Internet-Draft Conveying a CSR in an SZTP Request October 2021
Watsen, et al. Expires 25 April 2022 [Page]
Workgroup:
NETCONF Working Group
Internet-Draft:
draft-ietf-netconf-sztp-csr-09
Updates:
8572 (if approved)
Published:
Intended Status:
Standards Track
Expires:
Authors:
K. Watsen
Watsen Networks
R. Housley
Vigil Security, LLC
S. Turner
sn3rd

Conveying a Certificate Signing Request (CSR) in a Secure Zero Touch Provisioning (SZTP) Bootstrapping Request

Abstract

This draft extends the "get-bootstrapping-data" RPC defined in RFC 8572 to include an optional certificate signing request (CSR), enabling a bootstrapping device to additionally obtain an identity certificate (e.g., an LDevID, from IEEE 802.1AR) as part of the "onboarding information" response provided in the RPC-reply.

Editorial Note (To be removed by RFC Editor)

This draft contains many placeholder values that need to be replaced with finalized values at the time of publication. This note summarizes all of the substitutions that are needed. No other RFC Editor instructions are specified elsewhere in this document.

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Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on 25 April 2022.

Table of Contents

1. Introduction

1.1. Overview

This draft extends the "get-bootstrapping-data" RPC defined in [RFC8572] to include an optional certificate signing request (CSR) [RFC2986], enabling a bootstrapping device to additionally obtain an identity certificate (e.g., an LDevID [Std-802.1AR-2018]) as part of the "onboarding information" response provided in the RPC-reply.

The ability to provision an identity certificate that is purpose-built for a production environment during the bootstrapping process removes reliance on the manufacturer CA, and it also enables the bootstraped device to join the production environment with an appropriate identity and other attributes in its LDevID certificate.

Two YANG [RFC7950] modules are defined. The "ietf-ztp-types" module defines three YANG groupings for the various messages defined in this document. The "ietf-sztp-csr" module augments two groupings into the "get-bootstrapping-data" RPC and defines a YANG Data Structure [RFC8791] around the third grouping.

1.2. Terminology

This document uses the following terms from [RFC8572]:

  • Bootstrap Server
  • Bootstrapping Data
  • Conveyed Information
  • Device
  • Manufacturer
  • Onboarding Information
  • Signed Data

This document defines the following new terms:

SZTP-client
The term "SZTP-client" refers to a "device" that is using a "bootstrap server" as a source of "bootstrapping data".
SZTP-server
The term "SZTP-server" is an alternative term for "bootstrap server" that is symmetric with the "SZTP-client" term.

1.3. Requirements Language

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.

1.4. Conventions

Various examples used in this document use a placeholder value for binary data that has been base64 encoded (e.g., "BASE64VALUE="). This placeholder value is used as real base64 encoded structures are often many lines long and hence distracting to the example being presented.

2. The "ietf-sztp-csr" Module

The "ietf-sztp-csr" module is a YANG 1.1 [RFC7950] module that augments the "ietf-sztp-bootstrap-server" module defined in [RFC8572] and defines a YANG "structure" that is to be conveyed in the "error-info" node defined in Section 7.1 of [RFC8040].

2.1. Data Model Overview

The following tree diagram [RFC8340] illustrates the "ietf-sztp-csr" module.

module: ietf-sztp-csr

  augment /sztp-svr:get-bootstrapping-data/sztp-svr:input:
    +---w (msg-type)?
       +--:(csr-support)
       |  +---w csr-support
       |     +---w key-generation!
       |     |  +---w supported-algorithms
       |     |     +---w algorithm-identifier*   binary
       |     +---w csr-generation
       |        +---w supported-formats
       |           +---w format-identifier*   identityref
       +--:(csr)
          +---w (csr-type)
             +--:(p10-csr)
             |  +---w p10-csr?   ct:csr
             +--:(cmc-csr)
             |  +---w cmc-csr?   binary
             +--:(cmp-csr)
                +---w cmp-csr?   binary
module: ietf-sztp-csr

  structure: csr-request
     +--ro key-generation!
     |  +--ro selected-algorithm
     |     +--ro algorithm-identifier    binary
     +--ro csr-generation
     |  +--ro selected-format
     |     +--ro format-identifier    identityref
     +--ro cert-req-info?    ct:csr-info

  augment /sztp-svr:get-bootstrapping-data/sztp-svr:input:
    +---w (msg-type)?
       +--:(csr-support)
       |  +---w csr-support
       |     +---w key-generation!
       |     |  +---w supported-algorithms
       |     |     +---w algorithm-identifier*   binary
       |     +---w csr-generation
       |        +---w supported-formats
       |           +---w format-identifier*   identityref
       +--:(csr)
          +---w (csr-type)
             +--:(p10-csr)
             |  +---w p10-csr?   ct:csr
             +--:(cmc-csr)
             |  +---w cmc-csr?   binary
             +--:(cmp-csr)
                +---w cmp-csr?   binary

The augmentation defines two kinds of parameters that an SZTP-client can send to an SZTP-server. The YANG structure defines one collection of parameters that an SZTP-server can send to an SZTP-client.

In the order of their intended use:

To further illustrate how the augmentation and structure defined by the "ietf-sztp-csr" module are used, below are two additional tree diagrams showing these nodes placed where they are used.

The following tree diagram [RFC8340] illustrates SZTP's "get-bootstrapping-data" RPC with the augmentation in place.

