Protecting EST Payloads with OSCORE
draft-ietf-ace-coap-est-oscore-06
| Document | Type | Active Internet-Draft (ace WG) | |
|---|---|---|---|
| Authors | Göran Selander , Shahid Raza , Martin Furuhed , Mališa Vučinić , Timothy Claeys | ||
| Last updated | 2024-10-21 | ||
| Replaces | draft-selander-ace-coap-est-oscore | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-ace-coap-est-oscore-06
ACE Working Group G. Selander
Internet-Draft Ericsson AB
Intended status: Standards Track S. Raza
Expires: 24 April 2025 RISE
M. Furuhed
Nexus
M. Vučinić
Inria
T. Claeys
21 October 2024
Protecting EST Payloads with OSCORE
draft-ietf-ace-coap-est-oscore-06
Abstract
Enrollment over Secure Transport (EST) is a certificate provisioning
protocol over HTTPS [RFC7030] or CoAPs [RFC9148]. This document
specifies how to carry EST over the Constrained Application Protocol
(CoAP) protected with Object Security for Constrained RESTful
Environments (OSCORE). The specification builds on the EST-coaps
[RFC9148] specification, but uses OSCORE and Ephemeral Diffie-Hellman
over COSE (EDHOC) instead of DTLS. The specification also leverages
the certificate structures defined in
[I-D.ietf-cose-cbor-encoded-cert], which can be optionally used
alongside X.509 certificates.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Authentication and
Authorization for Constrained Environments Working Group mailing list
(ace@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/ace/.
Source for this draft and an issue tracker can be found at
https://github.com/EricssonResearch/EST-OSCORE.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 24 April 2025.
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Copyright (c) 2024 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/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Operational Differences with EST-coaps . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Authentication . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. EDHOC . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Certificate-based Authentication . . . . . . . . . . . . 6
3.3. Channel Binding . . . . . . . . . . . . . . . . . . . . . 6
3.4. Optimizations . . . . . . . . . . . . . . . . . . . . . . 7
4. Protocol Design and Layering . . . . . . . . . . . . . . . . 8
4.1. Discovery and URI . . . . . . . . . . . . . . . . . . . . 8
4.2. Mandatory/optional EST Functions . . . . . . . . . . . . 9
4.3. Payload formats . . . . . . . . . . . . . . . . . . . . . 9
4.4. Message Bindings . . . . . . . . . . . . . . . . . . . . 14
4.5. CoAP response codes . . . . . . . . . . . . . . . . . . . 14
4.6. Message Fragmentation . . . . . . . . . . . . . . . . . . 14
4.7. Delayed Responses . . . . . . . . . . . . . . . . . . . . 15
4.8. Enrollment of Static DH Keys . . . . . . . . . . . . . . 15
5. HTTP-CoAP Proxy . . . . . . . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
6.1. Server-generated Private Keys . . . . . . . . . . . . . . 17
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6.2. Considerations on Channel Binding . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Normative References . . . . . . . . . . . . . . . . . . 18
8.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Example Enrollment With Optimizations . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
One of the challenges with deploying a Public Key Infrastructure
(PKI) for the Internet of Things (IoT) is certificate enrollment,
because existing enrollment protocols are not optimized for
constrained environments [RFC7228].
One optimization of certificate enrollment targeting IoT deployments
is specified in EST-coaps [RFC9148], which defines a version of
Enrollment over Secure Transport [RFC7030] for transporting EST
payloads over CoAP [RFC7252] and DTLS [RFC9147], instead of HTTP and
TLS [RFC8446].
This document describes a method for protecting EST payloads over
CoAP or HTTP with OSCORE [RFC8613]. OSCORE specifies an extension to
CoAP which protects messages at the application layer and can be
applied independently of how CoAP messages are transported. OSCORE
can also be applied to CoAP-mappable HTTP which enables end-to-end
security for mixed CoAP and HTTP transfer of application layer data.
Hence EST payloads can be protected end-to-end independent of the
underlying transport and through proxies translating between CoAP and
HTTP.
OSCORE is designed for constrained environments, building on IoT
standards such as CoAP, CBOR [RFC8949], and COSE [RFC9052] [RFC9053],
and has in particular gained traction in settings where message sizes
and the number of exchanged messages need to be kept at a minimum,
such as 6TiSCH [RFC9031], or for securing CoAP group messages
[I-D.ietf-core-oscore-groupcomm]. Where OSCORE is implemented and
used for communication security, the reuse of OSCORE for other
purposes, such as enrollment, reduces the code footprint.
