What is the difference between SSH and SSL? Which one is more secure, if you can compare them together?
Which has more potential vulnerabilities?
SSL and SSH both provide the cryptographic elements to build a tunnel for confidential data transport with checked integrity. For that part, they use similar techniques, and may suffer from the same kind of attacks, so they should provide similar security (i.e. good security) assuming they are both properly implemented. That both exist is a kind of NIH syndrome: the SSH developers should have reused SSL for the tunnel part (the SSL protocol is flexible enough to accommodate many variations, including not using certificates).
They differ on the things which are around the tunnel. SSL traditionally uses X.509 certificates for announcing server and client public keys; SSH has its own format. Also, SSH comes with a set of protocols for what goes inside the tunnel (multiplexing several transfers, performing password-based authentication within the tunnel, terminal management...) while there is no such thing in SSL, or, more accurately, when such things are used in SSL they are not considered to be part of SSL (for instance, when doing password-based HTTP authentication in a SSL tunnel, we say that it is part of "HTTPS", but it really works in a way similar to what happens with SSH).
Conceptually, you could take SSH and replace the tunnel part with the one from SSL. You could also take HTTPS and replace the SSL thing with SSH-with-data-transport and a hook to extract the server public key from its certificate. There is no scientific impossibility and, if done properly, security would remain the same. However, there is no widespread set of conventions or existing tools for that.
So we do not use SSL and SSH for the same things, but that's because of what tools historically came with the implementations of those protocols, not due to a security related difference. And whoever implements SSL or SSH would be well advised to look at what kind of attacks were tried on both protocols.
This isn't a reasonable comparison to make. SSL is a general method for protecting data transported over a network, whereas SSH is a network application for logging in and sharing data with a remote computer.
The transport layer protection in SSH is similar in capability to SSL, so which is "more secure" depends on what your specific threat model calls for and whether the implementations of each address the issues you're trying to deal with.
SSH then has a user authentication layer which SSL lacks (because it doesn't need it - SSL just needs to authenticate the two connecting interfaces which SSH can also do). In UTF-8 art:
| Nothing | | RFC4254 | Connection multiplexing
| Nothing | | RFC4252 | User authentication
| RFC5246 | | RFC4253 | Encrypted data transport
Regarding the issue of which there are more potential attacks against, it seems clear that SSH has a larger attack surface. But that's just because SSH has a whole application built into it: the attack surface of SSL + whatever application you need to provide cannot be compared because we don't have enough information.
From a strict cryptographic point of view, they both provide authenticated encryption, but in two different ways.
SSH uses the so-called Encrypt-and-MAC, that is the ciphered message is juxtaposed to a message authentication code (MAC) of the clear message to add integrity. This is not proven to be always fully secure (even if in practical cases it should be enough).
SSL uses MAC-then-Encrypt: a MAC is juxtaposed to the clear text, then they are both encrypted. This is not the best either, as with some block cipher modes parts of the MAC can be guessable and reveal something on the cipher. This led to vulnerabilities in TLS 1.0 (BEAST attack).
So they have both potential theoretical weaknesses. The strongest method is Encrypt-then-MAC (add a MAC of the ciphered message), which is implemented, e.g., in IPsec ESP.
I think there is one aspect of this comparison that was overlooked. user185 came close but didn't quite get there. I agree that these are apples and oranges and feel a better apples to apples comparison to be HTTPS and SSH. HTTPS and SSH utilize different layers of the OSI model and therefore encrypt the data at different times in the transmission. Then the real questions one should be asking would be along the lines of when is this data encrypted and unencrypted during the transmission. This will reveal your potential attack surfaces. With HTTPS, once the packet is received by a device in the destination network (Web Server, Border Router, load balancer, etc...)it is un-encrypted and spends the rest of its journey in plain text. Many would argue that this is not a big deal since the traffic is internal at this time, but if the payload contains sensitive data, it is being stored un-encrypted in the log files of every network device it passes through until it gets to its final destination. With SSH, typically, the destination device is specified and the transmission is encrypted until it reaches this device. There are ways of re-encrypting the HTTPS data but these are extra steps that most forget to take when implementing an HTTPS solution in their environment.
SSL is a protocol layer that is abstracted from the content being tunneled. SSH is a secure version of shell (SH), it was not designed to contain an abstract layer underneath it, it was designed specifically to carry shell traffic. So though crypto operations are used in both, and those crypto operations could even be the same, the purpose, and therefor overall design is quite different.
Keep in mind there are specific differences (as have been cited above), but most if not all of those differences are rooted in the purpose of the different protocols.
If you really looking for SSH vs SSL(TLS) then the answer is SSH.
For one reason why SSH wins over SSL is the way it performs Authentication. Because of this reason when using FTP use SSH protocol (SFTP) rather then FTPS (FTP over SSL).
SSH is used in corporate networks for:
- providing secure access for users and automated processes
- interactive and automated file transfers
- issuing remote commands
The problem is two-fold, it's not just the strength and weakness of the encryption. But it's the ease and convenience of the delivery. So from business perspective SSL/TLS is is more convenient and easier because it just requires a browser and either a public or private Cert.
And SSH requires either the application or thin client installed to use it. That is more of a problem from a user Internet client perspective and support.