YANG Groupings for SSH Clients and SSH Servers

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>
     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) 2020 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 EEEE
     (https://www.rfc-editor.org/info/rfcEEEE); 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 2021-02-10 {
    description
      "Initial version";
    reference
      "RFC EEEE: 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.";
    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;
    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 {
    if-feature "ssh-ecc and ssh-sha2";
    base public-key-alg-base;
    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 {
    if-feature "ssh-ecc and ssh-sha2";
    base public-key-alg-base;
    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 {
    if-feature "ssh-ecc and ssh-sha2";
    base public-key-alg-base;
    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 {
    if-feature "ssh-x509-certs";
    base public-key-alg-base;
    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.";
    reference
      "RFC 6187: X.509v3 Certificates for Secure Shell
                 Authentication";
  }

  identity x509v3-rsa2048-sha256 {
    if-feature "ssh-x509-certs and ssh-sha2";
    base public-key-alg-base;
    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 {
    if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
    base public-key-alg-base;
    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 {
    if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
    base public-key-alg-base;
    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 {
    if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
    base public-key-alg-base;
    description
      "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 {
    if-feature "ssh-dh-group-exchange";
    base key-exchange-alg-base;
    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 {
    if-feature "ssh-dh-group-exchange and ssh-sha2";
    base key-exchange-alg-base;
    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 {
    if-feature "ssh-ecc and ssh-sha2";
    base key-exchange-alg-base;
    description
      "Elliptic Curve Diffie-Hellman (ECDH) key exchange 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 ecdh-sha2-nistp384 {
    if-feature "ssh-ecc and ssh-sha2";
    base key-exchange-alg-base;
    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 {
    if-feature "ssh-ecc and ssh-sha2";
    base key-exchange-alg-base;
    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";
  }

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

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

  identity aes256-ctr {
    if-feature "ssh-ctr";
    base encryption-alg-base;
    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
      "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 {
    if-feature "ssh-sha2";
    base mac-alg-base;
    description
      "HMAC-SHA2-256";
    reference
      "RFC 6668: SHA-2 Data Integrity Verification for the
                 Secure Shell (SSH) Transport Layer Protocol";
  }

  identity hmac-sha2-512 {
    if-feature "ssh-sha2";
    base mac-alg-base;
    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
          "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 EEEE for
           valid combinations.

           If this leaf-list is not configured (has zero elements)
           the acceptable host key algorithms are implementation-
           defined.";
        reference
          "RFC EEEE: 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 {
      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.";
      }
    }
  }
}

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

<ssh-client
  xmlns="urn:ietf:params:xml:ns:yang:ietf-ssh-client"
  xmlns:ct="urn:ietf:params:xml:ns:yang:ietf-crypto-types"
  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>
      <local-definition>
        <public-key-format>ct:ssh-public-key-format</public-key-form\
at>
        <public-key>base64encodedvalue==</public-key>
        <private-key-format>ct:rsa-private-key-format</private-key-f\
ormat>
        <cleartext-private-key>base64encodedvalue==</cleartext-priva\
te-key>
      </local-definition>
    </public-key>
  </client-identity>

  <!-- which host keys will this client trust -->
  <server-authentication>
    <ssh-host-keys>
      <local-definition>
        <public-key>
          <name>corp-fw1</name>
          <public-key-format>ct:ssh-public-key-format</public-key-fo\
rmat>
          <public-key>base64encodedvalue==</public-key>
        </public-key>
        <public-key>
          <name>corp-fw2</name>
          <public-key-format>ct:ssh-public-key-format</public-key-fo\
rmat>
          <public-key>base64encodedvalue==</public-key>
        </public-key>
      </local-definition>
    </ssh-host-keys>
    <ca-certs>
      <local-definition>
        <certificate>
          <name>Server Cert Issuer #1</name>
          <cert-data>base64encodedvalue==</cert-data>
        </certificate>
        <certificate>
          <name>Server Cert Issuer #2</name>
          <cert-data>base64encodedvalue==</cert-data>
        </certificate>
      </local-definition>
    </ca-certs>
    <ee-certs>
      <local-definition>
        <certificate>
          <name>My Application #1</name>
          <cert-data>base64encodedvalue==</cert-data>
        </certificate>
        <certificate>
          <name>My Application #2</name>
          <cert-data>base64encodedvalue==</cert-data>
        </certificate>
      </local-definition>
    </ee-certs>
  </server-authentication>

