added to deobfuscation

reverse engineering/docs/deobfuscation.md

@@ -1,4 +1,97 @@
 # Deobfuscation
 
+## Principles of Obfuscation
+
+* Software obfuscation may be divided into a theoretical layered approach, done by [Hui Xu et. al](https://cybersecurity.springeropen.com/track/pdf/10.1186/s42400-020-00049-3.pdf)
+
+* These layers and what's obfuscated are:
+    * __Code Element__
+        * Layout
+        * Controls
+        * Data
+        * Classes
+        * Methods
+    * __Software Component__
+    * __Inter Component__
+        * Library calls
+        * Used Resources
+    * __Application__
+        * DRM System
+        * Neural Networks
+
+## Evade Statical Rules
+
+* Critical data is obfuscated by the __Code Element__ layer which contains the following methods of obfuscation
+    * __Array Transformation__
+    * __Data Encoding__
+    * __Data Procedurization__
+    * __Data Splitting & Merging__
+
+### Splitting & Merging of Strings
+
+* Breaking signature by modifying data distribution inside the code
+* This may be done by modifying strings and functions through following measures
+
+* __Joining__
+```python
+"CAFFEE" + "BABE"
+```
+
+* __Reordering__
+```python
+a = "BABE"
+b = "CAFFEE"
+f"{b}{a}"
+```
+
+* __Whitespaces of functions which are not interpreted__
+```c
+int main ( void ) {
+    printf ( "The answer is %d", 42 ) ;
+}
+```
+
+* __Adding ticks which are not interpreted__
+
+* __Change `uPpER aNd loWeRcAsE oF cHaRaCtErS iN tHe StRinG`__
+
+### Adding Unnecessary Instructions
+
+* Obfuscation of layout and controls inside the code
+* __Junk Stubs__
+* __Separation of Related Code__
+* __Stripping Redundant Symbols__
+* __Meaningless Identifiers__
+* __Converting Explicit to Implicit Instructions__
+* __Dispatcher Based Controls Executed During Runtime__
+* __Probabilistic Control Flows__
+* __Bogus Control Flows__
+
+
+### Control Flow
+
+* Changing or adding to the flow of the code through change of conditions
+* Changes may be set to arbitrary code segments by __Opaque Predicates__
+* An __Opaque Predicate__ is a control path and value known by the obfuscater and hard to find out by the reverse engineer
+
+### Protecting Data
+
+* Stripping and protecting
+    * __Code Structure__
+    * __Object names__
+    * __File & Compilation Properties__
+
+* To strip symbols
+```sh
+strip --strip-all <binary>
+```
+
+* Check via
+```sh
+nm <binary>
+```
+
+## Usage
+
 * Find a deobfuscator like [de4dot](https://github.com/de4dot/de4dot.git) for e.g. deobfuscating dotfuscator 
-* In case of dotnet: Do not only use ghidra for reversing, use [ILSpy](https://github.com/icsharpcode/ILSpy.git) as well
+* In case of dotnet: __Do not only use ghidra for reversing, use [ILSpy](https://github.com/icsharpcode/ILSpy.git) as well__