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SEC lamps SubjectAltName SAN Certificate Extensions Certificate Signing Request This document updates RFC 7030, "Enrollment over Secure Transport" (EST), clarifying how the Certificate Signing Request (CSR) Attributes Response can be used by an EST server to specify both CSR attribute Object Identifiers (OIDs) and CSR attribute values, particularly X.509 extension values, that the server expects the client to include in a subsequent CSR request. RFC 9148 is derived from RFC 7030 and is also updated. RFC 7030 is ambiguous in its specification of the CSR Attributes Response. This has resulted in implementation challenges and implementor confusion because there was no universal understanding of what was specified. This document clarifies the encoding rules. This document also provides a new straightforward approach: using a template for CSR contents that may be partially filled in by the server. This also allows an EST server to specify a subject Distinguished Name (DN).
Introduction This document updates RFC 7030 and clarifies how the Certificate Signing Request (CSR) Attributes Response can be used by an EST server to specify both CSR attribute OIDs and CSR attribute values. In particular, the server needs to be able to specify X.509 extension values that it expects the client to include in the subsequent CSR. "Enrollment over Secure Transport" has been used in a wide variety of applications. In particular, and describe a way to use it in order to build out an Autonomic Control Plane (ACP) . When bootstrapping the ACP, there is a requirement that each node be given a very specific subjectAltName. In , the ACP specification, the EST server is specified to make use of the CSR Attributes ("/csrattrs") Request (specified in ) to convey the actual subjectAltName to the EST client that needs to go into its CSR and thus ultimately into its End Entity (EE) certificate. As a result of some implementation challenges, it came to light that this particular way of using the CSR Attributes was not universally agreed upon, and it was suggested that it went contrary to . says that the CSR Attributes "can provide additional descriptive information that the EST server cannot access itself". This is extended to describe how the EST server can provide values that it demands be used. After significant discussion, it has been determined that is sufficiently difficult to read and ambiguous to interpret, so clarification is needed. Also, is extended to clarify the use of the existing ASN.1 syntax . This covers all uses and is fully backward compatible with existing use, including addressing the needs of and .
Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.
CSR Attributes Handling
Extensions to RFC 7030, Section 2.6 This document replaces the second paragraph in with the following text:
These attributes can provide additional information that the EST server cannot access itself, such as the Media Access Control (MAC) address of an interface of the EST client. The EST server can also provide concrete values that it tells the client to include in the CSR, such as a specific X.509 Subject Alternative Name extension. Moreover, these attributes can indicate the type of the included public key or which crypto algorithms to use for the self-signature, such as a specific elliptic curve or a specific hash function that the client is expected to use when generating the CSR.
Extensions to RFC 7030, Section 4.5.2 The ASN.1 syntax for CSR Attributes as defined in EST () is as follows: This remains unchanged, such that bits-on-the-wire compatibility is maintained. Key parts that were unclear were which OID to use in the 'type' field and that the 'values' field can contain an entire sequence of X.509 extensions. The OID to use for such attributes in the 'type' field MUST be id-ExtensionReq, which has the value 1.2.840.113549.1.9.14. Note that this is the same as pkcs-9-at-extensionRequest defined in PKCS#9 . There MUST be only one such attribute. The 'values' field of this attribute MUST contain a set with exactly one element, and this element MUST be of type Extensions, as per : An Extension comprises the OID of the specific X.509 extension (extnID), optionally the 'critical' bit, and the extension value (extnValue). An Extensions structure, which is a sequence of elements of type Extension, MUST NOT include more than one element with a particular extnID. When not using the template-based approach of , specifying the requirement for a public key of a specific type and optionally its size and other parameters MUST be done as follows: Include exactly one Attribute with the type field being the OID specifying the type of the key, such as ecPublicKey or rsaEncryption. The 'values' field MAY be empty to indicate no further requirements on the key. Otherwise, it MUST contain suitable parameters for the given key type, such as a singleton set containing the OID of an elliptic curve (EC) (e.g., secp384r1) or containing an integer value for the RSA key size (e.g., 4096). Many examples for this are given in .
