Can you tell me if there are any differences between POP3S and POP3 with STARTTLS?
- What's the safest?
- Are there any known attacks?
- If you could choose about them, which would you pick?
The only difference is that with old-style SSL, it is implied that you will start SSL negotiation immediately upon connection. With STARTTLS, you go through the same process, but only after issuing/receiving a STARTTLS command. The protocols are designed to throw away any information discovered or negotiated before the connection was secured, and start over again from scratch.