When facing a binary for which Ghidra lacks the SLEIGH specifications required to disassemble it, you hit a problem that the vast majority of reverse engineers will never encounter.
Out of an estimated 25,000 reverse engineers in the United States, only about 500 (~2%) work on this specific domain.
The Reality of Undocumented ISAs A processor without a documented instruction set is never completely exotic.
Recreating an Instruction Set Architecture (ISA) from scratch is far too expensive.
Consequently, almost all undocumented processors are derivatives of major existing families: Base architectures with stripped-down instructions Custom instructions added for dedicated workloads Scrambled or non-standard opcodes due to the confidentiality of underlying operations To master this niche, hands-on experience is everything.
CTFs with custom VMs are a good start, but nothing beats generating your own custom processors and sharpening your tools against custom-built challenges.
The Pitfalls of Processor Generation Building a custom processor generator sounds straightforward, until you run into data extraction issues: retrieving valid opcode/instruction tables for major architectures.
Note on LLMs: For ancient 8-bit architectures with reduced instruction sets, LLMs can extract tables flawlessly (see my previous research on dev.to/ddupard).
However, as soon as you target modern, heavy ISAs, LLMs break down.
The Reliable Approach: The most robust solution consists of extracting, cleaning, and parsing the files (such as ) directly from binutils-gdb.
The Custom Processor Toolchain To generate a custom processor, you need a core triplet.
Example in ARM 32-bit: | | This triplet determines the possible permutations applicable to both opcode and operand bits, as well as the exact search space.
Once you possess a clean table, the process becomes a pure pipeline problem: Create a Transcoding Table: Map native instructions to your custom ISA (optionally dropping specific instructions).
Transcode the Target Binary: Process a standard binary compiled for the native ISA through your map.
Inject Custom Instructions: Insert new logic while recalculating jump, branch, and call offsets.
Fix Binary Headers: Re-align sections (e.g., within ELF structures).
Re-assemble & Output: Regenerate a valid, clean custom binary.
You now have a fully functional custom binary ready for analysis practice.
Taking It to the Hardware Level (FPGA Obfuscation) This software approach also maps directly to hardware logic.
You can easily derive a Verilog file from your pipeline to deploy an FPGA dedicated to this new processor.
If your goal is to make analysis virtually impossible, you can add layers of defense: Two-Layer Encryption: One static key combined with one dynamic key.
LUT Geometry Alteration: Tweak Lookup Table geometry to break standard ASIC/FPGA reversing.
Redundancy & Voting Systems: Implement hardware voting modules.
Anti-Tamper Physical Actions: Remove JTAG access and blow physical security fuses.
By removing hardware debug interfaces, standard chip-attack techniques fail — leaving the analyst with pure, unassisted combinatorial complexity.