Prior to running EST-oscore, the protocol defined in this
specification, there must exist a trust relation between the EST-
oscore client and the EST-oscore server. This trust relation may be
based on the pre-shared OSCORE security context, or based on the
common root of trust. In case there is a pre-shared OSCORE security
context, the CoAP exchange carrying EST payloads can occur
immediately. In case there is a common root of trust, a security
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handshake based on the Ephemeral Diffie-Hellman over COSE (EDHOC,
[RFC9528]) protocol needs to occur prior to running CoAP. How this
trust relation is established is out of scope of this document.
How the EST-oscore server verifies the identity of the client prior
to issuing a certificate is also out of scope of this specification.
EST-oscore defines a number of optimizations with respect to EST-
coaps. The performance of certificate enrollment and certificate-
based authentication described in this document includes the use of:
* Compact CBOR representations of X.509 certificates (see
[I-D.ietf-cose-cbor-encoded-cert])
* Certificates by reference (see [RFC9360])
* Compact, CBOR representations of EST payloads (see
[I-D.ietf-cose-cbor-encoded-cert])
1.1. Operational Differences with EST-coaps
The protection of EST payloads defined in this document builds on
EST-coaps [RFC9148] but transport layer security is replaced, or
complemented, by protection of the transfer- and application layer
data (i.e., CoAP message fields and payload). This specification
deviates from EST-coaps in the following respects:
* The DTLS record layer is replaced by, or complemented with,
OSCORE.
* The DTLS handshake is replaced by, or complemented with, the
lightweight authenticated key exchange protocol EDHOC [RFC9528],
and makes use of the following features:
- Authentication based on certificates is complemented with
authentication based on raw public keys.
- Authentication based on signature keys is complemented with
authentication based on static Diffie-Hellman keys, for
certificates/raw public keys.
- Authentication based on certificate by value is complemented
with authentication based on certificate/raw public keys by
reference.
* The EST payloads protected by OSCORE can be proxied between
constrained networks supporting CoAP/CoAPs and non-constrained
networks supporting HTTP/HTTPs with a CoAP-HTTP proxy protection
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without any security processing in the proxy (see Section 5). The
concept "Registrar" and its required trust relation with the EST
server as described in Section 5 of [RFC9148] is therefore not
applicable.
So, while the same authentication scheme (Diffie-Hellman key exchange
authenticated with transported certificates) and the same EST
payloads as EST-coaps also apply to EST-oscore, the latter specifies
other authentication schemes. The reason for these deviations is
that a significant overhead can be removed in terms of message sizes
and round trips by using a different handshake, public key type or
transported credential, and those are independent of the actual
enrollment procedure.
2. 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.
This document uses terminology from [RFC9148] which in turn is based
on [RFC7030] and, in turn, on [RFC5272].
The term "Trust Anchor" follows the terminology of [RFC6024]: "A
trust anchor represents an authoritative entity via a public key and
associated data. The public key is used to verify digital
signatures, and the associated data is used to constrain the types of
information for which the trust anchor is authoritative."
Apart from enrolling signature keys, this document also specifies how
to enroll static DH keys. Instead of signing, possession of the
private static DH key may be proved by generating a MAC given the
recipient's public DH key. Therefore this document extends the
definition of the term "Trust Anchor": the corresponding public key
can also be used for MAC generation for static DH proof of possession
procedures.
3. Authentication
This specification replaces, or complements, the DTLS handshake in
EST-coaps with the lightweight authenticated key exchange protocol
EDHOC [RFC9528]. During initial enrollment, the EST-oscore client
and server run EDHOC [RFC9528] to authenticate and establish the
OSCORE Security Context used to protect the messages carrying EST
payloads.
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The EST-oscore client MUST play the role of the EDHOC Initiator. The
EST-oscore server MUST play the role of the EDHOC Responder.
The EST-oscore clients and servers must perform mutual
authentication. The EST server and EST client are responsible for
ensuring that an acceptable cipher suite is negotiated. The client
must authenticate the server before accepting any server response.
The server must authenticate the client. These requirements are
fullfilled when using EDHOC [RFC9528].
The server must also provide relevant information to the CA to
support its decision about issuing a certificate.
