]> Ericsson AB

goran.selander@ericsson.com

RISE

shahid.raza@ri.se

Nexus

martin.furuhed@nexusgroup.com

INRIA

malisa.vucinic@inria.fr

timothy.claeys@gmail.com

Security ACE Working Group Internet-Draft This document specifies public-key certificate enrollment procedures protected with lightweight application-layer security protocols suitable for Internet of Things (IoT) deployments. The protocols leverage payload formats defined in Enrollment over Secure Transport (EST) and existing IoT standards including the Constrained Application Protocol (CoAP), Concise Binary Object Representation (CBOR) and the CBOR Object Signing and Encryption (COSE) format.

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 . One optimization of certificate enrollment targeting IoT deployments is specified in EST-coaps (), which defines a version of Enrollment over Secure Transport for transporting EST payloads over CoAP and DTLS , instead of secured HTTP. This document describes a method for protecting EST payloads over CoAP or HTTP with OSCORE . OSCORE specifies an extension to CoAP which protects the application layer message 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 underlying transport and through proxies translating between between CoAP and HTTP. OSCORE is designed for constrained environments, building on IoT standards such as CoAP, CBOR and COSE , and has in particular gained traction in settings where message sizes and the number of exchanged messages needs to be kept at a minimum, such as 6TiSCH , or for securing multicast CoAP messages . Where OSCORE is implemented and used for communication security, the reuse of OSCORE for other purposes, such as enrollment, reduces the code footprint. In order to protect certificate enrollment with OSCORE, the necessary keying material (notably, the OSCORE Master Secret, see ) needs to be established between EST-oscore client and EST-oscore server. For this purpose we assume the use of the lightweight authenticated key exchange protocol EDHOC . Other methods for key establishment are described in . Other ways to optimize the performance of certificate enrollment and certificate based authentication described in this draft include the use of: Compact representations of X.509 certificates (see ) Certificates by reference (see ) Compact representations of EST payloads (see )

The protection of EST payloads defined in this document builds on EST-coaps 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, or complemented, with OSCORE. The DTLS handshake is replaced, or complemented, with the lightweight authenticated key exchange protocol EDHOC , 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. One new EST function, /rpks, is defined for installation of compact explicit TAs in the EST client. 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 without any security processing in the proxy (see ). The concept “Registrar” and its required trust relation with EST server as described in Section 6 of is therefore redundant. 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 and a new matching EST function. 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. discusses yet other authentication and secure communication methods.

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in . These words may also appear in this document in lowercase, absent their normative meanings. This document uses terminology from which in turn is based on and, in turn, on . The term “Trust Anchor” follows the terminology of : “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.” One example of specifying more compact alternatives to X.509 certificates for exchanging trust anchor information is provided by the TrustAnchorInfo structure of , the mandatory parts of which essentially is the SubjectPublicKeyInfo structure , i.e., an algorithm identifier followed by a public key.

This specification replaces the DTLS handshake in EST-coaps with the lightweight authenticated key exchange protocol EDHOC . During initial enrollment the EST-oscore client and server run EDHOC to authenticate and establish the OSCORE security context with which the EST payloads are protected. 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 and provide relevant information to the CA for decision about issuing a certificate.

EDHOC supports authentication with certificates/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 to the credential (e.g., x5t, see ) or some other identifier which enables the receiving endpoint to retrieve the credential.

EST-oscore, like EST-coaps, supports certificate-based authentication between EST client and server. In this case the client MUST be configured with an Implicit or Explicit Trust Anchor (TA) 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 . The EST client and/or EST server certificate MAY be a (natively signed) CBOR certificate .

The 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 signature key, a proof-of-possession is generated by the client when it signs the PKCS#10 Request during the enrollment phase. Connection-based proof-of-possession is OPTIONAL for EST-oscore clients and servers. When desired the client can use the EDHOC-Exporter API to extract channel-binding information and provide a connection-based proof-of possession. Channel-binding information is obtained as follows edhoc-unique = EDHOC-Exporter(“EDHOC Unique”, length), where length equals the desired length of the edhoc-unique byte string. The client then adds the edhoc-unique byte string as a challengePassword (see Section 5.4.1 of ) in the attributes section of the PKCS#10 Request to prove to the server that the authenticated EDHOC client is in possession of the private key associated with the certification request, and signed the certification request after the EDHOC session was established.

The last 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 enrolment request and response) in two round trips . The certificates MAY be compressed, e.g. using the CBOR encoding defined in . The certificate MAY be referenced instead of transported . 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 be a reference to the enrolled certificate rather than the certificate itself. The EST-oscore server MAY in the enrolment response to the EST-oscore client include a pointer to a directory or database where the certificate can be retrieved.

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 ), 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 . A client MAY request an unsigned trust anchors using the /rpks function (see ). Client authentication can be performed with long-lived RPKs installed by the manufacturer. Re-enrollment requests can be authenticated through a valid certificate issued previously by the EST-oscore server or by using the key material available in the Implicit TA database. TODO: Sanity check this. Review the use of Implicit TA vs. Explicit TA.

