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https://en.wikipedia.org/wiki/Checksum
https://zh.wikipedia.org/wiki/校验和
A checksum is a small-sized datum derived from a block of digital data for the purpose of detecting errors which may have been introduced during its transmission or storage. It is usually applied to an installation file after it is received from the download server. By themselves, checksums are often used to verify data integrity but are not relied upon to verify data authenticity.
The actual procedure which yields the checksum from a data input is called a checksum function or checksum algorithm. Depending on its design goals, a good checksum algorithm will usually output a significantly different value, even for small changes made to the input. This is especially true of cryptographic hash functions, which may be used to detect many data corruption errors and verify overall data integrity; if the computed checksum for the current data input matches the stored value of a previously computed checksum, there is a very high probability the data has not been accidentally altered or corrupted.
Checksum functions are related to hash functions, fingerprints, randomization functions, and cryptographic hash functions. However, each of those concepts has different applications and therefore different design goals.
校验和（英语：Checksum）是冗余校验的一种形式。 它是通过错误检测方法，对经过空间（如通信）或者时间（如计算机存储）传送的数据的完整性进行检查的一种简单方法。
计算机领域常见的校验和的方法有循环冗余校验（CRC）、MD5、SHA家族等。
Checksum 一个良好的校验和算法通常会对进行很小的修改的输入数据都会输出一个显著不同的值
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原文地址：http://www.cnblogs.com/yuanjiangw/p/6692535.html