3.1. EDHOC
EDHOC supports authentication with certificates or raw public keys
(referred to as "credentials"), and the credentials may either be
transported in the protocol, or referenced. This is determined by
the identifier of the credential of the endpoint, ID_CRED_x for x=
Initiator/Responder, which is transported in an EDHOC message. This
identifier may be the credential itself (in which case the credential
is transported), or a pointer such as a URI of the credential (e.g.,
x5u, see [RFC9360]) or some other identifier which enables the
receiving endpoint to retrieve the credential.
3.2. Certificate-based Authentication
EST-oscore, like EST-coaps, supports certificate-based authentication
between the EST client and server. The client MUST be configured
with an Implicit or Explicit Trust Anchor (TA) [RFC7030] database,
enabling the client to authenticate the server. During the initial
enrollment the client SHOULD populate its Explicit TA database and
use it for subsequent authentications.
The EST client certificate SHOULD conform to [RFC7925]. The EST
client and/or EST server certificate MAY be a (natively signed) CBOR
certificate [I-D.ietf-cose-cbor-encoded-cert].
3.3. Channel Binding
The [RFC5272] specification describes proof-of-possession as the
ability of a client to prove its possession of a private key which is
linked to a certified public key. In case of a signature key, a
proof-of-possession is generated by the client when it signs the
PKCS#10 Request during the enrollment phase. In case of a static DH
key, a proof-of-possession is generated by the client when it
generates a MAC and includes it in the PKCS#10 request, as per
Section 4.8.
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Connection-based channel binding refers to the security binding
between the PKCS#10 object and the underlying secure transport layer.
This is typically achieved by including the challengePassword
attribute in the PKCS#10 object that is dependent on the underlying
security session. Connection-based proof-of-possession using the
challengePassword attribute of the PKCS#10 object is OPTIONAL, see
Section 6.
3.4. Optimizations
* The third message of the EDHOC protocol, message_3, MAY be
combined with an OSCORE request, enabling authenticated Diffie-
Hellman key exchange and a protected CoAP request/response (which
may contain an enrollment request and response) in two round trips
[I-D.ietf-core-oscore-edhoc].
* The enrolled certificates MAY be the CBOR-encoded certificates
defined in [I-D.ietf-cose-cbor-encoded-cert].
* The enrolled client certificate MAY be referenced instead of
transported [RFC9360]. The EST-oscore server MAY use information
in the credential identifier field of the EDHOC message
(ID_CRED_x) to access the EST-oscore client certificate, e.g., in
a directory or database provided by the issuer. In this case the
certificate may not need to be transported over a constrained link
between EST client and server.
* Conversely, the response to the PKCS#10 request MAY specify a
reference to the enrolled certificate rather than the certificate
itself. The EST-oscore server MAY in the enrollment response to
the EST-oscore client include a pointer to a directory or database
where the certificate can be retrieved.
* The PKCS#10 object MAY request a certificate for a static DH key
instead of a signature key. This may result in a more compact
request because the use of static DH keys may imply a proof-of-
posession using a MAC, which is shorter than a signature.
Additionally, subsequent EDHOC sessions using static DH keys for
authentication have less overhead than key exchange protocols
using signature-based authentication credentials.
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4. Protocol Design and Layering
EST-oscore uses CoAP [RFC7252] and Block-Wise transfer [RFC7959] to
transfer EST messages in the same way as [RFC9148]. Instead of DTLS
record layer, OSCORE [RFC8613] is used to protect the messages
conveying the EST payloads. External Authorization Data (EAD) fields
of EDHOC are intentionally not used to carry EST payloads because
EDHOC needs not be executed in the case of re-enrollment. The DTLS
handshake is complemented by or replaced with EDHOC [RFC9528].
Figure 1 below shows the layered EST-oscore architecture. Note that
Figure 1 does not illustrate the potential use of DTLS. Protocol
design also allows that OSCORE and EDHOC messages are carried within
the same CoAP message, as per [I-D.ietf-core-oscore-edhoc].
+----------------+
| EST messages |
+------------+----------------+
| EDHOC | OSCORE |
+------------+----------------+
| CoAP or HTTP |
+-----------------------------+
Figure 1: The stack diagram of EST protected with OSCORE.
EST-oscore follows much of the EST-coaps and EST design.
