YANG Groupings for SSH Clients and SSH Servers
draft-ietf-netconf-ssh-client-server-14

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NETCONF Working Group                                          K. Watsen
Internet-Draft                                           Watsen Networks
Intended status: Standards Track                                   G. Wu
Expires: December 9, 2019                                  Cisco Systems
                                                                  L. Xia
                                                                  Huawei
                                                            June 7, 2019

             YANG Groupings for SSH Clients and SSH Servers
                draft-ietf-netconf-ssh-client-server-14

Abstract

   This document defines three YANG modules: the first defines groupings
   for a generic SSH client, the second defines groupings for a generic
   SSH server, and the third defines common identities and groupings
   used by both the client and the server.  It is intended that these
   groupings will be used by applications using the SSH protocol.

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.

   This document contains references to other drafts in progress, both
   in the Normative References section, as well as in body text
   throughout.  Please update the following references to reflect their
   final RFC assignments:

   o  I-D.ietf-netconf-trust-anchors

   o  I-D.ietf-netconf-keystore

   Artwork in this document contains shorthand references to drafts in
   progress.  Please apply the following replacements:

   o  "XXXX" --> the assigned RFC value for this draft

   o  "YYYY" --> the assigned RFC value for I-D.ietf-netconf-trust-
      anchors

   o  "ZZZZ" --> the assigned RFC value for I-D.ietf-netconf-keystore

   Artwork in this document contains placeholder values for the date of
   publication of this draft.  Please apply the following replacement:

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   o  "2019-06-07" --> the publication date of this draft

   The following Appendix section is to be removed prior to publication:

   o  Appendix A.  Change Log

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/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 9, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  The SSH Client Model  . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Example Usage . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  The SSH Server Model  . . . . . . . . . . . . . . . . . . . .  14
     4.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . . . .  14

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     4.2.  Example Usage . . . . . . . . . . . . . . . . . . . . . .  15
     4.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  17
   5.  The SSH Common Model  . . . . . . . . . . . . . . . . . . . .  26
     5.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . . . .  28
     5.2.  Example Usage . . . . . . . . . . . . . . . . . . . . . .  29
     5.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  29
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  39
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  40
     7.1.  The IETF XML Registry . . . . . . . . . . . . . . . . . .  40
     7.2.  The YANG Module Names Registry  . . . . . . . . . . . . .  41
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  41
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  41
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  43
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  45
     A.1.  00 to 01  . . . . . . . . . . . . . . . . . . . . . . . .  45
     A.2.  01 to 02  . . . . . . . . . . . . . . . . . . . . . . . .  45
     A.3.  02 to 03  . . . . . . . . . . . . . . . . . . . . . . . .  45
     A.4.  03 to 04  . . . . . . . . . . . . . . . . . . . . . . . .  45
     A.5.  04 to 05  . . . . . . . . . . . . . . . . . . . . . . . .  46
     A.6.  05 to 06  . . . . . . . . . . . . . . . . . . . . . . . .  46
     A.7.  06 to 07  . . . . . . . . . . . . . . . . . . . . . . . .  46
     A.8.  07 to 08  . . . . . . . . . . . . . . . . . . . . . . . .  46
     A.9.  08 to 09  . . . . . . . . . . . . . . . . . . . . . . . .  46
     A.10. 09 to 10  . . . . . . . . . . . . . . . . . . . . . . . .  47
     A.11. 10 to 11  . . . . . . . . . . . . . . . . . . . . . . . .  47
     A.12. 11 to 12  . . . . . . . . . . . . . . . . . . . . . . . .  47
     A.13. 12 to 13  . . . . . . . . . . . . . . . . . . . . . . . .  47
     A.14. 13 to 14  . . . . . . . . . . . . . . . . . . . . . . . .  47
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  48
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  48

1.  Introduction

   This document defines three YANG 1.1 [RFC7950] modules: the first
   defines a grouping for a generic SSH client, the second defines a
   grouping for a generic SSH server, and the third defines identities
   and groupings common to both the client and the server.  It is
   intended that these groupings will be used by applications using the
   SSH protocol [RFC4252], [RFC4253], and [RFC4254].  For instance,
   these groupings could be used to help define the data model for an
   OpenSSH [OPENSSH] server or a NETCONF over SSH [RFC6242] based
   server.

   The client and server YANG modules in this document each define one
   grouping, which is focused on just SSH-specific configuration, and
   specifically avoids any transport-level configuration, such as what
   ports to listen on or connect to.  This affords applications the
   opportunity to define their own strategy for how the underlying TCP

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   connection is established.  For instance, applications supporting
   NETCONF Call Home [RFC8071] could use the "ssh-server-grouping"
   grouping for the SSH parts it provides, while adding data nodes for
   the TCP-level call-home configuration.

   The modules defined in this document use groupings defined in
   [I-D.ietf-netconf-keystore] enabling keys to be either locally
   defined or a reference to globally configured values.

   The modules defined in this document optionally support [RFC6187]
   enabling X.509v3 certificate based host keys and public keys.

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.  The SSH Client Model

3.1.  Tree Diagram

   This section provides a tree diagram [RFC8340] for the "ietf-ssh-
   client" module that does not have groupings expanded.

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   =========== NOTE: '\' line wrapping per BCP XX (RFC XXXX) ===========

   module: ietf-ssh-client

     grouping ssh-client-grouping
       +-- client-identity
       |  +-- username?            string
       |  +-- (auth-type)
       |     +--:(password)
       |     |  +-- password?      string
       |     +--:(public-key)
       |     |  +-- public-key
       |     |     +---u ks:local-or-keystore-asymmetric-key-grouping
       |     +--:(certificate)
       |        +-- certificate {sshcmn:ssh-x509-certs}?
       |           +---u ks:local-or-keystore-end-entity-cert-with-key-\
   grouping
       +-- server-authentication
       |  +-- ssh-host-keys?   ts:host-keys-ref {ts:ssh-host-keys}?
       |  +-- ca-certs?        ts:certificates-ref
       |  |       {sshcmn:ssh-x509-certs,ts:x509-certificates}?
       |  +-- server-certs?    ts:certificates-ref
       |          {sshcmn:ssh-x509-certs,ts:x509-certificates}?
       +-- transport-params {ssh-client-transport-params-config}?
       |  +---u sshcmn:transport-params-grouping
       +-- keepalives! {ssh-client-keepalives}?
          +-- max-wait?       uint16
          +-- max-attempts?   uint8

3.2.  Example Usage

   This section presents two examples showing the ssh-client-grouping
   populated with some data.  These examples are effectively the same
   except the first configures the client identity using a local key
   while the second uses a key configured in a keystore.  Both examples
   are consistent with the examples presented in Section 2 of
   [I-D.ietf-netconf-trust-anchors] and Section 3.2 of
   [I-D.ietf-netconf-keystore].

   The following example configures the client identity using a local
   key:

   =========== NOTE: '\' line wrapping per BCP XX (RFC XXXX) ===========

   <ssh-client
     xmlns="urn:ietf:params:xml:ns:yang:ietf-ssh-client"
     xmlns:algs="urn:ietf:params:xml:ns:yang:ietf-ssh-common">

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     <!-- how this client will authenticate itself to the server -->
     <client-identity>
       <username>foobar</username>
       <public-key>
         <local-definition>
           <algorithm xmlns:ct="urn:ietf:params:xml:ns:yang:ietf-crypto\
   -types">ct:rsa2048</algorithm>
           <private-key>base64encodedvalue==</private-key>
           <public-key>base64encodedvalue==</public-key>
         </local-definition>
       </public-key>
     </client-identity>

     <!-- which host-keys will this client trust -->
     <server-authentication>
       <ssh-host-keys>explicitly-trusted-ssh-host-keys</ssh-host-keys>
     </server-authentication>

