SSH is an encrypted Telnet protocol. Fritz!Box uses a program called dropbear to connect through SSH. I have added SFTP to my own version. SFTP is an encrypted FTP protocol. As you can see from the code you need also another busybox.
ATTENTION: Binary- recomended!
Security update 2012.55 — releases from 0.52 to 2011.54 are potentially vulnerable
to code execution as root by an authenticated user if a command="..." option is used
in authorized_keys. Release 2012.55 fixes the problem.