What a Passkey Is and How to Implement It

Passkeys are one of the most important shifts in application authentication in a long time.

If you have built login systems before, you already know the usual tradeoffs. Passwords are familiar, but they are weak, reused, phished, leaked, and expensive to support. Magic links are often easier to use, but they still depend on email and are not always ideal for high-assurance authentication. SMS OTP is widely understood, but it comes with real security and delivery concerns.

Passkeys are a better direction.

They let users sign in with the same device-level gesture they already trust, like Face ID, Touch ID, Windows Hello, or a device PIN, while the application authenticates them with public key cryptography instead of a shared secret. That change matters. It removes the password from the flow entirely and makes phishing much harder.

This post is for implementers. We will cover what a passkey actually is, how the underlying authentication flow works, why more teams should adopt them, and what implementation looks like in a real application.

What is a passkey?

A passkey is a credential based on the WebAuthn and FIDO2 standards that allows a user to authenticate with a public-private key pair instead of a password.

At a high level:

the authenticator creates a key pair
the private key stays on the user's device or secure authenticator
the public key is shared with your server and stored for future verification
when the user signs in, your server sends a challenge
the device signs that challenge with the private key
your server verifies the signature with the public key it already has

That means your application never stores a password hash for that login method, and the user never needs to type a secret that can be phished or reused.

In practice, a passkey usually feels like this:

A user clicks "Sign in with passkey"
The browser asks the platform authenticator to verify the user
The user uses Face ID, Touch ID, Windows Hello, Android biometrics, or a device PIN
The browser returns a signed assertion
Your backend verifies it and signs the user in

The important thing is that biometrics are not the credential your app receives. The fingerprint or face scan is only used locally to unlock the private key. Your server sees a cryptographic proof, not biometric data.

Why passkeys are a big deal

Passkeys are attractive for two reasons at the same time: security and user experience.

Passwords are often weak because humans have to remember them. As a result, many security rules are really patches around that fact. Complexity rules, resets, lockouts, MFA prompts, anti-phishing training, credential stuffing detection, and breach monitoring all exist partly because the first factor is fragile.

Passkeys improve the base layer.

Security benefits

Passkeys help because they are:

resistant to phishing, since the credential is scoped to the relying party and cannot be replayed on a fake domain
not shared secrets, which means there is no password to steal from the user and no password hash to steal from your database
unique per application, reducing credential reuse
protected by secure hardware or platform key storage in many environments
hard to brute force because the login is based on signed challenges rather than guessed secrets

User experience benefits

Passkeys also reduce friction:

no password creation step
no password reset flow
no need to remember another secret
faster repeat sign-in on trusted devices
often fewer support tickets around login issues

This is one of the rare places in software where the more secure option can also be the more pleasant one.

The mental model implementers should use

A lot of confusion around passkeys comes from treating them like "a better password field."

They are not that.

A passkey system is closer to a key registration and challenge verification system.

There are two main flows:

registration, where the user creates a new credential for your app
authentication, where the user proves possession of that credential

The passkey registration flow

During registration, your backend prepares a PublicKeyCredentialCreationOptions object and sends it to the browser. This includes things like:

the relying party ID and name
the user ID and username
a cryptographic challenge
the algorithms you support
authenticator preferences
optional attestation settings

The browser passes those options into navigator.credentials.create().

If the user completes the platform prompt successfully, the browser returns a new credential object. Your frontend sends that to your backend. The backend verifies the result and stores the credential ID, public key, sign counter, and metadata associated with the user.

The passkey authentication flow

Authentication is similar, but instead of creating a key pair, the authenticator uses the existing private key to sign a challenge.

Your backend creates PublicKeyCredentialRequestOptions. The browser passes those options into navigator.credentials.get(). The authenticator verifies the user locally, signs the challenge, and returns the assertion. Your backend verifies the signature against the stored public key for that credential.

The pieces you need in an application

A good passkey implementation usually has these pieces:

On the frontend

a registration button and sign-in button
endpoints to fetch registration and authentication options
calls to navigator.credentials.create() and navigator.credentials.get()
serialization logic for binary values like ArrayBuffers
error handling for cancellation and unsupported browsers

On the backend

endpoints to generate registration options
endpoints to verify registration responses
endpoints to generate authentication options
endpoints to verify authentication responses
persistent storage for passkey credentials
session creation after successful verification

In your database

You generally need to store:

user ID
credential ID
public key
sign count
transports if you want them
credential device type if relevant to your auth model
backup eligibility and backup state if your library exposes them
created at and last used timestamps

What the browser code looks like

You can build the raw WebAuthn flow yourself, but most teams use a library to avoid subtle bugs in binary encoding, challenge handling, and verification. A common setup is:

frontend: browser WebAuthn APIs or a helper package
backend: a server-side WebAuthn library for generating options and verifying responses

Here is a simplified frontend example.