     <transport-params>
       <host-key>
         <host-key-alg>algs:ssh-rsa</host-key-alg>
       </host-key>
       <key-exchange>
         <key-exchange-alg>
           algs:diffie-hellman-group-exchange-sha256
         </key-exchange-alg>
       </key-exchange>
       <encryption>
         <encryption-alg>algs:aes256-ctr</encryption-alg>
         <encryption-alg>algs:aes192-ctr</encryption-alg>
         <encryption-alg>algs:aes128-ctr</encryption-alg>
         <encryption-alg>algs:aes256-cbc</encryption-alg>
         <encryption-alg>algs:aes192-cbc</encryption-alg>
         <encryption-alg>algs:aes128-cbc</encryption-alg>
       </encryption>
       <mac>
         <mac-alg>algs:hmac-sha2-256</mac-alg>
         <mac-alg>algs:hmac-sha2-512</mac-alg>
       </mac>
     </transport-params>

     <keepalives>
       <max-wait>30</max-wait>
       <max-attempts>3</max-attempts>
     </keepalives>

   </ssh-client>

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   The following example configures the client identity using a key from
   the keystore:

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   <ssh-client
     xmlns="urn:ietf:params:xml:ns:yang:ietf-ssh-client"
     xmlns:algs="urn:ietf:params:xml:ns:yang:ietf-ssh-common">

     <!-- how this client will authenticate itself to the server -->
     <client-identity>
       <username>foobar</username>
       <public-key>
         <keystore-reference>ex-rsa-key</keystore-reference>
       </public-key>
     </client-identity>

     <!-- which host-keys will this client trust -->
     <server-authentication>
       <ssh-host-keys>explicitly-trusted-ssh-host-keys</ssh-host-keys>
     </server-authentication>

     <transport-params>
       <host-key>
         <host-key-alg>algs:ssh-rsa</host-key-alg>
       </host-key>
       <key-exchange>
         <key-exchange-alg>
           algs:diffie-hellman-group-exchange-sha256
         </key-exchange-alg>
       </key-exchange>
       <encryption>
         <encryption-alg>algs:aes256-ctr</encryption-alg>
         <encryption-alg>algs:aes192-ctr</encryption-alg>
         <encryption-alg>algs:aes128-ctr</encryption-alg>
         <encryption-alg>algs:aes256-cbc</encryption-alg>
         <encryption-alg>algs:aes192-cbc</encryption-alg>
         <encryption-alg>algs:aes128-cbc</encryption-alg>
       </encryption>
       <mac>
         <mac-alg>algs:hmac-sha2-256</mac-alg>
         <mac-alg>algs:hmac-sha2-512</mac-alg>
       </mac>
     </transport-params>

     <keepalives>
       <max-wait>30</max-wait>
       <max-attempts>3</max-attempts>
     </keepalives>

   </ssh-client>

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3.3.  YANG Module

   This YANG module has normative references to
   [I-D.ietf-netconf-trust-anchors], and [I-D.ietf-netconf-keystore].

   <CODE BEGINS> file "ietf-ssh-client@2019-06-07.yang"
   module ietf-ssh-client {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-ssh-client";
     prefix sshc;

     import ietf-ssh-common {
       prefix sshcmn;
       revision-date 2019-06-07; // stable grouping definitions
       reference
         "RFC XXXX: YANG Groupings for SSH Clients and SSH Servers";
     }

     import ietf-truststore {
       prefix ts;
       reference
         "RFC YYYY: A YANG Data Model for a Truststore";
     }

     import ietf-keystore {
       prefix ks;
       reference
         "RFC ZZZZ: A YANG Data Model for a Keystore";
     }

     import ietf-netconf-acm {
       prefix nacm;
       reference
         "RFC 8341: Network Configuration Access Control Model";
     }

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

     contact
       "WG Web:   <http://datatracker.ietf.org/wg/netconf/>
        WG List:  <mailto:netconf@ietf.org>
        Author:   Kent Watsen <mailto:kent+ietf@watsen.net>
        Author:   Gary Wu <mailto:garywu@cisco.com>";

     description
       "This module defines reusable groupings for SSH clients that
        can be used as a basis for specific SSH client instances.

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        Copyright (c) 2019 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 2019-06-07 {
       description
         "Initial version";
       reference
         "RFC XXXX: YANG Groupings for SSH Clients and SSH Servers";
     }

     // Features

     feature ssh-client-transport-params-config {
       description
         "SSH transport layer parameters are configurable on an SSH
          client.";
     }

     feature ssh-client-keepalives {
       description
         "Per socket SSH keepalive parameters are configurable for
          SSH clients on the server implementing this feature.";
     }

     // Groupings

     grouping ssh-client-grouping {
       description
         "A reusable grouping for configuring a SSH client without
          any consideration for how an underlying TCP session is

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          established.

          Note that this grouping uses fairly typical descendent
          node names such that a stack of 'uses' statements will
          have name conflicts.  It is intended that the consuming
          data model will resolve the issue (e.g., by wrapping
          the 'uses' statement in a container called
          'ssh-client-parameters').  This model purposely does
          not do this itself so as to provide maximum flexibility
          to consuming models.";

       container client-identity {
         nacm:default-deny-write;
         description
           "The credentials used by the client to authenticate to
            the SSH server.";
         leaf username {
           type string;
           description
             "The username of this user.  This will be the username
              used, for instance, to log into an SSH server.";
         }
         choice auth-type {
           mandatory true;
           description
             "The authentication type.";
           leaf password {
             nacm:default-deny-all;
             type string;
             description
               "A password to be used for client authentication.";
           }
           container public-key {
             uses ks:local-or-keystore-asymmetric-key-grouping;
             description
               "A locally-defined or referenced asymmetric key
                pair to be used for client authentication.";
             reference
               "RFC ZZZZ: YANG Data Model for a Centralized
                          Keystore Mechanism";
           }
           container certificate {
             if-feature "sshcmn:ssh-x509-certs";
             uses
               ks:local-or-keystore-end-entity-cert-with-key-grouping;
             description
               "A locally-defined or referenced certificate
                to be used for client authentication.";

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             reference
               "RFC ZZZZ: YANG Data Model for a Centralized
                          Keystore Mechanism";
           }
         }
       } // container client-identity

       container server-authentication {
         nacm:default-deny-write;
         must 'ssh-host-keys or ca-certs or server-certs';
         description
           "Trusted server identities.";
         leaf ssh-host-keys {
           if-feature "ts:ssh-host-keys";
           type ts:host-keys-ref;
           description
             "A reference to a list of SSH host keys used by the
              SSH client to authenticate SSH server host keys.
              A server host key is authenticated if it is an
              exact match to a configured SSH host key.";
           reference
             "RFC YYYY: YANG Data Model for Global Trust Anchors";
         }
         leaf ca-certs {
           if-feature "sshcmn:ssh-x509-certs";
           if-feature "ts:x509-certificates";
           type ts:certificates-ref;
           description
             "A reference to a list of certificate authority (CA)
              certificates used by the SSH client to authenticate
              SSH server certificates.  A server certificate is
              authenticated if it has a valid chain of trust to
              a configured CA certificate.";
           reference
             "RFC YYYY: YANG Data Model for Global Trust Anchors";
         }
         leaf server-certs {
           if-feature "sshcmn:ssh-x509-certs";
           if-feature "ts:x509-certificates";
           type ts:certificates-ref;
           description
             "A reference to a list of server certificates used by
              the SSH client to authenticate SSH server certificates.
              A server certificate is authenticated if it is an
              exact match to a configured server certificate.";
           reference
             "RFC YYYY: YANG Data Model for Global Trust Anchors";
         }

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       } // container server-authentication

       container transport-params {
         nacm:default-deny-write;
         if-feature "ssh-client-transport-params-config";
         description
           "Configurable parameters of the SSH transport layer.";
         uses sshcmn:transport-params-grouping;
       } // container transport-parameters

       container keepalives {
         nacm:default-deny-write;
         if-feature "ssh-client-keepalives";
         presence "Indicates that keepalives are enabled.";
         description
           "Configures the keep-alive policy, to proactively test
            the aliveness of the SSH server.  An unresponsive TLS
            server is dropped after approximately max-wait *
            max-attempts seconds.";
         leaf max-wait {
           type uint16 {
             range "1..max";
           }
           units "seconds";
           default "30";
           description
             "Sets the amount of time in seconds after which if
              no data has been received from the SSH server, a
              TLS-level message will be sent to test the
              aliveness of the SSH server.";
         }
         leaf max-attempts {
           type uint8;
           default "3";
           description
             "Sets the maximum number of sequential keep-alive
              messages that can fail to obtain a response from
              the SSH server before assuming the SSH server is
              no longer alive.";
         }
       } // container keepalives
     } // grouping ssh-client-grouping
   }
   <CODE ENDS>

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4.  The SSH Server Model

4.1.  Tree Diagram

   This section provides a tree diagram [RFC8340] for the "ietf-ssh-
   server" module that does not have groupings expanded.

