Protecting EST Payloads with OSCORE

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 . 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 HTTP and TLS . This document describes a method for protecting EST payloads over CoAP or HTTP with OSCORE . 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 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 need to be kept at a minimum, such as 6TiSCH , or for securing CoAP group 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 the EST-oscore client and EST-oscore server. For this purpose we assume by default the use of the lightweight authenticated key exchange protocol EDHOC , although pre-shared OSCORE keying material would also be an option. 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, CBOR representations of EST payloads (see )

Operational Differences with EST-coaps 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 by, or complemented with, OSCORE.
The DTLS handshake is replaced by, 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.
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 the EST server as described in Section 5 of 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 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.

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 when, and only when, they appear in all capitals, as shown here. 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.

Authentication This specification replaces, or complements, 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 used to protect the messages conveying EST payloads. 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 . The server must also provide relevant information to the CA for decision about issuing a certificate.

EDHOC 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., x5u, see ) or some other identifier which enables the receiving endpoint to retrieve the credential.

Certificate-based Authentication EST-oscore, like EST-coaps, supports certificate-based authentication between the 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 .

Channel Binding 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 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 . 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 .

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 enrolment request and response) in two round trips .
The enrolled certificates MAY be compressed, e.g., using the CBOR encoding defined in .
The enrolled client 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 specify 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.
The PKCS#10 object MAY request a certificate for a static DH key instead of a signature key. This results in a more compact request because the use of static DH keys implies 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.

Protocol Design and Layering 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 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 . below shows the layered EST-oscore architecture. Note that 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 .

EST protected with OSCORE. EST-oscore follows much of the EST-coaps and EST design.

Discovery and URI The discovery of EST resources and the definition of the short EST-coaps URI paths specified in Section 4.1 of , as well as the new Resource Type defined in Section 8.2 of apply to EST-oscore. Support for OSCORE is indicated by the "osc" attribute defined in Section 9 of . Example: ; 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.

Mandatory/optional EST Functions The EST-oscore specification has the same set of required-to-implement functions as EST-coaps. The content of is adapted from Section 4.2 in and uses the updated URI paths (see ). Mandatory and optional EST-oscore functions

EST functions	EST-oscore implementation
/crts	MUST
/sen	MUST
/sren	MUST
/skg	OPTIONAL
/skc	OPTIONAL
/att	OPTIONAL

/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 ), 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 .

Payload formats 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 identifiers when transferring EST requests and responses. summarizes the information from Section 4.3 in . EST functions and the associated CoAP Content-Format identifiers

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

Content-Format 281 MUST be supported by EST-oscore servers. Servers MAY also support Content-Format 287. It is up to the client to support only Content-Format 281, 287 or both. As indicated in , 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. The generated response for /skg and /skc requests contains two parts: certificate and the corresponding private key. 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. summarizes the Content-Format identifiers used in responses to /skg and /skc. Response Content-Format identifiers for /skg and /skc

Function	Response, Part 1	Response, Part 2
/skg	284	281
/skc	284	287

Message Bindings Note that the EST-oscore message characteristics are identical to those specified in Section 4.4 of . 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".

CoAP response codes See Section 4.5 in .

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 ). Together with various measures listed in this document such as CBOR-encoded payloads , CBOR certificates , certificates by reference (), and trust anchors without signature (), 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. It is recommended to prevent IP 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.

Delayed Responses See Section 4.7 in .

Enrollment of Static DH Keys This section specifies how the EST client enrolls a static DH key. Because a DH key pair cannot be used for signing operations, the EST client attempting to enroll a DH key must use an alternative proof-of-possession algorithm. The EST client prepares the PKCS#10 object and computes a MAC, replacing the signature, over the certification request information by following the steps in . 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 , 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 , the client MUST use the public ephemeral key of the EDHOC Responder, G_Y, as the recipient public key in the algorithm outlined in . 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 , the client has already in EDHOC message_2 obtained the ephemeral key G_Y of the server.

HTTP-CoAP Proxy As noted in Section 5 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 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 . Therefore the concept "Registrar" and its required trust relation with EST server as described in Section 5 of is not applicable. The mappings between CoAP and HTTP referred to in Section 8.1 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 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 .

CoAP-to-HTTP proxy at the CoAP boundary. | CoAP-to-HTTP |<-------->| EST Client| | |Server| (TLS) | Proxy | (DTLS) '-----------' | '------' '-----------------' | | | <------------------------------------------------> | OSCORE | | | | '-----------------------------' ]]>

Security Considerations

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 , 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.

Considerations on Channel Binding 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. In other cases, including pre-shared OSCORE contexts, this specification makes explicit channel binding based on the challengePassword attribute in PKCS#10 requests OPTIONAL. 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.

Privacy Considerations TBD

IANA Considerations

EDHOC Exporter Label Registry IANA is requested to register the following entry in the "EDHOC Exporter Label" registry under the group name "Ephemeral Diffie-Hellman Over COSE (EDHOC). EDHOC Exporter Label.

Label	Description	Response
TBD1	EDHOC unique	[[this document]]

Acknowledgments The authors would like to thank Marco Tiloca and John Mattsson for providing a review of the document.