=============== NOTE: '\' line wrapping per RFC 8792 ================

module: ietf-sztp-bootstrap-server

  rpcs:
    +---x get-bootstrapping-data
       +---w input
       |  +---w signed-data-preferred?          empty
       |  +---w hw-model?                       string
       |  +---w os-name?                        string
       |  +---w os-version?                     string
       |  +---w nonce?                          binary
       |  +---w (sztp-csr:msg-type)?
       |     +--:(sztp-csr:csr-support)
       |     |  +---w sztp-csr:csr-support
       |     |     +---w sztp-csr:key-generation!
       |     |     |  +---w sztp-csr:supported-algorithms
       |     |     |     +---w sztp-csr:algorithm-identifier*   bina\
ry
       |     |     +---w sztp-csr:csr-generation
       |     |        +---w sztp-csr:supported-formats
       |     |           +---w sztp-csr:format-identifier*   identit\
yref
       |     +--:(sztp-csr:csr)
       |        +---w (sztp-csr:csr-type)
       |           +--:(sztp-csr:p10-csr)
       |           |  +---w sztp-csr:p10-csr?   ct:csr
       |           +--:(sztp-csr:cmc-csr)
       |           |  +---w sztp-csr:cmc-csr?   binary
       |           +--:(sztp-csr:cmp-csr)
       |              +---w sztp-csr:cmp-csr?   binary
       +--ro output
          +--ro reporting-level?    enumeration {onboarding-server}?
          +--ro conveyed-information    cms
          +--ro owner-certificate?      cms
          +--ro ownership-voucher?      cms

The following tree diagram [RFC8340] illustrates RESTCONF's "errors" RPC-reply message with the "csr-request" structure in place.

module: ietf-restconf
  +--ro errors
     +--ro error* []
        +--ro error-type       enumeration
        +--ro error-tag        string
        +--ro error-app-tag?   string
        +--ro error-path?      instance-identifier
        +--ro error-message?   string
        +--ro error-info
           +--ro csr-request
              +--ro key-generation!
              |  +--ro selected-algorithm
              |     +--ro algorithm-identifier    binary
              +--ro csr-generation
              |  +--ro selected-format
              |     +--ro format-identifier    identityref
              +--ro cert-req-info?    ct:csr-info

2.2. Example Usage

An SZTP-client implementing this specification would signal to the bootstrap server its willingness to generate a CSR by including the "csr-support" node in its "get-bootstrapping-data" RPC, as illustrated below.

REQUEST

=============== NOTE: '\' line wrapping per RFC 8792 ================

POST /restconf/operations/ietf-sztp-bootstrap-server:get-bootstrappi\
ng-data HTTP/1.1
HOST: example.com
Content-Type: application/yang.data+json

{
  "ietf-sztp-bootstrap-server:input" : {
    "hw-model": "model-x",
    "os-name": "vendor-os",
    "os-version": "17.3R2.1",
    "nonce": "extralongbase64encodedvalue=",
    "ietf-sztp-csr:csr-support": {
      "key-generation": {
        "supported-algorithms": {
          "algorithm-identifier": [
            "BASE64VALUE1",
            "BASE64VALUE2",
            "BASE64VALUE3"
          ]
        }
      },
      "csr-generation": {
        "supported-formats": {
          "format-identifier": [
            "ietf-ztp-types:p10-csr",
            "ietf-ztp-types:cmc-csr",
            "ietf-ztp-types:cmp-csr"
          ]
        }
      }
    }
  }
}

Assuming the SZTP-server wishes to prompt the SZTP-client to provide a CSR, then it would respond with an HTTP 400 Bad Request error code:

RESPONSE

HTTP/1.1 400 Bad Request
Date: Sat, 31 Oct 2015 17:02:40 GMT
Server: example-server
Content-Type: application/yang.data+json

{
  "ietf-restconf:errors" : {
    "error" : [
      {
        "error-type": "application",
        "error-tag": "missing-attribute",
        "error-message": "Missing input parameter",
        "error-info": {
          "ietf-sztp-csr:csr-request": {
            "key-generation": {
              "selected-algorithm": {
                "algorithm-identifier": "BASE64VALUE="
              }
            },
            "csr-generation": {
              "selected-format": {
                "format-identifier": "ietf-ztp-types:p10-csr"
              }
            },
            "cert-req-info": "BASE64VALUE="
          }
        }
      }
    ]
  }
}

Upon being prompted to provide a CSR, the SZTP-client would POST another "get-bootstrapping-data" request, but this time including one of the "csr" nodes to convey its CSR to the SZTP-server:

REQUEST

=============== NOTE: '\' line wrapping per RFC 8792 ================

POST /restconf/operations/ietf-sztp-bootstrap-server:get-bootstrappi\
ng-data HTTP/1.1
HOST: example.com
Content-Type: application/yang.data+json

{
  "ietf-sztp-bootstrap-server:input" : {
    "hw-model": "model-x",
    "os-name": "vendor-os",
    "os-version": "17.3R2.1",
    "nonce": "extralongbase64encodedvalue=",
    "ietf-sztp-csr:p10-csr": "BASE64VALUE="
  }
}

At this point, it is expected that the SZTP-server, perhaps in conjunction with other systems, such as a backend CA or RA, will validate the CSR's origin and proof-of-possession and, assuming the CSR is approved, issue a signed certificate for the bootstrapping device.

The SZTP-server responds with "onboarding-information" (encoded inside the "conveyed-information" node, shown below) containing a signed identity certificate for the CSR provided by the SZTP-client:

RESPONSE

HTTP/1.1 200 OK
Date: Sat, 31 Oct 2015 17:02:40 GMT
Server: example-server
Content-Type: application/yang.data+json

{
  "ietf-sztp-bootstrap-server:output" : {
    "reporting-level": "verbose",
    "conveyed-information": "BASE64VALUE="
  }
}

How the signed certificate is conveyed inside the onboarding information is outside the scope of this document. Some implementations may choose to convey it inside a script (e.g., SZTP's "pre-configuration-script"), while other implementations may choose to convey it inside the SZTP "configuration" node. SZTP onboarding information is described in Section 2.2 of [RFC8572].

Following are two examples of conveying the signed certificate inside the "configuration" node. Both examples assume that the SZTP-client understands the "ietf-keystore" module defined in [I-D.ietf-netconf-keystore].