   =========== NOTE: '\' line wrapping per BCP XX (RFC XXXX) ===========

   module: ietf-ssh-server

     grouping ssh-server-grouping
       +-- server-identity
       |  +-- host-key* [name]
       |     +-- name?                string
       |     +-- (host-key-type)
       |        +--:(public-key)
       |        |  +-- public-key
       |        |     +---u ks:local-or-keystore-asymmetric-key-grouping
       |        +--:(certificate)
       |           +-- certificate {sshcmn:ssh-x509-certs}?
       |              +---u ks:local-or-keystore-end-entity-cert-with-k\
   ey-grouping
       +-- client-authentication
       |  +-- supported-authentication-methods
       |  |  +-- publickey?   empty
       |  |  +-- passsword?   empty
       |  |  +-- hostbased?   empty
       |  |  +-- none?        empty
       |  |  +-- other*       string
       |  +-- (local-or-external)
       |     +--:(local) {local-client-auth-supported}?
       |     |  +-- users
       |     |     +-- user* [name]
       |     |        +-- name?             string
       |     |        +-- password?         ianach:crypt-hash
       |     |        +-- authorized-key* [name]
       |     |           +-- name?        string
       |     |           +-- algorithm    string
       |     |           +-- key-data     binary
       |     +--:(external) {external-client-auth-supported}?
       |        +-- client-auth-defined-elsewhere?   empty
       +-- transport-params {ssh-server-transport-params-config}?
       |  +---u sshcmn:transport-params-grouping
       +-- keepalives! {ssh-server-keepalives}?
          +-- max-wait?       uint16
          +-- max-attempts?   uint8

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4.2.  Example Usage

   This section presents two examples showing the ssh-server-grouping
   populated with some data.  These examples are effectively the same
   except the first configures the server identity using a local key
   while the second uses a key configured in a keystore.  Both examples
   are consistent with the examples presented in Section 2 of
   [I-D.ietf-netconf-trust-anchors] and Section 3.2 of
   [I-D.ietf-netconf-keystore].

   The following example configures the server identity using a local
   key:

   =========== NOTE: '\' line wrapping per BCP XX (RFC XXXX) ===========

   <ssh-server
     xmlns="urn:ietf:params:xml:ns:yang:ietf-ssh-server"
     xmlns:algs="urn:ietf:params:xml:ns:yang:ietf-ssh-common">

     <!-- which host-keys will this SSH server present -->
     <server-identity>
       <host-key>
         <name>deployment-specific-certificate</name>
         <public-key>
           <local-definition>
             <algorithm xmlns:ct="urn:ietf:params:xml:ns:yang:ietf-cryp\
   to-types">ct:rsa2048</algorithm>
             <private-key>base64encodedvalue==</private-key>
             <public-key>base64encodedvalue==</public-key>
           </local-definition>
         </public-key>
       </host-key>
     </server-identity>

     <!-- which client credentials will this SSH server trust -->
     <client-authentication>
       <supported-authentication-methods>
         <publickey/>
       </supported-authentication-methods>
       <!--<local-definition>-->
         <users>
           <user>
             <name>mary</name>
           </user>
         </users>
         <!--</local-definition>-->
       <!--
       <ca-certs>explicitly-trusted-client-ca-certs</ca-certs>

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       <client-certs>explicitly-trusted-client-certs</client-certs>
       -->
     </client-authentication>

     <transport-params>
       <host-key>
         <host-key-alg>algs:ssh-rsa</host-key-alg>
       </host-key>
       <key-exchange>
         <key-exchange-alg>
           algs:diffie-hellman-group-exchange-sha256
         </key-exchange-alg>
       </key-exchange>
       <encryption>
         <encryption-alg>algs:aes256-ctr</encryption-alg>
         <encryption-alg>algs:aes192-ctr</encryption-alg>
         <encryption-alg>algs:aes128-ctr</encryption-alg>
         <encryption-alg>algs:aes256-cbc</encryption-alg>
         <encryption-alg>algs:aes192-cbc</encryption-alg>
         <encryption-alg>algs:aes128-cbc</encryption-alg>
       </encryption>
       <mac>
         <mac-alg>algs:hmac-sha2-256</mac-alg>
         <mac-alg>algs:hmac-sha2-512</mac-alg>
       </mac>
     </transport-params>

   </ssh-server>

   The following example configures the server identity using a key from
   the keystore:

   <ssh-server
     xmlns="urn:ietf:params:xml:ns:yang:ietf-ssh-server"
     xmlns:algs="urn:ietf:params:xml:ns:yang:ietf-ssh-common">

     <!-- which host-keys will this SSH server present -->
     <server-identity>
       <host-key>
         <name>deployment-specific-certificate</name>
         <public-key>
           <keystore-reference>ex-rsa-key</keystore-reference>
         </public-key>
       </host-key>
     </server-identity>

     <!-- which client credentials will this SSH server trust -->
     <client-authentication>

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       <supported-authentication-methods>
         <publickey/>
       </supported-authentication-methods>
       <!--<local-definition>-->
         <users>
           <user>
             <name>mary</name>
           </user>
         </users>
         <!--</local-definition>-->
       <!--
       <ca-certs>explicitly-trusted-client-ca-certs</ca-certs>
       <client-certs>explicitly-trusted-client-certs</client-certs>
       -->
     </client-authentication>

     <transport-params>
       <host-key>
         <host-key-alg>algs:ssh-rsa</host-key-alg>
       </host-key>
       <key-exchange>
         <key-exchange-alg>
           algs:diffie-hellman-group-exchange-sha256
         </key-exchange-alg>
       </key-exchange>
       <encryption>
         <encryption-alg>algs:aes256-ctr</encryption-alg>
         <encryption-alg>algs:aes192-ctr</encryption-alg>
         <encryption-alg>algs:aes128-ctr</encryption-alg>
         <encryption-alg>algs:aes256-cbc</encryption-alg>
         <encryption-alg>algs:aes192-cbc</encryption-alg>
         <encryption-alg>algs:aes128-cbc</encryption-alg>
       </encryption>
       <mac>
         <mac-alg>algs:hmac-sha2-256</mac-alg>
         <mac-alg>algs:hmac-sha2-512</mac-alg>
       </mac>
     </transport-params>

   </ssh-server>

4.3.  YANG Module

   This YANG module has normative references to
   [I-D.ietf-netconf-trust-anchors] and [I-D.ietf-netconf-keystore] and
   informative references to [RFC4253] and [RFC7317].

   <CODE BEGINS> file "ietf-ssh-server@2019-06-07.yang"

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   module ietf-ssh-server {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-ssh-server";
     prefix sshs;

     import ietf-ssh-common {
       prefix sshcmn;
       revision-date 2019-06-07; // stable grouping definitions
       reference
         "RFC XXXX: YANG Groupings for SSH Clients and SSH Servers";
     }
   /*
     import ietf-truststore {
       prefix ta;
       reference
         "RFC YYYY: A YANG Data Model for a Truststore";
     }
   */
     import ietf-keystore {
       prefix ks;
       reference
         "RFC ZZZZ: A YANG Data Model for a Keystore";
     }

     import iana-crypt-hash {
       prefix ianach;
       reference
         "RFC 7317: A YANG Data Model for System Management";
     }

     import ietf-netconf-acm {
       prefix nacm;
       reference
         "RFC 8341: Network Configuration Access Control Model";
     }

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

     contact
       "WG Web:   <http://datatracker.ietf.org/wg/netconf/>
        WG List:  <mailto:netconf@ietf.org>
        Author:   Kent Watsen <mailto:kent+ietf@watsen.net>
        Author:   Gary Wu <mailto:garywu@cisco.com>";

     description
       "This module defines reusable groupings for SSH servers that
        can be used as a basis for specific SSH server instances.

