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Why does SSH send 100 packets per keystroke?

eieio.games

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Re: Why does SSH send 100 packets per keystroke?

#81

Post by eikenberry »

OneDeuxTriSeiGo wrote: post #56
eikenberry wrote: post #45

Earlier quoted context omitted.

It is to prevent timing attacks but there are many ssh use cases where it is 100% computer to computer communications where there is no key based timing attack possible.

There is an argument that if: - you are listening to an SSH session between devices - and you know what protocol is being talked over the connection (i.e. what they are talking about) - and the protocol is reasonably predictable then you gain enough information about the plaintext to start extracting information about the cipher and keys. It's a non-trivial attack by all means but it's totally feasible. Especially if…

I agree it is more nuanced than a simple 'good for computer-to-computer' and 'bad for person-to-computer'. I'm sure there are cases where both are wrong but I don't think that necessarily changes that it makes a reasonable baseline heuristic.

Re: Why does SSH send 100 packets per keystroke?

#82

Post by Veserv »

diath wrote: post #61
Veserv wrote: post #21

The really mysterious part is how ~10,000 packets per second costs ~20% of a core. That would mean SSH is bottlenecking in its code at ~50,000 packets per second per core which would be ~500 Mbps per core (assuming full packets) which is ludicrously slow. It is trivial to do 10x that packet per second rate. Is SSH really that poorly designed?

> It is trivial to do 10x that packet per second rate. When making this statement, are you taking into account that SSH encrypts the traffic by default?

I do not know where people get the idea that encryption is that slow. Standard AES hardware acceleration instructions do ~25 Gbps per core (on a 2023 CPU) which is ~50x that rate [1]. I have heard modern cores can do ~40-50 Gbps, but I have not been able to find any independent benchmarks of that. Even the Intel i5-2500, a CPU from 2011, averages ~10 Gbps which is ~20x that rate. Even unaccelerated encryption can do ~2-5 Gbps in pure software which is 4-10x the SSH rate.

And in this situation, the amount of encrypted payload in each packet is 36 bytes which is ~40x less than a full packet of ~1500 bytes. You would almost surely hit packet per second limits before you hit payload throughput limits at these small sizes.

Encryption is slow when compared to data throughput you can get with a properly designed transport stack, but that is because it is in comparison to 100 Gbps per core even with no hardware offload. Anything less than ~10 Gbps/1 million packets per second (ignoring other bottlenecks, so only the software transport is the limit) is not merely unoptimized, it is pessimized.

[1] https://calomel.org/aesni_ssl_performance.html

Re: Why does SSH send 100 packets per keystroke?

#83

Post by Romario77 »

OneDeuxTriSeiGo wrote: post #56
eikenberry wrote: post #45

Earlier quoted context omitted.

It is to prevent timing attacks but there are many ssh use cases where it is 100% computer to computer communications where there is no key based timing attack possible.

There is an argument that if: - you are listening to an SSH session between devices - and you know what protocol is being talked over the connection (i.e. what they are talking about) - and the protocol is reasonably predictable then you gain enough information about the plaintext to start extracting information about the cipher and keys. It's a non-trivial attack by all means but it's totally feasible. Especially if…

this only works for manually typed text, not computer to computer communication where you can't deduce much from what is being "typed" as it's not typed but produced by a program to which every letter is the same and there is no different delay in sending some letters (as people have when typing by hand)

Re: Why does SSH send 100 packets per keystroke?

#84

Post by Animats »

In 2023, ssh added keystroke timing obfuscation. The idea is that the speed at which you type different letters betrays some information about which letters you’re typing. So ssh sends lots of “chaff” packets along with your keystrokes to make it hard for an attacker to determine when you’re actually entering keys.

Now that's solving the problem the wrong way. If you really want that, send all typed characters at 50ms intervals, to bound the timing resolution.

Re: Why does SSH send 100 packets per keystroke?

