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<erlref>
  <header>
    <copyright>
      <year>2018</year><year>2022</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
      
      http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>
    
    <title>ssh_file</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module since="OTP 21.2">ssh_file</module>
  <modulesummary>Default callback module for the client's and server's database operations in the ssh application</modulesummary>
  <description>
    <p>This module is the default callback handler for the client's and the server's user and host "database" operations.
    All data, for instance key pairs, are stored in files in the normal file system. This page documents the files, where they
    are stored and configuration options for this callback module.
    </p>
    <p>The intention is to be compatible with the
    <url href="http://www.openssh.com">OpenSSH</url>
    storage in files. Therefore it mimics directories and filenames of
    <url href="http://www.openssh.com">OpenSSH</url>.
    </p>

    <p>Ssh_file implements the <seeerl marker="ssh:ssh_server_key_api">ssh_server_key_api</seeerl> and
    the <seeerl marker="ssh:ssh_client_key_api">ssh_client_key_api</seeerl>.
    This enables the user to make an own interface using for example a database handler.
    </p>
    <p>Such another callback module could be used by setting the option
    <seetype marker="ssh:ssh#key_cb_common_option"><c>key_cb</c></seetype>
    when starting a client or a server (with for example
    <seemfa marker="ssh:ssh#connect/3">ssh:connect</seemfa>,
    <seemfa marker="ssh:ssh#daemon/2">ssh:daemon</seemfa> of
    <seemfa marker="ssh:ssh#shell/1">ssh:shell</seemfa>
    ).
    </p>

    <note>
      <p>The functions are <i>Callbacks</i> for the SSH app. They are not intended to be called from the user's code!
      </p>
    </note>
  </description>

  <section>
    <title>Files, directories and who uses them</title>
    <section>
      <title>Daemons</title>
      <p>Daemons uses all files stored in the <seeerl marker="#SYSDIR">SYSDIR</seeerl> directory.
      </p>
      <p>Optionally, in case of <c>publickey</c> authorization, one or more of the remote user's public keys
      in the <seeerl marker="#USERDIR">USERDIR</seeerl> directory are used.
      See the files 
      <seeerl marker="#FILE-authorized_keys"><c>USERDIR/authorized_keys</c></seeerl> and
      <seeerl marker="#FILE-authorized_keys2"><c>USERDIR/authorized_keys2</c></seeerl>.
      </p>
    </section>

    <section>
      <title>Clients</title>
      <p>Clients uses all files stored in the <seeerl marker="#USERDIR">USERDIR</seeerl> directory.
      </p>
    </section>

    <section>
      <title>Directory contents</title>
      <taglist>
	<tag><marker id="LOCALUSER"/>LOCALUSER</tag>
	<item><p>The user name of the OS process running the Erlang virtual machine (emulator).</p>
	</item>

	<tag><marker id="SYSDIR"/>SYSDIR</tag>
	<item><p>This is the directory holding the server's files:</p>
	<list>
	  <item><marker id="FILE-ssh_host_STAR_key"/>
	        <marker id="FILE-ssh_host_dsa_key"/><c>ssh_host_dsa_key</c> - private dss host key (optional)</item>
	  <item><marker id="FILE-ssh_host_rsa_key"/><c>ssh_host_rsa_key</c> - private rsa host key (optional)</item>
	  <item><marker id="FILE-ssh_host_ecdsa_key"/><c>ssh_host_ecdsa_key</c> - private ecdsa host key (optional)</item>
	  <item><marker id="FILE-ssh_host_ed25519_key"/><c>ssh_host_ed25519_key</c> - private eddsa host key for curve 25519 (optional)</item>
	  <item><marker id="FILE-ssh_host_ed448_key"/><c>ssh_host_ed448_key</c> - private eddsa host key for curve 448 (optional)</item>
	</list>
	<p>The key files could be generated with OpenSSH's ssh-keygen command.</p>
	<p>At least one host key must be defined. The default value of SYSDIR is <marker id="#/etc/ssh"/><c>/etc/ssh</c>.
	</p>
	<p>For security reasons, this directory is normally accessible only to the root user.
	</p>
	<p>To change the SYSDIR, see the <seetype marker="#system_dir_daemon_option">system_dir</seetype> option.
	</p>
	</item>

