Why does SSH send 100 packets per keystroke?

I come from a world (yesteryear) where a computer had 1KB of RAM (ZX80). I've used links with modems rocking 1200 bps (1200 bits per second). I recall US Robotics modems getting to speeds of 56K - well that was mostly a fib worse than MS doing QA these days. Ooh I could chat with some bloke from Novell on Compuserve.

In 1994ish I was asked to look into this fancy new world wide web thing on the internet. I was working at a UK military college as an IT bod, I was 24. I had a Windows 3.1 PC. I telnetted into a local VAX, then onto the X25 PAD. I used JANET to get to somewhere in the US (NIST) and from there to Switzerland to where this www thing started off. I was using telnet and WAIS and Gopher and then I was apparently using something called "www".

I described this www thing as "a bit wank", which shows what a visionary I am!

This feels like a really niche use case for SSH. Exposing this more broadly could lead to set-it-and-forget-it scenarios and ultimately make someone less secure.

The proper fix would be adding option server-side to signal client it's not needed and have client side have option to accept or warn about that

For that matter, why does it need to be encrypted at all? What's the threat model?

If there really is a genuine need to encrypt and low latency is critical, consider using a stream cipher mode like AES-CTR to pregenerate keystream at times when the CPU is lightly loaded. Then when you need to encrypt (say) 128 bytes you peel off that many bytes of keystream and encrypt at close to zero cost. Just remember to also MAC the encrypted data, since AES-CTR provides zero integrity protection.

Versus...seeing there's a vulnerability, someone adding a one-line change to disable the vulnerable algorithm, compile, image update, test. And a lot less testing because you're not moving to a new version of the language / compiler.

The man has no practical experience in running a production network service, an ego the size of a small moon, and yet was a major contributor to a security protocol now in use by billions of people.

But hey, you can be a handbag designer and end up head of design at Apple soooooooo

https://news.ycombinator.com/item?id=37307708

Also, port 21 is often blocked.

Time flies

I've used Claude for debugging system behavior, and I kind of agree with the author. While Claude isn't always directly helpful (hallucinations remain, or at least outdated information), it helps me 1) spell out my understanding of the system (see [1]) and 2) help me keep momentum by supplying tasks.

[1] https://en.wikipedia.org/wiki/Rubber_duck_debugging

For people who don't feel like googling it:

1. You TCP_CORK a socket

2. You put data into it and the kernel buffers it

3. If you uncork the socket, or if the buffer hits MSS, the kernel sends the packet

Basically, the kernel waits until it has a full packet worth of data, or until you say you don't have any more data to send, and then it sends. Sort of an extreme TCP_YESDELAY.

See https://catonmat.net/tcp-cork for where I learned it all from.

I am aware of TCP_NODELAY (funny enough I recently posted about TCP_NODELAY to HN[1] when I was thinking about it for the same game that I wrote about here). But I think the latency hit from disabling it just doesn't work for me.

[1] https://news.ycombinator.com/item?id=46359120

(1) This sounds brittle. Are you really going to have a good mental model about what's secret when using ssh and reliably refrain from typing those things? Seems to kinda defeat the idea of securing the channel. Also, as a collection your activities might be more confidential to you than single inputs, or correlated with your other activities outside ssh, etc - it's hard to keep a mental model of this as well. Aka optimism is not a form of security.

(2) There isn't a reason to think this is a difficult attack that only a powerful adversary could mount. Seems like a college lab level thing to me. And very amenable to AI help as well. Also here optimism is not a form of security. It's a 25 year old attack[1] so there's a lot of existing research[2] around.

(3) Saving 100x bandwidth on single keystrokes on an internet dominated by video traffic just because it's 100x doesn't make sense. Also it's good to cultivate a mindset that steers away from trading off security in favour of trivial resource savings.

