It is generally required to obtain a visa to enter China. You should contact the local ambassy / consulant to go through the process.
More information can be found here. The time to obtain a visa can vary widely and it is advisable to begin the application process as early as possible.
In case that an invitation letter is needed, please fill this form. Please fill EVERY field correctly, otherwise you risk the rejection of visa application.
Any further questions, please contact: firstname.lastname@example.org.
We hope that you will be able to attend Asiacrypt2012 and help us make the conference a great success.