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[转载]http://www.rpatrick.com/tech/ssh-ssl/

SSH (Secure Shell) and SSL (Secure Sockets Layer) can both be used to secure communications across the Internet. This page tries to explain the differences between the two in easily understood terms.

SSL was designed to secure web sessions; it can do more, but that's the original intent.

SSH was designed to replace telnet and FTP; it can do more, but that's the original intent.

SSL is a drop-in with a number of uses. It front-ends HTTP to give you HTTPS. It can also do this for POP3, SMTP, IMAP, and just about any other well-behaved TCP application. It's real easy for most programmers who are creating network applications from scratch to just grab an SSL implementation and bundle it with their app to provide encryption when communicating across the network via TCP. Check out: stunnel.org.

SSH is a swiss-army-knife designed to do a lot of different things, most of which revolve around setting up a secure tunnel between hosts. Some implementations of SSH rely on SSL libraries - this is because SSH and SSL use many of the same encryption algorithms (i.e. TripleDES).

SSH is not based on SSL in the sense that HTTPS is based on SSL. SSH does much more than SSL, and they don't talk to each other - the two are different protocols, but have some overlap in how they accomplish similiar goals.

SSL by itself gives you nothing - just a handshake and encryption. You need an application to drive SSL to get real work done.

SSH by itself does a whole lot of useful stuff that allows users to perform real work. Two aspects of SSH are the console login (telnet replacement) and secure file transfers (ftp replacement), but you also get an ability to tunnel (secure) additional applications, enabling a user to run HTTP, FTP, POP3, and just about anything else THROUGH an SSH tunnel.

Without interesting traffic from an application, SSL does nothing. Without interesting traffic from an application, SSH brings up an encrypted tunnel between two hosts which allows you to get real work done through an interactive login shell, file transfers, etc.

Last comment: HTTPS does not extend SSL, it uses SSL to do HTTP securely. SSH does much more than SSL, and you can tunnel HTTPS through it! Just because both SSL and SSH can do TripleDES doesn't mean one is based on the other.