Security isn't a binary system. You can be compromised by the NSA and still be effectively safe. In the case of putty, you are trying to prevent yourself from

Security isn't a binary system. You can be compromised by the NSA and still be effectively safe. In the case of putty, you are trying to prevent yourself from downloading a compromised binary that steals credentials and send them on to black hats, script kiddies, bot-net farmers, people that do other illegal stuff and want to cover their traces.