EST-oscore uses CoAP and Block-Wise to transfer EST messages in the same way as . Instead of DTLS record layer, OSCORE is used to protect the EST payloads. below shows the layered EST-oscore architecture.

EST-oscore follows much of the EST-coaps and EST design.

The discovery of EST resources and the definition of the short EST-coaps URI paths specified in Section 5.1 of , as well as the new Resource Type defined in Section 9.1 of apply to EST-oscore. Support for OSCORE is indicated by the “osc” attribute defined in Section 9 of , for example:

; rt="ace.est";osc ]]>

The EST client can request a copy of the current CA public keys. TODO: Map relevant parts of section 4.1 of RFC 7030 and other EST function related content from RFC7030 and EST-coaps. RATIONALE: 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. Motivated by the specification of more compact trust anchors (see ) we define here the new EST function /rpks which returns a set of RPKs to be installed in the Explicit TA database.

The EST-oscore specification has the same set of required-to-implement functions as EST-coaps. The content of is adapted from Section 5.2 in and uses the updated URI paths (see ). EST functions EST-oscore implementation /crts MUST /sen MUST /sren MUST /skg OPTIONAL /skc OPTIONAL /att OPTIONAL TODO: Add /rpks OPTIONAL

Similar to EST-coaps, EST-oscore allows transport of the ASN.1 structure of a given Media-Type in binary format. In addition, EST-oscore uses the same CoAP Content-Format Options to transport EST requests and responses . summarizes the information from Section 5.3 in . URI Content-Format #IANA /crts N/A (req) -   application/pkix-cert (res) 287   application/pkcs-7-mime;smime-type=certs-only (res) 281 /sen application/pkcs10 (req) 286   application/pkix-cert (res) 287   application/pkcs-7-mime;smime-type=certs-only (res) 281 /sren application/pkcs10 (req) 286   application/pkix-cert (res) 287   application/pkcs-7-mime;smime-type=certs-only (res) 281 /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 NOTE: CBOR is becoming a de facto encoding scheme in IoT settings. There is already work in progress on CBOR encoding of X.509 certificates , and this can be extended to other EST messages, see .

The EST-oscore message characteristics are identical to those specified in Section 5.4 of . It is RECOMMENDED 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.

See Section 5.5 in .

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 ). Together with various measures listed in this document such as CBOR payloads (), CBOR certificates , certificates by reference (), and trust anchors without signature (), a significant reduction of message sizes can be achieved. Nevertheless, depending on application, the protocol messages may become larger than available frame size resulting in fragmentation and, in resource constrained networks such as IEEE 802.15.4 where throughput is limited, fragment loss can trigger costly retransmissions. It is RECOMMENDED to prevent IP fragmentation, since it involves an error-prone datagram reconstitution. To limit the size of the CoAP payload, this specification mandates the implementation of CoAP option Block1 and Block2 fragmentation mechanism as described in Section 5.6 of .

See Section 5.7 in .

As noted in Section 6 of , 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 ) the EST payloads can be protected end-to-end between EST client and EST server independent of transport protocol or potential transport layer security which may need to be terminated in the proxy, see . Therefore the concept “Registrar” and its required trust relation with EST server as described in Section 6 of is redundant. The mappings between CoAP and HTTP referred to in Section 9.1 of apply, and additional mappings resulting from the use of OSCORE are specified in Section 11 of . OSCORE provides end-to-end security between EST Server and EST Client. The use of TLS and DTLS is optional.

| CoAP-to-HTTP |<-------->| EST Client| || |Server| (TLS) | Proxy | (DTLS) '-----------' || '------' '-----------------' || || || <------------------------------------------------> || OSCORE || || |'--------------------------'| '----------------------------' ]]>

TBD

TBD

&RFC2119; &RFC8949; &RFC7252; &RFC7925; &RFC7959; &RFC8152; &RFC8613; &I-D.ietf-lake-edhoc; &I-D.ietf-ace-coap-est; &RFC2985; &RFC2986; &RFC5272; &RFC5280; &RFC5914; &RFC6024; &RFC6347; &RFC7228; &RFC7030; &RFC8392; &I-D.ietf-6tisch-minimal-security; &I-D.ietf-ace-oscore-profile; &I-D.ietf-ace-oauth-authz; &I-D.ietf-core-oscore-groupcomm; &I-D.ietf-cose-x509; &I-D.mattsson-cose-cbor-cert-compress; &I-D.palombini-core-oscore-edhoc;

In order to protect certificate enrollment with OSCORE, the necessary keying material (notably, the OSCORE Master Secret, see ) needs to be established between EST-oscore client and EST-oscore server. In this appendix we analyse alternatives to EDHOC, which was assumed in the body of this specification.