[1] https://www.usenix.org/conference/10th-usenix-security-sympo... (probably older stuff exists outside open literature)

[2] eg https://crzphil.github.io/posts/ssh-obfuscation-bypass/

That said, plenty of people disable the most useful security features of SSH, like verifying host key signatures, with no ill-effects (as far as they know). For the majority of users, using Telnet and unencrypted HTTP would make no difference, as nobody's trying to hack them, and who really cares about privacy anyway?

Did you know SSH has long-standing performance limitations due to its design that need patches to eliminate? It was never intended to be a high-performance tool. If you want really high performance, use Telnet. If you want real security, use SSH with all strong security options enabled plus a server using ContainerSSH with the OAuth2 plugin (SSH's keys are static, which can be captured and reused, which is bad). If you don't care either way, use SSH with the defaults.

One common secret that goes through a tty ssh connection is a sudo password. You are probably typing sudo command so without obfuscation the attacker can find out the sudo keystrokes, the command keystrokes and then the encrypted bytes of the password. They don't have the timing data to decode them as easily as the previous parts but if they record enough traffic they might be able to decrypt the password. But maybe they won't, because the ssh session key is probably different each time. Furthermore I don't know how many times they should capture your encrypted password to be able to decrypt it. Maybe it's unfeasible.

Anyway, in case of the sudo password, if the attacker gets it what would happen? The attacker is hopefully not able to get a shell into the server. If they do they have different ways to get root privileges.

By the way, I also copy and paste secrets from either the password manager or the clipboard, because nobody remembers long random strings. The only exceptions are the passwords of a few accounts.

Wouldn't this just change the packet interval from 20ms to 50ms? Or did you mean a constant stream of packets at 50ms intervals, nonstop?

I think the idea behind the current implementation is that the keystrokes are batched in 20ms intervals, with the optimization that a sufficiently long silence stops the chaff stream, so the keystroke timing is obfucated with an increased error bar of 20ms multiplied by number of chaff packets.

There are two issues with it:

- a primary is not a totality: if "security is the #1 consideration for SSH", that implies there's a #2, maybe even a #3 and so on consideration. So the question that follows becomes tautological: "but if the author doesn't need security, why use ssh?" -> surely for one or more of the #2, #3, etc. considerations, right?

- overabstraction (*): you ended up strawmanning the author. What they had issue with was keystroke timing obfuscation, which is a privacy feature. Timing attacks are (in part) a privacy concern, and privacy is a security concern, yes, but security is not just privacy concern, and privacy concerns are not just about timing attacks; these groups are not equal. For example, they might very well want the transmitted keypresses themselves to remain confidential, or they might very well want to retain cryptographic assurance of their integrity. These are security features they can continue to utilize by sticking with SSH.

All of this is to say, it's not even necessarily them using SSH for a hypothetical #2 or #3 (...etc...) reason, but likely because they still very much want to make use of large chunks of #1, which disabling keypress obfuscation does not actually rid SSH of, only at most weakens it in ways they clearly seem to be okay with.

(*) although if I zoom out enough, this is once again just "a primary is not a totality", just implicitly

This is technically incorrect, because Windows now includes SSH too!

The problems you run into when doing things you shouldn't do are often really fun.

[1] https://news.ycombinator.com/item?id=42342382

[2] https://news.ycombinator.com/item?id=37810144

[3] https://news.ycombinator.com/item?id=42674116

However, there are existing libraries for exactly this use case - see https://github.com/ValveSoftware/GameNetworkingSockets

I guess QUIC libraries would also work.

> I bet this mystery could gave been solved much quicker by simply looking at the packet capture in Wireshark.

For some people who are used to using Wireshark and who know what to look for, probably yes. For the vast majority of even technical people, probably not.

In my case, I did a packet capture of a single keystroke using tcpdump and imported it into Wireshark and I get just over 200 'Client: encrypted packet' and 'Server: encrypted packet' entries. Nothing useful there at all. If I tcpdump the entire SSH connection setup from scratch I get just as much useful information - nothing - but, oddly, fewer packets than my one keystroke triggered.

