Status of This Memo

This is an Internet Standards Track document.¶

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8951.¶

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Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.¶

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Table of Contents

1. Introduction

Enrollment over Secure Transport (EST) is defined in [RFC7030]. The EST specification defines a number of HTTP endpoints for certificate enrollment and management. The details of the transaction were defined in terms of MIME headers, as defined in [RFC2045], rather than in terms of the HTTP protocol, as defined in [RFC7230] and [RFC7231].¶

[RFC2616] and later Appendix A.5 of [RFC7231] have text specifically deprecating Content-Transfer-Encoding. However, [RFC7030] incorrectly uses this header.¶

Any updates to [RFC7030] to bring it in line with HTTP processing risk changing the on-wire protocol in a way that is not backwards compatible. However, reports from implementers suggest that many implementations do not send the Content-Transfer-Encoding, and many of them ignore it. The consequence is that simply deprecating the header would remain compatible with current implementations.¶

[BRSKI] extends [RFC7030], adding new functionality. Interop testing of the protocol has revealed that unusual processing called out in [RFC7030] causes confusion.¶

EST is currently specified as part of [IEC62351] and is widely used in government, utilities, and financial markets today.¶

This document, therefore, revises [RFC7030] to reflect the field reality, deprecating the extraneous field.¶

This document deals with errata numbers [errata4384], [errata5107], [errata5108], and [errata5904].¶

This document deals with [errata5107] and [errata5904] in Section 3. [errata5108] is dealt with in Section 5. [errata4384] is closed by correcting the ASN.1 Module in Section 4.¶

2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. Changes to EST Endpoint Processing

Sections 4.1.3 (CA Certificates Response, /cacerts), 4.3.1 and 4.3.2 (Full CMC, /fullcmc), 4.4.2 (Server-Side Key Generation, /serverkeygen), and 4.5.2 (CSR Attributes, /csrattrs) of [RFC7030] specify the use of base64 encoding with a Content-Transfer-Encoding for requests and responses.¶

This document updates [RFC7030] to require the POST request and payload response of all endpoints using base64 encoding, as specified in Section 4 of [RFC4648]. In both cases, the Distinguished Encoding Rules (DER) [X.690] are used to produce the input for the base64 encoding routine. This format is to be used regardless of any Content-Transfer-Encoding header, and any value in such a header MUST be ignored.¶

4. Clarification of ASN.1 for Certificate Attribute Set

Section 4.5.2 of [RFC7030] is to be replaced with the following text:¶

4.5.2 CSR Attributes Response¶

If locally configured policy for an authenticated EST client indicates a CSR Attributes Response is to be provided, the server response MUST include an HTTP 200 response code. An HTTP response code of 204 or 404 indicates that a CSR Attributes Response is not available. Regardless of the response code, the EST server and CA MAY reject any subsequent enrollment requests for any reason, e.g., incomplete CSR attributes in the request.¶

Responses to attribute request messages MUST be encoded as the content-type of "application/csrattrs" and are to be "base64" [RFC4648] encoded. The syntax for application/csrattrs body is as follows:¶

CsrAttrs ::= SEQUENCE SIZE (0..MAX) OF AttrOrOID

AttrOrOID ::= CHOICE {
  oid        OBJECT IDENTIFIER,
  attribute  Attribute {{AttrSet}} }

AttrSet ATTRIBUTE ::= { ... }

¶

An EST server includes zero or more OIDs or attributes [RFC2986] that it requests the client to use in the certification request. The client MUST ignore any OID or attribute it does not recognize. When the server encodes CSR attributes as an empty SEQUENCE, it means that the server has no specific additional information it desires in a client certification request (this is functionally equivalent to an HTTP response code of 204 or 404).¶

If the CA requires a particular cryptographic algorithm or use of a particular signature scheme (e.g., certification of a public key based on a certain elliptic curve or signing using a certain hash algorithm), it MUST provide that information in the CSR Attribute Response. If an EST server requires the linking of identity and POP information (see Section 3.5), it MUST include the challengePassword OID in the CSR Attributes Response.¶

The structure of the CSR Attributes Response SHOULD, to the greatest extent possible, reflect the structure of the CSR it is requesting. Requests to use a particular signature scheme (e.g., using a particular hash function) are represented as an OID to be reflected in the SignatureAlgorithm of the CSR. Requests to use a particular cryptographic algorithm (e.g., certification of a public key based on a certain elliptic curve) are represented as an attribute, to be reflected as the AlgorithmIdentifier of the SubjectPublicKeyInfo, with a type indicating the algorithm and the values indicating the particular parameters specific to the algorithm. Requests for descriptive information from the client are made by an attribute, to be represented as Attributes of the CSR, with a type indicating the [RFC2985] extensionRequest and the values indicating the particular attributes desired to be included in the resulting certificate's extensions.¶

The sequence is Distinguished Encoding Rules (DER) encoded [X.690] and then base64 encoded (Section 4 of [RFC4648]). The resulting text forms the application/csrattr body, without headers.¶

For example, if a CA requests that a client a) submit a certification request containing the challengePassword (indicating that linking of identity and POP information is requested; see Section 3.5), b) submit an extensionRequest with the Media Access Control (MAC) address [RFC2307] of the client, and c) use the secp384r1 elliptic curve to sign using the SHA384 hash function, then it takes the following:¶

      OID:        challengePassword (1.2.840.113549.1.9.7)

      Attribute:  type = extensionRequest (1.2.840.113549.1.9.14)
                  value = macAddress (1.3.6.1.1.1.1.22)

      Attribute:  type = id-ecPublicKey (1.2.840.10045.2.1)
                  value = secp384r1 (1.3.132.0.34)