This first example illustrates the case where the signed certificate is for the same asymmetric key used by the SZTP-client's manufacturer-generated identity certificate (e.g., an IDevID, from [Std-802.1AR-2018]). As such, the configuration needs to associate the newly signed certificate with the existing asymmetric key:

=============== NOTE: '\' line wrapping per RFC 8792 ================

{
  "ietf-keystore:keystore": {
    "asymmetric-keys": {
      "asymmetric-key": [
        {
          "name": "Manufacturer-Generated Hidden Key",
          "public-key-format": "ietf-crypto-types:subject-public-key\
-info-format",
          "public-key": "BASE64VALUE=",
          "hidden-private-key": [null],
          "certificates": {
            "certificate": [
              {
                "name": "Manufacturer-Generated IDevID Cert",
                "cert-data": "BASE64VALUE="
              },
              {
                "name": "Newly-Generated LDevID Cert",
                "cert-data": "BASE64VALUE="
              }
            ]
          }
        }
      ]
    }
  }
}

This second example illustrates the case where the signed certificate is for a newly generated asymmetric key. As such, the configuration needs to associate the newly signed certificate with the newly generated asymmetric key:

=============== NOTE: '\' line wrapping per RFC 8792 ================

{
  "ietf-keystore:keystore": {
    "asymmetric-keys": {
      "asymmetric-key": [
        {
          "name": "Manufacturer-Generated Hidden Key",
          "public-key-format": "ietf-crypto-types:subject-public-key\
-info-format",
          "public-key": "BASE64VALUE=",
          "hidden-private-key": [null],
          "certificates": {
            "certificate": [
              {
                "name": "Manufacturer-Generated IDevID Cert",
                "cert-data": "BASE64VALUE="
              }
            ]
          }
        },
        {
          "name": "Newly-Generated Hidden Key",
          "public-key-format": "ietf-crypto-types:subject-public-key\
-info-format",
          "public-key": "BASE64VALUE=",
          "hidden-private-key": [null],
          "certificates": {
            "certificate": [
              {
                "name": "Newly-Generated LDevID Cert",
                "cert-data": "BASE64VALUE="
              }
            ]
          }
        }
      ]
    }
  }
}

In addition to configuring the signed certificate, it is often necessary to also configure the Issuer's signing certificate so that the device (i.e., STZP-client) can authenticate certificates presented by peer devices signed by the same issuer as its own. While outside the scope of this document, one way to do this would be to use the "ietf-truststore" module defined in [I-D.ietf-netconf-trust-anchors].

2.3. YANG Module

This module augments an RPC defined in [RFC8572]. The module uses a data types and groupings defined in [RFC8572], [RFC8791], and [I-D.ietf-netconf-crypto-types]. The module has additional normative references to [RFC2986], [RFC5272], [RFC4210], and [ITU.X690.2015], and an informative reference to [Std-802.1AR-2018].

<CODE BEGINS> file "ietf-sztp-csr@2021-10-22.yang"

module ietf-sztp-csr {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-sztp-csr";
  prefix sztp-csr;

  import ietf-sztp-bootstrap-server {
    prefix sztp-svr;
    reference
      "RFC 8572: Secure Zero Touch Provisioning (SZTP)";
  }

  import ietf-yang-structure-ext {
    prefix sx;
    reference
      "RFC 8791: YANG Data Structure Extensions";
  }

  import ietf-ztp-types {
    prefix zt;
    reference
      "RFC XXXX: Conveying a Certificate Signing Request (CSR)
                 in a Secure Zero Touch Provisioning (SZTP)
                 Bootstrapping Request";
  }

  organization
    "IETF NETCONF (Network Configuration) Working Group";

  contact
    "WG Web:   http://tools.ietf.org/wg/netconf
     WG List:  <mailto:netconf@ietf.org>
     Authors:  Kent Watsen <mailto:kent+ietf@watsen.net>
               Russ Housley <mailto:housley@vigilsec.com>
               Sean Turner <mailto:sean@sn3rd.com>";

  description
    "This module augments the 'get-bootstrapping-data' RPC,
     defined in the 'ietf-sztp-bootstrap-server' module from
     SZTP (RFC 8572), enabling the SZTP-client to obtain a
     signed identity certificate (e.g., an LDevID from IEEE
     802.1AR) as part of the SZTP onboarding information
     response.

     Copyright (c) 2021 IETF Trust and the persons identified
     as authors of the code. All rights reserved.

     Redistribution and use in source and binary forms, with
     or without modification, is permitted pursuant to, and
     subject to the license terms contained in, the Simplified
     BSD License set forth in Section 4.c of the IETF Trust's
     Legal Provisions Relating to IETF Documents
     (https://trustee.ietf.org/license-info).

     This version of this YANG module is part of RFC XXXX
     (https://www.rfc-editor.org/info/rfcXXXX); see the RFC
     itself for full legal notices.

     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
     (RFC 2119) (RFC 8174) when, and only when, they appear
     in all capitals, as shown here.";

  revision 2021-10-22 {
    description
      "Initial version";
    reference
      "RFC XXXX: Conveying a Certificate Signing Request (CSR)
                 in a Secure Zero Touch Provisioning (SZTP)
                 Bootstrapping Request";
  }

  // Protocol-accessible nodes

  augment "/sztp-svr:get-bootstrapping-data/sztp-svr:input" {
    description
      "This augmentation adds the 'csr-support' and 'csr' nodes to
       the SZTP (RFC 8572) 'get-bootstrapping-data' request message,
       enabling the SZTP-client to obtain an identity certificate
       (e.g., an LDevID from IEEE 802.1AR) as part of the onboarding
       information response provided by the SZTP-server.

       The 'csr-support' node enables the SZTP-client to indicate
       that it supports generating certificate signing requests
       (CSRs), and to provide details around the CSRs it is able
       to generate.