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

</ssh-client>

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

<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>
    <!-- can an SSH client have move than one key?
    <public-key>
      <keystore-reference>ssh-rsa-key</keystore-reference>
    </public-key>
    -->
    <certificate>
      <keystore-reference>
        <asymmetric-key>ssh-rsa-key-with-cert</asymmetric-key>
        <certificate>ex-rsa-cert2</certificate>
      </keystore-reference>
    </certificate>
  </client-identity>

  <!-- which host-keys will this client trust -->
  <server-authentication>
    <ssh-host-keys>
      <truststore-reference>trusted-ssh-public-keys</truststore-refe\
rence>
    </ssh-host-keys>
    <ca-certs>
      <truststore-reference>trusted-server-ca-certs</truststore-refe\
rence>
    </ca-certs>
    <ee-certs>
      <truststore-reference>trusted-server-ee-certs</truststore-refe\
rence>
    </ee-certs>
  </server-authentication>

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

</ssh-client>

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

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

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

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

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

  import ietf-ssh-common {
    prefix sshcmn;
    revision-date 2021-02-10; // stable grouping definitions
    reference
      "RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
  }

  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.

     Copyright (c) 2020 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 EEEE
     (https://www.rfc-editor.org/info/rfcEEEE); 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 2021-02-10 {
    description
      "Initial version";
    reference
      "RFC EEEE: 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.";
  }

  feature client-identity-password {
    description
      "Indicates that the 'password' authentication type
       is supported for client identification.";
  }

  feature client-identity-publickey {
    description
      "Indicates that the 'publickey' authentication type
       is supported for client identification.

       The 'publickey' authentication type is required by
       RFC 4252, but common implementations enable it to
       be disabled.";
  }

  feature client-identity-hostbased {
    description
      "Indicates that the 'hostbased' authentication type
       is supported for client identification.";
  }

  feature client-identity-none {
    description
      "Indicates that the 'none' authentication type is
       supported for client identification.";
  }

  // Groupings

  grouping ssh-client-grouping {
    description
      "A reusable grouping for configuring a SSH client without
       any consideration for how an underlying TCP session is
       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;
      must
        'public-key or password or hostbased or none or certificate';
      description
        "The credentials that the client may use, pending
         the SSH server's requirements, by the SSH 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.";
      }
      container public-key {
        if-feature client-identity-publickey;
        presence
          "Indicates that publickey-based authentication
           is configured";
        description
          "A locally-defined or referenced asymmetric key
           pair to be used for client identification.";
        reference
          "RFC CCCC: A YANG Data Model for a Keystore";
        uses ks:local-or-keystore-asymmetric-key-grouping {
          refine "local-or-keystore/local/local-definition" {
            must 'public-key-format = "ct:ssh-public-key-format"';
          }
          refine "local-or-keystore/keystore/keystore-reference" {
            must 'deref(.)/../ks:public-key-format'
                 + ' = "ct:ssh-public-key-format"';
          }
        }
      }
      container password {
        if-feature client-identity-password;
        presence
          "Indicates that password-based authenitcation is
           configured.";
        description
          "A password to be used to authenicate the client's
           identity.";
        uses ct:password-grouping;
      }
      container hostbased {
        if-feature client-identity-hostbased;
        presence
          "Indicates that hostbased authentication is configured";
        description
          "A locally-defined or referenced asymmetric key
           pair to be used for host identification.";
        reference
          "RFC CCCC: A YANG Data Model for a Keystore";
        uses ks:local-or-keystore-asymmetric-key-grouping {
          refine "local-or-keystore/local/local-definition" {
            must 'public-key-format = "ct:ssh-public-key-format"';
          }
          refine "local-or-keystore/keystore/keystore-reference" {
            must 'deref(.)/../ks:public-key-format'
                 + ' = "ct:ssh-public-key-format"';
          }
        }
      }
      leaf none {
        if-feature client-identity-none;
        type empty;
        description
          "Indicates that 'none' algorithm is used for client
           identification.";
      }
      container certificate {
        if-feature "sshcmn:ssh-x509-certs";
        presence
          "Indicates that certificate-based authentication
           is configured";
        description
          "A locally-defined or referenced certificate
           to be used for client identification.";
        reference
          "RFC CCCC: A YANG Data Model for a Keystore";
        uses
        ks:local-or-keystore-end-entity-cert-with-key-grouping {
          refine "local-or-keystore/local/local-definition" {
            must
              'public-key-format'
              + ' = "ct:subject-public-key-info-format"';
          }
          refine "local-or-keystore/keystore/keystore-reference"
                 + "/asymmetric-key" {
            must 'deref(.)/../ks:public-key-format'
                 + ' = "ct:subject-public-key-info-format"';
          }
        }
      }
    } // container client-identity

