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  • So we did a video a while back on the SCP exploit in the secure copy, and I though it would be just a

  • sort of interesting to talk/have a look in a bit more detail about how SSH works.

  • SSH stands for 'Secure Shell' and it first appeared in the mid 90's as a sort of replacement for/way of connecting to a remote machine over the internet.

  • Up until that time the technologies you had to connect to a remote machine, Telnet, rlogin, RSH and so on, worked fine

  • but they transmitted all the data in the clear over the network. So if you logged into a remote machine

  • anyone with a packet sniffer between you and the remote machine could see everything you were doing on there.

  • When these protocols first appeared that wasn't a problem because the machines were probably only networked within their computer department of a university

  • or a company so the people that had access to do that were people who worked there and probably the system administrators who had access to these things.

  • Anyway, but as the machines got networked to other networks and you started to build the internet, if you had access to the network

  • Any network that the data was travelling over you could sniff the packets.

  • And see any of the data that was being transmitted, including passwords and things.

  • And so in 1995, Tatu Ylonen in Finland (I apologies if I pronounced that wrongly) was concerned about this

  • so he developed a protocol SSH 'Secure Shell' to sort of encrypt the data so that you couldn't sort of see how/what was being sent over the wire

  • You could see that data was being transmitted across, you could see how much data

  • You could see the frequency of it to a certain extent, but you couldn't see what the data was, so you could do

  • certain types of analysis to see whats happening, but you couldn't see the actual data.

  • And so SSH was developed as a way of encrypting the connection between two machines

  • but it actually does a lot more than that, because when you SSH to another machine

  • The first thing that happens is that you open up a TCP connection between those two machines

  • like any standard things.

  • Although it doesn't have to be a TCP connection. You can actually specify that SSH uses any sort of reliable

  • network connection to make that, so you could theoretically run it over an RS-232 connection.

  • You could run it over the top of web sockets and things like that.

  • And I know people who have done that sort of thing.

  • So you've got a reliable transport between the two machines and so SSH is sending data over there.

  • And what SSH does is it breaks the data down into a series of packets.

  • And just like any packet transfer, these things have a few fields at the beginning.

  • So at the top you have something that tells you how big the packet is, so you have the packet

  • length, there are 4 bytes saying the packet is 'this big'. After that, you have another byte, which tells you

  • how much padding you've got. So you've got the packet length. You've got how much padding there is

  • and then you've got the data you want - the payload.

  • Then you have the padding that follows that, and so what you do, with each packet after the payload

  • the data that you want to send that's part of the connection, you add a small amount of padding.

  • Just random bytes. They don't mean anything, but they sort of force the encryption to sort of make

  • it harder to detect what's going on because you've got random data in there.

  • [See some of Mike's videos, for more details on why you might want to do that]

  • And then, you have some sort of message authentication code there, so that

  • you know that the data here hasn't been 'monkied' around with

  • and it's actually what has been sent.

  • You can the apply compression if you want to - to the payload

  • so you can compress the payload using standard compression algorithms, such as zlib, or something to compress the data.

  • And then, the whole of that packet (excluding the length) is then encrypted.

  • The algorithms used for the encryption and for the message authentication code

  • are selected by the server and the client. They establish that at the beginning, which ones they offer

  • which ones they want to use and things, so they are established by the client and the server, so vary from connection to connection.

  • And you can offer many and you can choose the one that is perhaps best for that connection.

  • We'll ignore that for the purposes of this video.

  • So we leave the packet length unencrypted, because obviously we need to know how much data is coming.

  • We encrypt the padding length. We encrypt the payload. We encrypt the padding

  • and we send the packet out over the network. At the other end, that's decrypted by the server, and it then knows it's got the packet of data.

  • And it can piece it back together.

  • And then the same thing is done for this packet, and this packet, and every packet.

  • And the encryption is done, so that it's continuous.

  • You start encrypting this one with the vectors from this one, and so on, so it sort of encrypts it one after the other.