4.1. Discovery and URI
The discovery of EST resources and the definition of the short EST-
coaps URI paths specified in Section 4.1 of [RFC9148], as well as the
new Resource Type defined in Section 8.2 of [RFC9148] apply to EST-
oscore. Support for OSCORE is indicated by the "osc" attribute
defined in Section 9 of [RFC8613].
Example:
REQ: GET /.well-known/core?rt=ace.est.sen
RES: 2.05 Content
</est>; rt="ace.est.sen";osc
The use of the "osc" attribute is REQUIRED. In scenarios where
OSCORE and DTLS are combined, the absence of the "osc" attribute
might wrongly suggest that the EST server is actually using EST-
coaps, because of the scheme "coaps", when it is using EST-oscore.
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4.2. Mandatory/optional EST Functions
The EST-oscore specification has the same set of required-to-
implement functions as EST-coaps. The content of Table 1 is adapted
from Section 4.2 in [RFC9148] and uses the updated URI paths (see
Section 4.1).
+===============+===========================+
| EST functions | EST-oscore implementation |
+===============+===========================+
| /crts | MUST |
+---------------+---------------------------+
| /sen | MUST |
+---------------+---------------------------+
| /sren | MUST |
+---------------+---------------------------+
| /skg | OPTIONAL |
+---------------+---------------------------+
| /skc | OPTIONAL |
+---------------+---------------------------+
| /att | OPTIONAL |
+---------------+---------------------------+
Table 1: Mandatory and optional EST-
oscore functions.
4.2.1. /crts
EST-coaps provides the /crts operation. A successful request from
the client to this resource will be answered with a bag of
certificates which is subsequently installed in the Explicit TA.
A trust anchor is commonly a self-signed certificate of the CA public
key. In order to reduce transport overhead, the trust anchor could
be just the CA public key and associated data (see Section 2), e.g.,
the SubjectPublicKeyInfo, or a public key certificate without the
signature. In either case they can be compactly encoded, e.g. using
CBOR encoding [I-D.ietf-cose-cbor-encoded-cert].
4.3. Payload formats
Similar to EST-coaps, EST-oscore allows transport of DER-encoded
objects of a given Media-Type. When transporting DER-encoded
objects, EST-oscore uses the same CoAP Content-Format identifiers as
EST-coaps when transferring EST requests and responses. In addition,
EST-oscore allows the transport of CBOR-encoded objects, signaled via
their corresponding Media-Type.
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EST-oscore servers MUST support both the DER-encoded ASN.1 objects
and the CBOR-encoded objects. This means supporting formats detailed
in Section 4.3.1 and Section 4.3.2. It is up to the client to
support only DER-encoded ASN.1, CBOR encoding, or both. As a
reminder, Content-Format negotiation happens through CoAP's Accept
option present in the requests.
4.3.1. DER-encoded ASN.1 Objects
Table 2 summarizes the information from Section 4.3 in [RFC9148] in
what concerns the transport of DER-encoded ASN.1 objects.
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+=======+==================================+======+=======+
| URI | Media Type | Type | #IANA |
+=======+==================================+======+=======+
| /crts | N/A | req | - |
+-------+----------------------------------+------+-------+
| | application/pkix-cert | res | 287 |
+-------+----------------------------------+------+-------+
| | application/pkcs7-mime;smime- | res | 281 |
| | type=certs-only | | |
+-------+----------------------------------+------+-------+
| /sen | application/pkcs10 | req | 286 |
+-------+----------------------------------+------+-------+
| | application/pkix-cert | res | 287 |
+-------+----------------------------------+------+-------+
| | application/pkcs7-mime;smime- | res | 281 |
| | type=certs-only | | |
+-------+----------------------------------+------+-------+
| /sren | application/pkcs10 | req | 286 |
+-------+----------------------------------+------+-------+
| | application/pkix-cert | res | 287 |
+-------+----------------------------------+------+-------+
| | application/pkcs7-mime;smime- | res | 281 |
| | type=certs-only | | |
+-------+----------------------------------+------+-------+
| /skg | application/pkcs10 | req | 286 |
+-------+----------------------------------+------+-------+
| | application/multipart-core | res | 62 |
+-------+----------------------------------+------+-------+
| /skc | application/pkcs10 | req | 286 |
+-------+----------------------------------+------+-------+
| | application/multipart-core | res | 62 |
+-------+----------------------------------+------+-------+
| /att | N/A | req | - |
+-------+----------------------------------+------+-------+
| | application/csrattrs | res | 285 |
+-------+----------------------------------+------+-------+
Table 2: EST functions and the associated ASN.1 CoAP
Content-Format identifiers.
