说明：  SHA-256源代码，三个对外的函数：void SHA256Init (SHA256Context *sc) void SHA256Update (SHA256Context *sc, const void *data, uint32_t len) void SHA256Final (SHA256Context *sc, uint8_t hash[SHA256_HASH_SIZE]) 第一个函数初始化，第二个函数输入要hash的字符串，第三个函数输出结果
(SHA-256 source code, three external function: void SHA256Init (SHA256Context* sc) void SHA256Update (SHA256Context* sc, const void* data, uint32_t len) void SHA256Final (SHA256Context* sc, uint8_t hash [SHA256_HASH_SIZE]) The first function is initialized, the second hash function, enter the string, the third function of output)