2022-11-17
"A detailed analysis of obfuscation techniques found in modern ECUs and how to mask external function calls."
In many post-2020 generation Denso-based ECUs, logic structures that are not directly referenced by standard map calls, but instead hidden by various bitmasks and chained conditional branches, have begun to attract attention. In this article, we explain how we analyze these advanced structures underlying some seemingly simple ECU firmware files and what methods we use to produce solutions.
Beyond the Visible Structure: Why Simple Hooks Are Not Enough?
Many software experts think that it will be enough to insert external logic by directly modifying the address after finding a specific map call. However, things are not that simple in the new generation Denso ECUs. Map pointers are generally not used directly. Instead, the values loaded by the instructions are made difficult to analyze with bit-level masks and conditional jumps such as andi, cmp, and bne.
This approach is used not only to hide the code, but also to trigger different paths according to different runtime conditions. The map usage logic can change dynamically depending on factors such as load, temperature, boost threshold, etc. This makes it impossible to determine map relationships with superficial analysis.
Real Use of Encryption and Obfuscation Techniques
The following structures are obvious obfuscation techniques that we often encounter in Denso and similar systems:
- Branching with bitmask: A single byte is used to redirect 4-5 different jarl calls.
- Conditional logic chains: Before the map control, 3-4 different control blocks are triggered and the points where the hook will be placed are distributed.
- Variable jump tables: Real function calls are formed at runtime with the use of jmp.
- Decision tree based on EEPROM or RAM flags: Map ID or user selections are processed indirectly, not directly.
This situation can lead to serious time loss and error risks, especially for reverse engineering teams. These systems, which are invisible on the surface but noticed when delving deeper, can only be understood with advanced dynamic analysis and full memory monitoring.
Where are Such Protection Mechanisms Seen?
It has been determined that such encryption and masked logic structures are actively used on the following platforms:
Denso-based heavy vehicle control systems such as Isuzu, Hino, Some Mazda SkyActiv ECUs, New generation Subaru Denso controllers
Especially vehicles from production year use this logic more intensively to determine internal firmware checkpoints.
Such structures, which are not obvious in the firmware but are revealed by behavioral analysis, have become standard in modern ECU software. In such cases, the classic “hook and pass” approach does not work. Real solutions are possible with a holistic analysis.
Our team has extensive experience in solving such encrypted logic structures. If you have encountered a similar situation, contact us we are ready to support you for the right solution and safe intervention.
Case Study: Obfuscation and Encrypted Logic in Subaru ECUs – Lessons from Legacy Firmware
In the evolving landscape of ECU reverse engineering, one recurring theme is the increasing use of obfuscation and dynamic encryption within firmware logic. Subaru platforms particularly Denso-based control units have quietly adopted similarly challenging protection mechanisms. This article presents a case study examining how certain Subaru ECUs implement fragmented control logic, encrypted dispatch routines, and bitmasked access conditions, and how we approached the problem.
Background: What Makes Subaru ECUs Different
Unlike platforms with easily traceable function calls or pointer references, specific Subaru ECUs often found in late-2010s turbocharged models employ a hybrid structure where calibration maps are accessed not directly, but through a layered execution flow. In these units:
- Function access is often gated by conditionally masked variables.
- Core maps are dispatched via indirect jump tables derived from runtime bitfields.
- There is no single, static reference to any map structure rendering traditional patch-and-test workflows ineffective.
These characteristics make it difficult to hook logic based on superficial patterns or rely on binary diffing with similar models. In short, appearances can be deceiving.
Problem Overview: Invisible Hooks and Encoded Conditions
In one analysis, a fuel limiter table appeared to be referenced by a known address. However, attempting to patch the call site resulted in no behavioral change during runtime. Upon further inspection using step-by-step tracing, it became evident that the value read from this address was not immediately used instead, it was masked, passed through several compare-and-branch operations, and then redirected to a handler that selected the map only if specific runtime flags aligned.
This effectively means:
- Hooks placed based on static assumptions may never trigger.
- The logic path that reaches the map is decided on-the-fly.
- Encoded flags decide which limiter variant is selected.
These layers of abstraction were not documented in any service material, nor visible in tuning tools.
Reverse Engineering Approach
To address this, we employed a two-step strategy:
- Behavioral Logging: By logging access to suspected memory locations under different conditions (idle, load, cold start), we established a matrix of trigger conditions.
- Decoding the Dispatch Table: Eventually, we reconstructed the internal logic responsible for dispatching eacriant of the map and identified the master switchpoint responsible for selection.
This process allowed us to safely place a logic hook that preserved stock behavior when conditions were unmet and redirected flow when tuning flags were present.
Subaru’s approach is often stealthier—executing obfuscation through dispatch indirection and runtime masking. These techniques do not only hinder unauthorized tampering but also challenge professional modification efforts that rely on deterministic behavior.
Subaru ECUs using Denso architecture represent an ongoing evolution in control logic design—one that requires reverse engineers to think beyond static analysis and adopt more dynamic, context-driven methods.
Our team has successfully analyzed, documented, and modified encrypted logic across a wide range of platforms including Subaru, Honda, and Mazda. If your current project is facing invisible hooks, encrypted logic chains, or unclear map routing, we’re here to help.
Contact us to schedule a review of your firmware.
Let's Work Together
Need Professional Assistance with Reverse Engineering or Cybersecurity Solutions? Our Team is Ready To Help You Tackle Complex Technical Challenges.