Content-Format 281 and Content-Format 287 MUST be supported by EST-
oscore servers. It is up to the client to support only Content-
Format 281, 287 or both. As indicated in Section 4.3 of [RFC9148],
the client will use a 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.
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The generated response for /skg and /skc requests contains two parts:
certificate and the corresponding private key. Section 4.8 of
[RFC9148] specifies that the private key in response to /skc request
may be either an encrypted (PKCS #7) or unencrypted (PKCS #8) key,
depending on whether the CSR request included SMIMECapabilities.
Due to the use of OSCORE, which protects the communication between
the EST client and the EST server end-to-end, it is possible to
return the private key to /skc or /skg as an unencrypted PKCS #8
object (Content-Format identifier 284). Therefore, when making the
CSR to /skc or /skg, the EST client MUST NOT include
SMIMECapabilities. As a consequence, the private key part of the
response to /skc or /skg is an unencrypted PKCS #8 object.
+==========+====================================+===================+
| Function | DER-encoded ASN.1 | DER-encoded ASN.1 |
| | Response, Part 1 | Response, Part 2 |
+==========+====================================+===================+
| /skg | 284 | 281 |
+----------+------------------------------------+-------------------+
| /skc | 284 | 287 |
+----------+------------------------------------+-------------------+
Table 3: Response Content-Format identifiers for /skg and /skc in
case of DER- encoded ASN.1 objects.
4.3.2. CBOR-encoded Objects
Table 4 presents the equivalent information to Section 4.3.1 when
CBOR-encoded objects are in use.
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+=======+==============================+======+=======+
| URI | Media Type | Type | #IANA |
+=======+==============================+======+=======+
| /crts | N/A | req | - |
+-------+------------------------------+------+-------+
| | application/cose-c509-cert | res | TBD6 |
+-------+------------------------------+------+-------+
| /sen | application/cose-c509-pkcs10 | req | TBD7 |
+-------+------------------------------+------+-------+
| | application/cose-c509-cert | res | TBD6 |
+-------+------------------------------+------+-------+
| /sren | application/cose-c509-pkcs10 | req | TBD7 |
+-------+------------------------------+------+-------+
| | application/cose-c509-cert | res | TBD6 |
+-------+------------------------------+------+-------+
| /skg | application/cose-c509-pkcs10 | req | TBD7 |
+-------+------------------------------+------+-------+
| | application/multipart-core | res | 62 |
+-------+------------------------------+------+-------+
| /skc | N/A | req | - |
+-------+------------------------------+------+-------+
| | N/A | res | - |
+-------+------------------------------+------+-------+
| /att | N/A | req | - |
+-------+------------------------------+------+-------+
| | application/csrattrs | res | TBD5 |
+-------+------------------------------+------+-------+
Table 4: EST functions and the associated CBOR CoAP
Content-Format identifiers.
In case of CBOR-encoded objects, there is a single Content-Format,
TBD6, that MUST be supported by both the EST-oscore servers and
clients.
EDITOR NOTE: Specify the CDDL structure of /csrattrs and point to
appropriate document for its semantics.
In the case of CBOR-encoded request to /skg, the two parts of the
response are also CBOR encoded. The certificate part is encoded as
the application/cose-c509-cert object (Content-Format identifier
TBD6), while the corresponding private key is encoded as application/
cose-c509-privkey (Content-Format identifier TBD10). The function
/skc is not available when using CBOR-encoded objects, and for
server-side generated keys, clients MUST use the /skg function.
Table 5 summarizes the Content-Format identifiers used in responses
to the /skg function.
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+==========+=======================+======================+
| Function | CBOR Response, Part 1 | CBOR Response Part 2 |
+==========+=======================+======================+
| /skg | 101 | TBD6 |
+----------+-----------------------+----------------------+
Table 5: Response Content-Format identifiers for /skg
in case of CBOR-encoded objects.
4.4. Message Bindings
Note that the EST-oscore message characteristics are identical to
those specified in Section 4.4 of [RFC9148]. It is therefore
required that
* The EST-oscore endpoints support delayed responses
* The endpoints supports the following CoAP options: OSCORE, Uri-
Host, Uri-Path, Uri-Port, Content-Format, Block1, Block2, and
Accept.
