Lua

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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Lua

local chilkat = require("chilkat")

-- 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.
-- -------------------------------------------------------------------------------------------

local sftp = chilkat.SFtp.new()

local json = chilkat.JsonObject.new()

-- 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.
local allowed_kex = "curve25519-sha256@libssh.org,ecdh-sha2-nistp256"
local allowed_hostKey = "ssh-ed25519,ecdsa-sha2-nistp256"
local allowed_cipher = "chacha20-poly1305@openssh.com,aes256-ctr"
local allowed_mac = "hmac-sha2-256,hmac-sha2-512"

json:update_string("kex", allowed_kex)
json:update_string("hostKey", allowed_hostKey)
json:update_string("cipher", allowed_cipher)
json:update_string("mac", allowed_mac)

-- Apply the allow-list before connecting.
local success = sftp:set_allowed_algorithms(json)
if not success then
    print(sftp.last_error_text)
    return
end

local port = 22
success = sftp:connect("sftp.example.com", port)
if not success then
    print(sftp.last_error_text)
    return
end

print("Connected.")

sftp:disconnect()