Update to RFC 9148 The updates to EST in this document equally apply when using the Constrained Application Protocol (CoAP) as a transport mechanism as described in . This document therefore adds the following paragraph after the second paragraph of :
EST over CoAP as specified in applies unchanged to , which is updated by RFC 9908. Hence, all references to in are assumed to indicate that is updated by RFC 9908.
Use of CSR Templates As an alternative to the unstructured inclusion of CSR Attributes specified in with its limitations and ambiguities, describes an approach using a CSR template. An entire CSR object is returned with various fields filled out and other fields waiting to be filled in. In that approach, a pKCS7PDU attribute includes a PKIData content type and that, in turn, includes a CSR or a Certificate Request Message Format (CRMF) formatted request (for details, see or of , respectively). One drawback to that approach, particularly for the CSR, is that some unused fields have to be included; specifically, the 'signature' field on the CSR is faked with an empty bit string. A similar method has been defined in "Internet X.509 Public Key Infrastructure -- Certificate Management Protocol (CMP)" and "Lightweight Certificate Management Protocol (CMP) Profile" () using a CSR template as defined for CRMF . Like the approach mentioned before, this method does not properly deal with absent Relative Distinguished Name (RDN) values, as it would encode them as invalid empty strings. Also, encoding an absent 'subjectPublicKey' value as an empty BIT STRING and an absent X.509 extension value as an empty OCTET STRING can cause issues with strict ASN.1 parsing and decoding. These drawbacks are avoided as follows: This specification defines a new Certificate Request Information Template attribute for CsrAttrs (as given in ) that is essentially a partially filled-in PKCS#10 CSR minus the signature wrapper: contains all details. The CertificationRequestInfoTemplate closely resembles the CertificationRequestInfo from : with the following differences:
  • The 'subject' field has been made OPTIONAL. It MUST be present if the server places any requirements on the RDNs of the subject name; otherwise, it MUST be absent.
  • RDNs in the 'subject' fields are allowed to have no value, which has been achieved by adding OPTIONAL to the 'value' field of SingleAttributeTemplate. If the client is expected to provide an RDN of a certain type such as commonName, the respective RDN MUST be present in the 'subject' field; otherwise, it MUST be absent. In addition, if the server gives an RDN value, this means that the client is expected to use this value for the RDN; otherwise, the client is expected to fill in a suitable value. The example at the end of this section has a 'subject' field that contains both forms of RDN specifications.
  • The 'subjectPKInfo' field has been made OPTIONAL. The field MUST be absent if the server places no requirements on the key; otherwise, it MUST be present, and the 'algorithm' field specifies the type of key pair the client is expected to use.
  • The 'subjectPublicKey' field contained in SubjectPublicKeyInfoTemplate has been made OPTIONAL because it is usually not needed. In case the server requires use of an RSA key and needs to specify its size, the field MUST be present and contain a placeholder public key value of the desired RSA modulus length; otherwise, the subjectPublicKey MUST be absent.
  • A new OID id-aa-extensionReqTemplate and the related ExtensionTemplate structure is defined where the 'extnValue' field has been made OPTIONAL. This is only needed to enable specifying partial extensions with values to be filled in by the client; otherwise, the id-ExtensionReq OID and the respective value of type ExtensionReq MUST be used for specifying requirements on X.509 extensions.
