A2 - Stop Typing Your Password

You are going to live on Onyx

Every project from P1 on runs on Onyx, and so do A4 and X1. That is hundreds of logins this semester, and right now every one of them starts with a password.

Typing it is slow, and it is also the weakest way to prove who you are. Today the door learns to recognize you, and along the way you see what SSH is doing every time you connect.

Before SSH, the password was on the wire

telnet and rlogin sent everything in plain text, including your password. Anyone with a packet sniffer anywhere on the path could read it.

In the spring of 1995, after a password sniffing attack on his university's network, Tatu Ylönen at Helsinki University of Technology wrote a replacement: SSH, the Secure Shell.

Port 22 is not an accident

• Telnet was port 23 and FTP was port 21, and SSH was meant to replace both. 22 sat between them, unassigned.
• Ylönen asked IANA for it in July 1995 and got it the next day.
• SSH 1.0.0 was released on July 12, 1995.

More than thirty years later, SSH still listens on port 22 by default.

SSH is just an application

Same client-server model as chapter 2:

• sshd is the server. It is always on, listening on TCP port 22.
• ssh is the client you run on your laptop.
• Underneath it is plain TCP, so everything you learn about TCP applies.

The protocol is three layers, each its own RFC (2006): transport (RFC 4253) encrypts and proves the server's identity, user authentication (RFC 4252) proves yours, and connection (RFC 4254) carries your shell and commands.

Two keys, two jobs 🔑

From chapter 8: public key crypto gives you a matched pair.

• The private key is yours alone. It never leaves your laptop.
• The public key can go to anyone. You can paste it in a chat window and nothing bad happens.
• What the private key signs, only the matching public key can verify.

That last line is the whole trick behind today.

Proving it is you without sending a secret

• Password login: your password travels to the server (encrypted) and the server compares it. The server sees it, and so does anything that has broken into the server.
• Key login: the server already has your public key. Your laptop signs data that is unique to this one session, and the server checks the signature.

The private key never crosses the network, and a recording of the exchange is useless next time, because the next session signs different data.

Ed25519

The key type you make today is Ed25519, an elliptic curve signature scheme.

Key	Public key line
Ed25519	about 80 characters
RSA, 3072 bit	about 550 characters

Same job, a fraction of the size. OpenSSH has supported it since 6.5 (January 2014), and since 9.5 (October 2023) it is what ssh-keygen makes by default.

The server has a key too

Your key proves who you are. The server has its own host key that proves it is really Onyx and not someone in the middle pretending to be.

• The first time you connect, ssh shows you the host key's fingerprint and asks whether to trust it.
• Say yes and it is saved in ~/.ssh/known_hosts.
• Every later connection checks it against that file.

This is called trust on first use. The first connection is the one that matters.

When the host key changes

March 24, 2023. GitHub's RSA host private key was briefly exposed in a public repository, so GitHub replaced it at about 05:00 UTC. Developers everywhere ran git pull and got this:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

If you know why it changed, remove the old entry and move on. If you do not know why, stop. That warning is exactly what a man in the middle looks like.

Unix permissions in 30 seconds

Every file has three sets of permission bits: owner, group and other. Each set is read (4), write (2) and execute (1), added up.

700  rwx------   only you can do anything (a directory you can enter)
600  rw-------   only you can read and write it
644  rw-r--r--   everyone can read it, only you can write

ls -l shows them on the left of every line.

SSH is picky on purpose

• sshd ignores authorized_keys if anyone else could write to it or to ~/.ssh. Anyone who can add a line to that file can log in as you.
• ssh refuses a private key that anyone else can read: Permissions 0644 for '...' are too open.

The server side does not always tell you why it ignored your key. It just asks for a password, which is the most confusing failure there is. Permissions are the first thing to check.

Every round trip costs time

The round trip time (RTT) is how long a packet takes to get there and back. ping measures it.

A protocol that needs N back and forth exchanges before it does anything useful pays N × RTT, no matter how fast your link is. Light in fiber covers about 200 km per millisecond, so more bandwidth does not help. Fewer round trips do.

Setup is the expensive part

You have seen this already in chapter 2:

• Non-persistent HTTP pays a TCP handshake for every object: 2 RTTs each, one to connect and one to fetch.
• Persistent HTTP pays it once and reuses the connection.

An SSH login stacks up more setup than HTTP: a TCP handshake, a version exchange, a key exchange, authentication, then your shell. How many round trips that adds up to is one of today's questions.

Reusing a connection

Connection multiplexing is SSH's version of persistent HTTP. The first connection stays open after you log out, and later sessions ride inside it instead of starting from scratch.

Same idea, same payoff: you skip the setup you already paid for. Today you measure exactly how much that is worth on the campus network.

Timing a login caught a backdoor

March 29, 2024. Andres Freund noticed that ssh logins on his Debian unstable machine were taking 0.807 s instead of 0.299 s.

He dug in and found a backdoor hidden in xz, a compression library that sshd loads on many Linux systems (CVE-2024-3094). Someone had spent years earning commit access to plant it. It was caught before it reached the stable releases of the big distributions, because one person timed a login and did not like the number.

A shell is a program that reads files

bash is just a program, and before it gives you a prompt it reads its configuration files.

• On Red Hat systems like Onyx, a login shell reads ~/.bash_profile, and the default one reads ~/.bashrc in turn.
• ssh onyx gives you an interactive shell: a prompt, waiting for you.
• ssh onyx uname -n is non-interactive: no prompt, one command, then it exits.

Which files a non-interactive shell reads, and what can stop it, is part of today.

Dotfiles

Files whose names start with a . are hidden from ls. Use ls -a to see them.

• ~/.ssh/config is your SSH client's settings
• ~/.bashrc is your shell's settings

They are plain text, you own them, and they follow you from machine to machine. A lot of developers keep theirs in a Git repository for exactly that reason.

Teaching the shell a new word

Anything you define at the prompt lives exactly as long as that shell. Close the terminal and it is gone.

Put the same definition in a file the shell reads at startup, and every new shell gets it for free. Bash gives you two ways to define a new word, an alias and a function. They are not interchangeable, and today you find out why.

vi in five keys

vi is a modal editor: most of the time, the keys you press are commands, not text.

Key	What it does
i	start typing (insert mode)
Esc	stop typing
:wq	save and quit
:q!	quit without saving
G	jump to the end of the file

Bill Joy wrote it at Berkeley in 1976, and it is on practically every Unix and Linux box you will ever ssh into.

Today

This one is instructor led. I do each step on the projector, you do it on your laptop, and we wait for every hand before moving on.

Sit with your A1 group and grab a worksheet. The first step is proving that the plain password login works:

ssh <username>@onyx.boisestate.edu