Protecting EST Payloads with OSCORE

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 [I-D.ietf-lake-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 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 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.¶

3.1. 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 [I-D.ietf-cose-x509]) 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. In this case 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 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, and it is supported when EDHOC is executed prior to enrollment. Connection-based proof-of-possession is not supported when pre-shared OSCORE context is used.¶

When EDHOC is executed prior to enrollment, 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(TBD1, "EDHOC Unique", length),¶

where TBD1 is a registered label from the EDHOC Exporter Label registry, length equals the desired length of the edhoc-unique byte string. Unless otherwise indicated by an application profile, the length SHOULD be set to 32 bytes. The client then adds the edhoc-unique byte string as a challengePassword (see Section 5.4.1 of [RFC2985]) in the attributes section of the PKCS#10 Request [RFC2986] 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.¶

3.4. Optimizations

• 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 [I-D.ietf-core-oscore-edhoc].¶
• The certificates MAY be compressed, e.g., using the CBOR encoding defined in [I-D.ietf-cose-cbor-encoded-cert].¶
• The client certificate MAY be referenced instead of transported [I-D.ietf-cose-x509]. 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.¶

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.¶

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).¶

4.3. 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. Table 2 summarizes the information from Section 4.3 in [RFC9148].¶

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 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.¶

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.¶

Table 3 summarizes the Content-Format identifiers used in responses to /skg and /skc.¶

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 [I-D.ietf-lake-edhoc]). 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.¶

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.¶

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. 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-possesion algorithm. The EST client obtained the CA certs including the CA's DH certificate using the /crts function. The certificate indicates the DH group parameters which MUST be respected by the EST client when generating its own DH key pair. The EST client prepares the PKCS #10 object and computes a MAC by following the steps in Section 4 of [RFC6955]. The Key Derivation Function (KDF) and the MAC MUST be set to the HDKF and HMAC algorithms used by OSCORE. As per [RFC8613], the HKDF MUST be one of the HMAC-based HKDF [RFC5869] algorithms defined for COSE [RFC9052]. 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.¶

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 channel binding is optional, and achieves it by including the tls-unique value in the CSR. As a rationale, Section 9 of [RFC9148] discusses the Triple SHAKE attack: the attack relies on the absence of the server certificate as a dependency in the tls-unique value in case of TLS 1.2. This was mitigated in TLS 1.2 with [RFC7627], and in TLS 1.3 through the tls-exporter API, which computes the value by taking into account the full handshake transcript. Similarly, this specification when used with EDHOC achieves channel binding through the EDHOC-Exporter interface, which also relies on the full handshake transcript. Therefore, authentication based on EDHOC is not susceptible to the same attack as the one considered in [RFC9148]. At the time of the writing, it seems to be safe not to require channel binding and the inclusion of EDHOC-Exporter value in CSR. However, this specification makes channel binding OPTIONAL, as a mitigation against any other attacks that might be discovered in future.¶

8.1. 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).¶

+-------------+------------------------------+-------------------+
| Label       | Description                  | Reference         |
+=============+==============================+===================+
| TBD1        | EDHOC unique                 | [[this document]] |
+-------------+------------------------------+-------------------+