	<tag><marker id="USERDIR"/>USERDIR</tag>
	<item><p>This is the directory holding the files:</p>
	<list>
	  <item><marker id="FILE-authorized_keys"/><c>authorized_keys</c>
	  and, as second alternative
	  <marker id="FILE-authorized_keys2"/><c>authorized_keys2</c> - 
	  the user's public keys are stored concatenated in one of those files.
	  <p>It is composed of lines as for
	  <url href="https://man.openbsd.org/sshd#AUTHORIZED_KEYS_FILE_FORMAT">OpenSSH</url>:</p>
	  <pre>(options)? keytype base64-encoded-key comment</pre>
	  <p>where</p>
	  <code>
options :: option(,option)*
option :: % All options are skipped
keytype :: 'ssh-dsa'
         | 'ssh-rsa'
         | 'ssh-ecdsa-nistp256'
	 | 'ssh-ecdsa-nistp384'
         | 'ssh-ecdsa-nistp521'
         | 'ssh-ed25519'
	 | 'ssh-ed448'
base64-encoded-key :: % The user's public key
comment :: % Comments are skipped
	  </code>
	  </item>
	  <item><marker id="FILE-known_hosts"/><c>known_hosts</c> - host keys from hosts visited
	  concatenated. The file is created and used by the client.
	  <p>It is composed of lines as for
	  <url href="https://man.openbsd.org/sshd#SSH_KNOWN_HOSTS_FILE_FORMAT">OpenSSH</url>:</p>
	  <pre>(option)? pattern(,pattern)* keytype key (comment)?</pre>
	  <p>where</p>
	  <code>
option :: '@revoked'
pattern :: host | '[' host ']:' port
host :: ip-address | hostname | '*'
port :: portnumber | '*'
keytype :: 'ssh-dsa'
         | 'ssh-rsa'
         | 'ssh-ecdsa-nistp256'
	 | 'ssh-ecdsa-nistp384'
         | 'ssh-ecdsa-nistp521'
         | 'ssh-ed25519'
	 | 'ssh-ed448'
key :: % encoded key from eg ssh_host_*.pub
</code>
	  </item>
	  <item><marker id="FILE-id_STAR"/>
	        <marker id="FILE-id_dsa"/><c>id_dsa</c> - private dss user key (optional)</item>
	  <item><marker id="FILE-id_rsa"/><c>id_rsa</c> - private rsa user key (optional)</item>
	  <item><marker id="FILE-id_ecdsa"/><c>id_ecdsa</c> - private ecdsa user key (optional)</item>
	  <item><marker id="FILE-id_ed25519"/><c>id_ed25519</c> - private eddsa user key for curve 25519 (optional)</item>
	  <item><marker id="FILE-id_ed448"/><c>id_ed448</c> - private eddsa user key for curve 448 (optional)</item>
	</list>
	<p>The key files could be generated with OpenSSH's ssh-keygen command.</p>
	<p>The default value of USERDIR is <c>/home/</c><seeerl marker="#LOCALUSER"><c>LOCALUSER</c></seeerl><c>/.ssh</c>.
	</p>
	<p>To change the USERDIR, see the <seetype marker="#user_dir_common_option">user_dir</seetype> option
	</p>
	</item>
      </taglist>
    </section>
  </section>

  <datatypes>
    <datatype_title>Options for the default ssh_file callback module</datatype_title>
    <datatype>
      <name name="user_dir_common_option"/>
      <desc>
	<p>Sets the <seeerl marker="#USERDIR">user directory</seeerl>.</p>
      </desc>
    </datatype>

    <datatype>
      <name name="user_dir_fun_common_option"/>
      <name name="user2dir"/>
      <desc>
	<p>Sets the <seeerl marker="#USERDIR">user directory</seeerl> dynamically
	by evaluating the <c>user2dir</c> function.
	</p>
      </desc>
    </datatype>

    <datatype>
      <name name="system_dir_daemon_option"/>
      <desc>
        <p>Sets the <seeerl marker="#SYSDIR">system directory</seeerl>.</p>
      </desc>
    </datatype>

    <datatype>
      <name name="pubkey_passphrase_client_options"/>
      <desc>
	<p>If the user's DSA, RSA or ECDSA key is protected by a passphrase, it can be
	supplied with thoose options.
	</p>
	<p>Note that EdDSA passhrases (Curves 25519 and 448) are not implemented.</p>
      </desc>
    </datatype>

