Taking the message you want to authenticate and combining it with a secret key creates a "message authentication code" (MAC). HMAC stands for "Hash-based Message Authentication Code".

Here's is how you calculate a hash using openssl cli:

openssl dgst \
  -sha1 \
  -mac HMAC \
  -macopt hexkey:$(echo -n "<secret key string>" | od -A n -t x1 | sed 's/ *//g') \
  <file path>

1. Echo the secret key to standard output without a trailing newline character (-n).
2. od stands for "octal dump".

• -A n - don't print the address of the input data
• -t x1 - output the input data as a series of hexadecimal bytes

3. 's/ *//g' - removes any spaces between the hexadecimal bytes output