    container server-authentication {
      nacm:default-deny-write;
      must 'ssh-host-keys or ca-certs or ee-certs';
      description
        "Specifies how the SSH client can authenticate SSH servers.
         Any combination of credentials is additive and unordered.";
      container ssh-host-keys {
        presence
          "Indicates that the client can authenticate servers
           using the configured SSH host keys.";
        description
          "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 BBBB: A YANG Data Model for a Truststore";
        uses ts:local-or-truststore-public-keys-grouping {
          refine
            "local-or-truststore/local/local-definition/public-key" {
            must 'public-key-format = "ct:ssh-public-key-format"';
          }
          refine
            "local-or-truststore/truststore/truststore-reference" {
            must 'deref(.)/../*/ts:public-key-format'
                  + ' = "ct:ssh-public-key-format"';
          }
        }
      }
      container ca-certs {
        if-feature "sshcmn:ssh-x509-certs";
        presence
          "Indicates that the client can authenticate servers
           using the configured trust anchor certificates.";
        description
          "A set of certificate authority (CA) certificates used by
           the SSH client to authenticate SSH servers.  A server
           is authenticated if its certificate  has a valid chain
           of trust to a configured CA certificate.";
         reference
           "RFC BBBB: A YANG Data Model for a Truststore";
        uses ts:local-or-truststore-certs-grouping;
      }
      container ee-certs {
        if-feature "sshcmn:ssh-x509-certs";
        presence
          "Indicates that the client can authenticate servers
           using the configured end-entity certificates.";
        description
          "A set of end-entity certificates used by the SSH client
           to authenticate SSH servers.  A server is authenticated
           if its certificate is an exact match to a configured
           end-entity certificate.";
         reference
           "RFC BBBB: A YANG Data Model for a Truststore";
        uses ts:local-or-truststore-certs-grouping;
      }
    } // 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 the SSH client proactively tests the
         aliveness of the remote SSH server.";
      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.  Per Section 4 of RFC 4254,
         the SSH client SHOULD send an SSH_MSG_GLOBAL_REQUEST
         message with a purposely nonexistent 'request name'
         value (e.g., keepalive@ietf.org) and the 'want reply'
         value set to '1'.";
      reference
        "RFC 4254: The Secure Shell (SSH) Connection Protocol";
      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
} // module ietf-ssh-client

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

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

  <!-- the host-key this SSH server will present -->
  <server-identity>
    <host-key>
      <name>my-pubkey-based-host-key</name>
      <public-key>
        <local-definition>
          <public-key-format>ct:ssh-public-key-format</public-key-fo\
rmat>
          <public-key>base64encodedvalue==</public-key>
          <private-key-format>ct:rsa-private-key-format</private-key\
-format>
          <cleartext-private-key>base64encodedvalue==</cleartext-pri\
vate-key>
        </local-definition>
      </public-key>
    </host-key>
    <host-key>
      <name>my-cert-based-host-key</name>
      <certificate>
        <local-definition>
          <public-key-format>ct:subject-public-key-info-format</publ\
ic-key-format>
          <public-key>base64encodedvalue==</public-key>
          <private-key-format>ct:rsa-private-key-format</private-key\
-format>
          <cleartext-private-key>base64encodedvalue==</cleartext-pri\
vate-key>
          <cert-data>base64encodedvalue==</cert-data>
        </local-definition>
      </certificate>
    </host-key>
  </server-identity>

  <!-- the client credentials this SSH server will trust -->
  <client-authentication>
    <supported-authentication-methods>
      <publickey/>
    </supported-authentication-methods>
    <users>
      <user>
        <name>mary</name>
        <password>$0$secret</password>
        <public-keys>
          <local-definition>
            <!--<ssh-public-key>-->
            <public-key>
              <name>User A</name>
              <public-key-format>ct:ssh-public-key-format</public-ke\
y-format>
              <public-key>base64encodedvalue==</public-key>
              <!--</ssh-public-key>
            <ssh-public-key>-->
            </public-key>
            <public-key>
              <name>User B</name>
              <public-key-format>ct:ssh-public-key-format</public-ke\
y-format>
              <public-key>base64encodedvalue==</public-key>
            </public-key>
            <!--</ssh-public-key>-->
          </local-definition>
        </public-keys>
      </user>
    </users>
    <ca-certs>
      <local-definition>
        <certificate>
          <name>Identity Cert Issuer #1</name>
          <cert-data>base64encodedvalue==</cert-data>
        </certificate>
        <certificate>
          <name>Identity Cert Issuer #2</name>
          <cert-data>base64encodedvalue==</cert-data>
        </certificate>
      </local-definition>
    </ca-certs>
    <ee-certs>
      <local-definition>
        <certificate>
          <name>Application #1</name>
          <cert-data>base64encodedvalue==</cert-data>
        </certificate>
        <certificate>
          <name>Application #2</name>
          <cert-data>base64encodedvalue==</cert-data>
        </certificate>
      </local-definition>
    </ee-certs>
  </client-authentication>