Frontend registration example

type RegistrationOptionsResponse = {
  options: PublicKeyCredentialCreationOptionsJSON
}

type VerifyRegistrationResponse = {
  verified: boolean
}

async function registerPasskey(email: string) {
  const optionsResp = await fetch("/auth/passkey/register/options", {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
    },
    body: JSON.stringify({ email }),
  })

  if (!optionsResp.ok) {
    throw new Error("Failed to get registration options")
  }

  const { options }: RegistrationOptionsResponse = await optionsResp.json()

  const credential = await startRegistration(options)

  const verifyResp = await fetch("/auth/passkey/register/verify", {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
    },
    body: JSON.stringify({ email, credential }),
  })

  if (!verifyResp.ok) {
    throw new Error("Failed to verify registration")
  }

  const result: VerifyRegistrationResponse = await verifyResp.json()

  if (!result.verified) {
    throw new Error("Registration could not be verified")
  }

  return result
}

Frontend authentication example

type AuthenticationOptionsResponse = {
  options: PublicKeyCredentialRequestOptionsJSON
}

type VerifyAuthenticationResponse = {
  verified: boolean
  redirectTo?: string
}

async function loginWithPasskey(email?: string) {
  const optionsResp = await fetch("/auth/passkey/authenticate/options", {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
    },
    body: JSON.stringify({ email }),
  })

  if (!optionsResp.ok) {
    throw new Error("Failed to get authentication options")
  }

  const { options }: AuthenticationOptionsResponse = await optionsResp.json()

  const credential = await startAuthentication(options)

  const verifyResp = await fetch("/auth/passkey/authenticate/verify", {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
    },
    body: JSON.stringify({ credential }),
  })

  if (!verifyResp.ok) {
    throw new Error("Failed to verify authentication")
  }

  const result: VerifyAuthenticationResponse = await verifyResp.json()

  if (!result.verified) {
    throw new Error("Authentication could not be verified")
  }

  return result
}

In the examples above, startRegistration and startAuthentication are helper functions many teams import from a WebAuthn library rather than writing the low-level binary conversion manually.

What the backend code looks like

Here is a simplified Node and Express example using a WebAuthn helper library. The exact function names may vary depending on the package you choose, but the structure is consistent.

Generate registration options

import express from "express"
import {
  generateRegistrationOptions,
} from "@simplewebauthn/server"

const router = express.Router()

router.post("/auth/passkey/register/options", async (req, res) => {
  const { email } = req.body

  const user = await findOrCreateUserByEmail(email)

  const existingCredentials = await getUserPasskeys(user.id)

  const options = await generateRegistrationOptions({
    rpName: "Your App",
    rpID: "yourapp.com",
    userName: user.email,
    userID: user.id,
    attestationType: "none",
    excludeCredentials: existingCredentials.map((credential) => ({
      id: credential.credentialID,
      transports: credential.transports,
    })),
    authenticatorSelection: {
      residentKey: "preferred",
      userVerification: "preferred",
    },
  })

  await saveRegistrationChallenge(user.id, options.challenge)

  res.json({ options })
})

Verify registration response

import {
  verifyRegistrationResponse,
} from "@simplewebauthn/server"

router.post("/auth/passkey/register/verify", async (req, res) => {
  const { email, credential } = req.body

  const user = await findUserByEmail(email)
  if (!user) {
    return res.status(404).json({ verified: false, error: "User not found" })
  }

  const expectedChallenge = await getRegistrationChallenge(user.id)

  const verification = await verifyRegistrationResponse({
    response: credential,
    expectedChallenge,
    expectedOrigin: "https://yourapp.com",
    expectedRPID: "yourapp.com",
  })

  if (!verification.verified || !verification.registrationInfo) {
    return res.status(400).json({ verified: false })
  }

  const { credentialPublicKey, credentialID, counter } =
    verification.registrationInfo

  await saveUserPasskey({
    userId: user.id,
    credentialID,
    publicKey: credentialPublicKey,
    counter,
  })

  await clearRegistrationChallenge(user.id)

  return res.json({ verified: true })
})

Generate authentication options

import {
  generateAuthenticationOptions,
} from "@simplewebauthn/server"

router.post("/auth/passkey/authenticate/options", async (req, res) => {
  const { email } = req.body

  let allowCredentials: Array<{
    id: Uint8Array
    transports?: AuthenticatorTransportFuture[]
  }> = []

  if (email) {
    const user = await findUserByEmail(email)
    if (user) {
      const credentials = await getUserPasskeys(user.id)
      allowCredentials = credentials.map((credential) => ({
        id: credential.credentialID,
        transports: credential.transports,
      }))
    }
  }

  const options = await generateAuthenticationOptions({
    rpID: "yourapp.com",
    userVerification: "preferred",
    allowCredentials,
  })

  await saveAuthenticationChallenge(options.challenge)

  res.json({ options })
})