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        Copyright (c) 2019 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 2019-06-07 {
       description
         "Initial version";
       reference
         "RFC XXXX: YANG Groupings for SSH Clients and SSH Servers";
     }

     // Features

     feature ssh-server-transport-params-config {
       description
         "SSH transport layer parameters are configurable on an SSH
          server.";
     }

     feature ssh-server-keepalives {
       description
         "Per socket SSH keepalive parameters are configurable for
          SSH servers on the server implementing this feature.";
     }

     feature local-client-auth-supported {
       description
         "Indicates that the SSH server supports local configuration
          of client credentials.";
     }

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     feature external-client-auth-supported {
       description
         "Indicates that the SSH server supports external configuration
          of client credentials.";
     }

     // Groupings

     grouping ssh-server-grouping {
       description
         "A reusable grouping for configuring a SSH server without
          any consideration for how underlying TCP sessions are
          established.

          Note that this grouping uses fairly typical descendent
          node names such that a stack of 'uses' statements will
          have name conflicts.  It is intended that the consuming
          data model will resolve the issue (e.g., by wrapping
          the 'uses' statement in a container called
          'ssh-server-parameters').  This model purposely does
          not do this itself so as to provide maximum flexibility
          to consuming models.";

       container server-identity {
         nacm:default-deny-write;
         description
           "The list of host-keys the SSH server will present when
            establishing a SSH connection.";
         list host-key {
           key "name";
           min-elements 1;
           ordered-by user;
           description
             "An ordered list of host keys the SSH server will use to
              construct its ordered list of algorithms, when sending
              its SSH_MSG_KEXINIT message, as defined in Section 7.1
              of RFC 4253.";
           reference
             "RFC 4253: The Secure Shell (SSH) Transport Layer
                        Protocol";
           leaf name {
             type string;
             description
               "An arbitrary name for this host-key";
           }
           choice host-key-type {
             mandatory true;
             description

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               "The type of host key being specified";
             container public-key {
               uses ks:local-or-keystore-asymmetric-key-grouping;
               description
                 "A locally-defined or referenced asymmetric key pair
                  to be used for the SSH server's host key.";
               reference
                 "RFC ZZZZ: YANG Data Model for a Centralized
                            Keystore Mechanism";
             }
             container certificate {
               if-feature "sshcmn:ssh-x509-certs";
               uses
               ks:local-or-keystore-end-entity-cert-with-key-grouping;
               description
                 "A locally-defined or referenced end-entity
                  certificate to be used for the SSH server's
                  host key.";
               reference
                 "RFC ZZZZ: YANG Data Model for a Centralized
                            Keystore Mechanism";
             }
           }
         }
       } // container server-identity

       container client-authentication {
         nacm:default-deny-write;
         description
           "Specifies if SSH client authentication is required or
            optional, and specifies if the SSH client authentication
            credentials are configured locally or externally.";
         container supported-authentication-methods {
           description
             "Indicates which authentication methods the server
              supports.";
           leaf publickey {
             type empty;
             description
               "Indicates that the 'publickey' method is supported.
                Note that RFC 6187 X.509v3 Certificates for SSH uses
                the 'publickey' method name.";
             reference
               "RFC 4252: The Secure Shell (SSH) Authentication
                          Protocol.
                RFC 6187: X.509v3 Certificates for Secure Shell
                          Authentication.";
           }

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           leaf passsword {
             type empty;
             description
               "Indicates that the 'password' method is supported.";
             reference
               "RFC 4252: The Secure Shell (SSH) Authentication
                          Protocol.";
           }
           leaf hostbased {
             type empty;
             description
               "Indicates that the 'hostbased' method is supported.";
             reference
               "RFC 4252: The Secure Shell (SSH) Authentication
                          Protocol.";
           }
           leaf none {
             type empty;
             description
               "Indicates that the 'none' method is supported.";
             reference
               "RFC 4252: The Secure Shell (SSH) Authentication
                          Protocol.";
           }
           leaf-list other {
             type string;
             description
               "Indicates a supported method name not defined by
                RFC 4253.";
             reference
               "RFC 4252: The Secure Shell (SSH) Authentication
                          Protocol.";
           }
         }
         choice local-or-external {
           mandatory true;
           description
             "Indicates if the client credentials are configured
              locally or externally.";
           case local {
             if-feature "local-client-auth-supported";
             description
               "Client credentials are configured locally.";
             container users {
               description
                 "A list of locally configured users.";
               list user {
                 key name;

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                 description
                   "The list of local users configured on this device.";

                 leaf name {
                   type string;
                   description
                    "The user name string identifying this entry.";
                 }
                 leaf password {
                   type ianach:crypt-hash;
                   description
                     "The password for this entry.";
                 }
                 list authorized-key {
                   key name;
                   description
                     "A list of public SSH keys for this user.  These
                      keys are allowed for SSH authentication, as
                      described in RFC 4253.";
                   reference
                     "RFC 4253: The Secure Shell (SSH) Transport Layer
                                Protocol";
                   leaf name {
                     type string;
                     description
                       "An arbitrary name for the SSH key.";
                   }
                   leaf algorithm {
                     type string;
                     mandatory true;
                     description
                       "The public key algorithm name for this SSH key.

                        Valid values are the values in the IANA 'Secure
                        Shell (SSH) Protocol Parameters' registry,
                        Public Key Algorithm Names.";
                     reference
                       "IANA 'Secure Shell (SSH) Protocol Parameters'
                        registry, Public Key Algorithm Names";
                   }
                   leaf key-data {
                     type binary;
                     mandatory true;
                     description
                       "The binary public key data for this SSH key, as
                        specified by RFC 4253, Section 6.6, i.e.:

                          string    certificate or public key format

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                                    identifier
                          byte[n]   key/certificate data.";
                     reference
                       "RFC 4253: The Secure Shell (SSH) Transport Layer
                                  Protocol";
                   }
                 }
               } // list user
   /*
               if-feature "sshcmn:ssh-x509-certs";
               description
                 "A reference to a list of certificate authority
                  (CA) certificates and a reference to a list of
                  client certificates.";
               leaf ca-certs {
                 if-feature "ts:x509-certificates";
                 type ts:certificates-ref;  // local or remote
                 description
                   "A reference to a list of certificate authority (CA)
                    certificates used by the SSH server to authenticate
                    SSH client certificates.  A client certificate is
                    authenticated if it has a valid chain of trust to
                    a configured CA certificate.";
                 reference
                   "RFC YYYY: YANG Data Model for Global Trust Anchors";
               }
               leaf client-certs {
                 if-feature "ts:x509-certificates";
                 type ts:certificates-ref;  // local or remote
                 description
                   "A reference to a list of client certificates
                    used by the SSH server to authenticate SSH
                    client certificates.  A clients certificate
                    is authenticated if it is an exact match to
                    a configured client certificate.";
                 reference
                   "RFC YYYY: YANG Data Model for Global Trust Anchors";
               }
   */
             } // container users
           } // case local
           case external {
             if-feature "external-client-auth-supported";
             description
               "Client credentials are configured externally, such
                as via RADIUS, RFC 7317, or another mechanism.";
             leaf client-auth-defined-elsewhere {
               type empty;

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               description
                 "Indicates that client credentials are configured
                  elsewhere.";
             }
           }
         } // choice local-or-external
       } // container client-authentication

       container transport-params {
         nacm:default-deny-write;
         if-feature "ssh-server-transport-params-config";
         description
           "Configurable parameters of the SSH transport layer.";
         uses sshcmn:transport-params-grouping;
       } // container transport-params

       container keepalives {
         nacm:default-deny-write;
         if-feature "ssh-server-keepalives";
         presence "Indicates that keepalives are enabled.";
         description
           "Configures the keep-alive policy, to proactively test
            the aliveness of the SSL client.  An unresponsive SSL
            client is dropped after approximately max-wait *
            max-attempts seconds.";
         leaf max-wait {
           type uint16 {
             range "1..max";
           }
           units "seconds";
           default "30";
           description
             "Sets the amount of time in seconds after which
              if no data has been received from the SSL client,
              a SSL-level message will be sent to test the
              aliveness of the SSL client.";
         }
         leaf max-attempts {
           type uint8;
           default "3";
           description
             "Sets the maximum number of sequential keep-alive
              messages that can fail to obtain a response from
              the SSL client before assuming the SSL client is
              no longer alive.";
         }
       } // container keepalives
     } // grouping server-identity-grouping

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   }
   <CODE ENDS>

5.  The SSH Common Model

   The SSH common model presented in this section contains identities
   and groupings common to both SSH clients and SSH servers.  The
   transport-params-grouping can be used to configure the list of SSH
   transport algorithms permitted by the SSH client or SSH server.  The
   lists of algorithms are ordered such that, if multiple algorithms are
   permitted by the client, the algorithm that appears first in its list
   that is also permitted by the server is used for the SSH transport
   layer connection.  The ability to restrict the algorithms allowed is
   provided in this grouping for SSH clients and SSH servers that are
   capable of doing so and may serve to make SSH clients and SSH servers
   compliant with security policies.