For each extension of type Extension or ExtensionTemplate provided by the server, the client is expected to include an extension of the type given by the extnID. If the 'critical' field is present, the client SHOULD use it in the extension as well. If the 'extnValue' is present (which is always the case when type Extension is used), the client SHOULD use the given extension value in its CSR. When the type ExtensionTemplate is used, the 'extnValue' can be absent, and then the client SHOULD provide an extension value in an Extension with the given extnID. For instance, if the server includes an ExtensionTemplate with the extnID 'subjectAltName' but without an extnValue, the client SHOULD include a SAN extension with a suitable value. In case the server includes an ExtensionTemplate with the extnID 'subjectAltName' and a partially filled-in extnValue, such as a 'directoryName' choice containing the NULL-DN (i.e., an empty sequence of RDNs) or the 'iPAddress' choice with an empty OCTET STRING, it means that the client SHOULD fill in the respective GeneralName value. The 'version' field of the CertificationRequestInfoTemplate MUST contain v1 (0). The 'attributes' field MUST NOT contain multiple id-aa-extensionReqTemplate attributes and MUST NOT contain both id-ExtensionReq and id-aa-extensionReqTemplate attributes. The 'values' field of an id-aa-extensionReqTemplate attribute MUST contain a set with exactly one element, and this element MUST be of type ExtensionTemplate. Suppose the server requires that the CSR will contain:
  • the 'subject' field with a common name to be filled in by the EE and two organizational unit names with given values "myDept" and "myGroup",
  • the 'publicKey' field with an Elliptic Curve Cryptography (ECC) public key on curve secp256r1,
  • the 'subjectAltName' extension with two entries; one DNS entry with name "www.myServer.com" and IP entry that is empty for the IP address to be filled in,
  • the 'keyUsage' extension marked critical with the values digitalSignature and keyAgreement, and
  • the 'extKeyUsage' extension with the value to be filled in by the EE.
Then, the CertificationRequestInfo structure constructed by the server will be as follows:
Coexistence with Existing Implementations EST servers with legacy clients MAY continue to use the unstructured list of attribute/value pairs as described in and MAY also include the template style described in for newer clients. Clients that understand both MUST use the template only, and ignore all other CsrAttrs elements. Older clients will ignore the new CertificationRequestInfoTemplate element.
Examples Using the Original Approach in RFC 7030 Each example has a high-level (English) explanation of what is expected. Some mapping back to the Attribute and Extension definitions above are included. The base64 DER encoding is then shown. The output of "dumpasn1" is then provided to detail what the contents are.
Require an RFC 8994 / ACP subjectAltName with Specific otherName A single subjectAltName extension is specified in a single CsrAttrs attribute with an OID 'id-ExtensionReq' indicating type Extensions. This is what might be created by a Registrar that is asking for AcpNodeName with format 'otherNames'.
Base64-Encoded Example The Base64:
ASN.1 DUMP Output There is a single subjectAltName Extension with an Attribute with Extension type. 0 104: SEQUENCE { <30 66> 2 102: SEQUENCE { <06 09> 4 9: OBJECT IDENTIFIER : extensionRequest (1 2 840 113549 1 9 14) : (PKCS #9 via CRMF) <31 59> 15 89: SET { <30 57> 17 87: SEQUENCE { <30 55> 19 85: SEQUENCE { <06 03> 21 3: OBJECT IDENTIFIER : subjectAltName (2 5 29 17) : (X.509 extension) <01 01> 26 1: BOOLEAN TRUE <04 4B> 29 75: OCTET STRING, encapsulates { <30 49> 31 73: SEQUENCE { 33 71: [0] { <06 08> 35 8: OBJECT IDENTIFIER '1 3 6 1 5 5 7 8 10' 45 59: [0] { <16 39> 47 57: IA5String : 'rfc8994+fd739fc23c34401122334455' : '00000000+@acp.example.com' : } : } : } : } : } : } : } : } : }]]>
Original Example in RFC 7030 In this example, taken from , a few different attributes are included. The original encoding of the 'macAddress' part in the example is NOT CORRECT. It was not aligned with the definition of the Extension Request attribute as specified in . The revised encoding given here does not use an 'id-ExtensionReq' attribute because the MAC Address is not an X.509 certificate extension by itself and because the server provides its OID without a value, which is not allowed syntactically within a structure of type 'Extension'.
Base64-Encoded Example The Base64:
ASN.1 DUMP Output The CsrAttrs structure contains:
  1. The challengePassword attribute to indicate that the CSR should include this value.
  2. An ecPublicKey OID with the value secp384r1 to indicate what kind of public key should be submitted.
  3. The macAddress OID 1.3.6.1.1.1.1.22 to indicate that the CSR is expected to include (in a subjectDirectoryAttributes extension) a MAC address value.
