What is Firmware Signing? Best Practices for Firmware Signing and Security

Most people don’t think about the software running inside their devices. But your washing machine, your car, even your electric toothbrush, they all run code. And not just any code firmware, the invisible layer that controls how hardware behaves.

We live in a world where physical things are now digital. A thermostat can call home. A pacemaker can receive updates. But with that power comes a simple, unsettling question.

How do you know the firmware running inside your device is the original and not a silent, malicious impostor?

One rogue firmware update, and your smart camera becomes a spy. Your industrial robot stops building and starts breaking. The solution to this invisible but growing problem is firmware signing, a cryptographic handshake between the code and the hardware, saying: “Yes, I am who I claim to be. You can trust me.” Let’s unpack what firmware signing is, why it matters more than ever, and how to get it right.

What is Firmware Signing?

Firmware is the code that tells hardware what to do. It’s not your app, and it’s not your operating system. It’s the invisible layer beneath them that makes the hardware work. A smart thermostat, a drone, and a router all rely on firmware. Without it, they’re just hunks of metal and plastic.

The problem is that firmware is very powerful. If someone can modify it, they can make the hardware do anything, spy, break, or lie. That’s where firmware signing comes in.

Think of it like sealing an envelope with a wax stamp. When the manufacturer writes firmware, they sign it using a private cryptographic key, a key that only they control. This signature acts like a wax seal. If the firmware is changed, the seal breaks.

When the device receives new firmware, it checks the signature using a public key, like holding up a known seal to the envelope and verifying it matches. If the seal is broken or the signature is missing, the device rejects the firmware.

In simpler terms, signing says “this code hasn’t been tampered with,” and verification says “this code came from someone we trust.” It’s how a piece of hardware knows whether to accept new instructions or throw them out.

Why is Firmware Signing Important?

Picture you are updating firmware for 10,000 smart meters in the field. They are all over neighbourhoods, in buildings, cities, quietly doing exactly what they are meant to do. You write new code and test it, and deploy it.

And now, a hacker quietly takes over that update channel. They don’t need to hack into each device. All they do is wait for the update, substitute it themselves, and let your infrastructure serve the malware for them. One update and they control your entire truck.

This is the reason why firmware signing is important. Without it, your device will cheerfully execute anything it is told to, with no questions asked. However, the device will be able to respond with, Wait, is this you? It halts the installation when there is a mismatch between the signature and the installer.

The most terrifying type of attack is avoided with firmware signing. The unseen, where all seems good until it turns out not to be.

This is what it safeguards you against:

Unauthorised Code Execution

Devices are wired only to execute programs signed by a trusted source. When rogue firmware is attempted to be sneaked in, it is rejected.

Backdoors and Malware

No signature? No execution. That is the rule. The malware has no use without a valid signature, even when it has made it to the device.

MITM Attacks on Updates

Signing guarantees that updates to firmware are performed by trusted sources, rather than by an evildoer relay server in the middle.

Regulatory Compliance

Firmware signing is necessary in many industries to be compliant and as part of a component of secure development and deployment.

How Firmware Signing Works?

The theory of firmware signing is quite complex, but the essential theory behind it is quite simple and the analogue of a lock-and-key trick, just math. Here’s how it works:

Key Generation

It all starts with a key pair, one private, one public. These are not physical keys, of course, they’re long strings of cryptographic data. The private key is kept secret by the firmware publisher. The public key is shared freely with all devices.

Signing

Once the firmware is developed, the developer confirms it with the signature using the private key. This does not encrypt all the firmware. Rather, it generates a small, distinctive cryptographic digest (signature) out of the contents of the firmware.

With a single change in one bit, e.g., a hacker introduces some malicious payload, the signature is considered invalid.

Verification

When a device is updated with a new firmware, it does not believe it by default. It does its check with the publicly known and trusted public key that it holds.

Execution

Only after successful verification does the device install or run the firmware. Otherwise, it halts the process, logs the error, or reverts to a known-good state. This last step, refusing to execute unsigned or invalid firmware, is the whole point. It turns the device from a passive recipient to an active gatekeeper.

How to Create or Get a Firmware Signing Certificate?

Before you can securely sign firmware, you need a firmware signing certificate, a digital certificate that proves the identity of the signer and enables the generation of trusted signatures. Selecting the right type and source of certificate is critical for both security and compatibility.

Firmware is typically signed using a Code Signing Certificate, which verifies the authenticity and integrity of the firmware code. There are two main types:

Standard Code Signing Certificates:

Suitable for basic use cases, these are issued to verified individuals or organizations. Standard Code Signing Certs are widely accepted for signing firmware, drivers, and software.

Extended Validation (EV) Code Signing Certificates:

It is used when regular trust isn’t enough. They require stricter identity checks, not just email and a form, but real vetting.

Platforms like Windows demand EV Code Signing for things like kernel-mode drivers. It’s slower and more locked down by design.

Where to Get a Firmware Signing Certificate?

You have two main options:

Self-Signed Certificates

You generate your own private/public key pair, and you sign your firmware. No outside help, no certificate authority (CA), no cost. This is great for internal testing or lab environments. It’s a bit like writing your passport and expecting other countries to let you in.

But it has the following disadvantages.