      OID:        ecdsaWithSHA384 (1.2.840.10045.4.3.3)

¶

and encodes them into an ASN.1 SEQUENCE to produce:¶

 30 41 06 09 2a 86 48 86 f7 0d 01 09 07 30 12 06 07 2a 86 48 ce 3d
 02 01 31 07 06 05 2b 81 04 00 22 30 16 06 09 2a 86 48 86 f7 0d 01
 09 0e 31 09 06 07 2b 06 01 01 01 01 16 06 08 2a 86 48 ce 3d 04 03
 03

¶

and then base64 encodes the resulting ASN.1 SEQUENCE to produce:¶

 MEEGCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAiMBYGCSqGSIb3DQEJDjEJ
 BgcrBgEBAQEWBggqhkjOPQQDAw==

¶

TESTING -- For example, if a CA requests that a client a) submit a certification request containing the challengePassword (indicating that linking of identity and POP information is requested; see Section 3.5), b) submit an extensionRequest with the Media Access Control (MAC) address [RFC2307] of the client, and c) use the secp384r1 elliptic curve to sign using the SHA384 hash function, then it takes the following:¶

      OID:        challengePassword (1.2.840.113549.1.9.7)

      Attribute:  type = extensionRequest (1.2.840.113549.1.9.14)
                  value = macAddress (1.3.6.1.1.1.1.22)

      Attribute:  type = id-ecPublicKey (1.2.840.10045.2.1)
                  value = secp384r1 (1.3.132.0.34)

      OID:        ecdsaWithSHA384 (1.2.840.10045.4.3.3)

and encodes them into an ASN.1 SEQUENCE to produce:¶

 30 41 06 09 2a 86 48 86 f7 0d 01 09 07 30 12 06 07 2a 86 48 ce 3d
 02 01 31 07 06 05 2b 81 04 00 22 30 16 06 09 2a 86 48 86 f7 0d 01
 09 0e 31 09 06 07 2b 06 01 01 01 01 16 06 08 2a 86 48 ce 3d 04 03
 03

and then base64 encodes the resulting ASN.1 SEQUENCE to produce:¶

 MEEGCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAiMBYGCSqGSIb3DQEJDjEJ
 BgcrBgEBAQEWBggqhkjOPQQDAw==

5. Clarification of Error Messages for Certificate Enrollment Operations

[errata5108] clarifies what format the error messages are to be in. Previously, a client might be confused into believing that an error returned with type text/plain was not intended to be an error.¶

6. Privacy Considerations

This document does not disclose any additional identities that either an active or passive observer would see with [RFC7030].¶

7. Security Considerations

This document clarifies an existing security mechanism. It does not create any new protocol mechanisms.¶

All security considerations from [RFC7030] also apply to the clarifications described in this document.¶

8. IANA Considerations

The ASN.1 module in Appendix A of this document makes use of object identifiers (OIDs).¶

IANA has registered an OID for id-mod-est-2019 (1.3.6.1.5.5.7.0.98) in the "SMI Security for PKIX Module Identifier" registry for the ASN.1 module.¶

The OID for the Asymmetric Decryption Key Identifier (1.2.840.113549.1.9.16.2.54) was previously defined in [RFC7030]. IANA has updated the Reference column for the Asymmetric Decryption Key Identifier attribute to also include a reference to this document.¶

Appendix A. ASN.1 Module

This annex provides the normative ASN.1 definitions for the structures described in this specification using ASN.1 as defined in [X.680], [X.681], [X.682], and [X.683].¶

The ASN.1 modules makes imports from the ASN.1 modules in [RFC5912] and [RFC6268].¶

There is no ASN.1 Module in [RFC7030]. This module has been created by combining the lines that are contained in the document body.¶

PKIXEST-2019
     { iso(1) identified-organization(3) dod(6)
       internet(1) security(5) mechanisms(5) pkix(7)
       id-mod(0) id-mod-est-2019(98) }

DEFINITIONS IMPLICIT TAGS ::=
BEGIN

-- EXPORTS ALL --

IMPORTS

Attribute
FROM CryptographicMessageSyntax-2010  -- [RFC6268]
      { iso(1) member-body(2) us(840) rsadsi(113549)
        pkcs(1) pkcs-9(9) smime(16) modules(0)
         id-mod-cms-2009(58) }

ATTRIBUTE
FROM PKIX-CommonTypes-2009 -- [RFC5912]
    { iso(1) identified-organization(3) dod(6) internet(1)
      security(5) mechanisms(5) pkix(7) id-mod(0)
      id-mod-pkixCommon-02(57) } ;


-- CSR Attributes

CsrAttrs ::= SEQUENCE SIZE (0..MAX) OF AttrOrOID

AttrOrOID ::= CHOICE {
   oid        OBJECT IDENTIFIER,
   attribute  Attribute {{AttrSet}} }

AttrSet ATTRIBUTE ::= { ... }


-- Asymmetric Decrypt Key Identifier Attribute

aa-asymmDecryptKeyID ATTRIBUTE ::=
    { TYPE AsymmetricDecryptKeyIdentifier
      IDENTIFIED BY id-aa-asymmDecryptKeyID }

id-aa-asymmDecryptKeyID OBJECT IDENTIFIER ::= { iso(1)
    member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
    smime(16) aa(2) 54 }

AsymmetricDecryptKeyIdentifier ::= OCTET STRING

END

Acknowledgements

Huawei Technologies supported the efforts of Wei Pan and Michael Richardson.¶

The ASN.1 Module was assembled by Russ Housley and formatted by Sean Turner. Russ Housley provided editorial review.¶

Authors' Addresses