My research aims to build physical infrastructures and the corresponding software interface to provide secure and private communications. The first part of this talk will focus on a stealth signal transmission system and its applications of private and anonymous communications. It is the first time that the amplifier noise has been deployed to carry signals. The signal is hidden in the noise with terahertz bandwidth. Since the bandwidth of the noise is far beyond the limit of any available receivers, the signal and noise cannot be directly detected. The stealth transmission system reaches Gigabit transmission rate and is compatible with the commercial optical communication networks. This technique can be applied for anonymous communication networks and introduces zero system redundancy. The second part of this talk includes several wide-band and ultra-fast signal-processing techniques for network security. A wide band radio frequency (RF) interference cancellation technique and its applications of signal anti-jamming as well as signal encryption will be discussed.