   [I-D.ietf-netconf-crypto-types] defines six categories of
   cryptographic algorithms (hash-algorithm, symmetric-key-encryption-
   algorithm, mac-algorithm, asymmetric-key-encryption-algorithm,
   signature-algorithm, key-negotiation-algorithm) and lists several
   widely accepted algorithms for each of them.  The SSH client and
   server models use one or more of these algorithms.  The SSH common
   model includes four parameters for configuring its permitted SSH
   algorithms, which are: host-key-alg, key-exchange-alg, encryption-alg
   and mac-alg.  The following tables are provided, in part, to define
   the subset of algorithms defined in the crypto-types model used by
   SSH and, in part, to ensure compatibility of configured SSH
   cryptographic parameters for configuring its permitted SSH algorithms
   ("sshcmn" representing SSH common model, and "ct" representing
   crypto-types model which the SSH client/server model is based on):

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     +-------------------------------+-------------------------------+
     |     sshcmn:host-key-alg       |      ct:signature-algorithm   |
     +-------------------------------+-------------------------------+
     | dsa-sha1                      | dsa-sha1                      |
     | rsa-pkcs1-sha1                | rsa-pkcs1-sha1                |
     | rsa-pkcs1-sha256              | rsa-pkcs1-sha256              |
     | rsa-pkcs1-sha512              | rsa-pkcs1-sha512              |
     | ecdsa-secp256r1-sha256        | ecdsa-secp256r1-sha256        |
     | ecdsa-secp384r1-sha384        | ecdsa-secp384r1-sha384        |
     | ecdsa-secp521r1-sha512        | ecdsa-secp521r1-sha512        |
     | x509v3-rsa-pkcs1-sha1         | x509v3-rsa-pkcs1-sha1         |
     | x509v3-rsa2048-pkcs1-sha256   | x509v3-rsa2048-pkcs1-sha1     |
     | x509v3-ecdsa-secp256r1-sha256 | x509v3-ecdsa-secp256r1-sha256 |
     | x509v3-ecdsa-secp384r1-sha384 | x509v3-ecdsa-secp384r1-sha384 |
     | x509v3-ecdsa-secp521r1-sha512 | x509v3-ecdsa-secp521r1-sha512 |
     +-------------------------------+-------------------------------+

             Table 1 The SSH Host-key-alg Compatibility Matrix

     +-------------------------------+-------------------------------+
     | sshcmn:key-exchange-alg       | ct:key-negotiation-algorithm  |
     +-------------------------------+-------------------------------+
     | diffie-hellman-group14-sha1   | diffie-hellman-group14-sha1   |
     | diffie-hellman-group14-sha256 | diffie-hellman-group14-sha256 |
     | diffie-hellman-group15-sha512 | diffie-hellman-group15-sha512 |
     | diffie-hellman-group16-sha512 | diffie-hellman-group16-sha512 |
     | diffie-hellman-group17-sha512 | diffie-hellman-group17-sha512 |
     | diffie-hellman-group18-sha512 | diffie-hellman-group18-sha512 |
     | ecdh-sha2-secp256r1           | ecdh-sha2-secp256r1           |
     | ecdh-sha2-secp384r1           | ecdh-sha2-secp384r1           |
     +-------------------------------+-------------------------------+

           Table 2 The SSH Key-exchange-alg Compatibility Matrix

     +-----------------------+---------------------------------------+
     | sshcmn:encryption-alg | ct:symmetric-key-encryption-algorithm |
     +-----------------------+---------------------------------------+
     | aes-128-cbc           | aes-128-cbc                           |
     | aes-192-cbc           | aes-192-cbc                           |
     | aes-256-cbc           | aes-256-cbc                           |
     | aes-128-ctr           | aes-128-ctr                           |
     | aes-192-ctr           | aes-192-ctr                           |
     | aes-256-ctr           | aes-256-ctr                           |
     +-----------------------+---------------------------------------+

            Table 3 The SSH Encryption-alg Compatibility Matrix

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                  +----------------+-------------------+
                  | sshcmn:mac-alg | ct:mac-algorithm  |
                  +----------------+-------------------+
                  | hmac-sha1      | hmac-sha1         |
                  | hmac-sha1-96   | hmac-sha1-96      |
                  | hmac-sha2-256  | hmac-sha2-256     |
                  | hmac-sha2-512  | hmac-sha2-512     |
                  +----------------+-------------------+

               Table 4 The SSH Mac-alg Compatibility Matrix

   As is seen in the tables above, the names of the "sshcmn" algorithms
   are all identical to the names of algorithms defined in
   [I-D.ietf-netconf-crypto-types].  While appearing to be redundant, it
   is important to realize that not all the algorithms defined in
   [I-D.ietf-netconf-crypto-types] are supported by SSH.  That is, the
   algorithms supported by SSH are a subset of the algorithms defined in
   [I-D.ietf-netconf-crypto-types].  The algorithms used by SSH are
   redefined in this document in order to constrain the algorithms that
   may be selected to just the ones used by SSH.

   Features are defined for algorithms that are OPTIONAL or are not
   widely supported by popular implementations.  Note that the list of
   algorithms is not exhaustive.  As well, some algorithms that are
   REQUIRED by [RFC4253] are missing, notably "ssh-dss" and "diffie-
   hellman-group1-sha1" due to their weak security and there being
   alternatives that are widely supported.

5.1.  Tree Diagram

   The following tree diagram [RFC8340] provides an overview of the data
   model for the "ietf-ssh-common" module.

   module: ietf-ssh-common

     grouping transport-params-grouping
       +-- host-key
       |  +-- host-key-alg*   identityref
       +-- key-exchange
       |  +-- key-exchange-alg*   identityref
       +-- encryption
       |  +-- encryption-alg*   identityref
       +-- mac
          +-- mac-alg*   identityref

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5.2.  Example Usage

   This following example illustrates how the transport-params-grouping
   appears when populated with some data.

   <transport-params
     xmlns="urn:ietf:params:xml:ns:yang:ietf-ssh-common"
     xmlns:algs="urn:ietf:params:xml:ns:yang:ietf-ssh-common">
     <host-key>
       <host-key-alg>algs:x509v3-rsa2048-sha256</host-key-alg>
       <host-key-alg>algs:ssh-rsa</host-key-alg>
     </host-key>
     <key-exchange>
       <key-exchange-alg>
         algs:diffie-hellman-group-exchange-sha256
       </key-exchange-alg>
     </key-exchange>
     <encryption>
       <encryption-alg>algs:aes256-ctr</encryption-alg>
       <encryption-alg>algs:aes192-ctr</encryption-alg>
       <encryption-alg>algs:aes128-ctr</encryption-alg>
       <encryption-alg>algs:aes256-cbc</encryption-alg>
       <encryption-alg>algs:aes192-cbc</encryption-alg>
       <encryption-alg>algs:aes128-cbc</encryption-alg>
     </encryption>
     <mac>
       <mac-alg>algs:hmac-sha2-256</mac-alg>
       <mac-alg>algs:hmac-sha2-512</mac-alg>
     </mac>
   </transport-params>

5.3.  YANG Module

   This YANG module has normative references to [RFC4253], [RFC4344],
   [RFC4419], [RFC5656], [RFC6187], and [RFC6668].

