The magic of syncing passkeys

Ever wonder why you can create a passkey on your iPhone and it just magically works on your MacBook? It feels like cheating, but it's actually just clever engineering.

Basically, a synced passkey is a fido credential that lives in your cloud account instead of being stuck on one physical chip. Think of it like this:

• Hardware-bound keys: These are locked to a specific device, like a YubiKey or a TPM chip in a laptop. If you lose the device, you're locked out.
• Synced passkeys: Your cloud provider (Apple, Google, or Microsoft) acts as a secure vault. They encrypt the private key and sync it across all your logged-in devices.
• The fido alliance role: They wrote the specs so different companies can actually talk to each other, making passkeys a real standard rather than a walled garden.

According to the FIDO Alliance, passkeys are designed to be easier than passwords while keeping the public-key cryptography that makes them secure.

In retail, this means a customer starts a purchase on their phone and finishes on a tablet without re-authenticating. It's smooth. Next, let's look at how the actual crypto stays safe during that sync process.

Is the cloud actually safe for my keys?

If you’re like me, you probably don't trust "the cloud" with your house keys, so why give it your digital ones? It sounds sketchy, but the math behind how Apple or Google handles this is actually pretty solid.

When your device syncs a passkey, it doesn't just upload a raw file. It uses end-to-end encryption (E2EE), meaning the cloud provider is just a blind messenger. They host the data, but they literally can't read it because they don't have the decryption key.

• The "Vault" is locked: Your private key is encrypted locally using a key derived from your device passcode or biometrics before it ever hits the web.
• Hardware security modules: Providers use hardened servers to manage the sync. Even if a rogue employee at a big tech firm tried to peek, the hardware is designed to wipe itself before giving up secrets.
• Recovery is the hard part: Since the provider doesn't have your key, if you forget your cloud password and lose all your devices, that passkey is gone forever. It's a "security vs. convenience" trade-off that's actually working in your favor.

In a healthcare setting, this is huge. A doctor can jump from a tablet in the ER to a desktop in their office and stay authenticated without risking a data breach if the cloud server gets hacked. According to Google in 2022, this architecture ensures that even a full server compromise doesn't expose user private keys.

So, the cloud is basically just a very secure, very dumb delivery truck for your keys. Next, we'll look at what happens when you actually lose your phone and need to get back in.

Implementing secure auth in your stack

Building your own webauthn flow is a total nightmare, honestly. You have to handle challenge-response loops, attestation formats, and making sure the public keys actually save to your database correctly without breaking everything.

I've seen teams spend weeks just trying to get a "simple" login button working. That is why using something like MojoAuth makes sense—it wraps all that fido complexity into a clean api so you can focus on your actual app.

Instead of wrestling with raw binary data, you get a structured workflow that handles the heavy lifting.

• Reduced Complexity: It abstracts the webauthn ceremony. You don't have to be a crypto expert to implement secure logins for a fintech app or a retail site.
• Faster Adoption: Users get a branded, smooth experience. A study by Ping Identity in 2023 showed that 57% of consumers are more likely to use a service that offers easy, passwordless options.
• Scalable CIAM: As you grow from 100 to 1 million users, managing identities and device syncing stays consistent across your stack.

Here is how a basic implementation looks when you're initializing the login:

// Quick example of triggering the mojoauth flow
mojoauth.signIn().then(response => {
  if (response.authenticated) {
    console.log("User is in! No passwords needed.");
  }
});

It’s way better than managing raw credentials yourself. Next, we'll talk about what happens when everything goes wrong—like losing your phone.

What happens when a device is breached?

So, you lost your phone at a bar or someone swiped your laptop. It’s a gut-wrenching feeling, but honestly, your passkeys are probably the safest thing you own right now.

Even if a thief gets past your lock screen, they still gotta hit the biometric gate. Passkeys rely on local hardware-level checks that don't just "leak" across the web.

For us engineers, the big win is that passkeys aren't vulnerable to remote stuffing attacks. A breach on your server doesn't matter because you only store public keys, which are useless to a hacker.

• Local biometrics: The private key won't sign anything unless the user provides a fingerprint or face scan. It’s a hardware-enforced gatekeeper.
• Attestation: In high-security apps—like a fintech platform—you can use attestation to verify the credential actually comes from a legit, secure device.
• Physical vs Remote: A thief needs physical access and your biometric/PIN. That is a way harder mission than just phoning in a credential reset.

A 2023 report by Verizon confirmed that over 80% of basic web app breaches involve stolen credentials, a vector that passkeys basically delete from existence.

Implementing this means your users in retail or finance don't have to panic. As previously discussed, using a solid api makes handling these edge cases—like revoking a compromised device—much simpler for your backend team.

Anyway, passkeys aren't perfect, but they're a massive leap over the "Password123" nightmare we've been living in. Focus on the implementation, and the security mostly takes care of itself.

*** This is a Security Bloggers Network syndicated blog from MojoAuth - Advanced Authentication & Identity Solutions authored by MojoAuth - Advanced Authentication & Identity Solutions. Read the original post at: https://mojoauth.com/blog/are-passkeys-safely-synced-across-multiple-devices