Security of CFB on a public channel Given a server that broadcasts all received data to all connected peers (except the sender), two participants can have an encrypted conversation using a self-synchronizing method like CFB and some error detection.

How secure is this given that anyone can not only listen but also paste in chosen text into the stream? Is it enough to prepend random padding to make it secure? Anything else that could be done?
There are two possible ways (I can think of) an attacker could mess you up here, but they both stem from very poor design. So I don't know how realistic they are.

Note: the following figure assumes a 64-bit blocksize




CFB is only self synchronizing against insertions/deletions of a specific length. The length is determined by the shift register. If the attacker can insert symbols that don't follow this length, then they can throw everything off in effect causing a DoS. Thus, when designing the system, you have to take into account the # of symbols that can be accidentially or maliciously inserted (deleted). If an attacker can inject down to a single symbol, then you'll have to set $k=1$ in the figure.
Another possible attack came to light in your comment about forcing CFB to behave like ECB. This can happen if you use the same instance of the block cipher to encrypt and decrypt. This is not how I would design the system. I'd use two separate instances of the block cipher, one for encrypting my messages to you and one for decrypting your messages to me. Furthermore, I would use different keys for each direction. If you do it this way, what the attacker injects will only mess up your decryption instance (which thanks to the self-synchronizing nature of the mode, is a problem from which you can recover).