They all help establish trust—but they authenticate different things
We Already Prove "It's Really Me" in Three Different Ways
Think about how you get into your own house. You punch in a door code, turn a physical key, or unlock the door with your fingerprint. Security professionals have long organized these into three factors of authentication: something you know, something you have, and something you are. A password is a knowledge factor. A key or a card is a possession factor. A fingerprint or iris scan is an inherence factor. Combine two or more, and authentication gets significantly stronger.
Passwords, security keys, and OTPs (one-time passwords) in the digital world map onto this same framework. A PUF (Physically Unclonable Function), though, operates at a different layer. A PUF's fundamental design purpose isn't proving who a person is — it's proving that a specific piece of hardware is genuine. So a PUF isn't a direct replacement for the other three. Instead, it provides a separate layer of trust that works alongside them and can be integrated into authentication devices.
What Each One Actually Proves
A password proves that a user knows a specific piece of information. It works on any device with no extra hardware required, which is why it remains the most widely used first-factor login method — However, password reuse and weak, easily guessed passwords create well-documented security risks. That's why NIST's digital identity guidelines recommend pairing passwords with multi-factor authentication (MFA) rather than relying on them alone (NIST SP 800-63B).
A hardware security key proves that a user physically possesses a specific device. Keys built on the FIDO2/WebAuthn standard are specifically designed to resist phishing — even if a user is tricked into visiting a fake login page, the credential itself can't be stolen the way a typed password can (FIDO Alliance, FIDO2). Being a physical object, though, means a lost or stolen key requires a re-issuance process.
An OTP (one-time password) generates a value that's only valid for a short window, making a leaked value hard to reuse. SMS-delivered OTPs, however, are exposed to a known attack pattern that exploits carrier account-recovery processes to intercept the message — which is why NIST guidelines classify SMS-based OTP as a "restricted" authenticator (see the same NIST SP 800-63B). App-based OTPs (from an authenticator app) are largely free of that particular carrier-channel risk.
A PUF starts from an entirely different premise. It exploits microscopic physical variations that occur naturally during semiconductor manufacturing to generate a value that's unique to that one chip. That value isn't something a person memorizes or carries — it's a physical characteristic embedded in the chip itself, so there's no "stored secret" to steal in the first place. What a PUF fundamentally proves is not "this is the right person" but "this is the expected hardware and, when combined with additional integrity mechanisms, has not been altered".
That said, when a PUF is embedded in a device issued to a specific individual — a security token or a smart card, for instance — that device itself can function as a possession factor tied to that person, becoming part of a broader user authentication system. Researchers have in fact published multi-factor user authentication protocols that combine PUFs with passwords and biometrics (IEEE, PUF-based authentication; PUFTAP-IoT: PUF-Based Three-Factor Authentication Protocol). In short: a PUF alone doesn't identify who's using a device, but it can extend into being a building block of a user authentication system.
All Four, Side by Side
| Password | Security Key (Hardware Token) | OTP | PUF |
|---|
| Authentication factor | Knowledge | Possession | Possession + time-based value | A physical uniqueness embedded in hardware |
| What it verifies | The person knows a piece of information | The person possesses a specific device | The person holds a value valid at that moment | The hardware can be uniquely identified and authenticated (extends to a possession factor when embedded in a personal device) |
| Key Strength | Works anywhere, no extra hardware | Strongly resists phishing (FIDO2) | Makes a leaked value hard to reuse | No stored secret, so there's nothing to clone or steal |
| Well-known limitation | Exposure risk from reuse or weak choices (NIST SP 800-63B) | Requires reissuance if lost or stolen | SMS delivery carries known carrier-channel risk (NIST SP 800-63B) | Doesn't identify "who" is using the device on its own — typically paired with a password or biometric for full user authentication |
| Typically use | Primary login | Second factor for high-security accounts | Second factor for login or payment | Device-level trust foundation (HRoT), security tokens/smart cards, IoT |
This table is a simplified conceptual comparison — actual security depends on implementation and operational practices. These methods generally aren't substitutes for one another; they're combined based on context.
How the Four Can Work Together
Consider online banking.
A user enters a password and then provides an OTP. Together, these steps help the bank determine whether the person attempting to sign in is the legitimate account holder. This is the user authentication layer.
But user authentication alone does not establish whether the smartphone, security token, or payment terminal involved in the process is a trusted device.
This is where a hardware root of trust (HRoT), potentially anchored in a PUF, can play an important role. It provides a hardware-based foundation for authenticating the device, deriving and protecting cryptographic keys, and supporting security functions such as secure boot and device integrity verification. For more on the relationship between PUFs and HRoT, see [What Is a Hardware Root of Trust? The One Foundation All Security Depends On].
Passwords, security keys, and OTPs primarily help answer, “Is this the authorized user?” A PUF provides a hardware-level foundation for answering, “Is this the expected device?” These two layers are not completely separate. When a PUF is embedded in a security token, smart card, or another device associated with a specific user, it can also support possession-based user authentication.
Wrapping Up
Passwords, security keys, and OTPs verify something about the user: what the user knows, possesses, or can provide at a particular moment. A PUF, by contrast, provides a way to derive trust from the unique physical characteristics of a semiconductor device.