  4. The ecdsaWithSHA384 OID to indicate what kind of hash is expected to be used for the self-signature in the PKCS#10 CSR.
0 50: SEQUENCE { <06 09> 2 9: OBJECT IDENTIFIER challengePassword (1 2 840 113549 1 9 7) : (PKCS #9) <30 12> 13 18: SEQUENCE { <06 07> 15 7: OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1) : (ANSI X9.62 public key type) <31 07> 24 7: SET { <06 05> 26 5: OBJECT IDENTIFIER secp384r1 (1 3 132 0 34) : (SECG (Certicom) named elliptic curve) : } : } <06 07> 33 7: OBJECT IDENTIFIER '1 3 6 1 1 1 1 22' <06 08> 42 8: OBJECT IDENTIFIER ecdsaWithSHA384 (1 2 840 10045 4 3 3) : (ANSI X9.62 ECDSA algorithm with SHA384) : }]]>
Require a Specific subjectAltName Extension This example is the same as the previous one except that instead of the OID for a macAddress, a subjectAltName is specified as the only Extension element.
Base64-Encoded Example The Base64:
ASN.1 DUMP Output The CsrAttrs structure contains:
  1. The challengePassword attribute to indicate that the CSR should include this value.
  2. An ecPublicKey OID with the value secp521r1 to indicate what kind of public key should be submitted.
  3. An extensionRequest container with a subjectAltName value containing the name potato@example.com.
  4. The ecdsaWithSHA512 OID to indicate the SHA-512 hash is expected to be used for the self-signature in the PKCS#10 CSR.
0 69: SEQUENCE { <06 09> 2 9: OBJECT IDENTIFIER challengePassword (1 2 840 113549 1 9 7) : (PKCS #9) <30 12> 13 18: SEQUENCE { <06 07> 15 7: OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1) : (ANSI X9.62 public key type) <31 07> 24 7: SET { <06 05> 26 5: OBJECT IDENTIFIER secp521r1 (1 3 132 0 35) : (SECG (Certicom) named elliptic curve) : } : } <06 09> 33 9: OBJECT IDENTIFIER friendlyName (for PKCS #12) (1 2 840 113549 1 9 20) : (PKCS #9 via PKCS #12) <06 0A> 44 10: OBJECT IDENTIFIER '0 9 2342 19200300 100 1 5' <06 03> 56 3: OBJECT IDENTIFIER serialNumber (2 5 4 5) : (X.520 DN component) <06 08> 61 8: OBJECT IDENTIFIER ecdsaWithSHA512 (1 2 840 10045 4 3 4) : (ANSI X9.62 ECDSA algorithm with SHA512) : }]]>
Require a Public Key of a Specific Size The CSR requires an RSA public key of a specific size.
Base64-Encoded Example The Base64:
ASN.1 DUMP Output Provide a CSR with an RSA key that's 4096 bits and use SHA256 as the hash algorithm within the signature. 0 41: SEQUENCE { <06 09> 2 9: OBJECT IDENTIFIER challengePassword (1 2 840 113549 1 9 7) : (PKCS #9) <30 11> 13 17: SEQUENCE { <06 09> 15 9: OBJECT IDENTIFIER rsaEncryption (1 2 840 113549 1 1 1) : (PKCS #1) <31 04> 26 4: SET { <02 02> 28 2: INTEGER 4096 : } : } <06 09> 32 9: OBJECT IDENTIFIER sha256WithRSAEncryption (1 2 840 113549 1 1 11) : (PKCS #1) : }]]>
Require a Public Key of a Specific Curve The CSR requires an ECC public key with a specific curve.
