Native Secure Enclave backed SSH keys on macOS

Check out `man sc_auth`. There's also an exportable variant where the private key is encrypted using the secure enclave as opposed to generated on the secure enclave:

    % sc_auth create-ctk-identity -l ssh-exportable -k p-256 -t bio
    % sc_auth list-ctk-identities
    p-256    A581E5404ED157C4C73FFDBDFC1339E0D873FCAE bio  ssh-exportable ssh-exportable               23.11.26, 19:50 YES  
    % sc_auth export-ctk-identity -h A581E5404ED157C4C73FFDBDFC1339E0D873FCAE -f ssh-exportable.pem
    Enter a password which will be used to protect the exported items:
    Verify password:

    % sc_auth import-ctk-identities -f ssh-exportable.pem.p12 -t bio
    Enter PKCS12 file password:

You're not really supposed to 'export' keys. Any time you move a key you risk exposing it. The idea of PKI is that only public keys move, the private key stays in one place, ideally never seen.

Correct. Set up multiple keys as backups. Thats also a positive, as nothing can leak the key.

Inability to export the private key is no different from using an YubiKey? You can't "backup" the private key they generate either.

Yes, that's the point, indeed. One key per device, impossible to extract, so you need to break into the device to use the key.

If you want to maintain backup access, you can use an SSH CA to sign your public SSH keys, then keep the private keys on your device. If you keep the CA keys safe (i.e. physically safe on a flash drive), this means you can even add new keys after you lose all your devices.

This way, you only need to trust your one CA on your servers (so you don't need to copy 20 public keys around for every server).

Plus, if you're setting up a (separate) SSH CA, you can also sign servers' host keys, so you don't need to rely on TOFU to prevent MITM attacks, if that's something you care about.

Strictly speaking people should be using multiple keys so if a device is lost/stolen, you're not left high and dry. Ideally one per device, especially if they don't support some kind of secure enclave.

I keep one in a yubikey protected by a PIN that sits in a safety deposit box, too. This way if I have my laptop, phone, and day-to-day yubikey is a house that suddenly burns down, I'm still ok.

You use multiple keys, if you need a key usable across different systems then buy a yubikey.

Is storing ssh-key in tpm possible on Linux?

A golang library is cool, but it doesn't give you a working ssh-agent.

I started working on one few years ago: https://github.com/Foxboron/ssh-tpm-agent

These aren't synced over iCloud

What you're thinking of are Passkeys. Which are synced. Somebody would have to write an SecurityKeyProvider that talks to the Passkey API instead.

Actually I don't think it's completely impossible. The only thing is that passkeys are origin-bound. They belong to a specific AppBundle ID or domain name. If say Secretive would add passkey support then that specific public/private keypair can't be used by another app. Though it does sync across instances of the app across devices.

I think this is an openssh limitation.

openssh only supports sk-ecdsa-sha2-nistp256 and sk-ed25519 security keys iirc

Only supports NIST curves and ECDSA yes.

I've heard people make the point before that EdDSA is not great for secure enclaves due to being suspictable to Fault Attacks which could lead to (partial) key extraction

Git commits can be signed with ssh keys.

Some Fido2 keys like the YubiKey and Nitrokeys support PGP keys as well. Works pretty nice as well and has the added bonus of your key not being tied to a pice of hardware that is as likely to break like a laptop (or be upgraded on a semi-regular basis)

You can (mis)use ssh keys for git signing, but GPG on gpg-card and S/MIME on PIV card are the two standards and their respective hardware implementations (for signing keys in general.)