown of the request is
Certificate Request:
Data:
Version: 0 (0x0)
Subject: C=US, ST=CA, L=LA, O=example Inc,
OU=IoT/serialNumber=Wt1234
Subject Public Key Info:
Public Key Algorithm: id-ecPublicKey
Public-Key: (256 bit)
pub:
04:c8:b4:21:f1:1c:25:e4:7e:3a:c5:71:23:bf:2d:
9f:dc:49:4f:02:8b:c3:51:cc:80:c0:3f:15:0b:f5:
0c:ff:95:8d:75:41:9d:81:a6:a2:45:df:fa:e7:90:
be:95:cf:75:f6:02:f9:15:26:18:f8:16:a2:b2:3b:
56:38:e5:9f:d9
ASN1 OID: prime256v1
NIST CURVE: P-256
Attributes:
challengePassword : <256-bit POP linking value>
Requested Extensions:
X509v3 Subject Alternative Name:
othername:<unsupported>
Signature Algorithm: ecdsa-with-SHA256
30:45:02:21:00:92:56:3a:54:64:63:bd:9e:cf:f1:70:d0:fd:
1f:2e:f0:d3:d0:12:16:0e:5e:e9:0c:ff:ed:ab:ec:9b:9a:38:
92:02:20:17:9f:10:a3:43:61:09:05:1a:ba:d1:75:90:a0:9b:
c8:7c:4d:ce:54:53:a6:fc:11:35:a1:e8:4e:ed:75:43:77
The CSR contained a ChallengePassword which is used for POP linking
(Section 6).
The breakdown of the issued certificate response is
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Certificate:
Data:
Version: 3 (0x2)
Serial Number: 9112578475118446130 (0x7e7661d7b54e4632)
Signature Algorithm: ecdsa-with-SHA256
Issuer: C=US, ST=CA, O=Example Inc, OU=certification,
CN=802.1AR CA
Validity
Not Before: Jan 31 11:29:16 2019 GMT
Not After : Dec 31 23:59:59 9999 GMT
Subject: C=US, ST=CA, L=LA, O=example Inc,
OU=IoT/serialNumber=Wt1234
Subject Public Key Info:
Public Key Algorithm: id-ecPublicKey
Public-Key: (256 bit)
pub:
04:c8:b4:21:f1:1c:25:e4:7e:3a:c5:71:23:bf:2d:
9f:dc:49:4f:02:8b:c3:51:cc:80:c0:3f:15:0b:f5:
0c:ff:95:8d:75:41:9d:81:a6:a2:45:df:fa:e7:90:
be:95:cf:75:f6:02:f9:15:26:18:f8:16:a2:b2:3b:
56:38:e5:9f:d9
ASN1 OID: prime256v1
NIST CURVE: P-256
X509v3 extensions:
X509v3 Basic Constraints:
CA:FALSE
X509v3 Subject Key Identifier:
96:60:0D:87:16:BF:7F:D0:E7:52:D0:AC:76:07:77:AD:66:5D:02:A0
X509v3 Authority Key Identifier:
keyid:
68:D1:65:51:F9:51:BF:C8:2A:43:1D:0D:9F:08:BC:2D:20:5B:11:60
X509v3 Key Usage: critical
Digital Signature, Key Encipherment
X509v3 Subject Alternative Name:
othername:<unsupported>
Signature Algorithm: ecdsa-with-SHA256
30:46:02:21:00:c0:d8:19:96:d2:50:7d:69:3f:3c:48:ea:a5:
ee:94:91:bd:a6:db:21:40:99:d9:81:17:c6:3b:36:13:74:cd:
86:02:21:00:a7:74:98:9f:4c:32:1a:5c:f2:5d:83:2a:4d:33:
6a:08:ad:67:df:20:f1:50:64:21:18:8a:0a:de:6d:34:92:36
C.3. serverkeygen
The following is the breakdown of the request example used.
