The paper introduces *template attacks*, the strongest form of side-channel attacks in an information-theoretic sense. These attacks can break implementations and countermeasures that assume adversaries cannot obtain more than a limited number of side-channel samples. Template attacks require an identical experimental device that can be programmed to the adversary's choice. The success of these attacks lies in handling noise within each sample, which is different from previous approaches that focused on reducing or eliminating noise. The authors describe how a single sample can be used to break an RC4 implementation, which is not susceptible to techniques like SPA and DPA. Other applications include attacks on DES implementations using DPA-resistant hardware and SSL accelerators, where electromagnetic emanations from RSA operations can be monitored from distances of 15 feet. The paper also discusses the theory behind template attacks, including the multivariate Gaussian model and pruning processes, and provides case studies to illustrate their effectiveness. Finally, it explores the implications and potential countermeasures against template attacks.The paper introduces *template attacks*, the strongest form of side-channel attacks in an information-theoretic sense. These attacks can break implementations and countermeasures that assume adversaries cannot obtain more than a limited number of side-channel samples. Template attacks require an identical experimental device that can be programmed to the adversary's choice. The success of these attacks lies in handling noise within each sample, which is different from previous approaches that focused on reducing or eliminating noise. The authors describe how a single sample can be used to break an RC4 implementation, which is not susceptible to techniques like SPA and DPA. Other applications include attacks on DES implementations using DPA-resistant hardware and SSL accelerators, where electromagnetic emanations from RSA operations can be monitored from distances of 15 feet. The paper also discusses the theory behind template attacks, including the multivariate Gaussian model and pruning processes, and provides case studies to illustrate their effectiveness. Finally, it explores the implications and potential countermeasures against template attacks.