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.
- Have NFTables checks for MSS, TCP SYN TTL, Window sizes, CIDR blocks permitted on the public ssh instances (throw away nodes) and behind a VPN on more important roles. If I have a need to keep netfilter disabled then I put sshd behind L4 haproxy with ACL's.
Your 2nd one is non-resident, the secret key material is encrypted in that sk file. But it is encrypted with a master key that is only inside your yk. So you need both your sk handle file and the yk to use it. This has the benefit of not consuming the limited number of memory slots in a yk.
In the first one the handle file that's created, can be recovered from the yk key itself, that's what resident is.
Having the only practical difference between the two is that, with the first one, you (or someone else) only needs the yk to use the credential. But in the latter case, one needs the yk and the handle file.
Resident keys have their "metadata" stored on the yubikey, in an exportable way. This means using the same ssh-key on a different computer is as easy as plugging in the yubikey and typing ssh-keygen -K.
With a non-resident key you’d have to copy the "privatekey", which contains the metadata needed to use the key, to the other computer first.
The downside with resident keys is that someone stealing the yubikey can see what resident keys are on it, and then possibly use those keys (if they aren’t protected). Newer firmware comes with ways to protect against this, but you’d have to enable it.
The first one sets "no-touch-required" and not the second one, is that expected?
Anyway, resident key means that you can extract the public key from the yubikey. Say someone steals your Yubikey Fido2 PIN and your Yubikey, they have everything.
Non-resident key means that the attacker needs to get that "useless" file you describe, too. And if you lose it, then you can't use the key anymore.
Another difference is that a resident key takes space on the Yubikey, so you can't have infinitely many. Whereas you can have infinitely many "useless" files.
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.
Running very similar setups. PAM handles FIDO / security key based auth on my local systems for root users and sudo commands, as well as LUKS decryption.
I have a handful of security keys, so I have a helper script to drop them into new hosts and another helper script on my systems to import keys straight to ssh-agent once detected.
Has worked very well and I don't think I'll ever go back to a "password manager as SSH agent" again, but it's probably the best alternative backup. I still have Bitwarden holding some backup keypairs for this reason, but haven't needed to use them in months.
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?
In my anecdotal experience Yubikeys tend to physically deteriorate long before this happens. I still have an early YubiKey 5 that must be 8 years old at this point that I use daily and it works fine.
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.
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.
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.
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)?
You can use agent forwarding (ssh -A) to make your local hardware-backed key available on the VPS.
If you're using tmux on the VPS this can get a little tricky, because by default your tmux panes will inherit your initial SSH client's SSH_AUTH_SOCK; when you re-attach tmux in a different SSH session you'll no longer be able to authenticate with the local YubiKey. But I wrote a shell script that hooks into tmux to keep SSH_AUTH_SOCK working correctly with this kind of setup: https://github.com/mshroyer/socklink
> 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.
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.
Except that's not really the network path that's going on, right? If I am on A and connect to B and then want to use the network to transfer a bunch of data from B to C over an ssh channel between them, using ProxyJump brings it all back to the bastion box. And if A is in the US and B & C are in Europe connected with a 40 GB LAN, using proxyjump will bring the connection from B->C into Europe->US->Europe. How do I do this with my credentials existing on A being used to log in to B and then the same credentials being used to connect from B to C while only having the one network connection is B->C and not to my bastion host?
If you're using -sk keys (i.e. setup via FIDO2 on the Yubikey like the article, not PIV) yes you can forward the keys loaded into your agent like normal, with ssh -A.
PIV keys can also be forwarded through an agent. The only thing to be aware of is that the agent process will effectively cache your PIN if you do this.
ssh-add -s <pkcs11.so>
To mitigate that you can add -c to the ssh-add command. That will make the agent prompt for confirmation (on the host with the PIV key) every time something asks to use it. Doesn't quite get you the same security as just not caching the PIN to start with but it's OK.
for most use cases they're functionally the same. the biggest difference for me is that nitrokey doesn't support doing hmac-sha1 challenge, so i can't use the nitrokey with keepassxc.
That's kind of the opposite of the approach presented here, though. With hardware-resident, non-exportable keys, you can know that no matter what you do, key hashes to some physical device are 1:1.
Whether you want or don't want that property is a different story.
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.
Yes, it's really nice. The fingerprint scanner on my Thinkpad is terrible (even when enrolling a fingerprint multiple times, etc.), whereas the YubiKey Bio work much better. So I use it as an alternative screen unlock.
- Put the public authorized keys in /etc/ssh/keys/%u immutable, monitored for checksum changes.
- Have CIDR restrictions in said public keys.
- Have NFTables checks for MSS, TCP SYN TTL, Window sizes, CIDR blocks permitted on the public ssh instances (throw away nodes) and behind a VPN on more important roles. If I have a need to keep netfilter disabled then I put sshd behind L4 haproxy with ACL's.`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?
In the first one the handle file that's created, can be recovered from the yk key itself, that's what resident is.
Having the only practical difference between the two is that, with the first one, you (or someone else) only needs the yk to use the credential. But in the latter case, one needs the yk and the handle file.
The downside with resident keys is that someone stealing the yubikey can see what resident keys are on it, and then possibly use those keys (if they aren’t protected). Newer firmware comes with ways to protect against this, but you’d have to enable it.
[1] has more details
1: https://developers.yubico.com/SSH/Securing_SSH_with_FIDO2.ht...
Anyway, resident key means that you can extract the public key from the yubikey. Say someone steals your Yubikey Fido2 PIN and your Yubikey, they have everything.
Non-resident key means that the attacker needs to get that "useless" file you describe, too. And if you lose it, then you can't use the key anymore.
Another difference is that a resident key takes space on the Yubikey, so you can't have infinitely many. Whereas you can have infinitely many "useless" files.
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.
I have a handful of security keys, so I have a helper script to drop them into new hosts and another helper script on my systems to import keys straight to ssh-agent once detected.
Has worked very well and I don't think I'll ever go back to a "password manager as SSH agent" again, but it's probably the best alternative backup. I still have Bitwarden holding some backup keypairs for this reason, but haven't needed to use them in months.
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.
Edit: well, maybe not exact. With -sk keys you’d have to type in a passphrase instead of a pin. But that passphrase could be a pin.
They mention twice that Yubikeys are expensive, but for SSH their cheaper FIDO2 keys work:
https://www.yubico.com/product/security-key-series/security-...
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.
I always worry what do you do when your hardware fails
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.
If you're using tmux on the VPS this can get a little tricky, because by default your tmux panes will inherit your initial SSH client's SSH_AUTH_SOCK; when you re-attach tmux in a different SSH session you'll no longer be able to authenticate with the local YubiKey. But I wrote a shell script that hooks into tmux to keep SSH_AUTH_SOCK working correctly with this kind of setup: https://github.com/mshroyer/socklink
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
ssh-add -s <pkcs11.so>
To mitigate that you can add -c to the ssh-add command. That will make the agent prompt for confirmation (on the host with the PIV key) every time something asks to use it. Doesn't quite get you the same security as just not caching the PIN to start with but it's OK.
fido, gpg, piv all work perfectly
Something about stealing an iPhone jailbreak years ago?
the ios thing was in 2009. the company that produces nitrokey security hardware was founded in 2015
Whether you want or don't want that property is a different story.
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.
1. Virtual terminals? The answer I can think of is usb hid attacks.
2. Serial Consoles? My answer the is encrypted connections on the wire.
Any other reasons than mines?
I'd rather have qubes than hardware resident keys.