1. https://www.techopedia.com/definition/6472/cipher
. Last accessed 29 October 2018
2. http://practicalcryptography.com/ciphers/
. Last accessed 29 October 2018
3. Jussien, N.: A to Z of Sudoku, 1st edn. Wiley, ISTE, USA (2007)
4. Felgenhauer, B., Jarvis, F.: Enumerating possible Sudoku grids. Math. Spect. 1–7 (2005)
5. Yato, T., Seta, T.: Complexity and completeness of finding another solution and its application to puzzle. IEICE Trans. Fundam. E86-A(50), 1052–1060 (2003)