• Not scalable, every device must be manually configured to trust your public key
• Not suitable for commercial products
• Risky if your private key leaks, you have no external authority to revoke or replace it

Recommended: Self-Signed vs. Publicly Trusted CA Code Signing Certificates: What to Choose?

Purchase from a Trusted Public CA

This is what most serious organisations use. Instead of writing your credentials, you go to a trusted Certificate Authority (CA) like DigiCert, Sectigo, Certera, Comodo or GlobalSign, and they issue you a code signing certificate.

Recommended: Buy Trusted Code Signing Certificates at Low Cost!

These certificates come with something called a root of trust, a chain of validation that devices already recognise. It’s like getting a passport from a well-known country: when your firmware shows up, the device says, “Ah, this signature comes from a known authority. Proceed.”

Best Practices for Managing Firmware Signing Certificates

So now you have obtained your firmware signing certificate. Great. However, you can have a certificate, but not enough. When you do it wrong, one bad throw, and you get a compromised product at best, or worse, a supply chain attack that everyone can use when hooking up your shipped devices.

This is how you can be safe, multiscale, and manage your signing infrastructure:

Store the Private Keys in HSMs

Your identity is your signing key. Do not put it in a folder named keys_backup on the hard drive of a machine. It prevents the critical document physically. Disallow tampering with the block remotes and sign only trusted applications.

Recommended: Top Best Practices for Storing X.509 Private Keys

Rotate the Certificates Before Expiry

Waiting until a certificate is about to expire to change it? That is an outage in the making. Always. Pre-configure auto notifications before expiration. Turn to certificates before the due date (a minimum of 30 to 60 days). To have in place an alternative as a key revocation plan in case of compromise.

Recommended: How to Avoid Code Signing Certificate Expired Issues?

Keep Track of Who Signs What And When

The process of code signing should be treated as a monetary transaction, and logging must be used. Put in place complete audit trails. Learn the signature of the firmware. Signing exception: Flag unexpected signing activity.

Timestamping of Long-Term validity

Certs are gone next week, and firmware will continue after four years in the field. The assurance is RFC 3161-compliant timestamping. The signature does not lose its validity even when the certificate has expired. Not only who signed it, but also when the firmware was signed, can be proven.

Restrict Access using the Role-Based Access Control (RBAC)

Not all developers should have signing authority. That is why we implement RBAC. Restrict the signing tools’ access. Separation of duties should be enforced. Minimise the risk posed by insiders.

Best Practices for Firmware Security in IoT Devices

IoT devices are often small, cheap, and scattered across the world. That makes them hard to secure and easy to exploit. If you don’t bake security into the firmware itself and the way it’s delivered, you’re just hoping things don’t go wrong. Here’s what works:

Use Secure Boot

The first step is refusing to boot anything untrusted. Secure Boot checks the firmware signature before the device even starts. If the signature’s missing or wrong, the device halts.

Encrypt Firmware at Rest and in Transit

Even signed firmware can leak. Attackers shouldn’t be able to read or reverse engineer your firmware just because they intercepted it.

Give Each Device a Unique Identity

Don’t use a shared key across your entire fleet. That’s one compromise away from total loss. Generate a unique key pair per device. Use it to authenticate and validate updates. Now, even if one device is breached, the others stay safe.

Sign and Validate OTA Updates

Over-the-air updates are a gift and also bad. You can fix bugs fast, or you can ship malware to every device with one mistake. All OTA updates should be signed. Devices must verify integrity before installing.

Update Firmware Regularly

No system stays secure forever. Bugs show up, libraries go stale, and protocols become outdated.

Monitor After Deployment

Even signed, encrypted firmware can go wrong in the wild. Something breaks, or worse, gets abused. Add runtime integrity checks. Watch for signs of tampering or abnormal behaviour.

Conclusion

Firmware is the invisible brain inside modern hardware. It runs the show quietly, constantly. But when that brain is compromised, the whole machine turns against you.  Firmware signing isn’t just about compliance or best practices. It’s about trust and security.

Need help getting started? Contact us to buy trusted code signing certificates and protect every device you ship from the bootloader to the last update.

FAQs

Can I use the same certificate for multiple devices?

Yes, although one cannot always do that, as it may not be such a good idea. Technically, it is possible, but this is a single point of failure since the same certificate is shared on all devices. Once that key becomes compromised, all devices can be compromised. Better is to think per-device keys or at least per-product line separation.

What happens if my signing certificate is compromised?

You lose confidence. The devices can be rejected when it comes to future updates, and the malicious firmware can be installed by the attacker and can be presented as legitimate. That is why revocation plans, audit logs, and quick key rotation routines are important. The best medicine is prevention. Getting healed is costly.

Recommended: Risks and Challenges with Compromised Code Signing Certificate

What’s the difference between code signing and firmware signing?

Firmware signing is a specific form of code signing. The key difference? Firmware often runs at the lowest trust level, sometimes before the OS even boots. That makes integrity even more critical. A bug in a signed app might crash a window. A bug in signed firmware might brick your device.

How do IoT devices validate firmware signatures without internet access?

They check against a securely embedded root of trust, or root of trust, usually a public key or certificate burned into secure hardware when it was manufactured. No use of the internet. It verifies whether the embedded key, which is a signature of the firmware, matches or not.

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