Base64-Encoded Example The Base64:
ASN.1 DUMP Output Provide a CSR with an ECC public key from p384, include your serial number, and use SHA384 as the hash algorithm within the signature. 0 46: SEQUENCE { <06 09> 2 9: OBJECT IDENTIFIER challengePassword (1 2 840 113549 1 9 7) : (PKCS #9) <30 12> 13 18: SEQUENCE { <06 07> 15 7: OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1) : (ANSI X9.62 public key type) <31 07> 24 7: SET { <06 05> 26 5: OBJECT IDENTIFIER secp384r1 (1 3 132 0 34) : (SECG (Certicom) named elliptic curve) : } : } <06 03> 33 3: OBJECT IDENTIFIER serialNumber (2 5 4 5) : (X.520 DN component) <06 08> 38 8: OBJECT IDENTIFIER ecdsaWithSHA384 (1 2 840 10045 4 3 3) : (ANSI X9.62 ECDSA algorithm with SHA384) : }]]>
Require Specific Extensions and Attributes The CSR is required to have an ECC public key, include a serial number, include a friendly name, include a favorite drink [OID 0.9.2342.19200300.100.1.5], and use SHA512 as the hash algorithm within the signature.
Base64-Encoded Example The Base64:
ASN.1 DUMP Output Provide a CSR with an ECC public key from sha521 and include your serial number, friendly name, and favorite drink, and hash it with SHA512. 0 69: SEQUENCE { <06 09> 2 9: OBJECT IDENTIFIER challengePassword (1 2 840 113549 1 9 7) : (PKCS #9) <30 12> 13 18: SEQUENCE { <06 07> 15 7: OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1) : (ANSI X9.62 public key type) <31 07> 24 7: SET { <06 05> 26 5: OBJECT IDENTIFIER secp521r1 (1 3 132 0 35) : (SECG (Certicom) named elliptic curve) : } : } <06 09> 33 9: OBJECT IDENTIFIER friendlyName (for PKCS #12) (1 2 840 113549 1 9 20) : (PKCS #9 via PKCS #12) <06 0A> 44 10: OBJECT IDENTIFIER '0 9 2342 19200300 100 1 5' <06 03> 56 3: OBJECT IDENTIFIER serialNumber (2 5 4 5) : (X.520 DN component) <06 08> 61 8: OBJECT IDENTIFIER ecdsaWithSHA512 (1 2 840 10045 4 3 4) : (ANSI X9.62 ECDSA algorithm with SHA512) : }]]>
Security Considerations The security considerations from are unchanged.
Identity and Privacy Considerations An EST server may use this mechanism to instruct the EST client about the identities it should include in the CSR it sends as part of enrollment. The client may only be aware of its Initial Device Identifier (IDevID) Subject, which includes a manufacturer serial number. The EST server can use this mechanism to tell the client to include a specific fully qualified domain name in the CSR in order to complete domain ownership proofs required by the CA. Additionally, the EST server may deem the manufacturer serial number in an IDevID as personally identifiable information and may want to specify a new random opaque identifier that the pledge should use in its CSR. This may be desirable if the CA and EST server have different operators.
IANA Considerations For the ASN.1 module in , IANA has assigned the following OID in the "SMI Security for S/MIME Module Identifier (1.2.840.113549.1.9.16.0)" registry:
Decimal Description Reference
82 id-mod-critemplate RFC 9908
For the Certification Request Information Template and Extension Request Template attributes in , IANA has assigned the following OIDs in the "SMI Security for S/MIME Attributes (1.2.840.113549.1.9.16.2)" registry:
Decimal Description Reference
61 id-aa-certificationRequestInfoTemplate RFC 9908
62 id-aa-extensionReqTemplate RFC 9908
References Normative References Information technology -- Abstract Syntax Notation One (ASN.1): Specification of basic notation ITU-T Information technology -- ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER) ITU-T Informative References Dump ASN drink(5) [other identifier: favouriteDrink] OID Repository OID 0.9.2342.19200300.100.1.5
ASN.1 Module This appendix provides an ASN.1 module for the Certification Request Information Template attribute and its sub-template structures. It follows the conventions established in , , and .
Acknowledgments crafted Example 2 using a different tool, and this helped debug other running code. provided major parts of the CertificationRequestInfoTemplate syntax declaration. conducted many reviews of the ASN.1 module and suggested many fixes. conducted the usual Area Director Review.