Windows provides a WritePrivateProfileStruct API which can be used to write binary data to an INI file, and a GetPrivateProfileStruct API which can be used to read that binary data back from the INI file.
The binary data is serialised in hex format, followed by a single additional byte which is a checksum. For example:
[ultravnc]
passwd=2AE0C448372D3C1CD2
in that case, the binary data is 2AE0C448372D3C1C and the checksum byte is D2.
How is that checksum calculated?
The checksum is calculated simply by adding up each byte, and using the result (mod 256) as the checksum. It is not using anything more sophisticated than that.