None of these approaches is universally superior to the others. They address different security questions at different layers, which is why they are often more effective when used together. A PUF can strengthen the hardware foundation beneath an authentication system, while passwords, security keys, biometrics, and OTPs verify the person using it.
The next time you enter a password or OTP, it may be worth asking another question as well: What establishes trust in the device handling that authentication?
☑️ Curious about HRoT/PUF architectures that physically guarantee device trust — or about applying PUF to authentication tokens? Reach out to ICTK to talk through a VIA PUF-based solution. → Contact us
☑️ Learn more about VIA PUF
References
They all help establish trust—but they authenticate different things
We Already Prove "It's Really Me" in Three Different Ways
Think about how you get into your own house. You punch in a door code, turn a physical key, or unlock the door with your fingerprint. Security professionals have long organized these into three factors of authentication: something you know, something you have, and something you are. A password is a knowledge factor. A key or a card is a possession factor. A fingerprint or iris scan is an inherence factor. Combine two or more, and authentication gets significantly stronger.
Passwords, security keys, and OTPs (one-time passwords) in the digital world map onto this same framework. A PUF (Physically Unclonable Function), though, operates at a different layer. A PUF's fundamental design purpose isn't proving who a person is — it's proving that a specific piece of hardware is genuine. So a PUF isn't a direct replacement for the other three. Instead, it provides a separate layer of trust that works alongside them and can be integrated into authentication devices.
What Each One Actually Proves
A password proves that a user knows a specific piece of information. It works on any device with no extra hardware required, which is why it remains the most widely used first-factor login method — However, password reuse and weak, easily guessed passwords create well-documented security risks. That's why NIST's digital identity guidelines recommend pairing passwords with multi-factor authentication (MFA) rather than relying on them alone (NIST SP 800-63B).
A hardware security key proves that a user physically possesses a specific device. Keys built on the FIDO2/WebAuthn standard are specifically designed to resist phishing — even if a user is tricked into visiting a fake login page, the credential itself can't be stolen the way a typed password can (FIDO Alliance, FIDO2). Being a physical object, though, means a lost or stolen key requires a re-issuance process.
An OTP (one-time password) generates a value that's only valid for a short window, making a leaked value hard to reuse. SMS-delivered OTPs, however, are exposed to a known attack pattern that exploits carrier account-recovery processes to intercept the message — which is why NIST guidelines classify SMS-based OTP as a "restricted" authenticator (see the same NIST SP 800-63B). App-based OTPs (from an authenticator app) are largely free of that particular carrier-channel risk.
A PUF starts from an entirely different premise. It exploits microscopic physical variations that occur naturally during semiconductor manufacturing to generate a value that's unique to that one chip. That value isn't something a person memorizes or carries — it's a physical characteristic embedded in the chip itself, so there's no "stored secret" to steal in the first place. What a PUF fundamentally proves is not "this is the right person" but "this is the expected hardware and, when combined with additional integrity mechanisms, has not been altered".
That said, when a PUF is embedded in a device issued to a specific individual — a security token or a smart card, for instance — that device itself can function as a possession factor tied to that person, becoming part of a broader user authentication system. Researchers have in fact published multi-factor user authentication protocols that combine PUFs with passwords and biometrics (IEEE, PUF-based authentication; PUFTAP-IoT: PUF-Based Three-Factor Authentication Protocol). In short: a PUF alone doesn't identify who's using a device, but it can extend into being a building block of a user authentication system.
All Four, Side by Side
This table is a simplified conceptual comparison — actual security depends on implementation and operational practices. These methods generally aren't substitutes for one another; they're combined based on context.
How the Four Can Work Together
Consider online banking.
A user enters a password and then provides an OTP. Together, these steps help the bank determine whether the person attempting to sign in is the legitimate account holder. This is the user authentication layer.
But user authentication alone does not establish whether the smartphone, security token, or payment terminal involved in the process is a trusted device.
This is where a hardware root of trust (HRoT), potentially anchored in a PUF, can play an important role. It provides a hardware-based foundation for authenticating the device, deriving and protecting cryptographic keys, and supporting security functions such as secure boot and device integrity verification. For more on the relationship between PUFs and HRoT, see [What Is a Hardware Root of Trust? The One Foundation All Security Depends On].
Passwords, security keys, and OTPs primarily help answer, “Is this the authorized user?” A PUF provides a hardware-level foundation for answering, “Is this the expected device?” These two layers are not completely separate. When a PUF is embedded in a security token, smart card, or another device associated with a specific user, it can also support possession-based user authentication.
Wrapping Up
Passwords, security keys, and OTPs verify something about the user: what the user knows, possesses, or can provide at a particular moment. A PUF, by contrast, provides a way to derive trust from the unique physical characteristics of a semiconductor device.
None of these approaches is universally superior to the others. They address different security questions at different layers, which is why they are often more effective when used together. A PUF can strengthen the hardware foundation beneath an authentication system, while passwords, security keys, biometrics, and OTPs verify the person using it.
The next time you enter a password or OTP, it may be worth asking another question as well: What establishes trust in the device handling that authentication?
☑️ Curious about HRoT/PUF architectures that physically guarantee device trust — or about applying PUF to authentication tokens? Reach out to ICTK to talk through a VIA PUF-based solution. → Contact us
☑️ Learn more about VIA PUF
References