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+<?xml version="1.0" encoding="utf-8" ?>
+<!DOCTYPE chapter SYSTEM "chapter.dtd">
+
+<chapter>
+ <header>
+ <copyright>
+ <year>2017</year>
+ <year>2017</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>Configuring algorithms in SSH</title>
+ <prepared></prepared>
+ <docno></docno>
+ <approved></approved>
+ <date></date>
+ <rev></rev>
+ <file>configure_algos.xml</file>
+ </header>
+
+ <section>
+ <marker id="introduction"/>
+ <title>Introduction</title>
+ <p>To fully understand how to configure the algorithms, we must understand partly both how the ssh protocol
+ works and how the OTP SSH app handles the corresponding items</p>
+
+ <p>The first subsection will give a short background about the ssh protocol while later sections describes
+ the implementation and provides many examples</p>
+
+ <section>
+ <title>Basics of the ssh protocol's algorithms handling</title>
+
+ <p>SSH uses different sets of algorithms in different phases of a session. Which
+ algorithms to use is negotiated by the client and the server at the beginning of a session.
+ See <url href="https://tools.ietf.org/html/rfc4253">RFC 4253</url>,
+ "The Secure Shell (SSH) Transport Layer Protocol" for details.
+ </p>
+
+ <p>The negotiation is simple: both peers sends their list of supported alghorithms to the other part.
+ The first algorithm on the client's list that also in on the server's list is selected. So it is the
+ client's orderering of the list that gives the priority for the algorithms.</p>
+
+ <p>There are five lists exchanged in the connection setup. Three of them are also divided in two
+ directions, to and from the server.</p>
+
+ <p>The lists are (named as in the SSH application's options):</p>
+ <taglist>
+ <tag><c>kex</c></tag>
+ <item>
+ <p>Key exchange.</p>
+ <p>An algorithm is selected for computing a secret encryption key. Among examples are:
+ the old nowadays week <c>'diffie-hellman-group-exchange-sha1'</c> and the very strong and modern
+ <c>'ecdh-sha2-nistp512'</c>.</p>
+ </item>
+
+ <tag><c>public_key</c></tag>
+ <item>
+ <p>Server host key</p>
+ <p>The asymetric encryption algorithm used in the server's private-public host key pair.
+ Examples include the well-known RSA <c>'ssh-rsa'</c> and elliptic curve <c>'ecdsa-sha2-nistp521'</c>.
+ </p>
+ </item>
+
+ <tag><c>cipher</c></tag>
+ <item>
+ <p>Symetric cipher algorithm used for the message encryption. This algorithm will use the key calculated
+ in the kex phase (together with other info) to genereate the actual key used. Examples are
+ tripple-DES <c>'3des-cbc'</c> and one of many AES variants <c>'aes192-ctr'</c>.
+ </p>
+ <p>This list is actually two - one for each direction server-to-client and client-to-server. Therefore it
+ is possible but rare to have different algorithms in the two directions in one connection.</p>
+ </item>
+
+ <tag><c>mac</c></tag>
+ <item>
+ <p>Message authentication code</p>
+ <p>"Check sum" of each message sent between the peers. Examples are SHA <c>'hmac-sha1'</c> and
+ SHA2 <c>'hmac-sha2-512'</c>.</p>
+ <p>This list is also divided into two for the both directions</p>
+ </item>
+
+ <tag><c>compression</c></tag>
+ <item>
+ <p>If and how to compress the message. Examples are <c>none</c>, that is, no compression and
+ <c>zlib</c>.</p>
+ <p>This list is also divided into two for the both directions</p>
+ </item>
+
+ </taglist>
+ </section>
+
+ <section>
+ <title>The SSH app's mechanism</title>
+ <p>The set of algorithms that the SSH app uses by default depends on the algoritms supported by the:</p>
+ <list>
+ <item><p><seealso marker="crypto:crypto">crypto</seealso> app,</p>
+ </item>
+ <item><p>The cryptolib OTP is linked with, usally the one the OS uses, probably OpenSSL,</p>
+ </item>
+ <item><p>and finaly what the SSH app implements</p>
+ </item>
+ </list>
+ <p>Due to this, it impossible to list in documentation what algorithms that are available in a certain installation.</p>
+ <p>There is an important commands to list the actual algorithms and their ordering:
+ <seealso marker="ssh#default_algorithms-0">ssh:default_algorithms/0</seealso>.</p>
+ <code type="erl">
+0> ssh:default_algorithms().
