The SFTP extension to get a hash (md5, sha1, ..) from a file (or part of a file) is very useful. There is already an open source implementation in ProFTP, but a implementation in openSSH would be better. You need to know the C programming language to solve this.

It would be great, if someone could implement this. First the server needs to be implemented. Then the scp command could be improved to copy only changed files.