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

</ssh-server>

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

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

  <!-- the host-key this SSH server will present -->
  <server-identity>
    <host-key>
      <name>my-pubkey-based-host-key</name>
      <public-key>
        <keystore-reference>ssh-rsa-key</keystore-reference>
      </public-key>
    </host-key>
    <host-key>
      <name>my-cert-based-host-key</name>
      <certificate>
        <keystore-reference>
          <asymmetric-key>ssh-rsa-key-with-cert</asymmetric-key>
          <certificate>ex-rsa-cert2</certificate>
        </keystore-reference>
      </certificate>
    </host-key>
  </server-identity>

  <!-- the client credentials this SSH server will trust -->
  <client-authentication>
    <supported-authentication-methods>
      <publickey/>
    </supported-authentication-methods>
    <users>
      <user>
        <name>mary</name>
        <password>$0$secret</password>
        <public-keys>
          <truststore-reference>SSH Public Keys for Application A</t\
ruststore-reference>
        </public-keys>
      </user>
    </users>
    <ca-certs>
      <truststore-reference>trusted-client-ca-certs</truststore-refe\
rence>
    </ca-certs>
    <ee-certs>
      <truststore-reference>trusted-client-ee-certs</truststore-refe\
rence>
    </ee-certs>
  </client-authentication>

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

</ssh-server>

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

  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";
  }

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

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

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

  import ietf-ssh-common {
    prefix sshcmn;
    revision-date 2021-02-10; // stable grouping definitions
    reference
      "RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
  }

  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.

     Copyright (c) 2020 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 EEEE
     (https://www.rfc-editor.org/info/rfcEEEE); 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 2021-02-10 {
    description
      "Initial version";
    reference
      "RFC EEEE: 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 client-auth-config-supported {
    description
      "Indicates that the configuration for how to authenticate
       clients can be configured herein, as opposed to in an
       application specific location.  That is, to support the
       consuming data models that prefer to place client
       authentication with client definitions, rather then
       in a data model principally concerned with configuring
       the transport.";
  }

  feature client-auth-publickey {
    description
      "Indicates that the 'publickey' authentication type
       is supported.

       The 'publickey' authentication type is required by
       RFC 4252, but common implementations enable it to
       be disabled.";
    reference
      "RFC 4252:
        The Secure Shell (SSH) Authentication Protocol";
  }

  feature client-auth-password {
    description
      "Indicates that the 'password' authentication type
       is supported.";
  }

  feature client-auth-hostbased {
    description
      "Indicates that the 'hostbased' authentication type
       is supported.";
  }

  feature client-auth-none {
    description
      "Indicates that the 'none' authentication type is
       supported.";
  }

  // 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
            "The type of host key being specified";
          container public-key {
            description
              "A locally-defined or referenced asymmetric key pair
               to be used for the SSH server's host key.";
            reference
              "RFC CCCC: A YANG Data Model for a Keystore";
            uses ks:local-or-keystore-asymmetric-key-grouping {
              refine "local-or-keystore/local/local-definition" {
                must
                  'public-key-format = "ct:ssh-public-key-format"';
              }
              refine "local-or-keystore/keystore/"
                     + "keystore-reference" {
                must 'deref(.)/../ks:public-key-format'
                     + ' = "ct:ssh-public-key-format"';
              }
            }
          }
          container certificate {
            if-feature "sshcmn:ssh-x509-certs";
            description
              "A locally-defined or referenced end-entity
               certificate to be used for the SSH server's
               host key.";
            reference
              "RFC CCCC: A YANG Data Model for a Keystore";
            uses
            ks:local-or-keystore-end-entity-cert-with-key-grouping {
              refine "local-or-keystore/local/local-definition" {
                must
                  'public-key-format'
                  + ' = "ct:subject-public-key-info-format"';
              }
              refine "local-or-keystore/keystore/keystore-reference"
                     + "/asymmetric-key" {
                must 'deref(.)/../ks:public-key-format'
                     + ' = "ct:subject-public-key-info-format"';
              }
            }
          }
        }
      }
    } // container server-identity