       The 'csr' node enables the SZTP-client to relay a CSR to
       the SZTP-server.";
    reference
      "IEEE 802.1AR: IEEE Standard for Local and metropolitan
                     area networks - Secure Device Identity
       RFC 8572: Secure Zero Touch Provisioning (SZTP)";
    choice msg-type {
      description
        "Messages are mutually exclusive.";
      case csr-support {
        description
          "Indicates how the SZTP-client supports generating CSRs.

           If present and a SZTP-server wishes to request the
           SZTP-client generate a CSR, the SZTP-server MUST
           respond with HTTP code 400 Bad Request with an
           'ietf-restconf:errors' message having the 'error-tag'
           value 'missing-attribute' and the 'error-info' node
           containing the 'csr-request' structure described
           in this module.";
        uses zt:csr-support-grouping;
      }
      case csr {
        description
          "Provides the CSR generated by the SZTP-client.

           When present, the SZTP-server SHOULD respond with
           an SZTP onboarding information message containing
           a signed certificate for the conveyed CSR.  The
           SZTP-server MAY alternatively respond with another
           HTTP error containing another 'csr-request', in
           which case the SZTP-client MUST invalidate the
           previously generated CSR.";
        uses zt:csr-grouping;
      }
    }
  }

  sx:structure csr-request {
    description
      "A YANG data structure, per RFC 8791, that specifies
       details for the CSR that the ZTP-client is to generate.";
    reference
      "RFC 8791: YANG Data Structure Extensions";
    uses zt:csr-request-grouping;
  }

}

<CODE ENDS>

3. The "ietf-ztp-types" Module

This section defines a YANG 1.1 [RFC7950] module that defines three YANG groupings, one each for messages sent between a ZTP-client and ZTP-server. This module is defines independently of the "ietf-sztp-csr" module so that it's groupings may be used by bootstrapping protocols other than SZTP [RFC8572].

3.1. Data Model Overview

The following tree diagram [RFC8340] illustrates the three groupings defined in the "ietf-ztp-types" module.

module: ietf-ztp-types

  grouping csr-support-grouping
    +-- csr-support
       +-- key-generation!
       |  +-- supported-algorithms
       |     +-- algorithm-identifier*   binary
       +-- csr-generation
          +-- supported-formats
             +-- format-identifier*   identityref
  grouping csr-request-grouping
    +-- key-generation!
    |  +-- selected-algorithm
    |     +-- algorithm-identifier    binary
    +-- csr-generation
    |  +-- selected-format
    |     +-- format-identifier    identityref
    +-- cert-req-info?    ct:csr-info
  grouping csr-grouping
    +-- (csr-type)
       +--:(p10-csr)
       |  +-- p10-csr?   ct:csr
       +--:(cmc-csr)
       |  +-- cmc-csr?   binary
       +--:(cmp-csr)
          +-- cmp-csr?   binary

3.2. YANG Module

This module uses a data types and groupings [RFC8791] and [I-D.ietf-netconf-crypto-types]. The module has additional normative references to [RFC2986], [RFC4210], [RFC5272], and [ITU.X690.2015], and an informative reference to [Std-802.1AR-2018].

<CODE BEGINS> file "ietf-ztp-types@2021-10-22.yang"

module ietf-ztp-types {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-ztp-types";
  prefix zt;

  import ietf-crypto-types {
    prefix ct;
    reference
      "RFC AAAA: YANG Data Types and Groupings for Cryptography";
  }

  organization
    "IETF NETCONF (Network Configuration) Working Group";

  contact
    "WG Web:   http://tools.ietf.org/wg/netconf
     WG List:  <mailto:netconf@ietf.org>
     Authors:  Kent Watsen <mailto:kent+ietf@watsen.net>
               Russ Housley <mailto:housley@vigilsec.com>
               Sean Turner <mailto:sean@sn3rd.com>";

  description
    "This module defines three groupings that enable
     bootstrapping devices to 1) indicate if and how they
     support generating CSRs, 2) obtain a request to
     generate a CSR, and 3) communicate the requested CSR.

     The terms 'IDevID' and 'LDevID' are used herein to
     mean 'initial device identifier' and 'local device
     identifer'.  These terms are defined consistent with
     the IEEE 802.1AR specification, though there is no
     requirement that a ZTP-client's identity certificate
     conform to IEEE 802.1AR.

     Copyright (c) 2021 IETF Trust and the persons identified
     as authors of the code. All rights reserved.

     Redistribution and use in source and binary forms, with
     or without modification, is permitted pursuant to, and
     subject to the license terms contained in, the Simplified
     BSD License set forth in Section 4.c of the IETF Trust's
     Legal Provisions Relating to IETF Documents
     (https://trustee.ietf.org/license-info).

     This version of this YANG module is part of RFC XXXX
     (https://www.rfc-editor.org/info/rfcXXXX); see the RFC
     itself for full legal notices.

     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
     (RFC 2119) (RFC 8174) when, and only when, they appear
     in all capitals, as shown here.";

  revision 2021-10-22 {
    description
      "Initial version";
    reference
      "RFC XXXX: Conveying a Certificate Signing Request (CSR)
                 in a Secure Zero Touch Provisioning (SZTP)
                 Bootstrapping Request";
  }

  identity certificate-request-format {
    description
      "A base identity for the request formats supported
       by the ZTP-client.

