3 Configuring algorithms in SSH
3.1 Introduction
To fully understand how to configure the algorithms, it is essential to have a basic understanding of the SSH protocol and how OTP SSH app handles the corresponding items
The first subsection will give a short background of the SSH protocol while later sections describes the implementation and provides some examples
Basics of the ssh protocol's algorithms handling
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 RFC 4253, "The Secure Shell (SSH) Transport Layer Protocol" for details.
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.
There are five lists exchanged in the connection setup. Three of them are also divided in two directions, to and from the server.
The lists are (named as in the SSH application's options):
 kex

Key exchange.
An algorithm is selected for computing a secret encryption key. Among examples are: the old nowadays week 'diffiehellmangroupexchangesha1' and the very strong and modern 'ecdhsha2nistp512'.
 public_key

Server host key
The asymetric encryption algorithm used in the server's privatepublic host key pair. Examples include the wellknown RSA 'sshrsa' and elliptic curve 'ecdsasha2nistp521'.
 cipher

Symetric cipher algorithm used for the payload encryption. This algorithm will use the key calculated in the kex phase (together with other info) to genereate the actual key used. Examples are trippleDES '3descbc' and one of many AES variants 'aes192ctr'.
This list is actually two  one for each direction servertoclient and clienttoserver. Therefore it is possible but rare to have different algorithms in the two directions in one connection.
 mac

Message authentication code
"Check sum" of each message sent between the peers. Examples are SHA 'hmacsha1' and SHA2 'hmacsha2512'.
This list is also divided into two for the both directions
 compression

If and how to compress the message. Examples are none, that is, no compression and zlib.
This list is also divided into two for the both directions
The SSH app's mechanism
The set of algorithms that the SSH app uses by default depends on the algoritms supported by the:

crypto app,

The cryptolib OTP is linked with, usally the one the OS uses, probably OpenSSL,

and finaly what the SSH app implements
Due to this, it impossible to list in documentation what algorithms that are available in a certain installation.
There is an important command to list the actual algorithms and their ordering: ssh:default_algorithms/0.
0> ssh:default_algorithms(). [{kex,['ecdhsha2nistp384','ecdhsha2nistp521', 'ecdhsha2nistp256','diffiehellmangroupexchangesha256', 'diffiehellmangroup16sha512', 'diffiehellmangroup18sha512', 'diffiehellmangroup14sha256', 'diffiehellmangroup14sha1', 'diffiehellmangroupexchangesha1']}, {public_key,['ecdsasha2nistp384','ecdsasha2nistp521', 'ecdsasha2nistp256','sshrsa','rsasha2256', 'rsasha2512','sshdss']}, {cipher,[{client2server,['aes256gcm@openssh.com', 'aes256ctr','aes192ctr','aes128gcm@openssh.com', 'aes128ctr','aes128cbc','3descbc']}, {server2client,['aes256gcm@openssh.com','aes256ctr', 'aes192ctr','aes128gcm@openssh.com','aes128ctr', 'aes128cbc','3descbc']}]}, {mac,[{client2server,['hmacsha2256','hmacsha2512', 'hmacsha1']}, {server2client,['hmacsha2256','hmacsha2512', 'hmacsha1']}]}, {compression,[{client2server,[none,'zlib@openssh.com',zlib]}, {server2client,[none,'zlib@openssh.com',zlib]}]}]
To change the algorithm list, there are two options which can be used in ssh:connect/2,3,4 and ssh:daemon/2,3. The options could of course be used in all other functions that initiates connections.
The options are preferred_algorithms and modify_algorithms. The first one replaces the default set, while the latter modifies the default set.
3.2 Replacing the default set: preferred_algorithms
See the Reference Manual for details
Here follows a series of examples ranging from simple to more complex.
To forsee the effect of an option there is an experimental function ssh:chk_algos_opts(Opts). It mangles the options preferred_algorithms and modify_algorithms in the same way as ssh:dameon, ssh:connect and their friends does.
Example 1
Replace the kex algorithms list with the single algorithm 'diffiehellmangroup14sha256':
1> ssh:chk_algos_opts( [{preferred_algorithms, [{kex, ['diffiehellmangroup14sha256']} ] } ]). [{kex,['diffiehellmangroup14sha256']}, {public_key,['ecdsasha2nistp384','ecdsasha2nistp521', 'ecdsasha2nistp256','sshrsa','rsasha2256', 'rsasha2512','sshdss']}, {cipher,[{client2server,['aes256gcm@openssh.com', 'aes256ctr','aes192ctr','aes128gcm@openssh.com', 'aes128ctr','aes128cbc','3descbc']}, {server2client,['aes256gcm@openssh.com','aes256ctr', 'aes192ctr','aes128gcm@openssh.com','aes128ctr', 'aes128cbc','3descbc']}]}, {mac,[{client2server,['hmacsha2256','hmacsha2512', 'hmacsha1']}, {server2client,['hmacsha2256','hmacsha2512', 'hmacsha1']}]}, {compression,[{client2server,[none,'zlib@openssh.com',zlib]}, {server2client,[none,'zlib@openssh.com',zlib]}]}]
Note that the unmentioned lists (public_key, cipher, mac and compression) are unchanged.
Example 2
In the lists that are divided in two for the two directions (c.f cipher) it is possible to change both directions at once:
2> ssh:chk_algos_opts( [{preferred_algorithms, [{cipher,['aes128ctr']} ] } ]). [{kex,['ecdhsha2nistp384','ecdhsha2nistp521', 'ecdhsha2nistp256','diffiehellmangroupexchangesha256', 'diffiehellmangroup16sha512', 'diffiehellmangroup18sha512', 'diffiehellmangroup14sha256', 'diffiehellmangroup14sha1', 'diffiehellmangroupexchangesha1']}, {public_key,['ecdsasha2nistp384','ecdsasha2nistp521', 'ecdsasha2nistp256','sshrsa','rsasha2256', 'rsasha2512','sshdss']}, {cipher,[{client2server,['aes128ctr']}, {server2client,['aes128ctr']}]}, {mac,[{client2server,['hmacsha2256','hmacsha2512', 'hmacsha1']}, {server2client,['hmacsha2256','hmacsha2512', 'hmacsha1']}]}, {compression,[{client2server,[none,'zlib@openssh.com',zlib]}, {server2client,[none,'zlib@openssh.com',zlib]}]}]
Note that both lists in cipher has been changed to the provided value ('aes128ctr').
Example 3
In the lists that are divided in two for the two directions (c.f cipher) it is possible to change only one of the directions:
3> ssh:chk_algos_opts( [{preferred_algorithms, [{cipher,[{client2server,['aes128ctr']}]} ] } ]). [{kex,['ecdhsha2nistp384','ecdhsha2nistp521', 'ecdhsha2nistp256','diffiehellmangroupexchangesha256', 'diffiehellmangroup16sha512', 'diffiehellmangroup18sha512', 'diffiehellmangroup14sha256', 'diffiehellmangroup14sha1', 'diffiehellmangroupexchangesha1']}, {public_key,['ecdsasha2nistp384','ecdsasha2nistp521', 'ecdsasha2nistp256','sshrsa','rsasha2256', 'rsasha2512','sshdss']}, {cipher,[{client2server,['aes128ctr']}, {server2client,['aes256gcm@openssh.com','aes256ctr', 'aes192ctr','aes128gcm@openssh.com','aes128ctr', 'aes128cbc','3descbc']}]}, {mac,[{client2server,['hmacsha2256','hmacsha2512', 'hmacsha1']}, {server2client,['hmacsha2256','hmacsha2512', 'hmacsha1']}]}, {compression,[{client2server,[none,'zlib@openssh.com',zlib]}, {server2client,[none,'zlib@openssh.com',zlib]}]}]