    container client-authentication {
      nacm:default-deny-write;
      description
        "Specifies how the SSH server can authenticate SSH clients.";
      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.";
        }
        leaf password {
          if-feature client-auth-password;
          type empty;
          description
            "Indicates that the 'password' method is supported.";
          reference
            "RFC 4252: The Secure Shell (SSH) Authentication
                       Protocol.";
        }
        leaf hostbased {
          if-feature client-auth-hostbased;
          type empty;
          description
            "Indicates that the 'hostbased' method is supported.";
          reference
            "RFC 4252: The Secure Shell (SSH) Authentication
                       Protocol.";
        }
        leaf none {
          if-feature client-auth-none;
          type empty;
          description
            "Indicates that the 'none' method is supported.";
          reference
            "RFC 4252: The Secure Shell (SSH) Authentication
                       Protocol.";
        }
      }

      container users {
        if-feature "client-auth-config-supported";
        description
          "A list of locally configured users.";
        list user {
          key name;
          description
            "The list of local users configured on this device.";
          leaf name {
            type string;
            description
             "The user name string identifying this entry.";
          }
          container public-keys {
            if-feature client-auth-publickey;
            presence
              "Indicates that the server can authenticate this
               user using any of the configured SSH public keys.";
            description
              "A set of SSH public keys may be used by the SSH
               server to authenticate this user.  A user is
               authenticated if its public key is an exact
               match to a configured public key.";
            reference
              "RFC BBBB: A YANG Data Model for a Truststore";
            uses ts:local-or-truststore-public-keys-grouping {
              refine "local-or-truststore/local/local-definition"
                     + "/public-key" {
                must 'public-key-format'
                     + ' = "ct:ssh-public-key-format"';
              }
              refine "local-or-truststore/truststore/"
                     + "truststore-reference" {
                must 'deref(.)/../*/ts:public-key-format'
                     + ' = "ct:ssh-public-key-format"';
              }
            }
          }
          leaf password {
            if-feature client-auth-password;
            type ianach:crypt-hash;
            description
              "The password for this user.";
          }

          container hostbased {
            if-feature client-auth-hostbased;
            presence
              "Indicates that the server can authenticate this
               user's 'host' using any of the configured SSH
               host keys.";
            description
              "A set of SSH host keys may be used by the SSH
               server to authenticate this user's host.  A
               user's host is authenticated if its host key
               is an exact match to a configured host key.";
            reference
              "RFC 4253: The Secure Shell (SSH) Transport Layer
               RFC BBBB: A YANG Data Model for a Truststore";
            uses ts:local-or-truststore-public-keys-grouping {
              refine "local-or-truststore/local/local-definition"
                     + "/public-key" {
                must 'public-key-format'
                     + ' = "ct:ssh-public-key-format"';
              }
              refine "local-or-truststore/truststore"
                     + "/truststore-reference" {
                must 'deref(.)/../*/ts:public-key-format'
                     + ' = "ct:ssh-public-key-format"';
              }
            }
          }
          leaf none {
            if-feature client-auth-none;
            type empty;
            description
              "Indicates that the 'none' method is supported.";
            reference
              "RFC 4252: The Secure Shell (SSH) Authentication
                         Protocol.";
          }
        }
      }
      container ca-certs {
        if-feature "client-auth-config-supported";
        if-feature "sshcmn:ssh-x509-certs";
        presence
          "Indicates that the SSH server can authenticate SSH
           clients using configured certificate authority (CA)
           certificates.";
        description
          "A set 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 BBBB: A YANG Data Model for a Truststore";
        uses ts:local-or-truststore-certs-grouping;
      }
      container ee-certs {
        if-feature "client-auth-config-supported";
        if-feature "sshcmn:ssh-x509-certs";
        presence
          "Indicates that the SSH server can authenticate SSH
           clients using configured end-entity certificates.";
        description
          "A set of client certificates (i.e., end entity
           certificates) used by the SSH server to authenticate
           the certificates presented by SSH clients.  A client
           certificate is authenticated if it is an exact match
           to a configured end-entity certificate.";
        reference
          "RFC BBBB: A YANG Data Model for a Truststore";
        uses ts:local-or-truststore-certs-grouping;
      }
    } // 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 the SSH server proactively tests the
         aliveness of the remote SSH client.";
      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.  Per Section 4 of RFC 4254,
         the SSH server SHOULD send an SSH_MSG_GLOBAL_REQUEST
         message with a purposely nonexistent 'request name'
         value (e.g., keepalive@ietf.org) and the 'want reply'
         value set to '1'.";
      reference
        "RFC 4254: The Secure Shell (SSH) Connection Protocol";
      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.";
      }
    }
  } // grouping ssh-server-grouping
} // module ietf-ssh-server