       Additional derived identities MAY be defined by
       future efforts.";
  }

  identity p10-csr {
    base certificate-request-format;
    description
      "Indicates that the ZTP-client supports generating
       requests using the 'CertificationRequest' structure
       defined in RFC 2986.";
    reference
      "RFC 2986: PKCS #10: Certification Request Syntax
                 Specification Version 1.7";
  }

  identity cmp-csr {
    base certificate-request-format;
    description
      "Indicates that the ZTP-client supports generating
       requests using a constrained version of the PKIMessage
       containing a p10cr structure defined in RFC 4210.";
    reference
      "RFC 4210: Internet X.509 Public Key Infrastructure
                 Certificate Management Protocol (CMP)";
  }

  identity cmc-csr {
    base certificate-request-format;
    description
      "Indicates that the ZTP-client supports generating
       requests using a constrained version of the 'Full
       PKI Request' structure defined in RFC 5272.";
    reference
      "RFC 5272: Certificate Management over CMS (CMC)";
  }

  // Protocol-accessible nodes

  grouping csr-support-grouping {
    description
      "A grouping enabling use by other efforts.";
    container csr-support {
      description
      "Enables a ZTP-client to indicate that it supports
       generating certificate signing requests (CSRs) and
       provides details about the CSRs it is able to
       generate.";
      container key-generation {
        presence
          "Indicates that the ZTP-client is capable of
           generating a new asymmetric key pair.

           If this node is not present, the ZTP-server MAY
           request a CSR using the asymmetric key associated
           with the device's existing identity certificate
           (e.g., an IDevID from IEEE 802.1AR).";
        description
          "Specifies details for the ZTP-client's ability to
           generate a new asymmetric key pair.";
        container supported-algorithms {
          description
            "A list of public key algorithms supported by the
             ZTP-client for generating a new asymmetric key.";
          leaf-list algorithm-identifier {
            type binary;
            min-elements 1;
            description
              "An AlgorithmIdentifier, as defined in RFC 2986,
               encoded using ASN.1 distinguished encoding rules
               (DER), as specified in ITU-T X.690.";
            reference
              "RFC 2986: PKCS #10: Certification Request Syntax
                         Specification Version 1.7
               ITU-T X.690:
                 Information technology - ASN.1 encoding rules:
                 Specification of Basic Encoding Rules (BER),
                 Canonical Encoding Rules (CER) and Distinguished
                 Encoding Rules (DER).";
          }
        }
      }
      container csr-generation {
        description
          "Specifies details for the ZTP-client's ability to
           generate a certificate signing requests.";
        container supported-formats {
          description
            "A list of certificate request formats supported
             by the ZTP-client for generating a new key.";
          leaf-list format-identifier {
            type identityref {
              base zt:certificate-request-format;
            }
            min-elements 1;
            description
              "A certificate request format supported by the
               ZTP-client.";
          }
        }
      }
    }
  }

  grouping csr-request-grouping {
    description
      "A grouping enabling use by other efforts.";
    container key-generation {
      presence
        "Provided by a ZTP-server to indicate that it wishes
         the ZTP-client to generate a new asymmetric key.

         This statement is present so the mandatory descendant
         nodes do not imply that this node must be configured.";
      description
        "The key generation parameters selected by the ZTP-server.

         This leaf MUST only appear if the ZTP-client's
         'csr-support' included the 'key-generation' node.";
      container selected-algorithm {
        description
          "The key algorithm selected by the ZTP-server. The
           algorithm MUST be one of the algorithms specified by
           the 'supported-algorithms' node in the ZTP-client's
           message containing the 'csr-support' structure.";
        leaf algorithm-identifier {
          type binary;
          mandatory true;
          description
            "An AlgorithmIdentifier, as defined in RFC 2986,
             encoded using ASN.1 distinguished encoding rules
             (DER), as specified in ITU-T X.690.";
          reference
            "RFC 2986: PKCS #10: Certification Request Syntax
                       Specification Version 1.7
             ITU-T X.690:
               Information technology - ASN.1 encoding rules:
               Specification of Basic Encoding Rules (BER),
               Canonical Encoding Rules (CER) and Distinguished
               Encoding Rules (DER).";
        }
      }
    }
    container csr-generation {
      description
        "Specifies details for the CSR that the ZTP-client
         is to generate.";
      container selected-format {
        description
          "The CSR format selected by the ZTP-server. The
           format MUST be one of the formats specified by
           the 'supported-formats' node in the ZTP-client's
           request message.";
        leaf format-identifier {
          type identityref {
            base zt:certificate-request-format;
          }
          mandatory true;
          description
            "A certificate request format to be used by the
             ZTP-client.";
        }
      }
    }
    leaf cert-req-info {
      type ct:csr-info;
      description
        "A CertificationRequestInfo structure, as defined in
         RFC 2986, and modeled via a 'typedef' statement by
         RFC AAAA.

         Enables the ZTP-server to provide a fully-populated
         CertificationRequestInfo structure that the ZTP-client
         only needs to sign in order to generate the complete
         'CertificationRequest' structure to send to ZTP-server
         in its next 'get-bootstrapping-data' request message.

         When provided, the ZTP-client SHOULD use this
         structure to generate its CSR; failure to do so MAY
         result in a 400 Bad Request response containing
         another 'csr-request' structure.

         When not provided, the ZTP-client SHOULD generate a
         CSR using the same structure defined in its existing
         identity certificate (e.g., IDevID).

         It is an error if the 'AlgorithmIdentifier' field
         contained inside the 'SubjectPublicKeyInfo' field
         does not match the algorithm identified by the
         'selected-algorithm' node.";
      reference
        "RFC 2986:
           PKCS #10: Certification Request Syntax Specification
         RFC AAAA:
           YANG Data Types and Groupings for Cryptography";
    }
  }

  grouping csr-grouping {
    description
      "Enables a ZTP-client to convey a certificate signing
       request, using the encoding format selected by a
       ZTP-server's 'csr-request' response to the ZTP-client's
       previously sent 'get-bootstrapping-data' request
       containing the 'csr-support' node.";
    choice csr-type {
      mandatory true;
      description
        "A choice amongst certificate signing request formats.

         Additional formats MAY be augmented into this 'choice'
         statement by future efforts.";
      case p10-csr {
        leaf p10-csr {
          type ct:csr;
          description
            "A CertificationRequest structure, per RFC 2986.";
          reference
            "RFC 2986: PKCS #10: Certification
                       Request Syntax Specification";
        }
      }
      case cmc-csr {
        leaf cmc-csr {
          type binary;
          description
            "A constrained version of the 'Full PKI Request'
             message defined in RFC 5272, encoded using ASN.1
             distinguished encoding rules (DER), as specified
             in ITU-T X.690.

