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This paper describes a low power implementation of a secure EPC UHF Passive RFID Tag baseband system. To ensure the secure information transaction of the tag, traditionally the focus is on directly applying a low-complexity encryption engine. However, this approach could lead to the problem of known-plaintext attack (KPA). The attacker could make use of the known header to reveal the secret key. Our contributions are proposing a novel dataflow solution enforced by an AES cryptography engine embedded inside the passive RFID tag. Also, various low power design techniques are proposed to reduce the power consumption of the baseband of the passive tag. In particular, we propose a moving window PIE decoding algorithm and an improved Tausworthe sequence generator to reduce the power consumption. Other low power design techniques such as clock gating, optimal clock driving and parallel operations are extensively used in the design of the tag. The complete RFID tag which consists of an analog frontend, 136 bits one-time programmable (OTP) memory, charge pump, rectifier, clock divider, and the proposed baseband system, was designed using TSMC 0.18 mum process and verified. The area of the proposed baseband system is 0.446mm2 and from the power simulation, the overall power consumption of the baseband system with the AES encryption is about 4.695 uW.