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In cryptography, a timing attack is a side channel attack in which the attacker attempts to compromise a cryptosystem by analyzing the time taken to execute cryptographic algorithms. Every logical operation in a computer takes time to execute, and the time can differ based on the input; with precise measurements of the time for each operation, an attacker can work backwards to the input. Information can leak from a system through measurement of the time it takes to respond to certain queries. How much such information can help an attacker depends on many variables: crypto system design, the CPU running the system, the algorithms used, assorted implementation details, timing attack countermeasures, the accuracy of the timing measurements, etc. Timing attacks are often overlooked in the design phase because they are so dependent on the implementation. == Concept == A timing attack is an example of an attack that exploits the data-dependent behavioral characteristics of the implementation of an algorithm rather than the mathematical properties of the algorithm itself. Many cryptographic algorithms can be implemented (or masked by a proxy) in a way that reduces or eliminates data dependent timing information: consider an implementation in which every call to a subroutine always returns in exactly x seconds, where x is the maximum time it ever takes to execute that routine on every possible authorised input. In such an implementation, the timing of the algorithm leaks no information about the data supplied to that invocation. The downside of this approach is that the time to execute many invocations increases from the average performance of the function to the worst-case performance of the function. Timing attacks are practical in many cases: * Timing attacks can be applied to any algorithm that has data-dependent timing variation. Software run on a CPU with a data cache will exhibit data-dependent timing variations as a result of memory looks into the cache. Some operations, such as multiplication, may have varied execution time depending on the inputs. Removing timing-dependencies is difficult in some algorithms that use low-level operations that frequently exhibit varied execution time. * Finding secrets through timing information may be significantly easier than using cryptanalysis of known plaintext, ciphertext pairs. Sometimes timing information is combined with cryptanalysis to improve the rate of information leakage. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Timing attack」の詳細全文を読む スポンサード リンク
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