             For asymmetric key-based origin authentication of
             a CSR based on the IDevID's private key for the
             associated IDevID's public key, the PKIData
             contains one reqSequence element and no cmsSequence
             or otherMsgSequence elements. The reqSequence is
             the TaggedRequest and it is the tcr CHOICE. The
             tcr is the TaggedCertificationRequest and it a
             bodyPartId and the certificateRequest elements.
             The certificateRequest is signed with the IDevID's
             private key. The IDevID certificate and optionally
             its certificate chain is included in the SignedData
             certificates that encapsulates the PKIData.

             For asymmetric key-based origin authentication
             based on the IDevID's private key that signs the
             encapsulated CSR signed by the LDevID's private key,
             the PKIData contains one cmsSequence element and no
             otherMsgSequence element.  The cmsSequence is the
             TaggedContentInfo and it includes a bodyPartID
             element and a contentInfo.  The contentInfo is
             a SignedData encapsulating a PKIData with one
             reqSequence element and no cmsSequence or
             otherMsgSequence elements. The reqSequence is
             the TaggedRequest and it is the tcr CHOICE. The
             tcr is the TaggedCertificationRequest and it a
             bodyPartId and the certificateRequest elements.
             The certificateRequest is signed with the LDevID's
             private key. The IDevID certificate and optionally
             its certificate chain is included in the SignedData
             certificates that encapsulates the PKIData.

             For shared secret-based origin authentication of a
             CSR signed by the LDevID's private key, the PKIData
             contains one cmsSequence element and no reqSequence
             or otherMsgSequence elements. The cmsSequence is
             the TaggedContentInfo and it includes a bodyPartID
             element and a contentInfo.  The contentInfo is an
             AuthenticatedData encapsulating a PKIData with one
             reqSequence element and no cmsSequences or
             otherMsgSequence elements. The reqSequence is the
             TaggedRequest and it is the tcr CHOICE. The tcr is
             the TaggedCertificationRequest and it a bodyPartId
             and the certificateRequest elements.  The
             certificateRequest is signed with the LDevID's
             private key. The IDevID certificate and optionally
             its certificate chain is included in the SignedData
             certificates that encapsulates the PKIData.";
          reference
            "RFC 5272: Certificate Management over CMS (CMC)
             ITU-T X.690:
               Information technology - ASN.1 encoding rules:
               Specification of Basic Encoding Rules (BER),
               Canonical Encoding Rules (CER) and Distinguished
               Encoding Rules (DER).";
        }
      }
      case cmp-csr {
        leaf cmp-csr {
          type binary;
          description
            "A PKIMessage structure, as defined in RFC 4210,
             encoded using ASN.1 distinguished encoding rules
             (DER), as specified in ITU-T X.690.

             For asymmetric key-based origin authentication of
             a CSR based on the IDevID's private key for the
             associated IDevID's public key, PKIMessages
             contains one PKIMessage with the header and body
             elements, no protection element, and should contain
             the extraCerts element. The header element contains
             the pvno, sender, and recipient elements. The pvno
             contains cmp2000, and the sender contains the
             subject of the IDevID certificate. The body element
             contains a p10cr CHOICE of type CertificationRequet.
             It is signed with the IDevID's private key. The
             extraCerts element contains the IDevID certificate,
             optionally followed by its certificate chain
             excluding the trust anchor.

             For asymmetric key-based origin authentication
             based on the IDevID's private key that signs the
             encapsulated CSR signed by the LDevID's private
             key, PKIMessages contains one PKIMessage with the
             header, body, and protection elements, and should
             contain the extraCerts element. The header element
             contains the pvno, sender, recipient, protectionAlg,
             and optionally senderKID elements. The pvno contains
             cmp2000, the sender contains the subject of the
             IDevID certificate, the protectionAlg contains the
             AlgorithmIdentifier of the used signature algorithm,
             and the senderKID contains the subject key
             identifier of the IDevID certificate. The body
             element contains a p10cr CHOICE of type
             CertificationRequet. It is signed with the LDevID's
             private key.  The protection element contains the
             digital signature generated with the IDevID's
             private key. The extraCerts element contains the
             IDevID certificate, optionally followed by its
             certificate chain excluding the trust anchor.

             For shared secret-based origin authentication of a
             CSR signed by the LDevID's private key, PKIMessages
             contains one PKIMessage with the header, body, and
             protection element, and no extraCerts element. The
             header element contains the pvno, sender, recipient,
             protectionAlg, and senderKID elements. The pvno
             contains cmp2000, the protectionAlg contains the
             AlgorithmIdentifier of the used MAC algorithm, and
             the senderKID contains a reference the recipient
             can use to identify the shared secret. The body
             element contains a p10cr CHOICE of type
             CertificationRequet. It is signed with the LDevID's
             private key.  The protection element contains the
             MAC value generated with the shared secret.";
          reference
            "RFC 4210:
               Internet X.509 Public Key Infrastructure
               Certificate Management Protocol (CMP)
             ITU-T X.690:
               Information technology - ASN.1 encoding rules:
               Specification of Basic Encoding Rules (BER),
               Canonical Encoding Rules (CER) and Distinguished
               Encoding Rules (DER).";
        }
      }
    }
  }

}

<CODE ENDS>

4. Security Considerations

This document builds on top of the solution presented in [RFC8572] and therefore all the Security Considerations discussed in RFC 8572 apply here as well.

4.1. SZTP-Client Considerations

4.1.1. Ensuring the Integrity of Asymmetric Private Keys

The private key the SZTP-client uses for the dynamically-generated identity certificate MUST be protected from inadvertent disclosure in order to prevent identity fraud.

