I deleted all my SSH keys

i recently bought three yubikeys: a 5C nano to sit in my PC, a 5C NFC on my keychain and another 5C NFC as a backup. currently only using them as SSH keys to my servers. probably overkill but all the recent cybersecurity issues got me a little spooked.

i can't lie, the thought of removing authenticator/authy 2FA from my critical accounts is a little daunting.

I will stick with ssh keys, except that I:

- Put the public authorized keys in /etc/ssh/keys/%u immutable, monitored for checksum changes.

- Have CIDR restrictions in said public keys.

    from="x.x.x.x/n,x.x.x.x/n" ssh-ed25519 AAAA[snip] Text-Identifier

Hey I'm in the middle of doing a very similar thing, though I didn't go quite as far as OP. I'm slowly deprecating my old RSA SSH keys and replacing them with a combination of Yubikey, Password Manager, and alternative SSH auth:

1. -sk keys on Yubikeys (with touch enabled) are my primary keys, used for signing and always installed on every host I touch/etc. I initially planned on using the PIV slots on my Yubi's to store the SSH keys instead of the built-in FIDO2 setup, but I hated having to manage multiple moving parts (like getting pscd running on all my clients). The only main benefit of using the PIV slots was being able to set the "cache" touch policy: the YK would cache a touch for up to 15 seconds so doing something like "signing multiple commits during a rebase" would only take one touch instead of num_commits. The side benefit would have been getting to re-use the same SSH key across primary and backup Yubis. The simplicity of the FIDO setup won out and I decided to treat getting annoyed at pressing my Yubikey a bunch of times as a sign I should be doing some sort of alternative setup like a dedicated key. Most prominently, I've now properly setup my Ansible configs with dedicated deploy keys kept as secrets instead of reusing my personal key.

2. I use 1Pass and Bitwarden (work/personal) to store a secondary SSH key that's only used for basic Git auth or other low-risk tasks. This SSH key is loaded from the Password manager directly into ssh-agent via stdin so it never touches my disk; I use fnox to make this backend agnostic.

3. Work uses Teleport, personal uses Tailscale so I rely on those SSH auth most of the time. Nothing much to say here except that Tailscale ssh is really convenient :) .

I've also setup PAM login and sudo on my machines, which brings me back to the old Sun Ray/Smartcard days. None of this is really to prevent security breaches/dedicated attackers or anything, though it certainly makes me harder to compromise. I just wanted to make absolutely sure I couldn't accidentally upload an SSH private key somewhere.

What is the practical/security difference between

`ssh-keygen -t ed25519-sk -O resident -O no-touch-required -O application="ssh:nameyourkey" -f ~/.ssh/id_ed25519-sk`

and

`ssh-keygen -t ed25519-sk -f ~/.ssh/id_ed25519-sk`

?

The latter is what I always do. It creates a file that is useless without my yubikey being present, but is it not actually resident on the key?

YubiKeys with proof-of-presence for SSH auth are great, but I still use the YubiKey's OpenPGP slot for this because my preferred behavior is:

1. Require a PIN followed by touch on the first use after inserting the YubiKey

2. For every following authentication while the key remains connected, only require touch

The extra GnuPG setup needed to make this work is unfortunate, but AFAIK this still isn't possible with fido -sk keys on the YubiKey.

There is also a middle ground - it's possible to sign SSH certificate using yubikey as CA. By controlling expiration and set of principals, the cert you get on your machine is scope restricted, and quickly expires. Or you can load it into an ssh-agent and drop it immediately.

Really nice article!

They mention twice that Yubikeys are expensive, but for SSH their cheaper FIDO2 keys work:

https://www.yubico.com/product/security-key-series/security-...

Great article! I discovered ssh-tpm-agent last year and was surprised that all my ThinkPads have hardware TPM, even one from 2018! I've switched over to using it for everything and there are no more private keys on my devices.

I also have a Yubikey but it's been a few years since I tried it for ssh keys. I use it as a regular security key. Maybe I should try it for ssh keys as it sounds like the tooling has improved.

This is all, frankly, nonsense.

