In the world of cryptography, a small detail can make or break system security. One such detail is the nonce—a “number used once.” While simple in theory, nonces are foundational to ensuring secure communication, protecting firmware, and complying with FDA cybersecurity expectations in modern medical devices.
This article explains what nonces are, where they’re used in medical systems, and how they support encryption, authentication, and anti-replay defenses across the product lifecycle.
What Is a Nonce?
A nonce is a unique, single-use value used in cryptographic protocols to ensure that operations cannot be repeated or replayed maliciously. Think of it as a disposable digital token—a value that appears once and is never reused.
Types of Nonces:
- Random: Generated using secure random number generators (RNGs)
- Time-based: Derived from timestamps or counters
- Deterministic: Based on session-specific data or algorithms
Nonces are used in:
- Message authentication
- Secure communications (TLS, BLE, LoRa)
- Firmware signing and update protection
- Session tokens in web and device interfaces
Why Nonces Matter in Medical Devices
Medical devices are increasingly connected—via Bluetooth, Wi-Fi, cellular, or cloud. This connectivity opens them to a wide range of replay attacks, where attackers intercept and resend legitimate data or commands to cause unintended device behavior.
Nonce-Based Protections:
- Firmware Integrity: Each update includes a nonce in the signature to prevent reuse or rollback to older, vulnerable firmware.
- Telemetry Security: Nonces in wireless messages prevent spoofing or replay of prior legitimate data (e.g., falsifying a heart rate spike).
- Session Authentication: Secure session tokens incorporate nonces to prevent unauthorized re-access by sniffed credentials.
Risk Without Proper Nonce Use
🔁 Replay Attack Scenario:
A malicious actor captures a BLE command to initiate drug delivery on a wireless infusion pump. Without nonce validation, they replay the command to trigger an unauthorized dose—without needing to break encryption.
🔒 Real-World Mitigation:
Proper nonce handling ensures the command is rejected as a duplicate, preserving patient safety and system trust.
Nonce Implementation Best Practices
To securely implement nonces in your device architecture:
- Use a Secure RNG
Avoid low-entropy or pseudo-random sources. - Tie Nonces to Sessions or Counters
For deterministic systems, incorporate device uptime or firmware hash. - Store and Track Used Nonces
Implement memory or timestamps to prevent reuse during power cycles or network lag. - Include in Threat Modeling
Account for nonce misuse, collision, or exposure in design-phase threat models. - Document in SPDF and SBOM
Nonce handling should be logged in the Secure Product Development Framework and update pipeline.
Regulatory and Compliance Perspective
The FDA’s 2025 Cybersecurity Guidance explicitly requires protections against replay attacks—a use case where nonces are critical.
“Systems must validate the uniqueness of transmitted commands and data. Nonces or equivalent controls should be used to ensure operations cannot be replayed.” – FDA Cybersecurity Guidance (2025)
During premarket submissions and postmarket surveillance, regulators expect:
- Secure communication protocols using nonces
- Validation of firmware update authenticity
- Documentation of replay protection in risk analyses
Final Thoughts
In cybersecurity, there’s no such thing as a small detail. Nonces may seem like a simple tool—but for medical devices, they can mean the difference between secure operation and catastrophic misuse.
By implementing robust nonce handling, developers can safeguard firmware, telemetry, and user sessions—ensuring both compliance and trust.
Work With Blue Goat Cyber
At Blue Goat Cyber, we specialize in medical device cybersecurity—from firmware validation and threat modeling to FDA documentation. We’ll help you design nonce-secure systems that stand up to real-world attacks and regulatory scrutiny.
👉 Schedule a consultation to secure your connected device architecture.