  • At the server, or the client, which receives this you just reverse that process, so you decrypt the packets

  • you decompress the payload, and you can extract the data and sort of stitch it back together.

  • So you start off, you've got your TCP connection and that's unencrypted, and on top of that you've got these

  • packets, which have been encrypted by the SSH protocol flowing on top of that.

  • But that's not the end of the story. That's just encrypting the packets.

  • It's/that's not the connection that say if you're SSH and that's not you connection to the remote machine.

  • On top of that, you then open a series of channels that you send the data over.

  • And the reason for that is it enables you to multiplex multiple connections over that thing

  • so when I start a connection to my machine, it's creating a channel on top of this.

  • So if this is one computer over here, and this is another one over here, we're creating this channel here for

  • our shell connection where we can communicate and control things.

  • But this is actually being actually being represented by a connection through the SSH packets.

  • And we could have multiple ones of these between the same machines. Or we could have different types

  • of connections, so for example SSH also allows you to forward any sort of TCP connection over there

  • so you can connect to a remote service via the SSH tunnel, so you can tunnel that through

  • and its encrypted and then sent out at the other end. And if we make the connection with

  • verbose mode on, we can actually see it's setting up that connection. So what we've got here, it's reading the

  • configuration data, its local protocol string is.

  • So it connects to the remote machine. It sends back saying I support this version of SSH.

  • I've sent what my version is. We've then exchanged information about our compatibility

  • what method it is we are going to use for encrypting the messages - the packets over the network. And so on.

  • We've checked that we know the machine, and then we get down to here, we're now trying to authenticate

  • so it's now establishing, ok this is this user on that machine so I can put my password in.

  • And we're now connected, but if we look here 'Authentication succeeded'

  • we are authenticated to the remote machine and you see here it has created a new channel.

  • So it's creating a connection, which again is just going to be 'I send bytes down here' they appear at the other end.

  • You send bytes the other way, they come back to us over this channel.

  • And it's then setting up what type of connection it is. It's setting environment variables and things.

  • And this is done by sending packets of data wrapped up inside the SSH packets which establish that.

  • Once that's done, we can then communicate and have our interactive session.

  • So SSH is not just a sort of way of connecting your shell. It's actually got layers of things on top of that

  • which enable you to do lots of interesting things.

  • So you can have multiple connections made over this thing.

  • You don't just have to have one and there are ways you can set SSH up, so that actually, you make

  • the connection, and it stays connected. When you've finished, so if you make then make another

  • connection to the same server, it reuses that existing connection and multiplexes the two connections in between things.

  • It can start and stop forwarded TCP connections to other addresses, in both directions

  • while the connection is running, just by sending different things over this connection.

  • The reason why it's got all this functionality is that often

  • (particularly in the Unix environment, or the Linux environment) when you are connecting to to other

  • machines, you may just want a remote shell connection, where you just want to type commands and things

  • that's great. But also, the way the Unix X windows graphical user interface works

  • is that has a/normally done over a connection usually TCP/IP to port 6000, although it can be done over other things as well

  • and that could then be forwarded as another connection, so that you could login to another machine and send

  • the graphics commands back encrypted as well.

  • Occasionally you may want to connect to a service behind a firewall, and you can do that by tunnelling that

  • tunnelling it through the SSH connection. So there's lots of reasons why it was built this way.

  • But it's a nice feature, and I know I've been using this now for 20 odd years, 24 years probably now.

  • But actually you're still discovering new things you can do with it. For example

  • that you can keep a connection alive beyond when you've just disconnected from the shell

  • so they can reuse that TCP connection. The lower level bits, when you create a new connection on top.

So we did a video a while back on the SCP exploit in the secure copy, and I though it would be just a

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安全殼如何工作(SSH) - Computerphile (How Secure Shell Works (SSH) - Computerphile)

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    林宜悉 發佈於 2021 年 01 月 14 日
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