+[{kex,['ecdh-sha2-nistp384','ecdh-sha2-nistp521',
+ 'ecdh-sha2-nistp256','diffie-hellman-group-exchange-sha256',
+ 'diffie-hellman-group16-sha512',
+ 'diffie-hellman-group18-sha512',
+ 'diffie-hellman-group14-sha256',
+ 'diffie-hellman-group14-sha1',
+ 'diffie-hellman-group-exchange-sha1']},
+ {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
+ 'ecdsa-sha2-nistp256','ssh-rsa','rsa-sha2-256',
+ 'rsa-sha2-512','ssh-dss']},
+ {cipher,[{client2server,['[email protected]',
+ 'aes256-ctr','aes192-ctr','[email protected]',
+ 'aes128-ctr','aes128-cbc','3des-cbc']},
+ {server2client,['[email protected]','aes256-ctr',
+ 'aes192-ctr','[email protected]','aes128-ctr',
+ 'aes128-cbc','3des-cbc']}]},
+ {mac,[{client2server,['hmac-sha2-256','hmac-sha2-512',
+ 'hmac-sha1']},
+ {server2client,['hmac-sha2-256','hmac-sha2-512',
+ 'hmac-sha1']}]},
+ {compression,[{client2server,[none,'[email protected]',zlib]},
+ {server2client,[none,'[email protected]',zlib]}]}]
+
+ </code>
+ <p>To change this listing, there are two options which can be used in
+ <seealso marker="ssh#connect-3">ssh:default_algorithms/2,3,4</seealso>
+ and
+ <seealso marker="ssh#daemon-2">ssh:daemon/2,3</seealso>. The options could of course
+ be used in all other functions that initiates connections.</p>
+
+ <p>The options are <c>preferred_algorithms</c> and <c>modify_algorithms</c>. The first one
+ replaces the default set, while the latter modifies the default set.</p>
+ </section>
+ </section>
+
+ <section>
+ <title>Replacing the default set: preferred_algorithms</title>
+ <p>See the <seealso marker="ssh#option_preferred_algorithms">Reference Manual</seealso> for details</p>
+
+ <p>Here follows a series of examples ranging from simple to more complex.</p>
+
+ <p>The experimental function <c>ssh:chk_algos_opts(Opts)</c> mangles the options <c>preferred_algorithms</c>
+ and <c>modify_algorithms</c> as <c>ssh:dameon</c>, <c>ssh:connect</c> and others does.</p>
+
+ <section>
+ <title>Example 1</title>
+ <p>Replace the kex algorithms list with the single algorithm <c>'diffie-hellman-group14-sha256'</c>:</p>
+ <code>
+1> ssh:chk_algos_opts(
+ [{preferred_algorithms,
+ [{kex, ['diffie-hellman-group14-sha256']}
+ ]
+ }
+ ]).
