R.N.Shakirov. Concept of multi-platform class cBigNumber for implementation of unlimited integer arithmetic. The class works on a variety of platforms and compilers as it is implemented on standard C++. It uses additive, bitwise and shift operations to provide for all regular C++ operations on unlimited integer numbers. Extra functions are also supported: power, power by module, square root and Miller strong probable prime test. In order to implement robust computation, the class contains built-in tools for error correction and compensation, including reserved array element, run-time check of indexes, initial and tail conditions. These tools are proved to be effective as they prevent against 70% of errors discovered at the test phase. Algorithms are optimized for numbers containing from 500 to 20,000 bits, tests were carried out for more 20,000,000 random numbers containing up to 12,000,000 bits. The class is available for free downloading from page http://www.imach.uran.ru/cbignum.