Example 4
It is of course possible to change more than one list:
4> ssh:chk_algos_opts( [{preferred_algorithms, [{cipher,['aes128ctr']}, {mac,['hmacsha2256']}, {kex,['ecdhsha2nistp384']}, {public_key,['sshrsa']}, {compression,[{server2client,[none]}, {client2server,[zlib]}]} ] } ]). [{kex,['ecdhsha2nistp384']}, {public_key,['sshrsa']}, {cipher,[{client2server,['aes128ctr']}, {server2client,['aes128ctr']}]}, {mac,[{client2server,['hmacsha2256']}, {server2client,['hmacsha2256']}]}, {compression,[{client2server,[zlib]}, {server2client,[none]}]}]
Note that the ordering of the tuples in the lists didn't matter.
3.3 Modifying the default set: modify_algorithms
A situation where it might be useful to add an algorithm is when one need to use a supported but disabled one. An example is the 'diffiehellmangroup1sha1' which nowadays is very unsecure and therefore disabled. It is however still supported and might be used.
The option preferred_algorithms may be complicated to use for adding or removing single algorithms. First one has to list them with ssh:default_algorithms() and then do changes in the lists.
To facilitate addition or removal of algorithms the option modify_algorithms is available. See the Reference Manual for details.
The option takes a list with instructions to append, prepend or remove algorithms:
{modify_algorithms, [{append, ...}, {prepend, ...}, {rm, ...} ]}
Each of the ... can be a algs_list() as the argument to the preferred_algorithms option.
Example 5
As an example let's add the DiffieHellman Group1 first in the kex list. It is supported according to Supported algoritms.
5> ssh:chk_algos_opts( [{modify_algorithms, [{prepend, [{kex,['diffiehellmangroup1sha1']}] } ] } ]). [{kex,['diffiehellmangroup1sha1','ecdhsha2nistp384', 'ecdhsha2nistp521','ecdhsha2nistp256', 'diffiehellmangroupexchangesha256', 'diffiehellmangroup16sha512', 'diffiehellmangroup18sha512', 'diffiehellmangroup14sha256', 'diffiehellmangroup14sha1', 'diffiehellmangroupexchangesha1']}, {public_key,['ecdsasha2nistp384','ecdsasha2nistp521', 'ecdsasha2nistp256','sshrsa','rsasha2256', 'rsasha2512','sshdss']}, {cipher,[{client2server,['aes256gcm@openssh.com', 'aes256ctr','aes192ctr','aes128gcm@openssh.com', 'aes128ctr','aes128cbc','3descbc']}, {server2client,['aes256gcm@openssh.com','aes256ctr', 'aes192ctr','aes128gcm@openssh.com','aes128ctr', 'aes128cbc','3descbc']}]}, {mac,[{client2server,['hmacsha2256','hmacsha2512', 'hmacsha1']}, {server2client,['hmacsha2256','hmacsha2512', 'hmacsha1']}]}, {compression,[{client2server,[none,'zlib@openssh.com',zlib]}, {server2client,[none,'zlib@openssh.com',zlib]}]}]
And the result shows that the DiffieHellman Group1 is added at the head of the kex list
Example 6
In this example, we in put the 'diffiehellmangroup1sha1' first and also move the 'ecdhsha2nistp521' to the end in the kex list, that is, append it.
6> ssh:chk_algos_opts( [{modify_algorithms, [{prepend, [{kex, ['diffiehellmangroup1sha1']} ]}, {append, [{kex, ['ecdhsha2nistp521']} ]} ] } ]). [{kex,['diffiehellmangroup1sha1','ecdhsha2nistp384', 'ecdhsha2nistp256','diffiehellmangroupexchangesha256', 'diffiehellmangroup16sha512', 'diffiehellmangroup18sha512', 'diffiehellmangroup14sha256', 'diffiehellmangroup14sha1', 'diffiehellmangroupexchangesha1','ecdhsha2nistp521']}, {public_key,['ecdsasha2nistp384','ecdsasha2nistp521', ..... ]
Note that the appended algorithm is removed from its original place and then appended to the same list.
Example 7
In this example, we use both options (preferred_algorithms and modify_algorithms) and also try to prepend an unsupported algorithm. Any unsupported algorithm is quietly removed.
7> ssh:chk_algos_opts( [{preferred_algorithms, [{cipher,['aes128ctr']}, {mac,['hmacsha2256']}, {kex,['ecdhsha2nistp384']}, {public_key,['sshrsa']}, {compression,[{server2client,[none]}, {client2server,[zlib]}]} ] }, {modify_algorithms, [{prepend, [{kex, ['some unsupported algorithm']} ]}, {append, [{kex, ['diffiehellmangroup1sha1']} ]} ] } ]). [{kex,['ecdhsha2nistp384','diffiehellmangroup1sha1']}, {public_key,['sshrsa']}, {cipher,[{client2server,['aes128ctr']}, {server2client,['aes128ctr']}]}, {mac,[{client2server,['hmacsha2256']}, {server2client,['hmacsha2256']}]}, {compression,[{client2server,[zlib]}, {server2client,[none]}]}]
It is of course questionable why anyone would like to use the both these options together, but it is possible if an unforeseen need should arise.