* The EST URLs based on https:// are translated to coap://, but with
mandatory use of the CoAP OSCORE option. In case DTLS is
additionally used, the translation target is the scheme "coaps",
instead of "coap".
4.5. CoAP response codes
See Section 4.5 in [RFC9148].
4.6. Message Fragmentation
The EDHOC key exchange is optimized for message overhead, in
particular the use of static DH keys instead of signature keys for
authentication (e.g., method 3 of [RFC9528]). Together with various
measures listed in this document such as CBOR-encoded payloads
[RFC8949], CBOR certificates [I-D.ietf-cose-cbor-encoded-cert],
certificates by reference (Section 3.4), and trust anchors without
signature (Section 4.2.1), a significant reduction of message sizes
can be achieved.
Nevertheless, depending on the application, the protocol messages may
become larger than the available frame size thus resulting in
fragmentation and, in resource constrained networks such as IEEE
802.15.4 where throughput is limited, fragment loss can trigger
costly retransmissions.
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It is recommended to prevent 6LoWPAN fragmentation, since it involves
an error-prone datagram reassembly. To limit the size of the CoAP
payload, this document specifies the requirements on implementing
CoAP options Block1 and Block2. EST-oscore servers MUST implement
Block1 and Block2. EST-oscore clients MUST implement Block2 and MAY
implement Block1.
4.7. Delayed Responses
See Section 4.7 in [RFC9148].
4.8. Enrollment of Static DH Keys
This section specifies how the EST client enrolls a static DH key.
In general, a given key pair should only be used for a single
purpose, such as key establishment, digital signature, key transport.
The EST client attempting to enroll a DH key for a key usage
operation other than digital signature SHOULD use an alternative
proof-of-possession algorithm: The EST client SHOULD prepare the
PKCS#10 object and compute a MAC, replacing the signature, over the
certification request information by following the steps in Section 6
of [RFC6955]. The Key Derivation Function (KDF) and the MAC MUST be
set to the HDKF and HMAC algorithms used by OSCORE. The KDF and MAC
is thus defined by the hash algorithm used by OSCORE in HKDF and
HMAC, which by default is SHA-256. When EDHOC is used, then the hash
algorithm is the application hash algorithm of the selected cipher
suite.
To generate a MAC according to the algorithm outlined in Section 6 of
[RFC6955], the client needs to know the public DH key of the proof-
of-possession recipient/verifier, i.e. the EST server. In the
general case, the EST client MAY obtain the CA certs including the
CA's DH certificate using the /crts function using an explicit
request/response flow. The obtained certificate indicates the DH
group parameters which MUST be respected by the EST client when
generating its own DH key pair.
As an optimization, when EDHOC precedes the enrollment and combined
OSCORE-EDHOC flow is being used in EDHOC message_3 and message_4 per
[I-D.ietf-core-oscore-edhoc], the client MUST use the public
ephemeral key of the EDHOC Responder, G_Y, as the recipient public
key in the algorithm outlined in Section 6 of [RFC6955]. When
generating its DH key pair, the client uses the group parameters as
indicated by the EDHOC cipher suite in use in the EDHOC session.
Because the combined delivery is used per
[I-D.ietf-core-oscore-edhoc], the client has already in EDHOC
message_2 obtained the ephemeral key G_Y of the server.
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In some cases, it may be beneficial to exceptionally use the static
DH private key associated to the public key used in enrollment for a
one-time signing operation of the CSR. While a key pair should only
be used for a single purpose (e.g. key establishment or signing),
this exceptional use for one-time signing of the CSR is allowed, as
discussed in Section 5.6.3.2 of [SP-800-56A] and Section 5.2 of
[SP-800-57].
5. HTTP-CoAP Proxy
As noted in Section 5 of [RFC9148], 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 HTTP but not CoAP, thus requiring an
intermediary CoAP-to-HTTP proxy.
Since OSCORE is applicable to CoAP-mappable HTTP (see Section 11 of
[RFC8613]) the messages conveying the EST payloads can be protected
end-to-end between the EST client and EST server, irrespective of
transport protocol or potential transport layer security which may
need to be terminated in the proxy, see Figure 2. Therefore the
concept "Registrar" and its required trust relation with EST server
as described in Section 5 of [RFC9148] is not applicable.
The mappings between CoAP and HTTP referred to in Section 8.1 of
[RFC9148] apply, and additional mappings resulting from the use of
OSCORE are specified in Section 11 of [RFC8613].