    <datatype>
      <name name="optimize_key_lookup"/>
      <desc>
        <p>Make the handling of large files fast by setting <c>time</c>, but this will use more memory.
	The <c>space</c> variant shrinks the memory requirements, but with a higher time consumption.
	</p>
	<p>To set it, set the
	option <c>{key_cb, {ssh_file, [{optimize,TimeOrSpace}]}</c> in the call of
	<seemfa marker="ssh#connect/3">"ssh:connect/3</seemfa>,
	<seemfa marker="ssh#daemon/2">ssh:daemon/2</seemfa>
	or similar function call that initiates an ssh connection.
	</p>
      </desc>
    </datatype>

    <datatype>
      <name name="key"/>
      <desc>
	<p>The key representation</p>
      </desc>
    </datatype>

    <datatype>
      <name name="experimental_openssh_key_v1"/>
      <name name="openssh_key_v1_attributes"/>
      <desc>
	<p>Types for the experimental implementaition of the <c>openssh_key_v1</c> format.
	</p>
      </desc>
      </datatype>

  </datatypes>

  <funcs>
    <func>
      <name since="OTP 21.2" name="host_key" arity="2"/>
      <fsummary></fsummary>
      <desc>
	<p><strong>Types and description</strong></p>
	<p>See the api description in
	<seemfa marker="ssh:ssh_server_key_api#Module:host_key/2">ssh_server_key_api, Module:host_key/2</seemfa>.
	</p>
	<p><strong>Options</strong></p>
	<list>
	  <item><seetype marker="#system_dir_daemon_option">system_dir</seetype></item>
	  <!-- item>dsa_pass_phrase</item -->
	  <!-- item>rsa_pass_phrase</item -->
	  <!-- item>ecdsa_pass_phrase</item -->
	</list>
	<p><strong>Files</strong></p>
	<list>
	  <item><seeerl marker="#FILE-ssh_host_rsa_key"><c>SYSDIR/ssh_host_rsa_key</c></seeerl></item>
	  <item><seeerl marker="#FILE-ssh_host_dsa_key"><c>SYSDIR/ssh_host_dsa_key</c></seeerl></item>
	  <item><seeerl marker="#FILE-ssh_host_ecdsa_key"><c>SYSDIR/ssh_host_ecdsa_key</c></seeerl></item>
          <item><seeerl marker="#FILE-ssh_host_ed25519_key"><c>SYSDIR/ssh_host_ed25519_key</c></seeerl></item>
          <item><seeerl marker="#FILE-ssh_host_ed448_key"><c>SYSDIR/ssh_host_ed448_key</c>c></seeerl></item>
	</list>
      </desc>
    </func>

    <func>
      <name since="OTP 21.2" name="is_auth_key" arity="3"/>
      <fsummary></fsummary>
      <desc>
	<p><strong>Types and description</strong></p>
	<p>See the api description in
	<seemfa marker="ssh:ssh_server_key_api#Module:is_auth_key/3">ssh_server_key_api: Module:is_auth_key/3</seemfa>.
	</p>
	<p><strong>Options</strong></p>
	<list>
	  <item><seetype marker="#user_dir_fun_common_option">user_dir_fun</seetype></item>
	  <item><seetype marker="#user_dir_common_option">user_dir</seetype></item>
	</list>
	<p><strong>Files</strong></p>
	<list>
	  <item><seeerl marker="#FILE-authorized_keys"><c>USERDIR/authorized_keys</c></seeerl></item>
	  <item><seeerl marker="#FILE-authorized_keys2"><c>USERDIR/authorized_keys2</c></seeerl></item>
	</list>
	<p>This functions discards all options in the beginning of the lines of thoose files when reading them.
	</p>
      </desc>
    </func>

    <func>
      <name since="OTP 23.0" name="add_host_key" arity="4"/>
      <fsummary></fsummary>
      <desc>
	<p><strong>Types and description</strong></p>
	<p>See the api description in
	<seemfa marker="ssh:ssh_client_key_api#Module:add_host_key/4">ssh_client_key_api, Module:add_host_key/4</seemfa>.
	</p>
	<marker id="add_host_key-3"/>
	<p>Note that the alternative, the old
	<seemfa marker="ssh:ssh_client_key_api#Module:add_host_key/3">Module:add_host_key/3</seemfa>
	is no longer supported by <c>ssh_file</c>.
	</p>
	<p><strong>Option</strong></p>
	<list>
	  <item><seetype marker="#user_dir_common_option">user_dir</seetype></item>
	</list>
	<p><strong>File</strong></p>
	<list>
	  <item><seeerl marker="#FILE-known_hosts"><c>USERDIR/known_hosts</c></seeerl></item>
	</list>
      </desc>
    </func>