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Certificate Request:
Data:
Version: 0 (0x0)
Subject: O=skg example
Subject Public Key Info:
Public Key Algorithm: id-ecPublicKey
Public-Key: (256 bit)
pub:
04:1b:b8:c1:11:78:96:f9:8e:45:06:c0:3d:70:ef:
be:82:0d:8e:38:ea:97:e9:d6:5d:52:c8:46:0c:58:
52:c5:1d:d8:9a:61:37:0a:28:43:76:0f:c8:59:79:
9d:78:cd:33:f3:c1:84:6e:30:4f:17:17:f8:12:3f:
1a:28:4c:c9:9f
ASN1 OID: prime256v1
NIST CURVE: P-256
Attributes:
a0:00
Signature Algorithm: ecdsa-with-SHA256
30:44:02:20:38:7c:d4:e9:cf:62:8d:4a:f7:7f:92:eb:ed:48:
90:d9:d1:41:dc:a8:6c:d2:75:7d:d1:4c:bd:59:cd:f6:96:18:
02:20:2f:24:5e:82:8c:77:75:43:78:b6:66:60:a4:97:7f:11:
3c:ac:da:a0:cc:7b:ad:7d:14:74:a7:fd:15:5d:09:0d
The following is the breakdown of the private key content of the
server-side key generation response payload.
Private-Key: (256 bit)
priv:
0b:9a:67:78:5b:65:e0:73:60:b6:d2:8c:fc:1d:3f:
39:25:c0:75:57:99:de:ec:a7:45:37:2b:01:69:7b:
d8:a6
pub:
04:1b:b8:c1:11:78:96:f9:8e:45:06:c0:3d:70:ef:
be:82:0d:8e:38:ea:97:e9:d6:5d:52:c8:46:0c:58:
52:c5:1d:d8:9a:61:37:0a:28:43:76:0f:c8:59:79:
9d:78:cd:33:f3:c1:84:6e:30:4f:17:17:f8:12:3f:
1a:28:4c:c9:9f
ASN1 OID: prime256v1
NIST CURVE: P-256
The following is the breakdown of the certificate of the second part
of the server-side key generation response payload.
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Certificate:
Data:
Version: 3 (0x2)
Serial Number: 1327972925857878603 (0x126de8571518524b)
Signature Algorithm: ecdsa-with-SHA256
Issuer: O=skg example
Validity
Not Before: Jan 9 08:57:08 2019 GMT
Not After : Jan 4 08:57:08 2039 GMT
Subject: O=skg example
Subject Public Key Info:
Public Key Algorithm: id-ecPublicKey
Public-Key: (256 bit)
pub:
04:1b:b8:c1:11:78:96:f9:8e:45:06:c0:3d:70:ef:
be:82:0d:8e:38:ea:97:e9:d6:5d:52:c8:46:0c:58:
52:c5:1d:d8:9a:61:37:0a:28:43:76:0f:c8:59:79:
9d:78:cd:33:f3:c1:84:6e:30:4f:17:17:f8:12:3f:
1a:28:4c:c9:9f
ASN1 OID: prime256v1
NIST CURVE: P-256
X509v3 extensions:
X509v3 Basic Constraints:
CA:FALSE
Netscape Comment:
OpenSSL Generated Certificate
X509v3 Subject Key Identifier:
49:4B:E5:98:DC:8D:BC:0D:BC:07:1C:48:6B:77:74:60:E5:CC:E6:21
X509v3 Authority Key Identifier:
keyid:
49:4B:E5:98:DC:8D:BC:0D:BC:07:1C:48:6B:77:74:60:E5:CC:E6:21
Signature Algorithm: ecdsa-with-SHA256
30:46:02:21:00:a4:b1:67:d0:f9:ad:d9:20:28:10:e6:bf:6a:
29:0b:8c:fd:fc:9b:9c:9f:ea:2c:c1:c8:fc:3a:46:4f:79:f2:
c2:02:21:00:81:d3:1b:a1:42:75:1a:7b:4a:34:fd:1a:01:fc:
fb:08:71:6b:9e:b5:3b:da:ad:c9:ae:60:b0:8f:52:42:9c:0f
Authors' Addresses
Peter van der Stok
Consultant
Email: consultancy@vanderstok.org
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Panos Kampanakis
Cisco Systems
Email: pkampana@cisco.com
Michael C. Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
URI: http://www.sandelman.ca/
Shahid Raza
RISE SICS
Isafjordsgatan 22
Kista, Stockholm 16440
SE
Email: shahid@sics.se
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