Clarification of Enrollment over Secure Transport (EST): transfer encodings and ASN.1
draft-ietf-lamps-rfc7030est-clarify-09

Abstract

This document updates RFC7030: Enrollment over Secure Transport (EST) to resolve some errata that were reported, and which has proven to cause interoperability issues when RFC7030 was extended.

This document deprecates the specification of “Content-Transfer-Encoding” headers for EST endpoints. This document fixes some syntactical errors in ASN.1 that were presented.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on January 13, 2021.

Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Changes to EST endpoint processing
• 3.1. Whitespace processing
• 3.2. Changes sections 4 of RFC7030
• 3.2.1. Section 4.1.3
• 3.2.2. Section 4.3.1
• 3.2.3. Section 4.3.2
• 3.2.4. Section 4.4.2
• 3.2.5. Section 4.5.2
• 4. Clarification of ASN.1 for Certificate Attribute set.
• 5. Clarification of error messages for certificate enrollment operations
• 5.1. Updating section 4.2.3: Simple Enroll and Re-enroll Response
• 5.2. Updating section 4.4.2: Server-Side Key Generation Response
• 6. Privacy Considerations
• 7. Security Considerations
• 8. IANA Considerations
• 9. Acknowledgements
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Appendix A. ASN.1 Module
• Authors' Addresses

1. Introduction

Enrollment over Secure Transport (EST) is defined in [RFC7030]. The EST specification defines a number of HTTP end points 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 [RFC2616] and [RFC7230].

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

Any updates to [RFC7030] to bring it inline 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.

[I-D.ietf-anima-bootstrapping-keyinfra] extends [RFC7030], adding new functionality, and 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 explicitely with [errata5107] and [errata5904] in Section 3. [errata5108] is dealt with in section 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

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

This document updates [RFC7030] to require the POST request and payload response of all endpoints use 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.

3.1. Whitespace processing

Note that “base64” as used in the HTTP [RFC2616] does not permit CRLF, while the “base64” used in MIME [RFC2045] does. This specification clarifies that despite [RFC2616], that white space including CR, LF, spaces (ASCII 32) and, tabs (ASCII 9) SHOULD be tolerated by receivers. Senders are not required to insert any kind of white space.

3.2. Changes sections 4 of RFC7030

3.2.1. Section 4.1.3

Replace:

   A successful response MUST be a certs-only CMC Simple PKI Response,
   as defined in [RFC5272], containing the certificates described in the
   following paragraph.  The HTTP content-type of
   "application/pkcs7-mime" is used.  The Simple PKI Response is sent
   with a Content-Transfer-Encoding of "base64" [RFC2045].

with:

   A successful response MUST be a certs-only CMC Simple PKI Response,
   as defined in [RFC5272], containing the certificates described in the
   following paragraph.  The HTTP content-type of
   "application/pkcs7-mime" is used.  The CMC Simple PKI Response is
   encoded in base64 [RFC4648].

3.2.2. Section 4.3.1

Replace:

   If the HTTP POST to /fullcmc is not a valid Full PKI Request, the
   server MUST reject the message.  The HTTP content-type used is
   "application/pkcs7-mime" with an smime-type parameter "CMC-request",
   as specified in [RFC5273].  The body of the message is the binary
   value of the encoding of the PKI Request with a
   Content-Transfer-Encoding of "base64" [RFC2045].

with:

   If the HTTP POST to /fullcmc is not a valid Full PKI Request, the
   server MUST reject the message.  The HTTP content-type used is
   "application/pkcs7-mime" with an smime-type parameter "CMC-request",
   as specified in [RFC5273].  The body of the message is encoded
   in base64 [RFC4648].

3.2.3. Section 4.3.2

Replace:

   The body of the message is the binary value of the encoding of the
   PKI Response with a Content-Transfer-Encoding of "base64" [RFC2045].

with:

   The body of the message is the base64 [RFC4648] encoding of the
   PKI Response.

3.2.4. Section 4.4.2

Replace:

   An "application/pkcs8"
   part consists of the base64-encoded DER-encoded [X.690]
   PrivateKeyInfo with a Content-Transfer-Encoding of "base64"
   [RFC4648].

with:

   An "application/pkcs8" part consists of the base64-encoded
   DER-encoded [X.690] PrivateKeyInfo.

Replace:

   In all three additional encryption cases, the EnvelopedData is
   returned in the response as an "application/pkcs7-mime" part with an
   smime-type parameter of "server-generated-key" and a Content-
   Transfer-Encoding of "base64".

with:

   In all three additional encryption cases, the EnvelopedData is
   returned in the response as an "application/pkcs7-mime" part
   with an smime-type parameter of "server-generated-key". It is
   base64 encoded [RFC4648].

3.2.5. Section 4.5.2

This section is updated in its entirety in Section 4.

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” [RFC2045] 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 a client to submit a certification request containing the challengePassword (indicating that linking of identity and POP information is requested; see Section 3.5), an extensionRequest with the Media Access Control (MAC) address ([RFC2307]) of the client, and to use the secp384r1 elliptic curve and to sign with 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.

5.1. Updating section 4.2.3: Simple Enroll and Re-enroll Response

Replace:

    If the content-type is not set, the response data MUST be a
    plaintext human-readable error message containing explanatory
    information describing why the request was rejected (for
    example, indicating that CSR attributes are incomplete).

with:

    If the content-type is not set, the response data MUST be a
    plaintext human-readable error message containing explanatory
    information describing why the request was rejected (for
    example, indicating that CSR attributes are incomplete).
    Servers MAY use the "text/plain” content-type [RFC2046]
    for human-readable errors.

5.2. Updating section 4.4.2: Server-Side Key Generation Response

Replace:

    If the content-type is not set, the response data MUST be a
    plaintext human-readable error message.

with:

    If the content-type is not set, the response data must be a
    plaintext human-readable error message.
    Servers MAY use the "text/plain” content-type [RFC2046]
    for human-readable errors.

6. Privacy Considerations

This document does not disclose any additional identities to either 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 mechanism.

All security considerations from [RFC7030] applies also for 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). This document requests that IANA register an OID in the SMI Security for PKIX Arc in the Module identifiers subarc (1.3.6.1.5.5.7.0) 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 is requested to update the “Reference” column for the Asymmetric Decryption Key Identifier attribute to also include a reference to this document.

9. Acknowledgements

This work was supported by Huawei Technologies.

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

10. References

10.1. Normative References

10.2. Informative References

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 [RFC5212] and [RFC6268].

There is no ASN.1 Module in RFC 7030. 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(TBD) }

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

Authors' Addresses