Software-based Code Attestation for Wireless Sensors

Introduction Securing sensors in critical applications is important Compromise of a sensor can enable attacker to inject false sensing information Compromise of shared keys can enable attacker to compromise secure communications 2

Attestation How to detect compromise? Attest! Ensure that the contents of the memory are unchanged – Detects sensor compromise that involves a modification of the program memory – Compute a checksum of the memory contents 3

Naïve Attestation Model Attestation routine reads memory and computes a checksum Attacker must offset memory reads to avoid detection – Offsets incur measurable delay in execution – Attester can measure execution time to detect compromise 4 MalcodeAttest Unmodified Copy Of Original Program Memory of Sensor

Limitations Suitable for directly connected devices – Slight execution delays can be accurately measured 5

Remote Attestation How can we adapt the attestation model to work in a remote setting? Prevent attacker from analyzing attestation code offline – Send the attestation routine to the sensor – Make it different each time Prevent attacker from modifying attestation code – Use techniques to make it difficult to statically analyze 6

Why Remote Attestation? Is remote attestation really necessary? Physical access to the sensors is not always feasible – Military setting - sensors are located in hostile, enemy territory – Building monitoring - sensors could be located in dangerous/inaccessible locations 7

Building Blocks Randomization Encryption Self-Modifying Code Obfuscation – Opaque Predicates/Pointer Aliasing – Junk Instructions 8

Opaque Predicates Conditions that always evaluate to true or always evaluate to false – Evaluation result is not obvious from static analysis Can be formed through pointer aliasing – known to be an NP-hard problem 9

Junk Instructions Full or partial machine code instructions – Full - distract analysis – Partial - confuse analysis 10

Attestation Protocol 11 Compare Results Measure Response Time Generate Attestation Routine Precompute Result Base Attestation Routine Checksum Result Execute Attestation Routine Sensor

Attestation Routine Overview Randomly step through program memory, adding values to the checksum result Loop repeats O(n log n) times to ensure complete coverage of the memory Routine will incorporate the building blocks to prevent attacks on the routine itself 12

New attacks: – Return oriented programming: does not need to inject malicious code – Manipulate the program stack – Completeness in the libc library – Not complete for sensor OS, but prototype has been demonstrated 13