   <CODE BEGINS> file "ietf-ssh-common@2019-06-07.yang"
   module ietf-ssh-common {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-ssh-common";
     prefix sshcmn;

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

     contact
       "WG Web:   <http://datatracker.ietf.org/wg/netconf/>
        WG List:  <mailto:netconf@ietf.org>

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        Author:   Kent Watsen <mailto:kent+ietf@watsen.net>
        Author:   Gary Wu <mailto:garywu@cisco.com>";

     description
       "This module defines a common features, identities, and
        groupings for Secure Shell (SSH).

        Copyright (c) 2019 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 2019-06-07 {
       description
         "Initial version";
       reference
         "RFC XXXX: YANG Groupings for SSH Clients and SSH Servers";
     }

     // Features

     feature ssh-ecc {
       description
         "Elliptic Curve Cryptography is supported for SSH.";
       reference
         "RFC 5656: Elliptic Curve Algorithm Integration in the
                    Secure Shell Transport Layer";
     }

     feature ssh-x509-certs {
       description
         "X.509v3 certificates are supported for SSH per RFC 6187.";

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       reference
         "RFC 6187: X.509v3 Certificates for Secure Shell
                    Authentication";
     }

     feature ssh-dh-group-exchange {
       description
         "Diffie-Hellman Group Exchange is supported for SSH.";
       reference
         "RFC 4419: Diffie-Hellman Group Exchange for the
                    Secure Shell (SSH) Transport Layer Protocol";
     }

     feature ssh-ctr {
       description
         "SDCTR encryption mode is supported for SSH.";
       reference
         "RFC 4344: The Secure Shell (SSH) Transport Layer
                    Encryption Modes";
     }

     feature ssh-sha2 {
       description
         "The SHA2 family of cryptographic hash functions is
          supported for SSH.";
       reference
         "FIPS PUB 180-4: Secure Hash Standard (SHS)";
     }

     // Identities

     identity public-key-alg-base {
       description
         "Base identity used to identify public key algorithms.";
     }

     identity ssh-dss {
       base public-key-alg-base;
       description
         "Digital Signature Algorithm using SHA-1 as the
          hashing algorithm.";
       reference
         "RFC 4253:
            The Secure Shell (SSH) Transport Layer Protocol";
     }

     identity ssh-rsa {
       base public-key-alg-base;

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       description
         "RSASSA-PKCS1-v1_5 signature scheme using SHA-1 as the
          hashing algorithm.";
       reference
         "RFC 4253:
            The Secure Shell (SSH) Transport Layer Protocol";
     }

     identity ecdsa-sha2-nistp256 {
       base public-key-alg-base;
       if-feature "ssh-ecc and ssh-sha2";
       description
         "Elliptic Curve Digital Signature Algorithm (ECDSA) using the
          nistp256 curve and the SHA2 family of hashing algorithms.";
       reference
         "RFC 5656: Elliptic Curve Algorithm Integration in the
                    Secure Shell Transport Layer";
     }

     identity ecdsa-sha2-nistp384 {
       base public-key-alg-base;
       if-feature "ssh-ecc and ssh-sha2";
       description
         "Elliptic Curve Digital Signature Algorithm (ECDSA) using the
          nistp384 curve and the SHA2 family of hashing algorithms.";
       reference
         "RFC 5656: Elliptic Curve Algorithm Integration in the
                    Secure Shell Transport Layer";
     }

     identity ecdsa-sha2-nistp521 {
       base public-key-alg-base;
       if-feature "ssh-ecc and ssh-sha2";
       description
         "Elliptic Curve Digital Signature Algorithm (ECDSA) using the
          nistp521 curve and the SHA2 family of hashing algorithms.";
       reference
         "RFC 5656: Elliptic Curve Algorithm Integration in the
                    Secure Shell Transport Layer";
     }

     identity x509v3-ssh-rsa {
       base public-key-alg-base;
       if-feature "ssh-x509-certs";
       description
         "RSASSA-PKCS1-v1_5 signature scheme using a public key stored
          in an X.509v3 certificate and using SHA-1 as the hashing
          algorithm.";

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       reference
         "RFC 6187: X.509v3 Certificates for Secure Shell
                    Authentication";
     }

     identity x509v3-rsa2048-sha256 {
       base public-key-alg-base;
       if-feature "ssh-x509-certs and ssh-sha2";
       description
         "RSASSA-PKCS1-v1_5 signature scheme using a public key stored
          in an X.509v3 certificate and using SHA-256 as the hashing
          algorithm.  RSA keys conveyed using this format MUST have a
          modulus of at least 2048 bits.";
       reference
         "RFC 6187: X.509v3 Certificates for Secure Shell
                    Authentication";
     }

     identity x509v3-ecdsa-sha2-nistp256 {
       base public-key-alg-base;
       if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
       description
         "Elliptic Curve Digital Signature Algorithm (ECDSA)
          using the nistp256 curve with a public key stored in
          an X.509v3 certificate and using the SHA2 family of
          hashing algorithms.";
       reference
         "RFC 6187: X.509v3 Certificates for Secure Shell
                    Authentication";
     }

     identity x509v3-ecdsa-sha2-nistp384 {
       base public-key-alg-base;
       if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
       description
         "Elliptic Curve Digital Signature Algorithm (ECDSA)
          using the nistp384 curve with a public key stored in
          an X.509v3 certificate and using the SHA2 family of
          hashing algorithms.";
       reference
         "RFC 6187: X.509v3 Certificates for Secure Shell
                    Authentication";
     }

     identity x509v3-ecdsa-sha2-nistp521 {
       base public-key-alg-base;
       if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
       description

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         "Elliptic Curve Digital Signature Algorithm (ECDSA)
          using the nistp521 curve with a public key stored in
          an X.509v3 certificate and using the SHA2 family of
          hashing algorithms.";
       reference
         "RFC 6187: X.509v3 Certificates for Secure Shell
                    Authentication";
     }

     identity key-exchange-alg-base {
       description
         "Base identity used to identify key exchange algorithms.";
     }

     identity diffie-hellman-group14-sha1 {
       base key-exchange-alg-base;
       description
         "Diffie-Hellman key exchange with SHA-1 as HASH and
          Oakley Group 14 (2048-bit MODP Group).";
       reference
         "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
     }

     identity diffie-hellman-group-exchange-sha1 {
       base key-exchange-alg-base;
       if-feature "ssh-dh-group-exchange";
       description
         "Diffie-Hellman Group and Key Exchange with SHA-1 as HASH.";
       reference
         "RFC 4419: Diffie-Hellman Group Exchange for the
                    Secure Shell (SSH) Transport Layer Protocol";
     }

     identity diffie-hellman-group-exchange-sha256 {
       base key-exchange-alg-base;
       if-feature "ssh-dh-group-exchange and ssh-sha2";
       description
         "Diffie-Hellman Group and Key Exchange with SHA-256 as HASH.";
       reference
         "RFC 4419: Diffie-Hellman Group Exchange for the
                    Secure Shell (SSH) Transport Layer Protocol";
     }

     identity ecdh-sha2-nistp256 {
       base key-exchange-alg-base;
       if-feature "ssh-ecc and ssh-sha2";
       description
         "Elliptic Curve Diffie-Hellman (ECDH) key exchange using the

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          nistp256 curve and the SHA2 family of hashing algorithms.";
       reference
         "RFC 5656: Elliptic Curve Algorithm Integration in the
                    Secure Shell Transport Layer";
     }

     identity ecdh-sha2-nistp384 {
       base key-exchange-alg-base;
       if-feature "ssh-ecc and ssh-sha2";
       description
         "Elliptic Curve Diffie-Hellman (ECDH) key exchange using the
          nistp384 curve and the SHA2 family of hashing algorithms.";
       reference
         "RFC 5656: Elliptic Curve Algorithm Integration in the
                    Secure Shell Transport Layer";
     }

     identity ecdh-sha2-nistp521 {
       base key-exchange-alg-base;
       if-feature "ssh-ecc and ssh-sha2";
       description
         "Elliptic Curve Diffie-Hellman (ECDH) key exchange using the
          nistp521 curve and the SHA2 family of hashing algorithms.";
       reference
         "RFC 5656: Elliptic Curve Algorithm Integration in the
                    Secure Shell Transport Layer";
     }

     identity encryption-alg-base {
       description
         "Base identity used to identify encryption algorithms.";
     }

     identity triple-des-cbc {
       base encryption-alg-base;
       description
         "Three-key 3DES in CBC mode.";
       reference
         "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
     }

     identity aes128-cbc {
       base encryption-alg-base;
       description
         "AES in CBC mode, with a 128-bit key.";
       reference
         "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
     }

