PHP

SFTP Set Allowed SSH Algorithms

Demonstrates the Chilkat SFtp.SetAllowedAlgorithms method, which configures the exact set of SSH algorithms permitted for the connection. A JsonObject supplies comma-separated allow-lists for the kex, hostKey, cipher, and mac categories. It must be called before Connect.

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

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 server. Otherwise the safer default is to let the library's ordering evolve as guidance changes.

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<?php

use Chilkat2\JsonObject;
use Chilkat2\SFtp;

// Demonstrates the SFtp.SetAllowedAlgorithms method, which configures the exact set of SSH
// algorithms permitted for the connection.  The only argument is a JsonObject.  It must be
// called before Connect.
//
// -------------------------------------------------------------------------------------------
// 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 can make 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.
// -------------------------------------------------------------------------------------------

$sftp = new SFtp();

$json = new JsonObject();

// The 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 (encryption):
//   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 (hash):
//   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 key-exchange, host-key, cipher (encryption), and mac (hash) algorithms, 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 before connecting.
try {
    $sftp->SetAllowedAlgorithms($json);

    $port = 22;
    $sftp->Connect('sftp.example.com', $port);
} catch (\Chilkat2\Exception $e) {
    echo $e->getMessage(), "\n";
    exit;
}

echo "Connected.\n";

$sftp->Disconnect();