+[{kex,['diffie-hellman-group14-sha256']},
+ {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
+ 'ecdsa-sha2-nistp256','ssh-rsa','rsa-sha2-256',
+ 'rsa-sha2-512','ssh-dss']},
+ {cipher,[{client2server,['[email protected]',
+ 'aes256-ctr','aes192-ctr','[email protected]',
+ 'aes128-ctr','aes128-cbc','3des-cbc']},
+ {server2client,['[email protected]','aes256-ctr',
+ 'aes192-ctr','[email protected]','aes128-ctr',
+ 'aes128-cbc','3des-cbc']}]},
+ {mac,[{client2server,['hmac-sha2-256','hmac-sha2-512',
+ 'hmac-sha1']},
+ {server2client,['hmac-sha2-256','hmac-sha2-512',
+ 'hmac-sha1']}]},
+ {compression,[{client2server,[none,'[email protected]',zlib]},
+ {server2client,[none,'[email protected]',zlib]}]}]
+ </code>
+ <p>Note that the unmentioned lists (<c>public_key</c>, <c>cipher</c>, <c>mac</c> and <c>compression</c>)
+ are un-changed.</p>
+ </section>
+
+ <section>
+ <title>Example 2</title>
+ <p>In the lists that are divided in two for the two directions (c.f <c>cipher</c>) it is possible
+ to change both directions at once:</p>
+ <code>
+2> ssh:chk_algos_opts(
+ [{preferred_algorithms,
+ [{cipher,['aes128-ctr']}
+ ]
+ }
+ ]).
+[{kex,['ecdh-sha2-nistp384','ecdh-sha2-nistp521',
+ 'ecdh-sha2-nistp256','diffie-hellman-group-exchange-sha256',
+ 'diffie-hellman-group16-sha512',
+ 'diffie-hellman-group18-sha512',
+ 'diffie-hellman-group14-sha256',
+ 'diffie-hellman-group14-sha1',
+ 'diffie-hellman-group-exchange-sha1']},
+ {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
+ 'ecdsa-sha2-nistp256','ssh-rsa','rsa-sha2-256',
+ 'rsa-sha2-512','ssh-dss']},
+ {cipher,[{client2server,['aes128-ctr']},
+ {server2client,['aes128-ctr']}]},
+ {mac,[{client2server,['hmac-sha2-256','hmac-sha2-512',
+ 'hmac-sha1']},
+ {server2client,['hmac-sha2-256','hmac-sha2-512',
+ 'hmac-sha1']}]},
+ {compression,[{client2server,[none,'[email protected]',zlib]},
+ {server2client,[none,'[email protected]',zlib]}]}]
+ </code>
+ <p>Note that both lists in <c>cipher</c> has been changed to the provided value (<c>'aes128-ctr'</c>).</p>
+ </section>
+
+ <section>
+ <title>Example 3</title>
+ <p>In the lists that are divided in two for the two directions (c.f <c>cipher</c>) it is possible
+ to change only one of the directions:</p>
+ <code>
+3> ssh:chk_algos_opts(
+ [{preferred_algorithms,
+ [{cipher,[{client2server,['aes128-ctr']}]}
+ ]
+ }
+ ]).
+[{kex,['ecdh-sha2-nistp384','ecdh-sha2-nistp521',
+ 'ecdh-sha2-nistp256','diffie-hellman-group-exchange-sha256',
+ 'diffie-hellman-group16-sha512',
+ 'diffie-hellman-group18-sha512',
+ 'diffie-hellman-group14-sha256',
+ 'diffie-hellman-group14-sha1',
+ 'diffie-hellman-group-exchange-sha1']},
+ {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
+ 'ecdsa-sha2-nistp256','ssh-rsa','rsa-sha2-256',
+ 'rsa-sha2-512','ssh-dss']},
+ {cipher,[{client2server,['aes128-ctr']},
+ {server2client,['[email protected]','aes256-ctr',
+ 'aes192-ctr','[email protected]','aes128-ctr',
+ 'aes128-cbc','3des-cbc']}]},
+ {mac,[{client2server,['hmac-sha2-256','hmac-sha2-512',
+ 'hmac-sha1']},
+ {server2client,['hmac-sha2-256','hmac-sha2-512',
+ 'hmac-sha1']}]},
+ {compression,[{client2server,[none,'[email protected]',zlib]},
+ {server2client,[none,'[email protected]',zlib]}]}]
+ </code>
+ </section>
+
+ <section>
+ <title>Example 4</title>
+ <p>It is of course possible to change more than one list:</p>
+ <code>
+4> ssh:chk_algos_opts(
+ [{preferred_algorithms,
+ [{cipher,['aes128-ctr']},
+ {mac,['hmac-sha2-256']},
+ {kex,['ecdh-sha2-nistp384']},
+ {public_key,['ssh-rsa']},
+ {compression,[{server2client,[none]},
+ {client2server,[zlib]}]}
+ ]
+ }
+ ]).
