Google leveled new charges against China this week, claiming that the country has interfered with some citizens' access to the Internet giant's Gmail service, disguising the interference as technical glitches.

Security experts say that China is most likely using invisible intermediary servers, or "transparent proxies," to intercept and relay network messages while rapidly modifying the contents of those communications. This makes it possible to block e-mail messages while making it appear as if Gmail is malfunctioning.

Companies regularly use transparent proxies to filter employees' Web access. Some ISPs have also used the technique to replace regular Web advertisements with those of their own. But it's becoming increasingly common for governments to use transparent proxies to censor and track dissidents and protestors. All traffic from a certain network is forced through the proxy, allowing communications to be monitored and modified on the fly. Intercepting and relaying traffic is known as a "man in the middle" attack.