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This Letter Proposes To Use Intelligent Reflecting Surface (IRS) As A Green Jammer To Attack A Legitimate Communication Without Using Any Internal Energy To Generate Jamming Signals. In Particular, The IRS Is Used To Intelligently Reflect The Signals From The Legitimate Transmitter To The Legitimate Receiver (LR) To Guarantee That The Received Signals From Direct And Reflecting Links Can Be Added Destructively, Which Thus Diminishes The Signal-to-Interference-plus-Noise Ratio (SINR) At The LR. To Minimize The Received Signal Power At The LR, We Consider The Joint Optimization Of Magnitudes Of Reflection Coefficients And Discrete Phase Shifts At The IRS. Based On The Block Coordinate Descent, Semidefinite Relaxation, And Gaussian Randomization Techniques, The Solution Can Be Obtained Efficiently. Through Simulation Results, We Show That By Using The IRS-based Jammer, We Can Reduce The Signal Power Received At The LR By Up To 99%. Interestingly, The Performance Of The Proposed IRS-based Jammer Is Even Better Than That Of The Conventional Active Jamming Attacks In Some Scenarios.