+[{kex,['ecdh-sha2-nistp384']},
+ {public_key,['ssh-rsa']},
+ {cipher,[{client2server,['aes128-ctr']},
+ {server2client,['aes128-ctr']}]},
+ {mac,[{client2server,['hmac-sha2-256']},
+ {server2client,['hmac-sha2-256']}]},
+ {compression,[{client2server,[zlib]},
+ {server2client,[none]}]}]
+
+ </code>
+ <p>Note that the ordering of the tuples in the lists didn't matter.</p>
+ </section>
+ </section>
+
+ <section>
+ <title>Modifying the default set: modify_algorithms</title>
+ <p>The option <c>preferred_algorithms</c> is complicated to use for adding or removing single algorithms. One has
+ to first list them with <c>ssh:default_algorithms()</c> and then do substitutions in the lists. A situation
+ when it might be useful to add an algorithm is when one need to use a supported but disabled one. An example
+ is the kex <c>'diffie-hellman-group1-sha1'</c> which nowadays is very unsecure and therefore disabled. It is
+ however still supported and might be used.</p>
+
+ <p>To facilitate addition or removal of algorithms the option <c>modify_algorithms</c> is available.
+ See the <seealso marker="ssh#option_modify_algorithms">Reference Manual</seealso> for details.</p>
+
+ <p>The option takes a list with instructions to append, prepend or remove algorithms:</p>
+ <code type="erl">
+{modify_algorithms, [{append, ...},
+ {prepend, ...},
+ {rm, ...}
+ ]}
+ </code>
+ <p>Each of the <c>...</c> can be a <c>algs_list()</c> as the argument to the <c>preferred_algorithms</c> option.</p>
+ <section>
+ <title>Example 5</title>
+ <p>As an example let's add the Diffie-Hellman Group1 first in the kex list. It is supported according to
+ <seealso marker="SSH_app#supported_algos">Supported algoritms</seealso>.</p>
+ <code type="erl">
+5> ssh:chk_algos_opts(
+ [{modify_algorithms,
+ [{prepend,
+ [{kex,['diffie-hellman-group1-sha1']}]
+ }
+ ]
+ }
+ ]).
+[{kex,['diffie-hellman-group1-sha1','ecdh-sha2-nistp384',
+ 'ecdh-sha2-nistp521','ecdh-sha2-nistp256',
+ 'diffie-hellman-group-exchange-sha256',
+ 'diffie-hellman-group16-sha512',
+ 'diffie-hellman-group18-sha512',
+ 'diffie-hellman-group14-sha256',
+ 'diffie-hellman-group14-sha1',
+ 'diffie-hellman-group-exchange-sha1']},
+ {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
+ 'ecdsa-sha2-nistp256','ssh-rsa','rsa-sha2-256',
+ 'rsa-sha2-512','ssh-dss']},
+ {cipher,[{client2server,['[email protected]',
+ 'aes256-ctr','aes192-ctr','[email protected]',
+ 'aes128-ctr','aes128-cbc','3des-cbc']},
+ {server2client,['[email protected]','aes256-ctr',
+ 'aes192-ctr','[email protected]','aes128-ctr',
+ 'aes128-cbc','3des-cbc']}]},
+ {mac,[{client2server,['hmac-sha2-256','hmac-sha2-512',
+ 'hmac-sha1']},
+ {server2client,['hmac-sha2-256','hmac-sha2-512',
+ 'hmac-sha1']}]},
+ {compression,[{client2server,[none,'[email protected]',zlib]},
+ {server2client,[none,'[email protected]',zlib]}]}]
+
+ </code>
+ <p>And the result shows that the Diffie-Hellman Group1 is added at the head of the kex list</p>
+ </section>
+
+ <section>
+ <title>Example 6</title>
+ <p>In next example, we also move the <c>'ecdh-sha2-nistp521'</c> to the end in the kex
+ list, that is, <c>append</c>.</p>
+ <code type="erl">
+6> ssh:chk_algos_opts(
+ [{modify_algorithms,
+ [{prepend,
+ [{kex, ['diffie-hellman-group1-sha1']}
+ ]},
+ {append,
+ [{kex, ['ecdh-sha2-nistp521']}
+ ]}
+ ]
+ }
+ ]).
