I have connected to a server via SFTP using FileZilla and accepted adding the server's SSH key to the key cache in FileZilla.
How can I extract this cached key to a keyfile so that may use it through other SFTP applications that require a keyfile be made available?
I have not been able to find anything in the FileZilla documentation related to this.
Unless I am misunderstanding you: you don't need to.
If you connect to the server with another application (ie: PuTTY) and it has not seen the server before then you will be prompted to accept the key.
I see why you might want to do this, but each application could have it's own way to store keys.
If you use the standard openssh console client (cygwin or from linux), host keys are stored, one-per-line, in ~/.ssh/known_hosts. From there, it's a simple matter of figuring out which bit of that host key is needed for your library.
Putty also stores host keys, but it appears to encode them in hex. Those can be found at HKCUR\Software\SimonTatham\PuTTY\SshHostKeys
Thomas was correct. FileZilla piggybacks on PuTTY's PSFTP program and stores the saved keys encoded in a hex format at the registry key he listed (HKCUR\Software\SimonTatham\PuTTY\SshHostKeys). I needed the key in known_hosts format, so I has able to install a windows version of openssh at his recommendation and used the ssh-keyscan tool to hit the server and save the key info out in the correct format:
ssh-keyscan -t rsa <my_ftp_ip_address> > c:\known_hosts
ssh-keyscan -t dsa <my_ftp_ip_address> > c:\known_hosts
Thank you Thomas and SO!