    <func>
      <name since="OTP 23.0" name="is_host_key" arity="5"/>
      <fsummary></fsummary>
      <desc>
	<p><strong>Types and description</strong></p>
	<p>See the api description in
	<seemfa marker="ssh:ssh_client_key_api#Module:is_host_key/5">ssh_client_key_api, Module:is_host_key/5</seemfa>.
	</p>
	<marker id="is_host_key-4"/>
	<p>Note that the alternative, the old
	<seemfa marker="ssh:ssh_client_key_api#Module:is_host_key/4">Module:is_host_key/4</seemfa>
	is no longer supported by <c>ssh_file</c>.
	</p>
	<p><strong>Option</strong></p>
	<list>
	  <item><seetype marker="#user_dir_common_option">user_dir</seetype></item>
	</list>
	<p><strong>File</strong></p>
	<list>
	  <item><seeerl marker="#FILE-known_hosts"><c>USERDIR/known_hosts</c></seeerl></item>
	</list>
      </desc>
    </func>

    <func>
      <name since="OTP 21.2" name="user_key" arity="2"/>
      <fsummary></fsummary>
      <desc>
	<p><strong>Types and description</strong></p>
	<p>See the api description in
	<seemfa marker="ssh:ssh_client_key_api#Module:user_key/2">ssh_client_key_api, Module:user_key/2</seemfa>.
	</p>
	<p><strong>Options</strong></p>
	<list>
	  <item><seetype marker="#user_dir_common_option">user_dir</seetype></item>
	  <item><seetype marker="#pubkey_passphrase_client_options">dsa_pass_phrase</seetype></item>
	  <item><seetype marker="#pubkey_passphrase_client_options">rsa_pass_phrase</seetype></item>
	  <item><seetype marker="#pubkey_passphrase_client_options">ecdsa_pass_phrase</seetype></item>
	</list>
	<p>Note that EdDSA passhrases (Curves 25519 and 448) are not implemented.</p>
	<p><strong>Files</strong>
	</p>
	<list>
	  <item><seeerl marker="#FILE-id_dsa"><c>USERDIR/id_dsa</c></seeerl></item>
	  <item><seeerl marker="#FILE-id_rsa"><c>USERDIR/id_rsa</c></seeerl></item>
	  <item><seeerl marker="#FILE-id_ecdsa"><c>USERDIR/id_ecdsa</c></seeerl></item>
	  <item><seeerl marker="#FILE-id_ed25519"><c>USERDIR/id_ed25519</c></seeerl></item>
	  <item><seeerl marker="#FILE-id_ed448"><c>USERDIR/id_ed448</c></seeerl></item>
	</list>
      </desc>
    </func>

    <func>
      <name since="OTP 24.0" name="decode" arity="2"/>
      <fsummary></fsummary>
      <desc>
	<p>Decodes an SSH file-binary.</p>
	<p>If <c>Type</c> is <c>public_key</c> the binary can be either
	an RFC4716 public key or an OpenSSH public key.</p>
	<note>
	  <p>The following key types have been renamed from the deprecated
	  <c>public_key:ssh_decode/2</c>:</p>
	  <list>
	    <item>rfc4716_public_key -> rfc4716_key</item>
	    <item>openssh_public_key -> openssh_key</item>
	  </list>
	</note>
	<note><p>The implementation of the <c>openssh_key_v1</c> format is still experimental.</p></note>
      </desc>
    </func>
    
    <func>
      <name since="OTP 24.0" name="encode" arity="2"/>
      <fsummary></fsummary>
      <desc>
	<p>Encodes a list of SSH file entries (public keys and attributes) to a binary.</p>
	<note>
	  <p>The following key types have been renamed from the removed <c>public_key:ssh_encode/2</c>:</p>
	  <list>
	    <item>rfc4716_public_key -> rfc4716_key</item>
	    <item>openssh_public_key -> openssh_key</item>
	  </list>
	</note>
	<note><p>The implementation of the <c>openssh_key_v1</c> format is still experimental.</p></note>
      </desc>
    </func>


    <func>
      <name since="OTP 25.0" name="extract_public_key" arity="1"/>
      <fsummary></fsummary>
      <desc>
	<p>Fetches the public key from a private key.</p>
      </desc>
    </func>

  </funcs>

</erlref>