If you're using userspace software to decrypt the keys into memory then it's all just security theatre. Any sufficiently motivated actor, ot a comprised package in the software chain, can capture your private key in the clear. Especially if using ssh-agent that keeps it resident in memory 'for convenience'.

Proper HSM integration with SSH requires challenges signed by the HSM without the private key ever being emitted.

Just keep your SSH keys on disk, one per remote host, with a different encryption password for each target host. If you're worried about threats that can defeat that then they'll just come to your door and beat the passwords out of you.

What is the benefit of using yubikeys instead of ssh keys (ie a plain text file)? My main concern is losing the device or damaging it.

> You will notice though that the private keyfile held on disk uses a .tpm extension this time. This is where an important caveat of the TPM method comes in – Unlike the YubiKey approach, the private key material is actually being stored on disk here. This is because TPM devices do not natively support enrolling SSH keys. So what is happening is that the private key material is being stored on disk in encrypted form, and the TPM is being used to decrypt it.

While this is true, you can create static hierarchy keys that would not resolve into separate key files for the TPM. The issue here is that anyone with access to your TPM can recreate the keys, unless you set a owner password.

https://man.archlinux.org/man/ssh-tpm-agent.1#Hierarchy_keys

Do yubikeys suffer from flash memory bit rot? If so, it seems it would best have several backups that rotate once a month to being the primary so that the memory gets used?

There's also an alternative way to using ssh-tpm-agent - the tpm2-pkcs11 stack provides it, here's a guide for setting it up: https://jade.fyi/blog/tpm-ssh/ Not related to the author, but using that setup personally. Similar considerations as in the OPs TPM notes.

What is the longevity and migration/upgrade story for a yubikey?

I always worry what do you do when your hardware fails

I like the idea of using my MacBook's Secure Enclave for ssh keys but what do I do if I develop on a remote VPS (purely because I'd rather have the AI accidentally rm -rf / on a VPS than my own computer)?

My sticking point with these has always been SSH agent support: the last time I looked, I couldn't use my Yubikey to authenticate the second hop in an A->B->C sequence with a jump box in the middle. Is that resolved now? This blog doesn't address that use case.

How are those compared to nitrokeys? I know an IT guy that prefers those, I think he mentioned those are open hardware?

all the commandline magic makes me shy away. why is security always combinde with lines over lines of comamndline

Way too much work for my taste. I use the bitwarden CLI to fetch/store the key. Opted in for ed25519 on day 1. Works great.

Speaking of which I do something cool with my main desktop (an AMD 7700X, but whatever). It's running Debian and there's zero executable with the suid bit set. There's no sudo command installed. Using "su" to su to root is technically possible but it's like a 40 characters long password, meant to never be entered (and I never ever entered it using su).

The only way I can get root (short of using a local privilege exploitation exploit to gain root) is by logging, using a Yubikey, over ssh, from a dedicated laptop that has nothing but a SSH client. So the Yubikey is plugged into the laptop.

That dedicated laptop has no wifi drivers and is on a dedicated LAN and that dedicated LAN only has two machines on it: my desktop and that laptop.

The laptop serves no purpose other than: "log in as root on the desktop".

Of course both that laptop and the desktop have firewalls.

This means, yes, I allow log in as root using SSH. But password logins are disabled and the only way to log in as root using SSH is to use the Yubikey. And the firewall only allow SSH attempts to port 22 from the MAC address, IP, and dedicated LAN that corresponds to the laptop.

In addition to that my main desktop has multiple user accounts: but none of those users are allowed to su or sudo.

Now, I know, I could use Qubes or whatever.

But what I explained above is how I do it: even myself I cannot get root on my own desktop without my laptop + Yubikey (I'd need to find and enter the 40 chars long root password).

When I really need to do something as root (and once becoming root becomes extremely hard, suddenly you realize how rare are the occasions when you really need to be root), I open my laptop and boot it (and because that laptop has no GUI and basically nothing besides a SSH client, it boots in three seconds or so).

Haters gonna hate but that's been working totally fine for me since years and years.

Can yubikeys be used for logging onto Linux consoles?

Unfortunately hardware resident keys are basically useless under qubes.

I'd rather have qubes than hardware resident keys.