]> Consultant

consultancy@vanderstok.org

Cisco Systems

pkampana@cisco.com

Sandelman Software Works

mcr+ietf@sandelman.ca http://www.sandelman.ca/

RISE SICS

Isafjordsgatan 22 Kista Stockholm 16440 SE shahid@sics.se

Security ACE Enrollment over Secure Transport (EST) is used as a certificate provisioning protocol over HTTPS. Low-resource devices often use the lightweight Constrained Application Protocol (CoAP) for message exchanges. This document defines how to transport EST payloads over secure CoAP (EST-coaps), which allows constrained devices to use existing EST functionality for provisioning certificates.

EDNOTE: Remove this section before publication -18 IESG Reviews fixes. Removed spurious lines introduced in v-17 due to xml2rfc v3. -17 v16 remnants by Ben K. Typos. -16 Updates to address Yaron S.'s Secdir review. Updates to address David S.'s Gen-ART review. -15 Updates to addressed Ben's AD follow up feedback. -14 Updates to complete Ben's AD review feedback and discussions. -13 Updates based on AD's review and discussions Examples redone without password -12 Updated section 5 based on Esko's comments and nits identified. Nits and some clarifications for Esko's new review from 5/21/2019. Nits and some clarifications for Esko's new review from 5/28/2019. -11 Updated Server-side keygen to simplify motivation and added paragraphs in Security considerations to point out that random numbers are still needed (feedback from Hannes). -10 Addressed WGLC comments More consistent request format in the examples. Explained root resource difference when there is resource discovery Clarified when the client is supposed to do discovery Fixed nits and minor Option length inaccurracies in the examples. -09 WGLC comments taken into account consensus about discovery of content-format added additional path for content-format selection merged DTLS sections -08 added application/pkix-cert Content-Format TBD287. discovery text clarified Removed text on ct negotiation in connection to multipart-core removed text that duplicates or contradicts RFC7252 (thanks Klaus) Stated that well-known/est is compulsory Use of response codes clarified. removed bugs: Max-Age and Content-Format Options in Request Accept Option explained for est/skg and added in enroll example Added second URI /skc for server-side key gen and a simple cert (not PKCS#7) Persistence of DTLS connection clarified. Minor text fixes. -07: redone examples from scratch with openssl Updated authors. Added CoAP RST as a MAY for an equivalent to an HTTP 204 message. Added serialization example of the /skg CBOR response. Added text regarding expired IDevIDs and persistent DTLS connection that will start using the Explicit TA Database in the new DTLS connection. Nits and fixes Removed CBOR envelop for binary data Replaced TBD8 with 62. Added RFC8174 reference and text. Clarified MTI for server-side key generation and Content-Formats. Defined the /skg MTI (PKCS#8) and the cases where CMS encryption will be used. Moved Fragmentation section up because it was referenced in sections above it. -06: clarified discovery section, by specifying that no discovery may be needed for /.well-known/est URI. added resource type values for IANA added list of compulsory to implement and optional functions. Fixed issues pointed out by the idnits tool. Updated CoAP response codes section with more mappings between EST HTTP codes and EST-coaps CoAP codes. Minor updates to the MTI EST Functions section. Moved Change Log section higher. -05: repaired again TBD8 = 62 removed from C-F registration, to be done in CT draft. -04: Updated Delayed response section to reflect short and long delay options. -03: Removed observe and simplified long waits Repaired Content-Format specification -02: Added parameter discussion in section 8 Concluded Content-Format specification using multipart-ct draft examples updated -01: Editorials done. Redefinition of proxy to Registrar in . Explained better the role of https-coaps Registrar, instead of "proxy" Provide "observe" Option examples extended block message example. inserted new server key generation text in and motivated server key generation. Broke down details for DTLS 1.3 New Media-Type uses CBOR array for multiple Content-Format payloads provided new Content-Format tables new media format for IANA -00 copied from vanderstok-ace-coap-04

"Classical" Enrollment over Secure Transport (EST) is used for authenticated/authorized endpoint certificate enrollment (and optionally key provisioning) through a Certificate Authority (CA) or Registration Authority (RA). EST transports messages over HTTPS. This document defines a new transport for EST based on the Constrained Application Protocol (CoAP) since some Internet of Things (IoT) devices use CoAP instead of HTTP. Therefore, this specification utilizes DTLS and CoAP instead of TLS and HTTP . EST responses can be relatively large and for this reason this specification also uses CoAP Block-Wise Transfer to offer a fragmentation mechanism of EST messages at the CoAP layer. This document also profiles the use of EST to only support certificate-based client authentication. HTTP Basic or Digest authentication (as described in Section 3.2.3 of ) are not supported.

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. Many of the concepts in this document are taken from . Consequently, much text is directly traceable to .

This section describes how EST-coaps conforms to the profiles of low-resource devices described in . EST-coaps can transport certificates and private keys. Certificates are responses to (re-)enrollment requests or requests for a trusted certificate list. Private keys can be transported as responses to a server-side key generation request as described in Section 4.4 of (and subsections) and discussed in of this document. EST-coaps depends on a secure transport mechanism that secures the exchanged CoAP messages. DTLS is one such secure protocol. No other changes are necessary regarding the secure transport of EST messages.