OSCORE provides end-to-end security between EST Server and EST
Client. The additional use of TLS and DTLS is optional. If a secure
association is needed between the EST Client and the CoAP-to-HTTP
Proxy, this may also rely on OSCORE
[I-D.tiloca-core-oscore-capable-proxies].
Constrained-Node Network
.---------. .-----------------------------.
| CA | | |
'---------' | |
| | |
.------. HTTP .-----------------. CoAP .-----------. |
| EST |<------->| CoAP-to-HTTP |<-------->| EST Client| |
|Server| (TLS) | Proxy | (DTLS) '-----------' |
'------' '-----------------' |
| |
<------------------------------------------------> |
OSCORE | |
| |
'-----------------------------'
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Figure 2: CoAP-to-HTTP proxy at the CoAP boundary.
6. Security Considerations
6.1. Server-generated Private Keys
This document enables the EST client to request generation of private
keys and the enrollment of the corresponding public key through /skg
and /skc functions. As discussed in Section 9 of [RFC9148], the
transport of private keys generated at EST-server is inherently
risky. The use of server-generated private keys may lead to the
increased probability of digital identity theft. Therefore,
implementations SHOULD NOT use server-generated private key EST
functions.
A cryptographically secure pseudo-random number generator is required
to be available to generate good quality private keys on EST-clients.
A cryptographically secure pseudo-random number generator is also a
dependency of many security protocols. This includes the EDHOC
protocol, which EST-oscore uses for the mutual authentication of EST-
client and EST-server. If EDHOC is used and a secure pseudo-random
number generator is available, the EST-client MUST NOT use server-
generated private key EST functions. However, EST-oscore also allows
pre-shared OSCORE contexts to be used for authentication, meaning
that EDHOC may not necessarily be required in the protocol stack of
an EST-client. If EDHOC is not used for authentication, and the EST-
client device does not have a cryptographically secure pseudo-random
number generator, then the EST-client MAY use the server-generated
private key functions.
Although hardware random number generators are becoming dominantly
present in modern IoT devices, it has been shown that many available
hardware modules contain vulnerabilities and do not produce
cryptographically secure random numbers. It is therefore important
to use multiple randomness sources to seed the cryptographically
secure pseudo-random number generator.
6.2. Considerations on Channel Binding
Section 3 of [RFC9148] specifies that the use of connection-based
channel binding is optional, and achieves it by including the tls-
unique value in the CSR. This specification when used with EDHOC for
the enrollment of static DH keys achieves connection-based channel
binding by using the EDHOC ephemeral key of the responder as the
public key in the proof-of-possession algorithm which generates a
PKCS#10 MAC. Therefore, connection-based channel binding is in this
case achieved without any additional overhead.
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Other cases include pre-shared OSCORE contexts and the case where the
signature key used for signing the CSR is different from the key used
in the EDHOC run. In these other cases, this specification makes
explicit channel binding based on the challengePassword attribute in
PKCS#10 requests OPTIONAL. For example, the challengePassword
attribute could be used for freshness in the case of pre-shared
OSCORE contexts and a re-enrollment request.
How challengePassword is generated is outside of the scope of this
specification and can be specified by an application profile.
7. IANA Considerations
This document does not require any IANA registrations.
8. References
8.1. Normative References
[I-D.ietf-core-oscore-edhoc]
Palombini, F., Tiloca, M., Höglund, R., Hristozov, S., and
G. Selander, "Using Ephemeral Diffie-Hellman Over COSE
(EDHOC) with the Constrained Application Protocol (CoAP)
and Object Security for Constrained RESTful Environments
(OSCORE)", Work in Progress, Internet-Draft, draft-ietf-
core-oscore-edhoc-11, 9 April 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
oscore-edhoc-11>.
[I-D.ietf-cose-cbor-encoded-cert]
Mattsson, J. P., Selander, G., Raza, S., Höglund, J., and
M. Furuhed, "CBOR Encoded X.509 Certificates (C509
Certificates)", Work in Progress, Internet-Draft, draft-
ietf-cose-cbor-encoded-cert-11, 8 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-cose-
cbor-encoded-cert-11>.
[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>.
[RFC6955] Schaad, J. and H. Prafullchandra, "Diffie-Hellman Proof-
of-Possession Algorithms", RFC 6955, DOI 10.17487/RFC6955,
May 2013, <https://www.rfc-editor.org/info/rfc6955>.
