What is the difference between SSH and SSL? Which one is more secure, if you can compare them together?
Which has more potential vulnerabilities?
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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 poorly-drawn UTF-8 art:
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.