In accordance with sections 3.3 and 4.4 of , the mandatory cipher suite for DTLS in EST-coaps is TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 . Curve secp256r1 MUST be supported ; this curve is equivalent to the NIST P-256 curve. After the publication of , support for Curve25519 will likely be required in the future by (D)TLS Profiles for the Internet of Things . DTLS 1.2 implementations must use the Supported Elliptic Curves and Supported Point Formats Extensions in . Uncompressed point format must also be supported. DTLS 1.3 implementations differ from DTLS 1.2 because they do not support point format negotiation in favor of a single point format for each curve. Thus, support for DTLS 1.3 does not mandate point format extensions and negotiation. In addition, in DTLS 1.3 the Supported Elliptic Curves extension has been renamed to Supported Groups. CoAP was designed to avoid IP fragmentation. DTLS is used to secure CoAP messages. However, fragmentation is still possible at the DTLS layer during the DTLS handshake when using ECC ciphersuites. If fragmentation is necessary, "DTLS provides a mechanism for fragmenting a handshake message over several records, each of which can be transmitted separately, thus avoiding IP fragmentation" . The authentication of the EST-coaps server by the EST-coaps client is based on certificate authentication in the DTLS handshake. The EST-coaps client MUST be configured with at least an Implicit TA database which will enable the authentication of the server the first time before updating its trust anchor (Explicit TA) . The authentication of the EST-coaps client MUST be with a client certificate in the DTLS handshake. This can either be a previously issued client certificate (e.g., an existing certificate issued by the EST CA); this could be a common case for simple re-enrollment of clients. a previously installed certificate (e.g., manufacturer IDevID or a certificate issued by some other party). IDevID's are expected to have a very long life, as long as the device, but under some conditions could expire. In that case, the server MAY authenticate a client certificate against its trust store although the certificate is expired (). EST-coaps supports the certificate types and Trust Anchors (TA) that are specified for EST in Section 3 of . As described in Section 2.1 of proof-of-identity refers to a value that can be used to prove that an end-entity or client is in the possession of and can use the private key corresponding to the certified public key. Additionally, channel-binding information can link proof-of-identity with an established connection. Connection-based proof-of-possession is OPTIONAL for EST-coaps clients and servers. When proof-of-possession is desired, a set of actions are required regarding the use of tls-unique, described in Section 3.5 in . The tls-unique information consists of the contents of the first "Finished" message in the (D)TLS handshake between server and client . The client adds the "Finished" message as a ChallengePassword in the attributes section of the PKCS#10 Request to prove that the client is indeed in control of the private key at the time of the (D)TLS session establishment. In the case of handshake message fragmentation, if proof-of-possession is desired, the Finished message added as the ChallengePassword in the CSR is calculated as specified by the DTLS standards. We summarize it here for convenience. For DTLS 1.2, in the event of handshake message fragmentation, the Hash of the handshake messages used in the MAC calculation of the Finished message must be computed on each reassembled message, as if each message had not been fragmented (Section 4.2.6 of ). The Finished message is calculated as shown in Section 7.4.9 of . Similarly, for DTLS 1.3, the Finished message must be computed as if each handshake message had been sent as a single fragment (Section 5.8 of ) following the algorithm described in 4.4.4 of . In a constrained CoAP environment, endpoints can't always afford to establish a DTLS connection for every EST transaction. An EST-coaps DTLS connection MAY remain open for sequential EST transactions, which was not the case with . For example, if a /crts request is followed by a /sen request, both can use the same authenticated DTLS connection. However, when a /crts request is included in the set of sequential EST transactions, some additional security considerations apply regarding the use of the Implicit and Explicit TA database as explained in . Given that after a successful enrollment, it is more likely that a new EST transaction will not take place for a significant amount of time, the DTLS connections SHOULD only be kept alive for EST messages that are relatively close to each other. These could include a /sen immediatelly following a /crts when a device is getting bootstrapped. In some cases, like NAT rebinding, keeping the state of a connection is not possible when devices sleep for extended periods of time. In such occasions, negotiates a connection ID that can eliminate the need for new handshake and its additional cost; or DTLS session resumption provides a less costly alternative than re-doing a full DTLS handshake.

EST-coaps uses CoAP to transfer EST messages, aided by Block-Wise Transfer to avoid IP fragmentation. The use of Blocks for the transfer of larger EST messages is specified in . shows the layered EST-coaps architecture. The EST-coaps protocol design follows closely the EST design. The supported message types in EST-coaps are: CA certificate retrieval needed to receive the complete set of CA certificates. Simple enroll and re-enroll for a CA to sign client identity public key. Certificate Signing Request (CSR) attribute messages that informs the client of the fields to include in a CSR. Server-side key generation messages to provide a client identity private key when the client chooses so. While permits a number of the EST functions to be used without authentication, this specification requires that the client MUST be authenticated for all functions.

EST-coaps is targeted for low-resource networks with small packets. Two types of installations are possible: (1) rigid ones, where the address and the supported functions of the EST server(s) are known, and (2) a flexible one, where the EST server and its supported functions need to be discovered. For both types of installations, saving header space is important and short EST-coaps URIs are specified in this document. These URIs are shorter than the ones in . Two example EST-coaps resource path names are:

/.well-known/est/ coaps://example.com:/.well-known/est/ArbitraryLabel/ ]]>

The short-est strings are defined in . Arbitrary Labels are usually defined and used by EST CAs in order to route client requests to the appropriate certificate profile. Implementers should consider using short labels to minimize transmission overhead. The EST-coaps server URIs, obtained through discovery of the EST-coaps resource(s) as shown below, are of the form:

// coaps://example.com://ArbitraryLabel/ ]]>

Figure 5 in Section 3.2.2 of enumerates the operations and corresponding paths which are supported by EST. provides the mapping from the EST URI path to the shorter EST-coaps URI path. EST EST-coaps /cacerts /crts /simpleenroll /sen /simplereenroll /sren /serverkeygen /skg (PKCS#7) /serverkeygen /skc (application/pkix-cert) /csrattrs /att The /skg message is the EST /serverkeygen equivalent where the client requests a certificate in PKCS#7 format and a private key. If the client prefers a single application/pkix-cert certificate instead of PKCS#7, it will make an /skc request. In both cases (i.e., /skg, /skc) a private key MUST be returned. Clients and servers MUST support the short resource EST-coaps URIs. In the context of CoAP, the presence and location of (path to) the EST resources are discovered by sending a GET request to "/.well-known/core" including a resource type (RT) parameter with the value "ace.est*" . The example below shows the discovery over CoAPS of the presence and location of EST-coaps resources. Linefeeds are included only for readability.

;rt="ace.est.crts";ct="281 TBD287", ;rt="ace.est.sen";ct="281 TBD287", ;rt="ace.est.sren";ct="281 TBD287", ;rt="ace.est.att";ct=285, ;rt="ace.est.skg";ct=62, ;rt="ace.est.skc";ct=62 ]]>

The first three lines, describing ace.est.crts, ace.est.sen, and ace.est.sren, of the discovery response above MUST be returned if the server supports resource discovery. The last three lines are only included if the corresponding EST functions are implemented (see ). The Content-Formats in the response allow the client to request one that is supported by the server. These are the values that would be sent in the client request with an Accept option. Discoverable port numbers can be returned in the response payload. An example response payload for non-default CoAPS server port 61617 follows below. Linefeeds are included only for readability.

;rt="ace.est.crts"; ct="281 TBD287", ;rt="ace.est.sen"; ct="281 TBD287", ;rt="ace.est.sren"; ct="281 TBD287", ;rt="ace.est.att"; ct=285, ;rt="ace.est.skg"; ct=62, ;rt="ace.est.skc"; ct=62 ]]>

The server MUST support the default /.well-known/est root resource. The server SHOULD support resource discovery when it supports non-default URIs (like /est or /est/ArbitraryLabel) or ports. The client SHOULD use resource discovery when it is unaware of the available EST-coaps resources. Throughout this document the example root resource of /est is used.

This specification contains a set of required-to-implement functions, optional functions, and not specified functions. The unspecified functions are deemed too expensive for low-resource devices in payload and calculation times. specifies the mandatory-to-implement or optional implementation of the EST-coaps functions. Discovery of the existence of optional functions is described in . EST Functions EST-coaps implementation /cacerts MUST /simpleenroll MUST /simplereenroll MUST /fullcmc Not specified /serverkeygen OPTIONAL /csrattrs OPTIONAL

EST-coaps is designed for low-resource devices and hence does not need to send Base64-encoded data. Simple binary is more efficient (30% smaller payload for DER-encoded ASN.1) and well supported by CoAP. Thus, the payload for a given Media-Type follows the ASN.1 structure of the Media-Type and is transported in binary format. The Content-Format (HTTP Content-Type equivalent) of the CoAP message determines which EST message is transported in the CoAP payload. The Media-Types specified in the HTTP Content-Type header field (Section 3.2.2 of ) are specified by the Content-Format Option (12) of CoAP. The combination of URI-Path and Content-Format in EST-coaps MUST map to an allowed combination of URI and Media-Type in EST. The required Content-Formats for these requests and response messages are defined in . The CoAP response codes are defined in . Content-Format TBD287 can be used in place of 281 to carry a single certificate instead of a PKCS#7 container in a /crts, /sen, /sren or /skg response. Content-Format 281 MUST be supported by EST-coaps servers. Servers MAY also support Content-Format TBD287. It is up to the client to support only Content-Format 281, TBD287 or both. The client 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. Content-Format 286 is used in /sen, /sren and /skg requests and 285 in /att responses. A representation with Content-Format identifier 62 contains a collection of representations along with their respective Content-Format. The Content-Format identifies the Media-Type application/multipart-core specified in . For example, a collection, containing two representations in response to a EST-coaps server-side key generation /skg request, could include a private key in PKCS#8 with Content-Format identifier 284 (0x011C) and a single certificate in a PKCS#7 container with Content-Format identifier 281 (0x0119). Such a collection would look like [284,h'0123456789abcdef', 281,h'fedcba9876543210'] in diagnostic CBOR notation. The serialization of such CBOR content would be

When the client makes an /skc request the certificate returned with the private key is a single X.509 certificate (not a PKCS#7 container) with Content-Format identifier TBD287 (0x011F) instead of 281. In cases where the private key is encrypted with CMS (as explained in ) the Content-Format identifier is 280 (0x0118) instead of 284. The content format used in the response is summarized in . Function Response part 1 Response part 2 /skg 284 281 /skc 280 TBD287 The key and certificate representations are DER-encoded ASN.1, in its native binary form. An example is shown in .

The general EST-coaps message characteristics are: EST-coaps servers sometimes need to provide delayed responses which are preceded by an immediately returned empty ACK or an ACK containing response code 5.03 as explained in . Thus, it is RECOMMENDED for implementers to send EST-coaps requests in confirmable CON CoAP messages. The CoAP Options used are Uri-Host, Uri-Path, Uri-Port, Content-Format, Block1, Block2, and Accept. These CoAP Options are used to communicate the HTTP fields specified in the EST REST messages. The Uri-host and Uri-Port Options can be omitted from the COAP message sent on the wire. When omitted, they are logically assumed to be the transport protocol destination address and port respectively. Explicit Uri-Host and Uri-Port Options are typically used when an endpoint hosts multiple virtual servers and uses the Options to route the requests accordingly. Other COAP Options should be handled in accordance with . EST URLs are HTTPS based (https://), in CoAP these are assumed to be translated to CoAPS (coaps://) provides the mapping from the EST URI path to the EST-coaps URI path. includes some practical examples of EST messages translated to CoAP.

Section 5.9 of and Section 7 of specify the mapping of HTTP response codes to CoAP response codes. The success code in response to an EST-coaps GET request (/crts, /att), is 2.05. Similarly, 2.04 is used in successful response to EST-coaps POST requests (/sen, /sren, /skg, /skc). EST makes use of HTTP 204 or 404 responses when a resource is not available for the client. In EST-coaps 2.04 is used in response to a POST (/sen, /sren, /skg, /skc). 4.04 is used when the resource is not available for the client. HTTP response code 202 with a Retry-After header field in has no equivalent in CoAP. HTTP 202 with Retry-After is used in EST for delayed server responses. specifies how EST-coaps handles delayed messages with 5.03 responses with a Max-Age Option. Additionally, EST's HTTP 400, 401, 403, 404 and 503 status codes have their equivalent CoAP 4.00, 4.01, 4.03, 4.04 and 5.03 response codes in EST-coaps. summarizes the EST-coaps response codes. operation EST-coaps response code Description /crts, /att 2.05 Success. Certs included in the response payload. 4.xx / 5.xx Failure. /sen, /skg, /sren, /skc 2.04 Success. Cert included in the response payload. 5.03 Retry in Max-Age Option time. 4.xx / 5.xx Failure.

DTLS defines fragmentation only for the handshake and not for secure data exchange (DTLS records). states that to avoid using IP fragmentation, which involves error-prone datagram reconstitution, invokers of the DTLS record layer should size DTLS records so that they fit within any Path MTU estimates obtained from the record layer. In addition, invokers residing on a 6LoWPAN over IEEE 802.15.4 network are recommended to size CoAP messages such that each DTLS record will fit within one or two IEEE 802.15.4 frames. That is not always possible in EST-coaps. Even though ECC certificates are small in size, they can vary greatly based on signature algorithms, key sizes, and Object Identifier (OID) fields used. For 256-bit curves, common ECDSA cert sizes are 500-1000 bytes which could fluctuate further based on the algorithms, OIDs, Subject Alternative Names (SAN) and cert fields. For 384-bit curves, ECDSA certificates increase in size and can sometimes reach 1.5KB. Additionally, there are times when the EST cacerts response from the server can include multiple certificates that amount to large payloads. Section 4.6 of CoAP describes the possible payload sizes: "if nothing is known about the size of the headers, good upper bounds are 1152 bytes for the message size and 1024 bytes for the payload size". Section 4.6 of also suggests that IPv4 implementations may want to limit themselves to more conservative IPv4 datagram sizes such as 576 bytes. Even with ECC, EST-coaps messages can still exceed MTU sizes on the Internet or 6LoWPAN (Section 2 of ). EST-coaps needs to be able to fragment messages into multiple DTLS datagrams. To perform fragmentation in CoAP, specifies the Block1 Option for fragmentation of the request payload and the Block2 Option for fragmentation of the return payload of a CoAP flow. As explained in Section 1 of , block-wise transfers should be used in Confirmable CoAP messages to avoid the exacerbation of lost blocks. EST-coaps servers MUST implement Block1 and Block2. EST-coaps clients MUST implement Block2. EST-coaps clients MUST implement Block1 only if they are expecting to send EST-coaps requests with a packet size that exceeds the Path MTU. also defines Size1 and Size2 Options to provide size information about the resource representation in a request and response. EST-client and server MAY support Size1 and Size2 Options. Examples of fragmented EST-coaps messages are shown in .

Server responses can sometimes be delayed. According to Section 5.2.2 of , a slow server can acknowledge the request and respond later with the requested resource representation. In particular, a slow server can respond to an EST-coaps enrollment request with an empty ACK with code 0.00, before sending the certificate to the client after a short delay. If the certificate response is large, the server will need more than one Block2 block to transfer it. This situation is shown in . The client sends an enrollment request that uses N1+1 Block1 blocks. The server uses an empty 0.00 ACK to announce the delayed response which is provided later with 2.04 messages containing N2+1 Block2 Options. The first 2.04 is a confirmable message that is acknowledged by the client. Onwards, the client acknowledges all subsequent Block2 blocks. The notation of is explained in .

<-- (ACK) (1:0/1/256) (2.31 Continue) POST [2001:db8::2:1]:61616/est/sen (CON)(1:1/1/256) {CSR (frag# 2)} --> <-- (ACK) (1:1/1/256) (2.31 Continue) . . . POST [2001:db8::2:1]:61616/est/sen(CON)(1:N1/0/256){CSR (frag# N1+1)}--> <-- (0.00 empty ACK) | ... Short delay before the certificate is ready ... | <-- (CON) (1:N1/0/256)(2:0/1/256)(2.04 Changed) {Cert resp (frag# 1)} (ACK) --> POST [2001:db8::2:1]:61616/est/sen (CON)(2:1/0/256) --> <-- (ACK) (2:1/1/256) (2.04 Changed) {Cert resp (frag# 2)} . . . POST [2001:db8::2:1]:61616/est/sen (CON)(2:N2/0/256) --> <-- (ACK) (2:N2/0/256) (2.04 Changed) {Cert resp (frag# N2+1)} ]]>

If the server is very slow (for example, manual intervention is required which would take minutes), it SHOULD respond with an ACK containing response code 5.03 (Service unavailable) and a Max-Age Option to indicate the time the client SHOULD wait before sending another request to obtain the content. After a delay of Max-Age, the client SHOULD resend the identical CSR to the server. As long as the server continues to respond with response code 5.03 (Service Unavailable) with a Max-Age Option, the client will continue to delay for Max-Age and then resend the enrollment request until the server responds with the certificate or the client abandons the request for policy or other reasons. To demonstrate this scenario, shows a client sending an enrollment request that uses N1+1 Block1 blocks to send the CSR to the server. The server needs N2+1 Block2 blocks to respond, but also needs to take a long delay (minutes) to provide the response. Consequently, the server uses a 5.03 ACK response with a Max-Age Option. The client waits for a period of Max-Age as many times as it receives the same 5.03 response and retransmits the enrollment request until it receives a certificate in a fragmented 2.04 response.

<-- (ACK) (1:0/1/256) (2.31 Continue) POST [2001:db8::2:1]:61616/est/sen (CON)(1:1/1/256) {CSR (frag# 2)} --> <-- (ACK) (1:1/1/256) (2.31 Continue) . . . POST [2001:db8::2:1]:61616/est/sen(CON)(1:N1/0/256){CSR (frag# N1+1)}--> <-- (ACK) (1:N1/0/256) (5.03 Service Unavailable) (Max-Age) | | ... Client tries again after Max-Age with identical payload ... | | POST [2001:db8::2:1]:61616/est/sen(CON)(1:0/1/256){CSR (frag# 1)}--> <-- (ACK) (1:0/1/256) (2.31 Continue) POST [2001:db8::2:1]:61616/est/sen (CON)(1:1/1/256) {CSR (frag# 2)} --> <-- (ACK) (1:1/1/256) (2.31 Continue) . . . POST [2001:db8::2:1]:61616/est/sen(CON)(1:N1/0/256){CSR (frag# N1+1)}--> | ... Immediate response when certificate is ready ... | <-- (ACK) (1:N1/0/256) (2:0/1/256) (2.04 Changed){Cert resp (frag# 1)} POST [2001:db8::2:1]:61616/est/sen (CON)(2:1/0/256) --> <-- (ACK) (2:1/1/256) (2.04 Changed) {Cert resp (frag# 2)} . . . POST [2001:db8::2:1]:61616/est/sen (CON)(2:N2/0/256) --> <-- (ACK) (2:N2/0/256) (2.04 Changed) {Cert resp (frag# N2+1)} ]]>