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[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>.
[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>.
[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>.
[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>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/info/rfc9052>.
[RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
August 2022, <https://www.rfc-editor.org/info/rfc9053>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.
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[RFC9148] van der Stok, P., Kampanakis, P., Richardson, M., and S.
Raza, "EST-coaps: Enrollment over Secure Transport with
the Secure Constrained Application Protocol", RFC 9148,
DOI 10.17487/RFC9148, April 2022,
<https://www.rfc-editor.org/info/rfc9148>.
[RFC9360] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Header Parameters for Carrying and Referencing X.509
Certificates", RFC 9360, DOI 10.17487/RFC9360, February
2023, <https://www.rfc-editor.org/info/rfc9360>.
[RFC9528] Selander, G., Preuß Mattsson, J., and F. Palombini,
"Ephemeral Diffie-Hellman Over COSE (EDHOC)", RFC 9528,
DOI 10.17487/RFC9528, March 2024,
<https://www.rfc-editor.org/info/rfc9528>.
8.2. Informative References
[I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
and R. Höglund, "Group Object Security for Constrained
RESTful Environments (Group OSCORE)", Work in Progress,
Internet-Draft, draft-ietf-core-oscore-groupcomm-23, 26
September 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-oscore-groupcomm-23>.
[I-D.tiloca-core-oscore-capable-proxies]
Tiloca, M. and R. Höglund, "OSCORE-capable Proxies", Work
in Progress, Internet-Draft, draft-tiloca-core-oscore-
capable-proxies-07, 10 July 2023,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
oscore-capable-proxies-07>.
[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>.
[RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management
Requirements", RFC 6024, DOI 10.17487/RFC6024, October
2010, <https://www.rfc-editor.org/info/rfc6024>.
[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>.
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[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>.
[RFC9031] Vučinić, M., Ed., Simon, J., Pister, K., and M.
Richardson, "Constrained Join Protocol (CoJP) for 6TiSCH",
RFC 9031, DOI 10.17487/RFC9031, May 2021,
<https://www.rfc-editor.org/info/rfc9031>.
[SP-800-56A]
Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
Davis, "Recommendation for Pair-Wise Key-Establishment
Schemes Using Discrete Logarithm Cryptography",
NIST Special Publication 800-56A Revision 3, April 2018,
<https://doi.org/10.6028/NIST.SP.800-56Ar3>.
[SP-800-57]
Barker, E., "Recommendation for Key Management",
NIST Special Publication 800-57 Revision 5, May 2020,
<https://doi.org/10.6028/NIST.SP.800-57pt1r5>.
Appendix A. Example Enrollment With Optimizations
The message flow starts with the EST client sending EDHOC message_1.
The EDHOC handshake follows and concludes with the EDHOC message_3.
EDHOC message_3 is carried in the same message as the OSCORE
enrollment request, as specified in [I-D.ietf-core-oscore-edhoc].
The OSCORE enrollment request contains a CoAP POST to the /sen
endpoint. This POST request includes the Content-Format option set
to the value application/cose-c509-pkcs10, and the Accept option set
to the value application/cose-c509-cert, indicating the support for
CBOR-encoded objects. In response, the client receives the
application/cose-c509-cert object which contains the certificate.
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EST Client EST Server
| |
| EDHOC message_1 |
+------------------------------------>|
| EDHOC message_2 |
|<------------------------------------+
| EDHOC message_3 |
| + |
| OSCORE request to /sen |
+------------------------------------>|
| POST /.well-known/est/sen |
| Accept: TBD6 |
| Content-Format: TBD7 |
| Payload: Serialized TBD7 |
| |
| OSCORE response |
|<------------------------------------+
| 2.04 Changed |
| Content-Format: TBD6 |
| Payload: Serialized TBD6 |
Figure 3: Enrollment EST-oscore flow with optimizations.
Acknowledgments
The authors would like to thank Marco Tiloca, and John Mattsson for
providing a review of the document.
Authors' Addresses
Göran Selander
Ericsson AB
Email: goran.selander@ericsson.com
Shahid Raza
RISE
Email: shahid.raza@ri.se
Martin Furuhed
Nexus
Email: martin.furuhed@nexusgroup.com
Mališa Vučinić
Inria
Email: malisa.vucinic@inria.fr
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Timothy Claeys
Email: timothy.claeys@gmail.com
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