+[{kex,['diffie-hellman-group1-sha1','ecdh-sha2-nistp384',
+ 'ecdh-sha2-nistp256','diffie-hellman-group-exchange-sha256',
+ 'diffie-hellman-group16-sha512',
+ 'diffie-hellman-group18-sha512',
+ 'diffie-hellman-group14-sha256',
+ 'diffie-hellman-group14-sha1',
+ 'diffie-hellman-group-exchange-sha1','ecdh-sha2-nistp521']},
+ {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
+ .....
+]
+ </code>
+ <p>Note that the appended algorithm is removed from its original place and then appended.</p>
+ </section>
+
+ <section>
+ <title>Example 7</title>
+ <p>In next example, we also move the <c>'ecdh-sha2-nistp521'</c> to the end in the kex
+ list, that is, <c>append</c>.</p>
+ <code type="erl">
+7> ssh:chk_algos_opts(
+ [{modify_algorithms,
+ [{prepend,
+ [{kex, ['diffie-hellman-group1-sha1']}
+ ]},
+ {append,
+ [{kex, ['ecdh-sha2-nistp521']}
+ ]}
+ ]
+ }
+ ]).
+[{kex,['diffie-hellman-group1-sha1','ecdh-sha2-nistp384',
+ 'ecdh-sha2-nistp256','diffie-hellman-group-exchange-sha256',
+ 'diffie-hellman-group16-sha512',
+ 'diffie-hellman-group18-sha512',
+ 'diffie-hellman-group14-sha256',
+ 'diffie-hellman-group14-sha1',
+ 'diffie-hellman-group-exchange-sha1','ecdh-sha2-nistp521']},
+ {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
+ .....
+]
+ </code>
+ <p>Note that the appended algorithm first is removed from its original place and then appended.</p>
+ </section>
+
+
+ <section>
+ <title>Example 8</title>
+ <p>In this example, we use both options (<c>preferred_algorithms</c> and <c>modify_algorithms</c>) and
+ also try to prepend an unsupported algorithm. Any unsupported algorithm is quietly removed.</p>
+ <code type="erl">
+8> ssh:chk_algos_opts(
+ [{preferred_algorithms,
+ [{cipher,['aes128-ctr']},
+ {mac,['hmac-sha2-256']},
+ {kex,['ecdh-sha2-nistp384']},
+ {public_key,['ssh-rsa']},
+ {compression,[{server2client,[none]},
+ {client2server,[zlib]}]}
+ ]
+ },
+ {modify_algorithms,
+ [{prepend,
+ [{kex, ['some unsupported algorithm']}
+ ]},
+ {append,
+ [{kex, ['diffie-hellman-group1-sha1']}
+ ]}
+ ]
+ }
+ ]).
+[{kex,['ecdh-sha2-nistp384','diffie-hellman-group1-sha1']},
+ {public_key,['ssh-rsa']},
+ {cipher,[{client2server,['aes128-ctr']},
+ {server2client,['aes128-ctr']}]},
+ {mac,[{client2server,['hmac-sha2-256']},
+ {server2client,['hmac-sha2-256']}]},
+ {compression,[{client2server,[zlib]},
+ {server2client,[none]}]}]
+
+ </code>
+ <p>It is of course questionable why anyone would like to use the both options together, but it is possible
+ if the needed.</p>
+ </section>
+
+
+
+ </section>
+
+</chapter>