Trusted third party (TTP) based provisioning, such as the OSCORE profile of ACE assumes existing security associations between the client and the TTP, and between the server and the TTP. This setup allows for reduced message overhead and round trips compared to the full-fledged EDHOC key exchange. Following the ACE terminology the TTP plays the role of the Authorization Server (AS), the EST-oscore client corresponds to the ACE client and the EST-oscore server is the ACE Resource Server (RS).

| Trusted | | | | Third | | | <--(B)- Access Token ------- | Party (AS) | | | | | | | +------------+ | EST-oscore | | ^ | Client | (F) (E) |(ACE Client)| V | | | +------------+ | | | | | | -(C)- Token + EST Request --> | EST-oscore | | | | server (RS)| | | <--(D)--- EST response ------ | | | | | | +------------+ +------------+ ]]>

During initial enrollment the EST-oscore client uses its existing security association with the TTP, which replaces the Implicit TA database, to establish an authenticated secure channel. The ACE profile RECOMMENDS the use of OSCORE between client and TTP (AS), but TLS or DTLS MAY be used additionally or instead. The client requests an access token at the TTP corresponding the EST service it wants to access. If the client request was invalid, or not authorized according to the local EST policy, the AS returns an error response as described in Section 5.6.3 of . In its responses the TTP (AS) SHOULD signal that the use of OSCORE is REQUIRED for a specific access token as indicated in section 4.3 of . This means that the EST-oscore client MUST use OSCORE towards all EST-oscore servers (RS) for which this access token is valid, and follow Section 4.3 in to derive the security context to run OSCORE. The ACE OSCORE profile RECOMMENDS the use of CBOR web token (CWT) as specified in . The TTP (AS) MUST also provision an OSCORE security context to the EST-oscore client and EST-oscore server (RS), which is then used to secure the subsequent messages between the client and the server. The details on how to transfer the OSCORE contexts are described in section 3.2 of . Once the client has retrieved the access token it follows the steps in to install the OSCORE security context and presents the token to the EST-oscore server. The EST-oscore server installs the corresponding OSCORE context and can either verify the validity of the token locally or request a token introspection at the TTP. In either case EST policy decisions, e.g., which client can request enrollment or reenrollment, can be implemented at the TTP. Finally the EST-oscore client receives a response from the EST-oscore server.

Another method to bootstrap EST services requires a pre-shared OSCORE security context between the EST-oscore client and EST-oscore server. Authentication using the Implicit TA is no longer required since the shared security context authenticates both parties. The EST-oscore client and EST-oscore server need access to the same OSCORE Master Secret as well as the OSCORE identifiers (Sender ID and Recipient ID) from which an OSCORE security context can be derived, see . Some optional parameters may be provisioned if different from the default value: an ID context distinguishing between different OSCORE security contexts to use, an AEAD algorithm, an HKDF algorithm, a master salt, and a replay window size.

Current EST based specifications transport messages using the ASN.1 data type declaration. It would be favorable to use a more compact representation better suitable for constrained device implementations. In this appendix we list CBOR encodings of requests and responses of the mandatory EST functions (see ).

The EST client can request a copy of the current CA certificates. In EST-coaps and EST-oscore this is done using a GET request to /crts (with empty payload). The response contains a chain of certificates used to establish an Explicit Trust Anchor database for subsequent authentication of the EST server. CBOR encoding of X.509 certificates is specified in . CBOR encoding of certificate chains is specified below. This allows for certificates encoded using the CBOR certificate format, or as binary X.509 data wrapped as a CBOR byte string. CDDL:

Existing EST standards specify the enrollment request to be a PKCS#10 formated message . The essential information fields for the CA to verify are the following: Information about the subject, here condensed to the subject common name, subject public key, and signature made by the subject private key. CDDL:

The response to the enrollment request is the subject certificate, for which CBOR encoding is specified in . The same message content in request and response applies to re-enrollment. TODO: PKCS#10 allows inclusion of attributes, which can be used to specify extension requests, see Section 5.4.2 of . CBOR encoding of the challengePassword attribute needs to be defined (see ). What other attributes are relevant?

Here is an example of CBOR encoding of certificate request as defined in the previous section. 114 bytes: ( h’0123456789ABCDF0’, h’61eb80d2abf7d7e4139c86b87e42466f1b4220d3f7ff9d6a1ae298fb9adbb464’, h’30440220064348b9e52ee0da9f9884d8dd41248c49804ab923330e208a168172dcae1 27a02206a06c05957f1db8c4e207437b9ab7739cb857aa6dd9486627b8961606a2b68ae’ ) In the example above the signature is generated on an ASN.1 data structure. To validate this, the receiver needs to reconstruct the original data structure. Alternatively, in native mode, the signature is generated on the profiled data structure, in which case the overall overhead is further reduced.

A corresponding certificate request of the previous section using ASN.1 is shown in .

In Base64, 375 bytes:

In hex, 221 bytes: