RFC 9148: EST-coaps: Enrollment over Secure Transport with the Secure Constrained Application Protocol

4.1. Discovery and URIs

EST-coaps is targeted for low-resource networks with small packets. Two types of installations are possible: (1) a rigid one, 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 [RFC7030]. Two example EST-coaps resource path names are:¶

coaps://example.com:<port>/.well-known/est/<short-est>
coaps://example.com:<port>/.well-known/est/ArbitraryLabel/<short-est>

The short-est strings are defined in Table 1. 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:<port>/<root-resource>/<short-est>
coaps://example.com:<port>/<root-resource>/ArbitraryLabel/<short-est>

Figure 5 in Section 3.2.2 of [RFC7030] enumerates the operations and corresponding paths that are supported by EST. Table 1 provides the mapping from the EST URI path to the shorter EST-coaps URI path.¶

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*" [RFC6690]. The example below shows the discovery over CoAPS of the presence and location of EST-coaps resources. Linefeeds are included only for readability.¶

  REQ: GET /.well-known/core?rt=ace.est*

  RES: 2.05 Content
</est/crts>;rt="ace.est.crts";ct="281 287",
</est/sen>;rt="ace.est.sen";ct="281 287",
</est/sren>;rt="ace.est.sren";ct="281 287",
</est/att>;rt="ace.est.att";ct=285,
</est/skg>;rt="ace.est.skg";ct=62,
</est/skc>;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 Table 2). 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.¶

  REQ: GET /.well-known/core?rt=ace.est*

  RES: 2.05 Content
<coaps://[2001:db8:3::123]:61617/est/crts>;rt="ace.est.crts";
              ct="281 287",
<coaps://[2001:db8:3::123]:61617/est/sen>;rt="ace.est.sen";
              ct="281 287",
<coaps://[2001:db8:3::123]:61617/est/sren>;rt="ace.est.sren";
              ct="281 287",
<coaps://[2001:db8:3::123]:61617/est/att>;rt="ace.est.att";
              ct=285,
<coaps://[2001:db8:3::123]:61617/est/skg>;rt="ace.est.skg";
              ct=62,
<coaps://[2001:db8:3::123]:61617/est/skc>;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.¶

4.2. Mandatory/Optional EST Functions

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.¶

Table 2 specifies the mandatory-to-implement or optional implementation of the EST-coaps functions. Discovery of the existence of optional functions is described in Section 4.1.¶

4.3. Payload Formats

EST-coaps is designed for low-resource devices; hence, it 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 3.2.4 of [RFC7030]) 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 Section 8.1. The CoAP response codes are defined in Section 4.5.¶

Content-Format 287 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 287. It is up to the client to support only Content-Format 281, 287 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 [RFC8710]. For example, a collection, containing two representations in response to an EST-coaps server-side key generation /skg request, could include a private key in PKCS #8 [RFC5958] 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 Concise Binary Object Representation (CBOR) notation. The serialization of such CBOR content would be:¶

   84                  # array(4)
   19 011C             # unsigned(284)
   48                  # bytes(8)
      0123456789ABCDEF # "\x01#Eg\x89\xAB\xCD\xEF"
   19 0119             # unsigned(281)
   48                  # bytes(8)
      FEDCBA9876543210 # "\xFE\xDC\xBA\x98vT2\x10"

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 287 (0x011F) instead of 281. In cases where the private key is encrypted with Cryptographic Message Syntax (CMS) (as explained in Section 4.8), the Content-Format identifier is 280 (0x0118) instead of 284. The Content-Format used in the response is summarized in Table 3.¶

The key and certificate representations are DER-encoded ASN.1, in its binary form. An example is shown in Appendix A.3.¶

4.5. CoAP Response Codes

Section 5.9 of [RFC7252] and Section 7 of [RFC8075] 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 [RFC7030] has no equivalent in CoAP. HTTP 202 with Retry-After is used in EST for delayed server responses. Section 4.7 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. Table 4 summarizes the EST-coaps response codes.¶

4.6. Message Fragmentation

DTLS defines fragmentation only for the handshake and not for secure data exchange (DTLS records). [RFC6347] 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 6LoWPAN (IPv6 over Low-Power Wireless Personal Area Networks) over IEEE 802.15.4 networks [IEEE802.15.4] 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 Elliptic Curve Digital Signature Algorithm (ECDSA) cert sizes are 500-1000 bytes, which could fluctuate further based on the algorithms, OIDs, Subject Alternative Names (SANs), 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 [RFC7252] (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 [RFC7252] 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 [RFC4919] (Section 2 of [RFC7959]). EST-coaps needs to be able to fragment messages into multiple DTLS datagrams.¶

To perform fragmentation in CoAP, [RFC7959] 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 [RFC7959], 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.¶

[RFC7959] also defines Size1 and Size2 Options to provide size information about the resource representation in a request and response. The EST-coaps client and server MAY support Size1 and Size2 Options.¶

Examples of fragmented EST-coaps messages are shown in Appendix B.¶

4.7. Delayed Responses

Server responses can sometimes be delayed. According to Section 5.2.2 of [RFC7252], 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 Figure 3. 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 Figure 3 is explained in Appendix B.1.¶

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)}-->
   <-- (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 due to policy or other reasons.¶

To demonstrate this scenario, Figure 4 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.¶

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)}-->
  <-- (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)}

4.8. Server-Side Key Generation

Private keys can be generated on the server to support scenarios where server-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 (Section 9.1).¶

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 reuse 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 per [RFC7030]. The symmetric key or the asymmetric keypair establishment method is out of scope of this specification. An /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 [RFC8710] containing a CBOR array with four items (Section 4.3). 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 [RFC5958] (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 [RFC7030]. 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 a recipientEncryptedKeys attribute in a KeyAgreeRecipientInfo.¶

[RFC7030] 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 Figure 5). Though [RFC7030] 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.¶

8.1. Content-Formats Registry

IANA has registered the following Content-Formats given in Table 5 in the "CoAP Content-Formats" subregistry within the "CoRE Parameters" registry [CORE-PARAMS]. These have been registered in the IETF Review or IESG Approval range (256-9999).¶

8.2. Resource Type Registry

IANA has registered the following Resource Type (rt=) Link Target Attributes given in Table 6 in the "Resource Type (rt=) Link Target Attribute Values" subregistry under the "Constrained RESTful Environments (CoRE) Parameters" registry.¶

8.3. Well-Known URIs Registry

IANA has added an additional reference to the est URI in the "Well-Known URIs" registry:¶

URI Suffix:

est¶

Change Controller:

IETF¶

References:

[RFC7030] RFC 9148¶

Status:

permanent¶

Related Information:

Date Registered:

2013-08-16¶

Date Modified:

2020-04-29¶

9.1. EST Server Considerations

The security considerations in Section 6 of [RFC7030] 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 in [RFC7030] 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 [RFC7925]. 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 [PsQs], 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 [RSA-FACT].¶

As discussed in Section 6 of [RFC7030], it is¶

RECOMMENDED that the Implicit Trust Anchor database used for EST server authentication be carefully managed to reduce the chance of a third-party CA with poor certification practices from being trusted. Disabling the Implicit Trust Anchor database after successfully receiving the Distribution of CA certificates response ([RFC7030], Section 6) limits any vulnerability 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 (Section 3). 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 Initial Device Identifier (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 (Section 3). 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 [RFC7030], 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 [BCP195].¶

As described in Certificate Management over CMS (CMC), Section 6.7 of [RFC5272], "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". In (D)TLS 1.2, the inclusion of tls-unique in the certificate request links the proof-of-possession to the (D)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 [RFC5929]. The 3SHAKE attack [TRIPLESHAKE] poses a risk by allowing an on-path active attacker to leverage session resumption and renegotiation to inject itself between a client and server even when channel binding is in use. Implementers should use the Extended Master Secret Extension in DTLS [RFC7627] 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. In this specification, we still depend on the tls-unique mechanism defined in [RFC5929] for DTLS 1.2 because a 3SHAKE attack does not expose messages exchanged with EST-coaps. But for DTLS 1.3, [TLS13-CHANNEL-BINDINGS] is used instead to derive a 32-byte tls-exporter binding in place of the tls-unique value in the CSR. That would alleviate the risks from the 3SHAKE attack [TRIPLESHAKE].¶

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.¶

9.2. HTTPS-CoAPS Registrar Considerations

The Registrar proposed in Section 5 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 [RFC6402] 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 (Section 4.8) have no knowledge as to whether 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.¶

A.1. cacerts

In EST-coaps, a cacerts message can be the following:¶

GET example.com:9085/est/crts
(Accept:  281)

The corresponding CoAP header fields are shown below. The use of block and DTLS are shown in Appendix B.¶

  Ver = 1
  T = 0 (CON)
  Code = 0x01 (0.01 is GET)
  Token = 0x9a (client generated)
  Options
  Option (Uri-Host)
     Option Delta = 0x3  (option# 3)
     Option Length = 0xB
     Option Value = "example.com"
  Option (Uri-Port)
     Option Delta = 0x4  (option# 3+4=7)
     Option Length = 0x2
     Option Value = 9085
   Option (Uri-Path)
     Option Delta = 0x4   (option# 7+4=11)
     Option Length = 0x3
     Option Value = "est"
   Option (Uri-Path)
     Option Delta = 0x0   (option# 11+0=11)
     Option Length = 0x4
     Option Value = "crts"
   Option (Accept)
     Option Delta = 0x6   (option# 11+6=17)
     Option Length = 0x2
     Option Value = 281
  Payload = [Empty]

As specified in Section 5.10.1 of [RFC7252], 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 the following:¶

2.05 Content (Content-Format: 281)
   {payload with certificate in binary format}

With the following CoAP fields:¶

  Ver = 1
  T = 2 (ACK)
  Code = 0x45 (2.05 Content)
  Token = 0x9a   (copied from request by server)
  Options
    Option (Content-Format)
      Option Delta = 0xC  (option# 12)
      Option Length = 0x2
      Option Value = 281

  [ The hexadecimal representation below would NOT be transported
  in hex, but in binary. Hex is used because a binary representation
  cannot be rendered well in text. ]

  Payload =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The payload is shown in plain text in Appendix C.1.¶

A.3. serverkeygen

In a serverkeygen exchange, the CoAP POST request looks like the following:¶

POST 192.0.2.1:8085/est/skg
(Token: 0xa5)
(Accept: 62)
(Content-Format: 286)

[ The hexadecimal representation below would NOT be transported
in hex, but in binary. Hex is used because a binary representation
cannot be rendered well in text. ]

3081d03078020100301631143012060355040a0c0b736b67206578616d70
6c653059301306072a8648ce3d020106082a8648ce3d03010703420004c8
b421f11c25e47e3ac57123bf2d9fdc494f028bc351cc80c03f150bf50cff
958d75419d81a6a245dffae790be95cf75f602f9152618f816a2b23b5638
e59fd9a000300a06082a8648ce3d040302034800304502207c553981b1fe
349249d8a3f50a0346336b7dfaa099cf74e1ec7a37a0a760485902210084
79295398774b2ff8e7e82abb0c17eaef344a5088fa69fd63ee611850c34b
0a

The response would follow [RFC8710] and could look like the following:¶

2.04 Changed
(Token: 0xa5)
(Content-Format: 62)

[ The hexadecimal representations below would NOT be transported
in hex, but in binary. Hex is used because a binary representation
cannot be rendered well in text. ]

84                                   # array(4)
19 011C                              # unsigned(284)
58 8A                                # bytes(138)
308187020100301306072a8648ce3d020106082a8648ce3d030107046d30
6b020101042061336a86ac6e7af4a96f632830ad4e6aa0837679206094d7
679a01ca8c6f0c37a14403420004c8b421f11c25e47e3ac57123bf2d9fdc
494f028bc351cc80c03f150bf50cff958d75419d81a6a245dffae790be95
cf75f602f9152618f816a2b23b5638e59fd9
19 0119                              # unsigned(281)
59 01D3                              # bytes(467)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The private key in the response above is without CMS EnvelopedData and has no additional encryption beyond DTLS (Section 4.8).¶

The request and response is shown in plain text in Appendix C.3.¶

B.1. cacerts

This section provides a detailed example of the messages using DTLS and CoAP 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 [RFC7030] contains 639 bytes in binary in this example. The CoAP message adds around 10 bytes in this example, and 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.¶

   GET example.com:9085/est/crts (2:0/0/64)  -->
                 <--   (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 following:¶

  Ver = 1
  T = 0 (CON)
  Code = 0x01 (0.1 GET)
  Token = 0x9a    (client generated)
  Options
   Option (Uri-Host)
     Option Delta = 0x3  (option# 3)
     Option Length = 0xB
     Option Value = "example.com"
   Option (Uri-Port)
     Option Delta = 0x4   (option# 3+4=7)
     Option Length = 0x2
     Option Value = 9085
   Option (Uri-Path)
     Option Delta = 0x4    (option# 7+4=11)
     Option Length = 0x3
     Option Value = "est"
   Option (Uri-Path)Uri-Path)
     Option Delta = 0x0    (option# 11+0=11)
     Option Length = 0x4
     Option Value = "crts"
   Option (Accept)
     Option Delta = 0x6   (option# 11+6=17)
     Option Length = 0x2
     Option Value = 281
  Payload = [Empty]

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.¶

To provide further details on the CoAP headers, the first two and the last blocks are written out below. The header of the first Block2 response looks like the following:¶

  Ver = 1
  T = 2 (ACK)
  Code = 0x45 (2.05 Content)
  Token = 0x9a     (copied from request by server)
  Options
    Option
      Option Delta = 0xC  (option# 12 Content-Format)
      Option Length = 0x2
      Option Value = 281
    Option
      Option Delta = 0xB  (option# 12+11=23 Block2)
      Option Length = 0x1
      Option Value = 0x0A (block#=0, M=1, SZX=2)

  [ The hexadecimal representation below would NOT be transported
  in hex, but in binary. Hex is used because a binary representation
  cannot be rendered well in text. ]

  Payload =
3082027b06092a864886f70d010702a082026c308202680201013100300b
06092a864886f70d010701a082024e3082024a308201f0a0030201020209
009189bc

The header of the second Block2 response looks like the following:¶

  Ver = 1
  T = 2 (means ACK)
  Code = 0x45 (2.05 Content)
  Token = 0x9a     (copied from request by server)
  Options
    Option
      Option Delta = 0xC  (option# 12 Content-Format)
      Option Length = 0x2
      Option Value = 281
    Option
      Option Delta = 0xB  (option 12+11=23 Block2)
      Option Length = 0x1
      Option Value = 0x1A (block#=1, M=1, SZX=2)

  [ The hexadecimal representation below would NOT be transported
  in hex, but in binary. Hex is used because a binary representation
  cannot be rendered well in text. ]

  Payload =
df9c99244b300a06082a8648ce3d0403023067310b300906035504061302
5553310b300906035504080c024341310b300906035504070c024c413114
30120603

The header of the tenth and final Block2 response looks like the following:¶

  Ver = 1
  T = 2 (means ACK)
  Code = 0x45      (2.05 Content)
  Token = 0x9a     (copied from request by server)
  Options
    Option
      Option Delta = 0xC  (option# 12 Content-Format)
      Option Length = 0x2
      Option Value = 281
    Option
      Option Delta = 0xB  (option# 12+11=23 Block2 )
      Option Length = 0x1
      Option Value = 0x92 (block#=9, M=0, SZX=2)

  [ The hexadecimal representation below would NOT be transported
  in hex, but in binary. Hex is used because a binary representation
  cannot be rendered well in text. ]

  Payload =
2ec0b4af52d46f3b7ecc9687ddf267bcec368f7b7f1353272f022047a28a
e5c7306163b3c3834bab3c103f743070594c089aaa0ac870cd13b902caa1
003100

B.2. enroll / reenroll

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 of 256 is equal to (2^(SZX+4)), which gives SZX=4. The notation for block numbering is the same as in Appendix B.1. The header fields and the payload are omitted for brevity.¶

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  .........
                      |
  <-- (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 server, and N2+1 blocks have been transferred from server to client.¶

C.1. cacerts

The cacerts response containing one root CA certificate is presented in plain text in the following:¶

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 831953162763987486 (0xb8bb0fe604f6a1e)
    Signature Algorithm: ecdsa-with-SHA256
        Issuer: C=US, ST=CA, L=LA, O=Example Inc,
                  OU=certification, CN=Root CA
        Validity
            Not Before: Jan 31 11:27:03 2019 GMT
            Not After : Jan 26 11:27:03 2039 GMT
        Subject: C=US, ST=CA, L=LA, O=Example Inc,
                     OU=certification, CN=Root CA
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                pub:
                    04:0c:1b:1e:82:ba:8c:c7:26:80:97:3f:97:ed:b8:
                    a0:c7:2a:b0:d4:05:f0:5d:4f:e2:9b:99:7a:14:cc:
                    ce:89:00:83:13:d0:96:66:b6:ce:37:5c:59:5f:cc:
                    8e:37:f8:e4:35:44:97:01:1b:e9:0e:56:79:4b:d9:
                    1a:d9:51:ab:45
                ASN1 OID: prime256v1
                NIST CURVE: P-256
        X509v3 extensions:
            X509v3 Subject Key Identifier:
1D:F1:20:89:44:D7:7B:5F:1D:9D:CB:51:EE:24:4A:52:3F:3E:F5:DE
            X509v3 Authority Key Identifier:
                  keyid:
1D:F1:20:89:44:D7:7B:5F:1D:9D:CB:51:EE:24:4A:52:3F:3E:F5:DE

            X509v3 Basic Constraints: critical
                CA:TRUE
            X509v3 Key Usage: critical
                Certificate Sign, CRL Sign
            X509v3 Subject Alternative Name:
                email:certify@example.com
    Signature Algorithm: ecdsa-with-SHA256
         30:45:02:20:2b:89:1d:d4:11:d0:7a:6d:6f:62:19:47:63:5b:
         a4:c4:31:65:29:6b:3f:63:37:26:f0:2e:51:ec:f4:64:bd:40:
         02:21:00:b4:be:8a:80:d0:86:75:f0:41:fb:c7:19:ac:f3:b3:
         9d:ed:c8:5d:c9:2b:30:35:86:8c:b2:da:a8:f0:5d:b1:96

C.2. enroll / reenroll

The enrollment request is presented in plain text in the following:¶

Certificate Request:
    Data:
        Version: 0 (0x0)
        Subject: C=US, ST=CA, L=LA, O=example Inc,
                    OU=IoT/serialNumber=Wt1234
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                pub:
                    04:c8:b4:21:f1:1c:25:e4:7e:3a:c5:71:23:bf:2d:
                    9f:dc:49:4f:02:8b:c3:51:cc:80:c0:3f:15:0b:f5:
                    0c:ff:95:8d:75:41:9d:81:a6:a2:45:df:fa:e7:90:
                    be:95:cf:75:f6:02:f9:15:26:18:f8:16:a2:b2:3b:
                    56:38:e5:9f:d9
                ASN1 OID: prime256v1
                NIST CURVE: P-256
        Attributes:
            challengePassword:   <256-bit POP linking value>
        Requested Extensions:
            X509v3 Subject Alternative Name:
                othername:<unsupported>
    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 (Section 3). The CSR also contains an id-on-hardwareModuleName hardware identifier to customize the returned certificate to the requesting device (See [RFC7299] and [PKI-GUIDE]).¶

The issued certificate presented in plain text in the following:¶

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 9112578475118446130 (0x7e7661d7b54e4632)
    Signature Algorithm: ecdsa-with-SHA256
        Issuer: C=US, ST=CA, O=Example Inc,
                      OU=certification, CN=802.1AR CA
        Validity
            Not Before: Jan 31 11:29:16 2019 GMT
            Not After : Dec 31 23:59:59 9999 GMT
        Subject: C=US, ST=CA, L=LA, O=example Inc,
                OU=IoT/serialNumber=Wt1234
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                pub:
                    04:c8:b4:21:f1:1c:25:e4:7e:3a:c5:71:23:bf:2d:
                    9f:dc:49:4f:02:8b:c3:51:cc:80:c0:3f:15:0b:f5:
                    0c:ff:95:8d:75:41:9d:81:a6:a2:45:df:fa:e7:90:
                    be:95:cf:75:f6:02:f9:15:26:18:f8:16:a2:b2:3b:
                    56:38:e5:9f:d9
                ASN1 OID: prime256v1
                NIST CURVE: P-256
        X509v3 extensions:
            X509v3 Basic Constraints:
                CA:FALSE
            X509v3 Subject Key Identifier:
96:60:0D:87:16:BF:7F:D0:E7:52:D0:AC:76:07:77:AD:66:5D:02:A0
            X509v3 Authority Key Identifier:
                keyid:
68:D1:65:51:F9:51:BF:C8:2A:43:1D:0D:9F:08:BC:2D:20:5B:11:60

            X509v3 Key Usage: critical
                Digital Signature, Key Encipherment
            X509v3 Subject Alternative Name:
                othername:<unsupported>
    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

C.3. serverkeygen

The following is the server-side key generation request presented in plain text:¶

Certificate Request:
    Data:
        Version: 0 (0x0)
        Subject: O=skg example
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                pub:
                    04:c8:b4:21:f1:1c:25:e4:7e:3a:c5:71:23:bf:2d:
                    9f:dc:49:4f:02:8b:c3:51:cc:80:c0:3f:15:0b:f5:
                    0c:ff:95:8d:75:41:9d:81:a6:a2:45:df:fa:e7:90:
                    be:95:cf:75:f6:02:f9:15:26:18:f8:16:a2:b2:3b:
                    56:38:e5:9f:d9
                ASN1 OID: prime256v1
                NIST CURVE: P-256
        Attributes:
            a0:00
    Signature Algorithm: ecdsa-with-SHA256
         30:45:02:20:7c:55:39:81:b1:fe:34:92:49:d8:a3:f5:0a:03:
         46:33:6b:7d:fa:a0:99:cf:74:e1:ec:7a:37:a0:a7:60:48:59:
         02:21:00:84:79:29:53:98:77:4b:2f:f8:e7:e8:2a:bb:0c:17:
         ea:ef:34:4a:50:88:fa:69:fd:63:ee:61:18:50:c3:4b:0a

The following is the private key content of the server-side key generation response presented in plain text:¶

Private-Key: (256 bit)
priv:
    61:33:6a:86:ac:6e:7a:f4:a9:6f:63:28:30:ad:4e:
    6a:a0:83:76:79:20:60:94:d7:67:9a:01:ca:8c:6f:
    0c:37
pub:
    04:c8:b4:21:f1:1c:25:e4:7e:3a:c5:71:23:bf:2d:
    9f:dc:49:4f:02:8b:c3:51:cc:80:c0:3f:15:0b:f5:
    0c:ff:95:8d:75:41:9d:81:a6:a2:45:df:fa:e7:90:
    be:95:cf:75:f6:02:f9:15:26:18:f8:16:a2:b2:3b:
    56:38:e5:9f:d9
ASN1 OID: prime256v1
NIST CURVE: P-256

The following is the certificate in the server-side key generation response payload presented in plain text:¶

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            b3:31:3e:8f:3f:c9:53:8e
    Signature Algorithm: ecdsa-with-SHA256
        Issuer: O=skg example
        Validity
            Not Before: Sep  4 07:44:03 2019 GMT
            Not After : Aug 30 07:44:03 2039 GMT
        Subject: O=skg example
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                pub:
                    04:c8:b4:21:f1:1c:25:e4:7e:3a:c5:71:23:bf:2d:
                    9f:dc:49:4f:02:8b:c3:51:cc:80:c0:3f:15:0b:f5:
                    0c:ff:95:8d:75:41:9d:81:a6:a2:45:df:fa:e7:90:
                    be:95:cf:75:f6:02:f9:15:26:18:f8:16:a2:b2:3b:
                    56:38:e5:9f:d9
                ASN1 OID: prime256v1
                NIST CURVE: P-256
        X509v3 extensions:
            X509v3 Basic Constraints:
                CA:FALSE
            Netscape Comment:
                OpenSSL Generated Certificate
            X509v3 Subject Key Identifier:
96:60:0D:87:16:BF:7F:D0:E7:52:D0:AC:76:07:77:AD:66:5D:02:A0
            X509v3 Authority Key Identifier:
                keyid:
96:60:0D:87:16:BF:7F:D0:E7:52:D0:AC:76:07:77:AD:66:5D:02:A0

    Signature Algorithm: ecdsa-with-SHA256
         30:45:02:21:00:e9:5b:fa:25:a0:89:76:65:22:46:f2:d9:61:
         43:da:39:fc:e0:dc:4c:9b:26:b9:cc:e1:f2:41:64:cc:2b:12:
         b6:02:20:13:51:fd:8e:ea:65:76:4e:34:59:d3:24:e4:34:5f:
         f5:b2:a9:15:38:c0:49:76:11:17:96:b3:69:8b:f6:37:9c