The security of this private key is essential in order to ensure the associated identity certificate can be used as a root of trust.

It is RECOMMENDED that devices are manufactured with an HSM (hardware security module), such as a TPM (trusted platform module), to generate and forever contain the private key within the security perimeter of the HSM. In such cases, the private key, and its associated certificates, MAY have long validity periods.

In cases where the SZTP-client does not possess an HSM, or otherwise is unable to use an HSM for the private key, it is RECOMMENDED to regenerate the private key (and associated identity certificates) periodically. Details for how to generate a new private key and associate a new identity certificate are outside the scope of this document.

4.1.2. Reuse of a Manufacturer-generated Private Key

It is RECOMMENDED that a new private key is generated for each CSR described in this document.

This private key SHOULD be protected as well as the built-in private key associated with the SZTP-client's initial device identity certificate (e.g., the IDevID, from [Std-802.1AR-2018]).

In cases where it is not possible to generate a new private key that is protected as well as the built-in private key, it is RECOMMENDED to reuse the built-in private key rather than generate a new private key that is not as well protected.

4.1.3. Replay Attack Protection

This RFC enables an SZTP-client to announce an ability to generate a new key to use for its CSR.

When the SZTP-server responds with a request for the SZTP-client to generate a new key, it is essential that the SZTP-client actually generates a new key.

Generating a new key each time enables the random bytes used to create the key to also serve the dual-purpose of acting like a "nonce" used in other mechanisms to detect replay attacks.

When a fresh public/private key pair is generated for the request, confirmation to the SZTP-client that the response has not been replayed is enabled by the SZTP-client's fresh public key appearing in the signed certificate provided by the SZTP-server.

When a public/private key pair associated with the manufacturer-generated identity certificate (e.g., IDevID) is used for the request, there may not be confirmation to the SZTP-client that the response has not been replayed; however, the worst case result is a lost certificate that is associated to the private key known only to the SZTP-client.

4.1.4. Connecting to an Untrusted Bootstrap Server

[RFC8572] allows SZTP-clients to connect to untrusted SZTP-servers, by blindly authenticating the SZTP-server's TLS end-entity certificate.

As is discussed in Section 9.5 of [RFC8572], in such cases the SZTP-client MUST assert that the bootstrapping data returned is signed, if the SZTP-client is to trust it.

However, the HTTP error message used in this document cannot be signed data, as described in RFC 8572.

Therefore, the solution presented in this document cannot be used when the SZTP-client connects to an untrusted SZTP-server.

Consistent with the recommendation presented in Section 9.6 of [RFC8572], SZTP-clients SHOULD NOT pass the "csr-support" input parameter to an untrusted SZTP-server. SZTP-clients SHOULD pass instead the "signed-data-preferred" input parameter, as discussed in Appendix B of [RFC8572].

4.1.5. Selecting the Best Origin Authentication Mechanism

The origin of the CSR must be verified before a certificate is issued.

When generating a new key, it is important that the SZTP-client be able to provide additional proof that it was the entity that generated the key.

The CMP and CMC certificate request formats defined in this document support origin authentication. A raw PKCS#10 does not support origin authentication.

The CMP and CMC request formats support origin authentication using both PKI and shared secret.

Typically, only one possible origin authentication mechanism can possibly be used but, in the case that the SZTP-client authenticates itself using both TLS-level (e.g., IDevID) and HTTP-level credentials (e.g., Basic), as is allowed by Section 5.3 of [RFC8572], then the SZTP-client may need to choose between the two options.

In the case that the SZTP-client must choose between an asymmetric key option versus a shared secret for origin authentication, it is RECOMMENDED that the SZTP-client choose using the asymmetric key.

4.1.6. Clearing the Private Key and Associated Certificate

Unlike a manufacturer-generated identity certificate (e.g., IDevID), the deployment-generated identity certificate (e.g., LDevID) and the associated private key (assuming a new private key was generated for the purpose), are considered user data and SHOULD be cleared whenever the SZTP-client is reset to its factory default state, such as by the "factory-reset" RPC defined in [I-D.ietf-netmod-factory-default].

4.2. SZTP-Server Considerations

4.2.1. Verifying Proof of Possession

Regardless if using a new asymmetric key or the bootstrapping device's manufacturer-generated key (e.g., the IDevID key), the public key is placed in the CSR and the CSR is signed by that private key. Proof-of-possession of the private key is verified by ensuring the signature over the CSR using the public key placed in the CSR.

4.2.2. Verifying Proof of Origin

When the bootstrapping device's manufacturer-generated private key (e.g., the IDevID key) is reused for the CSR, proof-of-origin is verified by ensuring the CSR was signed by that key.

When a new asymmetric key is used, proof-or-origin is provided, in the CMP and CMC formats, by the CSR's parent ASN.1 structure containing origin authentication using either the manufacturer-generated private key or a shared secret.

When a new asymmetric key is used, with the CMP or CMC formats, the parent ASN.1 structure of the CSR provides origin authentication using either the manufacturer-generated private key or a shared secret. In this way the proof-of-possession of the CSR is directly linked to the proof-or-origin provided by the parent ASN.1 structure.

4.2.3. Supporting SZTP-Clients that don't trust the SZTP-Server

[RFC8572] allows SZTP-clients to connect to untrusted SZTP-servers, by blindly authenticating the SZTP-server's TLS end-entity certificate.

As is recommended in Section 4.1.4 in this document, in such cases, SZTP-clients SHOULD pass the "signed-data-preferred" input parameter.

The reciprocal of this statement is that SZTP-servers, wanting to support SZTP-clients that don't trust them, SHOULD support the "signed-data-preferred" input parameter, as discussed in Appendix B of [RFC8572].

4.3. Security Considerations for the "ietf-sztp-csr" YANG Module

The recommended format for documenting the Security Considerations for YANG modules is described in Section 3.7 of [RFC8407]. However, this module only augments two input parameters into the "get-bootstrapping-data" RPC in [RFC8572], and therefore only needs to point to the relevant Security Considerations sections in that RFC.