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     identity aes192-cbc {
       base encryption-alg-base;
       description
         "AES in CBC mode, with a 192-bit key.";
       reference
         "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
     }

     identity aes256-cbc {
       base encryption-alg-base;
       description
         "AES in CBC mode, with a 256-bit key.";
       reference
         "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
     }

     identity aes128-ctr {
       base encryption-alg-base;
       if-feature "ssh-ctr";
       description
         "AES in SDCTR mode, with 128-bit key.";
       reference
         "RFC 4344: The Secure Shell (SSH) Transport Layer Encryption
                    Modes";
     }

     identity aes192-ctr {
       base encryption-alg-base;
       if-feature "ssh-ctr";
       description
         "AES in SDCTR mode, with 192-bit key.";
       reference
         "RFC 4344: The Secure Shell (SSH) Transport Layer Encryption
                    Modes";
     }

     identity aes256-ctr {
       base encryption-alg-base;
       if-feature "ssh-ctr";
       description
         "AES in SDCTR mode, with 256-bit key.";
       reference
         "RFC 4344: The Secure Shell (SSH) Transport Layer Encryption
            Modes";
     }

     identity mac-alg-base {
       description

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         "Base identity used to identify message authentication
          code (MAC) algorithms.";
     }

     identity hmac-sha1 {
       base mac-alg-base;
       description
         "HMAC-SHA1";
       reference
         "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
     }

     identity hmac-sha2-256 {
       base mac-alg-base;
       if-feature "ssh-sha2";
       description
         "HMAC-SHA2-256";
       reference
         "RFC 6668: SHA-2 Data Integrity Verification for the
                    Secure Shell (SSH) Transport Layer Protocol";
     }

     identity hmac-sha2-512 {
       base mac-alg-base;
       if-feature "ssh-sha2";
       description
         "HMAC-SHA2-512";
       reference
         "RFC 6668: SHA-2 Data Integrity Verification for the
                    Secure Shell (SSH) Transport Layer Protocol";
     }

     // Groupings

     grouping transport-params-grouping {
       description
         "A reusable grouping for SSH transport parameters.";
       reference
         "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
       container host-key {
         description
           "Parameters regarding host key.";
         leaf-list host-key-alg {
           type identityref {
             base public-key-alg-base;
           }
           ordered-by user;
           description

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             "Acceptable host key algorithms in order of descending
              preference.  The configured host key algorithms should
              be compatible with the algorithm used by the configured
              private key.  Please see Section 5 of RFC XXXX for
              valid combinations.

              If this leaf-list is not configured (has zero elements)
              the acceptable host key algorithms are implementation-
              defined.";
           reference
             "RFC XXXX: YANG Groupings for SSH Clients and SSH Servers";
         }
       }
       container key-exchange {
         description
           "Parameters regarding key exchange.";
         leaf-list key-exchange-alg {
           type identityref {
             base key-exchange-alg-base;
           }
           ordered-by user;
           description
             "Acceptable key exchange algorithms in order of descending
              preference.

              If this leaf-list is not configured (has zero elements)
              the acceptable key exchange algorithms are implementation
              defined.";
         }
       }
       container encryption {
         description
           "Parameters regarding encryption.";
         leaf-list encryption-alg {
           type identityref {
             base encryption-alg-base;
           }
           ordered-by user;
           description
             "Acceptable encryption algorithms in order of descending
              preference.

              If this leaf-list is not configured (has zero elements)
              the acceptable encryption algorithms are implementation
              defined.";
         }
       }
       container mac {

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         description
           "Parameters regarding message authentication code (MAC).";
         leaf-list mac-alg {
           type identityref {
             base mac-alg-base;
           }
           ordered-by user;
           description
             "Acceptable MAC algorithms in order of descending
              preference.

              If this leaf-list is not configured (has zero elements)
              the acceptable MAC algorithms are implementation-
              defined.";
         }
       }
     }
   }
   <CODE ENDS>

6.  Security Considerations

   The YANG modules defined in this document are designed to be accessed
   via YANG based management protocols, such as NETCONF [RFC6241] and
   RESTCONF [RFC8040].  Both of these protocols have mandatory-to-
   implement secure transport layers (e.g., SSH, TLS) with mutual
   authentication.

   The NETCONF access control model (NACM) [RFC8341] provides the means
   to restrict access for particular users to a pre-configured subset of
   all available protocol operations and content.

   Since the modules in this document only define groupings, these
   considerations are primarily for the designers of other modules that
   use these groupings.

   There are a number of data nodes defined in the YANG modules that are
   writable/creatable/deletable (i.e., config true, which is the
   default).  These data nodes may be considered sensitive or vulnerable
   in some network environments.  Write operations (e.g., edit-config)
   to these data nodes without proper protection can have a negative
   effect on network operations.  These are the subtrees and data nodes
   and their sensitivity/vulnerability:

      *: All of the nodes defined by the grouping statement in both the
         "ietf-ssh-client" and "ietf-ssh-server" modules are sensitive
         to write operations.  For instance, the addition or removal of
         references to keys, certificates, trusted anchors, etc., or

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         even the modification of transport or keepalive parameters can
         dramatically alter the implemented security policy.  For this
         reason, all the nodes are protected the NACM extension
         "default-deny-write".

   Some of the readable data nodes in the YANG modules may be considered
   sensitive or vulnerable in some network environments.  It is thus
   important to control read access (e.g., via get, get-config, or
   notification) to these data nodes.  These are the subtrees and data
   nodes and their sensitivity/vulnerability:

      ssh-client-grouping/client-identity/:  This subtree in the "ietf-
         ssh-client" module contains nodes that are additionally
         sensitive to read operations such that, in normal use cases,
         they should never be returned to a client.  Specifically, the
         descendent nodes 'password', 'public-key/local-definition/
         private-key' and 'certificate/local-definition/private-key'.
         For this reason, all of these node are protected by the NACM
         extension "default-deny-all".

      ssh-server-grouping/server-identity/:  This subtree in the "ietf-
         ssh-server" module contains nodes that are additionally
         sensitive to read operations such that, in normal use cases,
         they should never be returned to a client.  Specifically, the
         descendent nodes 'host-key/public-key/local-definition/private-
         key' and 'host-key/certificate/local-definition/private-key'.
         For this reason, both of these node are protected by the NACM
         extension "default-deny-all".

   Some of the operations in this YANG module may be considered
   sensitive or vulnerable in some network environments.  It is thus
   important to control access to these operations.  These are the
   operations and their sensitivity/vulnerability:

      *: The groupings defined in this document include "action"
         statements that come from groupings defined in
         [I-D.ietf-netconf-crypto-types].  Please consult that document
         for the security considerations of the "action" statements
         defined by the "grouping" statements defined in this document.

7.  IANA Considerations

7.1.  The IETF XML Registry

   This document registers three URIs in the "ns" subregistry of the
   IETF XML Registry [RFC3688].  Following the format in [RFC3688], the
   following registrations are requested:

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

7.2.  The YANG Module Names Registry

   This document registers three YANG modules in the YANG Module Names
   registry [RFC6020].  Following the format in [RFC6020], the following
   registrations are requested:

      name:         ietf-ssh-client
      namespace:    urn:ietf:params:xml:ns:yang:ietf-ssh-client
      prefix:       sshc
      reference:    RFC XXXX

      name:         ietf-ssh-server
      namespace:    urn:ietf:params:xml:ns:yang:ietf-ssh-server
      prefix:       sshs
      reference:    RFC XXXX

      name:         ietf-ssh-common
      namespace:    urn:ietf:params:xml:ns:yang:ietf-ssh-common
      prefix:       sshcmn
      reference:    RFC XXXX

8.  References

8.1.  Normative References

   [I-D.ietf-netconf-crypto-types]
              Watsen, K. and H. Wang, "Common YANG Data Types for
              Cryptography", draft-ietf-netconf-crypto-types-06 (work in
              progress), April 2019.