Verify authentication response

import {
  verifyAuthenticationResponse,
} from "@simplewebauthn/server"

router.post("/auth/passkey/authenticate/verify", async (req, res) => {
  const { credential } = req.body

  const storedPasskey = await findPasskeyByCredentialId(credential.id)
  if (!storedPasskey) {
    return res.status(404).json({ verified: false, error: "Passkey not found" })
  }

  const expectedChallenge = await getAuthenticationChallenge()

  const verification = await verifyAuthenticationResponse({
    response: credential,
    expectedChallenge,
    expectedOrigin: "https://yourapp.com",
    expectedRPID: "yourapp.com",
    credential: {
      id: storedPasskey.credentialID,
      publicKey: storedPasskey.publicKey,
      counter: storedPasskey.counter,
      transports: storedPasskey.transports,
    },
  })

  if (!verification.verified) {
    return res.status(400).json({ verified: false })
  }

  await updatePasskeyCounter(
    storedPasskey.id,
    verification.authenticationInfo.newCounter,
  )

  await createUserSession(storedPasskey.userId, res)

  return res.json({ verified: true })
})

Important implementation details people often miss

Passkey implementations are not especially large, but they do have sharp edges. These are the details worth respecting.

1. Challenge handling matters

Challenges must be:

unique
short-lived
tied to the current flow
verified on the server
cleared after use

Do not trust the browser to enforce this for you. The server is responsible for challenge lifecycle.

2. Origin and RP ID checks are critical

The browser and authenticator operate in a context tied to your domain. Your server must verify that the expected origin and relying party ID match what you intended.

If you get sloppy here, you undermine one of the biggest benefits of passkeys.

3. Binary encoding is annoying

Credential IDs, public keys, and authenticator responses contain binary data. If you build this yourself, expect to handle base64url conversion carefully on both client and server.

This is one of the best reasons to use a mature library.

4. User verification policy is a product choice

You will often see options like:

required
preferred
discouraged

If you need stronger assurance, required may be appropriate. If you want the broadest compatibility, preferred is often the pragmatic place to start.

5. You still need account recovery thinking

Passkeys reduce pain, but they do not remove the need for recovery flows.

Users lose devices. Browsers change. Sync may fail. Enterprise users may switch laptops. You need a careful fallback strategy that does not quietly reintroduce weak security.

A common pattern is:

passkeys as the preferred primary method
verified email fallback for recovery
step-up verification for sensitive changes
the ability to register more than one passkey per account

Where passkeys fit relative to other auth methods

Passkeys are not the only useful auth method, but they are increasingly the best primary choice for many applications.

Here is a plain comparison:

Method	UX	Phishing resistance	Secret stored on server	Recovery complexity
Passwords	Familiar but frustrating	Low	Yes	Medium
Magic links	Simple	Medium	No password, but email is dependency	Medium
SMS OTP	Familiar	Low to medium	No password	Medium
Passkeys	Fast once enrolled	High	No shared secret	Medium

The reason passkeys stand out is that they improve the underlying security model, not just the surface experience.

Who should implement passkeys now

Passkeys are especially worth adopting if you are building:

SaaS products with repeat login behavior
admin surfaces with elevated privileges
consumer apps where password reset is a support burden
products that want to be modern and security-forward by default
apps that already support WebAuthn-capable browsers and devices

They are also a good fit for teams that want to build toward a passwordless future instead of continuing to harden a weak password-based past.

Common objections

"Our users will not understand passkeys"

Some will not at first. That is normal.

What helps is good product language. In many cases, users do not need a lecture on public key cryptography. They need clear prompts like:

Sign in with your device
Use Face ID or your fingerprint to continue
Save a passkey for faster, more secure sign-in next time

If the experience is clean, the concept lands quickly.

"It sounds hard to implement"

It is more specialized than a password field, but it is not unreasonable. Most of the complexity is in:

correct challenge generation and verification
handling browser credential objects
storing credential metadata properly
designing sane recovery flows

With a good library and a focused implementation, this is very manageable.

"We still need something else for fallback"

Yes, and that is okay.

A strong passkey implementation is usually part of a broader auth system, not the entire system in isolation.

If you are building this today

A good implementation plan looks like this:

Add passkey credential storage to your auth database
Build registration and authentication option endpoints
Build verification endpoints
Add frontend flows for create and sign-in
Create a recovery path that you trust
Let users enroll multiple passkeys
Start nudging new users toward passkey-first authentication

If you are already thinking in terms of passwordless architecture, passkeys are not just another feature. They are one of the clearest ways to reduce both user friction and security risk at the same time.

Final thought

A lot of authentication work over the last decade has been about compensating for the weaknesses of passwords.

Passkeys feel different because they improve the foundation itself.

They give implementers a cleaner model, users a better login experience, and security teams a stronger default story. That does not mean every edge case disappears. It does mean the direction is better, and for many modern applications, it is the direction worth taking now.

If you want to go deeper into building passwordless authentication systems and implementing flows like passkeys in real applications, take a look at the Seamless Auth docs. They are a good next step if you want to move from understanding the concept to actually wiring it into your stack.