MD5加密

+ (NSString *)md5:(NSString *)str{

    const char *original_str = [str UTF8String];

    unsigned char result[CC_MD5_DIGEST_LENGTH];

    CC_MD5(original_str, (unsigned int)strlen(original_str), result);

    NSMutableString *hash = [NSMutableString string];

    for (int i = 0; i < 16; i++)

        [hash appendFormat:@"%02x", result[i]];

    return hash;

}