So yeah, I dislike LLMs entirely and dislike the reliance on LLMs that we see today, but in this case the author learned a lot of interesting stuff and shared it with us, whereas without LLMs he might have just shrugged and moved on.

Seems because dumping the session keys is not at all a common thing. It's just a matter of effort though - if someone put in the time to improve the SSH story for dissectors, most of the groundwork is there.

The author didn't, and used a general tool to their aid - why is that unfortunate?

My thinking was:

  * Yes, I clearly know what tcpdump is / how to capture network traffic
  * It has been several years since I have looked at a pcap
  * I don't have wireshark installed on this computer
  * I've done the thing where you decrypt TLS with wireshark exactly once, years ago, and I found it frustrating for reasons I can't remember[1]. Wasn't sure if I could do this with ssh
  * When I started investigating this, I didn't remotely think that ssh was the root cause. I thought it was a quirk of my game
  * I *did* make a client that printed out all the data it was receiving, but it was useless because it was operating at the wrong layer (e.g. it connected over SSH and logged the bytes SSH handed it)
  * I'm experimenting with Claude Code a lot because it has a lot of hype and I would like to form an opinion
  * Looking up flags is annoying
  * Being able to tell an agent "look at this pcap and tell me what you see" is *cool*

And the big leap here was realizing that this was my SSH client and not a quirk of my game. The time at which I would have read the SSH spec was after I captured traffic from a regular SSH session and observed the same pattern; before that I was thinking about the problem wrong.

I don't think that this is unfortunate. In fact, I think I got what I wanted here (a better sense of Claude Code's strengths and weaknesses). You're right that an alternative approach would have taught me different things, and that's a worthy goal too.

[1] I suspect this is because I was doing it for an old job and I had to figure out how to run some application with keys I controlled? It would have been easier here. I don't remember.

Grok, ChatGPT, and Claude all have these tics, and even the pro versions will use their signature phrases multiple times in an answer. I have to wonder if it's deliberate, to make detecting AI easier?

https://pshapira.net/2024/03/31/delving-into-delve/

It's nauseating.

Maybe it has something to do with your profile/memories?

Oh shoot! A shooting.

So the TL;DR of this post is: don't change this setting unless you know what you're doing.

I've been thinking about some stuff like this! Not being able to put my game behind Cloudflare[1] is a bummer. Substantial architectural overhead though.

> The idea of letting Claude loose on my crypto[graphy] implementation is about the most frightening thing I've heard of in a while [though libnss is so craptastic, I can't see how it would hurt in that case.]

I hear you, but FWIW the patch I was reverting was trivial (and it's also in the go crypto library, which is pretty easy to read). It's a couple-of-line change[2], and Claude did almost exactly what I would have done (I was tired and would have forgotten to shrink the handshake payload).

[1] This isn't strictly true, Cloudflare spectrum exists, but its pricing is an insane $1/GB last I checked.

[2] https://cs.opensource.google/go/x/crypto/+/833695f0a57b30373...

2023 discussion about it here.

When making this statement, are you taking into account that SSH encrypts the traffic by default?

I assume this is done for novelty. There is also Terminal coffee which is a coffee company that takes orders for delivery over SSH.

The extension is "ping@openssh.com." It shows up in the blog reliably for me across several browsers and devices.

[1] https://people.eecs.berkeley.edu/~daw/papers/ssh-use01.pdf

[0]: https://www.openbsd.org/plat.html

[1]: https://www.openbsd.org/landisk.html#hardware

But you cannot just sue a company because their network connected software doesn't work well on slow networks. Let alone a project like OpenSSH. It would be like me suing a game studio because my PC doesn't meet their listed minimum requirements to play the game.

If you want a “1990s” mode, add it yourself or pay some to do it for you.

This is funny to me, because ADSL used to be the fast thing, as opposed to dialup modems.