CkPython

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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import sys
import chilkat

success = False

#  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 = chilkat.CkSFtp()

json = chilkat.CkJsonObject()

#  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.
success = sftp.SetAllowedAlgorithms(json)
if (success == False):
    print(sftp.lastErrorText())
    sys.exit()

port = 22
success = sftp.Connect("sftp.example.com",port)
if (success == False):
    print(sftp.lastErrorText())
    sys.exit()

print("Connected.")

sftp.Disconnect()