Decompilation

Decompilation is the process of transforming executable code, such as machine code or bytecode, back into a higher-level representation that resembles the original source code. It is the reverse of compilation and is used to analyze or modify software when the source is unavailable. Decompilers aim to reconstruct readable code that a human can understand, rather than produce an exact replica of the original source.

How it works: The decompiler analyzes control flow, data flow, and available type information to rebuild functions, loops, conditionals, and variable-like entities. It may use symbolic execution or pattern matching to identify common constructs. The output is usually pseudocode or source-like code in a target language. Because compilers optimize and may discard information such as local variable names and comments, the recovered source rarely matches the original.

Targets and approaches: Java bytecode and .NET intermediate language can often be decompiled to Java or C# with reasonable fidelity. Native binaries (such as x86 or ARM) are more challenging and typically yield C-like pseudocode. Decompilers vary in their use of static analysis, dynamic analysis, or hybrid methods, and some are specialized for particular platforms or languages.

Uses and limitations: Decompilation is used for software maintenance, interoperability, malware analysis, and vulnerability research. Readability depends on the original program and the optimizer; lacking symbols and with inlined or optimized code, the output can be hard to interpret or misleading.

Examples: Tools such as RetDec, Ghidra, and Snowman illustrate decompilation in practice, while language-specific options include CFR, Procyon, and JD-GUI for Java, and ILSpy or dnSpy for .NET.

Legal and ethical considerations: Decompilation is subject to copyright, license terms, and anti-circumvention laws that vary by jurisdiction. In some contexts, it is allowed for interoperability or security research; in others, it may be restricted. Consult applicable laws and licenses before proceeding.