Private keys can be generated on the server to support scenarios where serer-side key generation is needed. Such scenarios include those where it is considered more secure to generate the long-lived, random private key that identifies the client at the server, or where the resources spent to generate a random private key at the client are considered scarce, or where the security policy requires that the certificate public and corresponding private keys are centrally generated and controlled. As always, it is necessary to use proper random numbers in various protocols such as (D)TLS (). When requesting server-side key generation, the client asks for the server or proxy to generate the private key and the certificate, which are transferred back to the client in the server-side key generation response. In all respects, the server treats the CSR as it would treat any enroll or re-enroll CSR; the only distinction here is that the server MUST ignore the public key values and signature in the CSR. These are included in the request only to allow re-use of existing codebases for generating and parsing such requests. The client /skg request is for a certificate in a PKCS#7 container and private key in two application/multipart-core elements. Respectively, an /skc request is for a single application/pkix-cert certificate and a private key. The private key Content-Format requested by the client is indicated in the PKCS#10 CSR request. If the request contains SMIMECapabilities and DecryptKeyIdentifier or AsymmetricDecryptKeyIdentifier the client is expecting Content-Format 280 for the private key. Then this private key is encrypted symmetrically or asymmetrically as per . The symmetric key or the asymmetric keypair establishment method is out of scope of this specification. A /skg or /skc request with a CSR without SMIMECapabilities expects an application/multipart-core with an unencrypted PKCS#8 private key with Content-Format 284. The EST-coaps server-side key generation response is returned with Content-Format application/multipart-core containing a CBOR array with four items (). The two representations (each consisting of two CBOR array items) do not have to be in a particular order since each representation is preceded by its Content-Format ID. Depending on the request, the private key can be in unprotected PKCS#8 format (Content-Format 284) or protected inside of CMS SignedData (Content-Format 280). The SignedData, placed in the outermost container, is signed by the party that generated the private key, which may be the EST server or the EST CA. SignedData placed within the Enveloped Data does not need additional signing as explained in Section 4.4.2 of . In summary, the symmetrically encrypted key is included in the encryptedKey attribute in a KEKRecipientInfo structure. In the case where the asymmetric encryption key is suitable for transport key operations the generated private key is encrypted with a symmetric key. The symmetric key itself is encrypted by the client-defined (in the CSR) asymmetric public key and is carried in an encryptedKey attribute in a KeyTransRecipientInfo structure. Finally, if the asymmetric encryption key is suitable for key agreement, the generated private key is encrypted with a symmetric key. The symmetric key itself is encrypted by the client defined (in the CSR) asymmetric public key and is carried in an recipientEncryptedKeys attribute in a KeyAgreeRecipientInfo. recommends the use of additional encryption of the returned private key. For the context of this specification, clients and servers that choose to support server-side key generation MUST support unprotected (PKCS#8) private keys (Content-Format 284). Symmetric or asymmetric encryption of the private key (CMS EnvelopedData, Content-Format 280) SHOULD be supported for deployments where end-to-end encryption is needed between the client and a server. Such cases could include architectures where an entity between the client and the CA terminates the DTLS connection (Registrar in ). Although strongly recommends that clients request the use of CMS encryption on top of the TLS channel's protection, this document does not make such a recommendation; CMS encryption can still be used when mandated by the use-case.

In real-world deployments, the EST server will not always reside within the CoAP boundary. The EST server can exist outside the constrained network in which case it will support TLS/HTTP instead of CoAPS. In such environments EST-coaps is used by the client within the CoAP boundary and TLS is used to transport the EST messages outside the CoAP boundary. A Registrar at the edge is required to operate between the CoAP environment and the external HTTP network as shown in .

|EST-coaps-to-HTTPS|<------->| EST Client| || |Server|over TLS | Registrar | '-----------' || '------' '-----------------' || || || |'--------------------------'| '----------------------------' ]]>

The EST-coaps-to-HTTPS Registrar MUST terminate EST-coaps downstream and initiate EST connections over TLS upstream. The Registrar MUST authenticate and optionally authorize the client requests while it MUST be authenticated by the EST server or CA. The trust relationship between the Registrar and the EST server SHOULD be pre-established for the Registrar to proxy these connections on behalf of various clients. When enforcing Proof-of-Possession (PoP) linking, the DTLS tls-unique value of the (D)TLS session is used to prove that the private key corresponding to the public key is in the possession of the client and was used to establish the connection as explained in . The PoP linking information is lost between the EST-coaps client and the EST server when a Registrar is present. The EST server becomes aware of the presence of a Registrar from its TLS client certificate that includes id-kp-cmcRA extended key usage extension (EKU). As explained in Section 3.7 of , the "EST server SHOULD apply an authorization policy consistent with a Registrar client. For example, it could be configured to accept PoP linking information that does not match the current TLS session because the authenticated EST client Registrar has verified this information when acting as an EST server". contains the URI mappings between EST-coaps and EST that the Registrar MUST adhere to. of this specification and Section 7 of define the mappings between EST-coaps and HTTP response codes, that determine how the Registrar MUST translate CoAP response codes from/to HTTP status codes. The mapping from CoAP Content-Format to HTTP Content-Type is defined in . Additionally, a conversion from CBOR major type 2 to Base64 encoding MUST take place at the Registrar. If CMS end-to-end encryption is employed for the private key, the encrypted CMS EnvelopedData blob MUST be converted at the Registrar to binary CBOR type 2 downstream to the client. This is a format conversion that does not require decryption of the CMS EnvelopedData. A deviation from the mappings in could take place if clients that leverage server-side key generation preferred for the enrolled keys to be generated by the Registrar in the case the CA does not support server-side key generation. Such a Registrar is responsible for generating a new CSR signed by a new key which will be returned to the client along with the certificate from the CA. In these cases, the Registrar MUST use random number generation with proper entropy. Due to fragmentation of large messages into blocks, an EST-coaps-to-HTTP Registrar MUST reassemble the BLOCKs before translating the binary content to Base64, and consecutively relay the message upstream. The EST-coaps-to-HTTP Registrar MUST support resource discovery according to the rules in .

This section addresses transmission parameters described in sections 4.7 and 4.8 of . EST does not impose any unique values on the CoAP parameters in , but the setting of the CoAP parameter values may have consequence for the setting of the EST parameter values. Implementations should follow the default CoAP configuration parameters . However, depending on the implementation scenario, retransmissions and timeouts can also occur on other networking layers, governed by other configuration parameters. When a change in a server parameter has taken place, the parameter values in the communicating endpoints MUST be adjusted as necessary. Examples of how parameters could be adjusted include higher layer congestion protocols, provisioning agents and configurations included in firmware updates. Some further comments about some specific parameters, mainly from Table 2 in : NSTART: A parameter that controls the number of simultaneous outstanding interactions that a client maintains to a given server. An EST-coaps client is expected to control at most one interaction with a given server, which is the default NSTART value defined in . DEFAULT_LEISURE: This setting is only relevant in multicast scenarios, outside the scope of EST-coaps. PROBING_RATE: A parameter which specifies the rate of re-sending non-confirmable messages. In the rare situations that non-confirmable messages are used, the default PROBING_RATE value defined in applies. Finally, the Table 3 parameters in are mainly derived from Table 2. Directly changing parameters on one table would affect parameters on the other.

Although EST-coaps paves the way for the utilization of EST by constrained devices in constrained networks, some classes of devices will not have enough resources to handle the payloads that come with EST-coaps. The specification of EST-coaps is intended to ensure that EST works for networks of constrained devices that choose to limit their communications stack to DTLS/CoAP. It is up to the network designer to decide which devices execute the EST protocol and which do not.

Additions to the sub-registry "CoAP Content-Formats", within the "CoRE Parameters" registry are specified in . These have been registered provisionally in the IETF Review or IESG Approval range (256-9999). HTTP Content-Type ID Reference application/pkcs7-mime; smime-type=server-generated-key280 [ThisRFC] application/pkcs7-mime; smime-type=certs-only 281 [ThisRFC] application/pkcs8 284 [ThisRFC] application/csrattrs 285 application/pkcs10 286 [ThisRFC] application/pkix-cert TBD287 [ThisRFC] It is suggested that 287 is allocated to TBD287.

This memo registers new Resource Type (rt=) Link Target Attributes in the "Resource Type (rt=) Link Target Attribute Values" subregistry under the "Constrained RESTful Environments (CoRE) Parameters" registry. rt="ace.est.crts". This resource depicts the support of EST get cacerts. rt="ace.est.sen". This resource depicts the support of EST simple enroll. rt="ace.est.sren". This resource depicts the support of EST simple reenroll. rt="ace.est.att". This resource depicts the support of EST get CSR attributes. rt="ace.est.skg". This resource depicts the support of EST server-side key generation with the returned certificate in a PKCS#7 container. rt="ace.est.skc". This resource depicts the support of EST server-side key generation with the returned certificate in application/pkix-cert format.

A new additional reference is requested for the est URI in the Well-Known URIs registry: URI Suffix Change Controller References Status Related Information Date Registered Date Modified est IETF [RFC7030] [THIS RFC] permanent 2013-08-16 [THIS RFC's publication date]

The security considerations of Section 6 of are only partially valid for the purposes of this document. As HTTP Basic Authentication is not supported, the considerations expressed for using passwords do not apply. The other portions of the security considerations of continue to apply. Modern security protocols require random numbers to be available during the protocol run, for example for nonces and ephemeral (EC) Diffie-Hellman key generation. This capability to generate random numbers is also needed when the constrained device generates the private key (that corresponds to the public key enrolled in the CSR). When server-side key generation is used, the constrained device depends on the server to generate the private key randomly, but it still needs locally generated random numbers for use in security protocols, as explained in Section 12 of . Additionally, the transport of keys generated at the server is inherently risky. For those deploying server-side key generation, analysis SHOULD be done to establish whether server-side key generation increases or decreases the probability of digital identity theft. It is important to note that, as pointed out in , sources contributing to the randomness pool used to generate random numbers on laptops or desktop PCs, such as mouse movement, timing of keystrokes, or air turbulence on the movement of hard drive heads, are not available on many constrained devices. Other sources have to be used or dedicated hardware has to be added. Selecting hardware for an IoT device that is capable of producing high-quality random numbers is therefore important . As discussed in Section 6 of , it is "RECOMMENDED that the Implicit Trust Anchor database used for EST server authentication is carefully managed to reduce the chance of a third-party CA with poor certification practices jeopardizing authentication. Disabling the Implicit Trust Anchor database after successfuly receiving the Distribution of CA certificates response (Section 4.1.3) limits any risk to the first TLS exchange". Alternatively, in a case where a /sen request immediately follows a /crts, a client MAY choose to keep the connection authenticated by the Implicit TA open for efficiency reasons (). A client that interleaves EST-coaps /crts request with other requests in the same DTLS connection SHOULD revalidate the server certificate chain against the updated Explicit TA from the /crts response before proceeding with the subsequent requests. If the server certificate chain does not authenticate against the database, the client SHOULD close the connection without completing the rest of the requests. The updated Explicit TA MUST continue to be used in new DTLS connections. In cases where the IDevID used to authenticate the client is expired the server MAY still authenticate the client because IDevIDs are expected to live as long as the device itself (). In such occasions, checking the certificate revocation status or authorizing the client using another method is important for the server to raise its confidence that the client can be trusted. In accordance with , TLS cipher suites that include "_EXPORT_" and "_DES_" in their names MUST NOT be used. More recommendations for secure use of TLS and DTLS are included in . As described in CMC, Section 6.7 of , "For keys that can be used as signature keys, signing the certification request with the private key serves as a PoP on that key pair". The inclusion of tls-unique in the certificate request links the proof-of-possession to the TLS proof-of-identity. This implies but does not prove that only the authenticated client currently has access to the private key. What's more, CMC PoP linking uses tls-unique as it is defined in . The 3SHAKE attack poses a risk by allowing a man-in-the-middle to leverage session resumption and renegotiation to inject himself between a client and server even when channel binding is in use. Implementers should use the Extended Master Secret Extension in DTLS to prevent such attacks. In the context of this specification, an attacker could invalidate the purpose of the PoP linking ChallengePassword in the client request by resuming an EST-coaps connection. Even though the practical risk of such an attack to EST-coaps is not devastating, we would rather use a more secure channel binding mechanism. Such a mechanism could include an updated tls-unique value generation like the tls-unique-prf defined in by using a TLS exporter in TLS 1.2 or TLS 1.3's updated exporter (Section 7.5 of ) value in place of the tls-unique value in the CSR. Such mechanism has not been standardized yet. Adopting a channel binding value generated from an exporter would break backwards compatibility for an RA that proxies through to a classic EST server. Thus, in this specification we still depend on the tls-unique mechanism defined in , especially since a 3SHAKE attack does not expose messages exchanged with EST-coaps. Interpreters of ASN.1 structures should be aware of the use of invalid ASN.1 length fields and should take appropriate measures to guard against buffer overflows, stack overruns in particular, and malicious content in general.

