SFTP21 error = #4 (Algorithm negotiation failed)


  • CuteFTP Pro┬« 8.3.2 or earlier 


When attempting a connection using the SFTP protocol, the following error is encountered:

STATUS:>   Getting listing ""...

STATUS:>   Initializing SFTP21 module...

STATUS:>   Connecting to SFTP server... (ip =

ERROR:>    Algorithm negotiation failed.

ERROR:>    The requested operation failed for an unknown reason.

ERROR:>    Can't connect to SFTP21 error = #4.

STATUS:>   SFTP21 connection closed.


The SFTP server is using an encryption algorithm or cipher that is not supported by CuteFTP Pro version 8.3.2 or earlier.  The most common ciphers causing this error are AES128 or AES256. (That is what Algorithm negotiation failed indicates.)

You can also get this error just from providing the wrong password. #4 error is a generic error saying connection/login failed. It just so happens in the example above, it was caused by bad ciphers.


To resolve this issue, upgrade to CuteFTP Pro version 8.3.3 or later.  Support for AES128 and AES256 ciphers was added in CuteFTP Pro 8.3.3.  If an upgrade is not an option, ask the administrator of the remote FTP server to allow use of a different cipher.  The default configuration of earlier versions of CuteFTP Pro provided support for TripleDES, ARCFOUR, CAST128, Twofish and Blowfish automatically.  The allowed ciphers are accessible from the SSH2 Security Settings screen in Global Options.