Perl

SSH Set Allowed Algorithms

Demonstrates explicitly setting the algorithms allowed during SSH connection negotiation. A JsonObject supplies comma-separated allow-lists for the kex, hostKey, cipher, and mac categories, in order of preference, and is applied before connecting.

Important: You typically should not set allowed algorithms explicitly. By default Chilkat orders algorithms according to best practices and accounts for known vulnerabilities such as the Terrapin attack.

Background: SSH negotiates a mutually supported algorithm from each category at connection time, and pinning that choice makes an application brittle: if a server later drops a weak algorithm, or you point the code at a different server, the two sides may fail to agree and the connection breaks. The legitimate reasons to override are a security policy mandating specific algorithms, or a compatibility problem with an old device. Otherwise the safer default is to let the library's ordering evolve as cryptographic guidance changes.

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Perl

use chilkat();

$success = 0;

#  This example requires the Chilkat API to have been previously unlocked.
#  See Global Unlock Sample for sample code.

#  Demonstrates explicitly setting the algorithms allowed during SSH connection negotiation.
#  
#  -------------------------------------------------------------------------------------------
#  Note: You typically should NOT explicitly set allowed algorithms.
#  By default, Chilkat orders algorithms according to best practices and accounts for known
#  vulnerabilities such as the "Terrapin Attack".  Hard-coding algorithms makes an application
#  brittle over time: if a server later changes its allowed algorithms, or if you connect to a
#  different server, the client and server may fail to agree on a mutually supported set.
#  -------------------------------------------------------------------------------------------

$ssh = chilkat::CkSsh->new();

$json = chilkat::CkJsonObject->new();

#  Algorithms supported by Chilkat, by category:
#  
#  Key-exchange:  curve25519-sha256, curve25519-sha256@libssh.org, ecdh-sha2-nistp256,
#    ecdh-sha2-nistp384, ecdh-sha2-nistp521, diffie-hellman-group14-sha256,
#    diffie-hellman-group16-sha512, diffie-hellman-group18-sha512,
#    diffie-hellman-group-exchange-sha256, diffie-hellman-group1-sha1,
#    diffie-hellman-group14-sha1, diffie-hellman-group-exchange-sha1
#  
#  Host key:  ssh-ed25519, ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521,
#    rsa-sha2-256, rsa-sha2-512, ssh-rsa, ssh-dss
#  
#  Cipher:  chacha20-poly1305@openssh.com, aes128-ctr, aes256-ctr, aes192-ctr, aes128-cbc,
#    aes256-cbc, aes192-cbc, aes128-gcm@openssh.com, aes256-gcm@openssh.com, twofish256-cbc,
#    twofish128-cbc, blowfish-cbc
#  
#  MAC:  hmac-sha2-256, hmac-sha2-512, hmac-sha2-256-etm@openssh.com,
#    hmac-sha2-512-etm@openssh.com, hmac-sha1-etm@openssh.com, hmac-sha1, hmac-ripemd160,
#    hmac-sha1-96, hmac-md5

#  List the allowed algorithms for each category, in order of preference.
$allowed_kex = 'curve25519-sha256@libssh.org,ecdh-sha2-nistp256';
$allowed_hostKey = "ssh-ed25519,ecdsa-sha2-nistp256";
$allowed_cipher = 'chacha20-poly1305@openssh.com,aes256-ctr';
$allowed_mac = "hmac-sha2-256,hmac-sha2-512";

$json->UpdateString("kex",$allowed_kex);
$json->UpdateString("hostKey",$allowed_hostKey);
$json->UpdateString("cipher",$allowed_cipher);
$json->UpdateString("mac",$allowed_mac);

#  Apply the allow-list.  This must be done before connecting.
$success = $ssh->SetAllowedAlgorithms($json);
if ($success == 0) {
    print $ssh->lastErrorText() . "\r\n";
    exit;
}

$port = 22;
$success = $ssh->Connect("ssh.example.com",$port);
if ($success == 0) {
    print $ssh->lastErrorText() . "\r\n";
    exit;
}

print "Connected." . "\r\n";

$ssh->Disconnect();