The Registrar proposed in must be deployed with care, and only when direct client-server connections are not possible. When PoP linking is used the Registrar terminating the DTLS connection establishes a new TLS connection with the upstream CA. Thus, it is impossible for PoP linking to be enforced end-to-end for the EST transaction. The EST server could be configured to accept PoP linking information that does not match the current TLS session because the authenticated EST Registrar is assumed to have verified PoP linking downstream to the client. The introduction of an EST-coaps-to-HTTP Registrar assumes the client can authenticate the Registrar using its implicit or explicit TA database. It also assumes the Registrar has a trust relationship with the upstream EST server in order to act on behalf of the clients. When a client uses the Implicit TA database for certificate validation, it SHOULD confirm if the server is acting as an RA by the presence of the id-kp-cmcRA EKU in the server certificate. In a server-side key generation case, if no end-to-end encryption is used, the Registrar may be able see the private key as it acts as a man-in-the-middle. Thus, the client puts its trust on the Registrar not exposing the private key. Clients that leverage server-side key generation without end-to-end encryption of the private key () have no knowledge if the Registrar will be generating the private key and enrolling the certificates with the CA or if the CA will be responsible for generating the key. In such cases, the existence of a Registrar requires the client to put its trust on the registrar when it is generating the private key.

Martin Furuhed contributed to the EST-coaps specification by providing feedback based on the Nexus EST over CoAPS server implementation that started in 2015. Sandeep Kumar kick-started this specification and was instrumental in drawing attention to the importance of the subject.

The authors are very grateful to Klaus Hartke for his detailed explanations on the use of Block with DTLS and his support for the Content-Format specification. The authors would like to thank Esko Dijk and Michael Verschoor for the valuable discussions that helped in shaping the solution. They would also like to thank Peter Panburana for his feedback on technical details of the solution. Constructive comments were received from Benjamin Kaduk, Eliot Lear, Jim Schaad, Hannes Tschofenig, Julien Vermillard, John Manuel, Oliver Pfaff, Pete Beal and Carsten Bormann. Interop tests were done by Oliver Pfaff, Thomas Werner, Oskar Camezind, Bjorn Elmers and Joel Hoglund. Robert Moskowitz provided code to create the examples.

&RFC2119; &RFC2585; &RFC5246; &RFC5958; &RFC5967; &RFC6347; &RFC6690; &RFC7030; &RFC7252; &RFC7925; &RFC7959; &RFC8075; &RFC8174; &RFC8422; &RFC8446; &I-D.ietf-tls-dtls13; &I-D.ietf-core-multipart-ct; &I-D.ietf-lamps-rfc5751-bis; &RFC5272; &RFC5705; &RFC6402; &RFC7230; &RFC7228; &RFC7251; &RFC7299; &RFC7627; &RFC4919; &RFC5929; &RFC7748; &I-D.ietf-tls-dtls-connection-id; &I-D.moskowitz-ecdsa-pki; &I-D.josefsson-sasl-tls-cb;

This section shows similar examples to the ones presented in Appendix A of . The payloads in the examples are the hex encoded binary, generated with 'xxd -p', of the PKI certificates created following . Hex is used for visualization purposes because a binary representation cannot be rendered well in text. The hexadecimal representations would not be transported in hex, but in binary. The payloads are shown unencrypted. In practice the message content would be transferred over an encrypted DTLS channel. The certificate responses included in the examples contain Content-Format 281 (application/pkcs7). If the client had requested Content-Format TBD287 (application/pkix-cert) by querying /est/skc, the server would respond with a single DER binary certificate in the multipart-core container. These examples assume a short resource path of "/est". Even though omitted from the examples for brevity, before making the EST-coaps requests, a client would learn about the server supported EST-coaps resources with a GET request for /.well-known/core?rt=ace.est* as explained in . The corresponding CoAP headers are only shown in . Creating CoAP headers is assumed to be generally understood. The message content breakdown is presented in .

In EST-coaps, a cacerts message can be:

The corresponding CoAP header fields are shown below. The use of block and DTLS are worked out in .

As specified in Section 5.10.1 of , the Uri-Host and Uri-Port Options can be omitted if they coincide with the transport protocol destination address and port respectively. A 2.05 Content response with a cert in EST-coaps will then be

with CoAP fields

The breakdown of the payload is shown in .

During the (re-)enroll exchange the EST-coaps client uses a CSR (Content-Format 286) request in the POST request payload. The Accept option tells the server that the client is expecting Content-Format 281 (PKCS#7) in the response. As shown in , the CSR contains a ChallengePassword which is used for PoP linking ().