   [I-D.ietf-netconf-keystore]
              Watsen, K., "YANG Data Model for a Centralized Keystore
              Mechanism", draft-ietf-netconf-keystore-09 (work in
              progress), April 2019.

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   [I-D.ietf-netconf-trust-anchors]
              Watsen, K., "YANG Data Model for Global Trust Anchors",
              draft-ietf-netconf-trust-anchors-04 (work in progress),
              April 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4344]  Bellare, M., Kohno, T., and C. Namprempre, "The Secure
              Shell (SSH) Transport Layer Encryption Modes", RFC 4344,
              DOI 10.17487/RFC4344, January 2006,
              <https://www.rfc-editor.org/info/rfc4344>.

   [RFC4419]  Friedl, M., Provos, N., and W. Simpson, "Diffie-Hellman
              Group Exchange for the Secure Shell (SSH) Transport Layer
              Protocol", RFC 4419, DOI 10.17487/RFC4419, March 2006,
              <https://www.rfc-editor.org/info/rfc4419>.

   [RFC5656]  Stebila, D. and J. Green, "Elliptic Curve Algorithm
              Integration in the Secure Shell Transport Layer",
              RFC 5656, DOI 10.17487/RFC5656, December 2009,
              <https://www.rfc-editor.org/info/rfc5656>.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.

   [RFC6187]  Igoe, K. and D. Stebila, "X.509v3 Certificates for Secure
              Shell Authentication", RFC 6187, DOI 10.17487/RFC6187,
              March 2011, <https://www.rfc-editor.org/info/rfc6187>.

   [RFC6668]  Bider, D. and M. Baushke, "SHA-2 Data Integrity
              Verification for the Secure Shell (SSH) Transport Layer
              Protocol", RFC 6668, DOI 10.17487/RFC6668, July 2012,
              <https://www.rfc-editor.org/info/rfc6668>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
              Access Control Model", STD 91, RFC 8341,
              DOI 10.17487/RFC8341, March 2018,
              <https://www.rfc-editor.org/info/rfc8341>.

8.2.  Informative References

   [OPENSSH]  Project, T. O., "OpenSSH", 2016, <http://www.openssh.com>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

   [RFC4252]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252,
              January 2006, <https://www.rfc-editor.org/info/rfc4252>.

   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
              January 2006, <https://www.rfc-editor.org/info/rfc4253>.

   [RFC4254]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Connection Protocol", RFC 4254, DOI 10.17487/RFC4254,
              January 2006, <https://www.rfc-editor.org/info/rfc4254>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
              Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
              <https://www.rfc-editor.org/info/rfc6242>.

   [RFC7317]  Bierman, A. and M. Bjorklund, "A YANG Data Model for
              System Management", RFC 7317, DOI 10.17487/RFC7317, August
              2014, <https://www.rfc-editor.org/info/rfc7317>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

   [RFC8071]  Watsen, K., "NETCONF Call Home and RESTCONF Call Home",
              RFC 8071, DOI 10.17487/RFC8071, February 2017,
              <https://www.rfc-editor.org/info/rfc8071>.

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   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
              <https://www.rfc-editor.org/info/rfc8340>.

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Appendix A.  Change Log

A.1.  00 to 01

   o  Noted that '0.0.0.0' and '::' might have special meanings.

   o  Renamed "keychain" to "keystore".

A.2.  01 to 02

   o  Removed the groupings 'listening-ssh-client-grouping' and
      'listening-ssh-server-grouping'.  Now modules only contain the
      transport-independent groupings.

   o  Simplified the "client-auth" part in the ietf-ssh-client module.
      It now inlines what it used to point to keystore for.

   o  Added cipher suites for various algorithms into new 'ietf-ssh-
      common' module.

A.3.  02 to 03

   o  Removed 'RESTRICTED' enum from 'password' leaf type.

   o  Added a 'must' statement to container 'server-auth' asserting that
      at least one of the various auth mechanisms must be specified.

   o  Fixed description statement for leaf 'trusted-ca-certs'.

A.4.  03 to 04

   o  Change title to "YANG Groupings for SSH Clients and SSH Servers"

   o  Added reference to RFC 6668

   o  Added RFC 8174 to Requirements Language Section.

   o  Enhanced description statement for ietf-ssh-server's "trusted-ca-
      certs" leaf.

   o  Added mandatory true to ietf-ssh-client's "client-auth" 'choice'
      statement.

   o  Changed the YANG prefix for module ietf-ssh-common from 'sshcom'
      to 'sshcmn'.

   o  Removed the compression algorithms as they are not commonly
      configurable in vendors' implementations.

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   o  Updating descriptions in transport-params-grouping and the
      servers's usage of it.

   o  Now tree diagrams reference ietf-netmod-yang-tree-diagrams

   o  Updated YANG to use typedefs around leafrefs to common keystore
      paths

   o  Now inlines key and certificates (no longer a leafref to keystore)

A.5.  04 to 05

   o  Merged changes from co-author.

A.6.  05 to 06

   o  Updated to use trust anchors from trust-anchors draft (was
      keystore draft)

   o  Now uses new keystore grouping enabling asymmetric key to be
      either locally defined or a reference to the keystore.

A.7.  06 to 07

   o  factored the ssh-[client|server]-groupings into more reusable
      groupings.

   o  added if-feature statements for the new "ssh-host-keys" and
      "x509-certificates" features defined in draft-ietf-netconf-trust-
      anchors.

A.8.  07 to 08

   o  Added a number of compatibility matrices to Section 5 (thanks
      Frank!)

   o  Clarified that any configured "host-key-alg" values need to be
      compatible with the configured private key.

A.9.  08 to 09

   o  Updated examples to reflect update to groupings defined in the
      keystore -09 draft.

   o  Add SSH keepalives features and groupings.

   o  Prefixed top-level SSH grouping nodes with 'ssh-' and support
      mashups.

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   o  Updated copyright date, boilerplate template, affiliation, and
      folding algorithm.

A.10.  09 to 10

   o  Reformatted the YANG modules.

A.11.  10 to 11

   o  Reformatted lines causing folding to occur.

A.12.  11 to 12

   o  Collapsed all the inner groupings into the top-level grouping.

   o  Added a top-level "demux container" inside the top-level grouping.

   o  Added NACM statements and updated the Security Considerations
      section.

   o  Added "presence" statements on the "keepalive" containers, as was
      needed to address a validation error that appeared after adding
      the "must" statements into the NETCONF/RESTCONF client/server
      modules.

   o  Updated the boilerplate text in module-level "description"
      statement to match copyeditor convention.

A.13.  12 to 13

   o  Removed the "demux containers", floating the nacm:default-deny-
      write to each descendent node, and adding a note to model
      designers regarding the potential need to add their own demux
      containers.

   o  Fixed a couple references (section 2 --> section 3)

   o  In the server model, replaced <client-cert-auth> with <client-
      authentication> and introduced 'local-or-external' choice.

A.14.  13 to 14

   o  Updated to reflect changes in trust-anchors drafts (e.g., s/trust-
      anchors/truststore/g + s/pinned.//)

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Acknowledgements

   The authors would like to thank for following for lively discussions
   on list and in the halls (ordered by last name): Andy Bierman, Martin
   Bjorklund, Benoit Claise, Mehmet Ersue, Balazs Kovacs, David
   Lamparter, Alan Luchuk, Ladislav Lhotka, Radek Krejci, Tom Petch,
   Juergen Schoenwaelder, Phil Shafer, Sean Turner, Michal Vasko, and
   Bert Wijnen.

Authors' Addresses

   Kent Watsen
   Watsen Networks

   EMail: kent+ietf@watsen.net

   Gary Wu
   Cisco Systems

   EMail: garywu@cisco.com

   Liang Xia
   Huawei

   EMail: frank.xialiang@huawei.com

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