计算文件的SHA1 值的实现源码。

安全哈希算法（Secure Hash Algorithm）主要适用于数字签名标准 （Digital Signature Standard DSS）里面定义的数字签名算法（Digital Signature Algorithm DSA）。对于长度小于2^64位的消息，SHA1会产生一个160位的消息摘要。当接收到消息的时候，这个消息摘要可以用来验证数据的完整性。在传输的过程中，数据很可能会发生变化，那么这时候就会产生不同的消息摘要。 SHA1有如下特性：不可以从消息摘要中复原信息；两个不同的消息不会产生同样的消息摘要。-Secure Hash Algorithm (Secure Hash Algorithm) is mainly applied to the Digital Signature Standard (Digital Signature Standard DSS) which defines the Digital Signature Algorithm (Digital Signature Algorithm DSA). For the message length is less than 2 ^ 64 s, SHA1 produces a 160-bit message digest. When a message is received when the message digest can be used to verify the integrity of the data. During the transmission, the data is likely to change, then this time it will have a different message digest. SHA1 has the following features: You can not recover information　the message digest　two different messages do not produce the same message digest.