  • Security considerations for the "get-bootstrapping-data" RPC are described in Section 9.16 of [RFC8572].
  • Security considerations for the "input" parameters passed inside the "get-bootstrapping-data" RPC are described in Section 9.6 of [RFC8572].

4.4. Security Considerations for the "ietf-ztp-types" YANG Module

The recommended format for documenting the Security Considerations for YANG modules is described in Section 3.7 of [RFC8407]. However, this module does not define any protocol-accessible nodes (it only defines "identity" and "grouping" statements) and therefore there are no Security considerations to report.

5. IANA Considerations

5.1. The "IETF XML" Registry

This document registers two URIs in the "ns" subregistry of the IETF XML Registry [RFC3688] maintained at https://www.iana.org/assignments/xml-registry/xml-registry.xhtml#ns. Following the format in [RFC3688], the following registrations are requested:

URI: urn:ietf:params:xml:ns:yang:ietf-sztp-csr
Registrant Contact: The NETCONF WG of the IETF.
XML: N/A, the requested URI is an XML namespace.

URI: urn:ietf:params:xml:ns:yang:ietf-ztp-types
Registrant Contact: The NETCONF WG of the IETF.
XML: N/A, the requested URI is an XML namespace.

5.2. The "YANG Module Names" Registry

This document registers two YANG modules in the YANG Module Names registry [RFC6020] maintained at https://www.iana.org/assignments/yang-parameters/yang-parameters.xhtml. Following the format defined in [RFC6020], the below registrations are requested:

name:      ietf-sztp-csr
namespace: urn:ietf:params:xml:ns:yang:ietf-sztp-csr
prefix:    sztp-csr
reference: RFC XXXX

name:      ietf-ztp-types
namespace: urn:ietf:params:xml:ns:yang:ietf-ztp-types
prefix:    ztp-types
reference: RFC XXXX

6. References

6.1. Normative References

[I-D.ietf-netconf-crypto-types]
Watsen, K., "YANG Data Types and Groupings for Cryptography", Work in Progress, Internet-Draft, draft-ietf-netconf-crypto-types-21, , <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-crypto-types-21>.
[ITU.X690.2015]
International Telecommunication Union, "Information Technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1, , <https://www.itu.int/rec/T-REC-X.690/>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC2986]
Nystrom, M. and B. Kaliski, "PKCS #10: Certification Request Syntax Specification Version 1.7", RFC 2986, DOI 10.17487/RFC2986, , <https://www.rfc-editor.org/info/rfc2986>.
[RFC3688]
Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, , <https://www.rfc-editor.org/info/rfc3688>.
[RFC4210]
Adams, C., Farrell, S., Kause, T., and T. Mononen, "Internet X.509 Public Key Infrastructure Certificate Management Protocol (CMP)", RFC 4210, DOI 10.17487/RFC4210, , <https://www.rfc-editor.org/info/rfc4210>.
[RFC5272]
Schaad, J. and M. Myers, "Certificate Management over CMS (CMC)", RFC 5272, DOI 10.17487/RFC5272, , <https://www.rfc-editor.org/info/rfc5272>.
[RFC6020]
Bjorklund, M., Ed., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, , <https://www.rfc-editor.org/info/rfc6020>.
[RFC7950]
Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, , <https://www.rfc-editor.org/info/rfc7950>.
[RFC8040]
Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, , <https://www.rfc-editor.org/info/rfc8040>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC8572]
Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero Touch Provisioning (SZTP)", RFC 8572, DOI 10.17487/RFC8572, , <https://www.rfc-editor.org/info/rfc8572>.
[RFC8791]
Bierman, A., Björklund, M., and K. Watsen, "YANG Data Structure Extensions", RFC 8791, DOI 10.17487/RFC8791, , <https://www.rfc-editor.org/info/rfc8791>.

6.2. Informative References

[I-D.ietf-netconf-keystore]
Watsen, K., "A YANG Data Model for a Keystore", Work in Progress, Internet-Draft, draft-ietf-netconf-keystore-22, , <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-keystore-22>.
[I-D.ietf-netconf-trust-anchors]
Watsen, K., "A YANG Data Model for a Truststore", Work in Progress, Internet-Draft, draft-ietf-netconf-trust-anchors-15, , <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-trust-anchors-15>.
[I-D.ietf-netmod-factory-default]
Wu, Q., Lengyel, B., and Y. Niu, "A YANG Data Model for Factory Default Settings", Work in Progress, Internet-Draft, draft-ietf-netmod-factory-default-15, , <https://datatracker.ietf.org/doc/html/draft-ietf-netmod-factory-default-15>.
[RFC8340]
Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams", BCP 215, RFC 8340, DOI 10.17487/RFC8340, , <https://www.rfc-editor.org/info/rfc8340>.
[RFC8407]
Bierman, A., "Guidelines for Authors and Reviewers of Documents Containing YANG Data Models", BCP 216, RFC 8407, DOI 10.17487/RFC8407, , <https://www.rfc-editor.org/info/rfc8407>.
[Std-802.1AR-2018]
Group, W. -. H. L. L. P. W., "IEEE Standard for Local and metropolitan area networks - Secure Device Identity", , <http://standards.ieee.org/findstds/standard/802.1AR-2018.html>.

Acknowledgements

The authors would like to thank for following for lively discussions on list and in the halls (ordered by first name): David von Oheimb, Hendrik Brockhaus, Guy Fedorkow, Joe Clarke, Rich Salz, Rob Wilton, and Qin Wu.

Contributors

Special thanks goes to David von Oheimb and Hendrik Brockhaus for helping with the descriptions for the "cmc-csr" and "cmp-csr" nodes.

Authors' Addresses

Kent Watsen
Watsen Networks
Russ Housley
Vigil Security, LLC
Sean Turner
sn3rd