openssl hmac-sha256

使用openssl生成sha256的数字签名

命令如下：

openssl dgst -binary -sha256 -mac HMAC -macopt hexkey:$kSecret $filename

参考文档：
1.openssl hmac differ from python hmac
2.HMAC使用Linux内核加密api不一样,通过OpenSSL命令
3.openssl命令行入门
4.Using openssl to generate HMAC using a binary key
5.AWS Signature v4 in Bash
6.HMAC-SHA1 in bash
7.S3-AWSV4-upload.sh