refactor: Enhance docs, add more tests in `XORCipher`

[Hardvan]

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 66.80%. Comparing base (e499d3b) to head (99a0353).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5900   +/-   ##
=========================================
  Coverage     66.80%   66.80%           
  Complexity     4501     4501           
=========================================
  Files           611      611           
  Lines         16958    16962    +4     
  Branches       3275     3275           
=========================================
+ Hits          11328    11331    +3     
  Misses         5182     5182           
- Partials        448      449    +1     

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