After verification of the CSR by the server, a 2.04 Changed response with the issued certificate will be returned to the client.

The breakdown of the request and response is shown in .

In a serverkeygen exchange the CoAP POST request looks like

The response would follow and could look like

The private key in the response above is without CMS EnvelopedData and has no additional encryption beyond DTLS (). The breakdown of the request and response is shown in

Below is a csrattrs exchange

A 2.05 Content response should contain attributes which are relevant for the authenticated client. This example is copied from Section A.2 in , where the base64 representation is replaced with a hexadecimal representation of the equivalent binary format. The EST-coaps server returns attributes that the client can ignore if they are unknown to him.

Two examples are presented in this section: a cacerts exchange shows the use of Block2 and the block headers an enroll exchange shows the Block1 and Block2 size negotiation for request and response payloads. The payloads are shown unencrypted. In practice the message contents would be binary formatted and transferred over an encrypted DTLS tunnel. The corresponding CoAP headers are only shown in . Creating CoAP headers is assumed to be generally known.

This section provides a detailed example of the messages using DTLS and BLOCK option Block2. The example block length is taken as 64 which gives an SZX value of 2. The following is an example of a cacerts exchange over DTLS. The content length of the cacerts response in appendix A.1 of contains 639 bytes in binary in this example. The CoAP message adds around 10 bytes in this exmple, the DTLS record around 29 bytes. To avoid IP fragmentation, the CoAP Block Option is used and an MTU of 127 is assumed to stay within one IEEE 802.15.4 packet. To stay below the MTU of 127, the payload is split in 9 packets with a payload of 64 bytes each, followed by a last tenth packet of 63 bytes. The client sends an IPv6 packet containing a UDP datagram with DTLS record protection that encapsulates a CoAP request 10 times (one fragment of the request per block). The server returns an IPv6 packet containing a UDP datagram with the DTLS record that encapsulates the CoAP response. The CoAP request-response exchange with block option is shown below. Block Option is shown in a decomposed way (block-option:NUM/M/size) indicating the kind of Block Option (2 in this case) followed by a colon, and then the block number (NUM), the more bit (M = 0 in Block2 response means it is last block), and block size with exponent (2**(SZX+4)) separated by slashes. The Length 64 is used with SZX=2. The CoAP Request is sent confirmable (CON) and the Content-Format of the response, even though not shown, is 281 (application/pkcs7-mime; smime-type=certs-only). The transfer of the 10 blocks with partially filled block NUM=9 is shown below

<-- (2:0/1/64) 2.05 Content GET example.com:9085/est/crts (2:1/0/64) --> <-- (2:1/1/64) 2.05 Content | | | GET example.com:9085/est/crts (2:9/0/64) --> <-- (2:9/0/64) 2.05 Content ]]>

The header of the GET request looks like

The Uri-Host and Uri-Port Options can be omitted if they coincide with the transport protocol destination address and port respectively. Explicit Uri-Host and Uri-Port Options are typically used when an endpoint hosts multiple virtual servers and uses the Options to route the requests accordingly. For further detailing the CoAP headers, the first two and the last blocks are written out below. The header of the first Block2 response looks like

The second Block2:

The 10th and final Block2:

In this example, the requested Block2 size of 256 bytes, required by the client, is transferred to the server in the very first request message. The block size 256=(2**(SZX+4)) which gives SZX=4. The notation for block numbering is the same as in . The header fields and the payload are omitted for brevity.

<-- (ACK) (1:0/1/256) (2.31 Continue) POST [2001:db8::2:1]:61616/est/sen (CON)(1:1/1/256) {CSR (frag# 2)} --> <-- (ACK) (1:1/1/256) (2.31 Continue) . . . POST [2001:db8::2:1]:61616/est/sen (CON)(1:N1/0/256){CSR(frag# N1+1)}--> | ...........Immediate response ......... | <-- (ACK) (1:N1/0/256)(2:0/1/256)(2.04 Changed){Cert resp (frag# 1)} POST [2001:db8::2:1]:61616/est/sen (CON)(2:1/0/256) --> <-- (ACK) (2:1/1/256)(2.04 Changed) {Cert resp (frag# 2)} . . . POST [2001:db8::2:321]:61616/est/sen (CON)(2:N2/0/256) --> <-- (ACK) (2:N2/0/256) (2.04 Changed) {Cert resp (frag# N2+1)} ]]>

N1+1 blocks have been transferred from client to the server and N2+1 blocks have been transferred from server to client.

This appendix presents the breakdown of the hexadecimal dumps of the binary payloads shown in .

The breakdown of cacerts response containing one root CA certificate is

The breakdown of the enrollment request is

Requested Extensions: X509v3 Subject Alternative Name: othername: Signature Algorithm: ecdsa-with-SHA256 30:45:02:21:00:92:56:3a:54:64:63:bd:9e:cf:f1:70:d0:fd: 1f:2e:f0:d3:d0:12:16:0e:5e:e9:0c:ff:ed:ab:ec:9b:9a:38: 92:02:20:17:9f:10:a3:43:61:09:05:1a:ba:d1:75:90:a0:9b: c8:7c:4d:ce:54:53:a6:fc:11:35:a1:e8:4e:ed:75:43:77 ]]>

The CSR contains a ChallengePassword which is used for PoP linking (). The CSR also contains an id-on-hardwareModuleName hardware identifier to customize the returned certificate to the requesting device (See and ). The breakdown of the issued certificate is

Signature Algorithm: ecdsa-with-SHA256 30:46:02:21:00:c0:d8:19:96:d2:50:7d:69:3f:3c:48:ea:a5: ee:94:91:bd:a6:db:21:40:99:d9:81:17:c6:3b:36:13:74:cd: 86:02:21:00:a7:74:98:9f:4c:32:1a:5c:f2:5d:83:2a:4d:33: 6a:08:ad:67:df:20:f1:50:64:21:18:8a:0a:de:6d:34:92:36 ]]>

The following is the breakdown of the server-side key generation request.

Following is the breakdown of the private key content of the server-side key generation response.

The following is the breakdown of the certificate in the server-side key generation response payload.