#85

Post by PaulHoule »

starttoaster wrote: post #49
PaulHoule wrote: post #37

I find it disturbing. One thing you notice if you have ADSL is that some services are built as if slower connections matter and others are not. Like Google's voice and audio chat services work poorly but most of the others work well. Uploading images to Mastodon, Bluesky, Facebook, LinkedIn, Instagram and Nextdoor is reliable, but for Tumblr you have to try it twice. I don't what they are doing wrong but they are doi…

There's a good chance you have other options. Regardless of how you feel about the company's head, Starlink would probably be one of them, with likely better performance than you're dealing with on ADSL. 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 thei…

Hey, it is one thing to buy a new computer, it is another thing to ask people to move.

A better analogy is a bank redlining neighborhoods. The cost to run fiber to difficult rural locations pays itself easily if you look at a 25-year time span and is an order of magnitude less than building a new housing unit on the West Coast.

Re: Why does SSH send 100 packets per keystroke?

#86

Post by layer8 »

idontwantthis wrote: post #46

If security doesn’t matter then why not use telnet or something else besides ssh instead of forking a security library?

Telnet nowadays typically isn’t available by default for security reasons, and OP wants people to be able to play the game just by typing “ssh thegamehost”.

Re: Why does SSH send 100 packets per keystroke?

#87

Post by mkj »

eikenberry wrote: post #41
swiftcoder wrote: post #11

> Obviously forking go’s crypto library is a little scary, and I’m gonna have to do some thinking about how to maintain my little patch in a safe way This should really be upstreamed as an option on the ssh library. Its good to default to sending chaff in untrusted environments, but there are plenty of places where we might as well save the bandwidth

+1... Given how much SSH is used for computer-to-computer communication it seems like there really should be a way to disable this when it isn't necessary.

It looks like it is only applied for PTY sessions, which most computer-computer connections wouldn't be using.

https://github.com/openssh/openssh-portable/blob/d7950aca8ea...

Re: Why does SSH send 100 packets per keystroke?

#88

Post by PaulHoule »

bergen wrote: post #68
PaulHoule wrote: post #37

I find it disturbing. One thing you notice if you have ADSL is that some services are built as if slower connections matter and others are not. Like Google's voice and audio chat services work poorly but most of the others work well. Uploading images to Mastodon, Bluesky, Facebook, LinkedIn, Instagram and Nextdoor is reliable, but for Tumblr you have to try it twice. I don't what they are doing wrong but they are doi…

You just opened a huge nostalgia portal, never thought that Dreamweaver would still be around, I used that somewhere around 2003 I believe. Good memories

Frankly I wish there was an HTML editor that delivers on what it promised. I mean, markdown is almost as rife with edge cases as YAML and somehow the link syntax still eludes me. If we could “just” template by merging at the DOM level and had decent HTML editors the world would be a different place. But yeah, Adobe probably thinks Dreamweaver isn’t worth maintaining just as they seem to think Photoshop is barely worth maintaining (they keep adding AI features that sorta work but the foundations seem to be much worse than Illustrator)

Re: Why does SSH send 100 packets per keystroke?

#89

Post by thadt »

rubslopes wrote: post #58
eieio wrote: post #20

Earlier quoted context omitted.

Yes! While this post was written entirely by me, I wouldn't be surprised if I had "smoking gun" ready to go because I spent so much time debugging with Claude last night.

It's interesting how LLMs influence us, right? The opposite happened to me: I loved using em dashes, but AI ruined it for me.

I used to love using em dashes.

I still do - but I used to, too.

Re: Why does SSH send 100 packets per keystroke?

#90

Post by PaulHoule »

Refreeze5224 wrote: post #51
PaulHoule wrote: post #37

I find it disturbing. One thing you notice if you have ADSL is that some services are built as if slower connections matter and others are not. Like Google's voice and audio chat services work poorly but most of the others work well. Uploading images to Mastodon, Bluesky, Facebook, LinkedIn, Instagram and Nextdoor is reliable, but for Tumblr you have to try it twice. I don't what they are doing wrong but they are doi…

You're not ok with a security/privacy tool using defensive techniques because of ... the lack of fiber in Africa?

My backyard but people will take Africa more seriously than anywhere in the US 2 miles from the end of cable.
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