How to calculate HMAC programmatically

We are going to implement the hmac program that will calculate HMAC-SHA-256.

Our hmac program will need two command-line arguments:

1. The input filename
2. The secret key, hex-encoded

When it comes to the Application Programming Interface (API) for HMAC calculations, OpenSSL 3.0 provides three whole APIs:

• The deprecated legacy low-level API, consisting of functions with an HMAC_ prefix. A fun fact about this API is that even though it’s not an EVP API, it uses the EVP_MD API to access underlying message digest functions.
• The EVP_DigestSign API. This API is mostly used for digital signatures. It is possible to use this API for MAC, but such usage is not very intuitive.
• The EVP_MAC API. This new API was specially created for MAC and introduced in OpenSSL 3.0. We are going to use this API in our code.

The official documentation for the EVP_MAC API can be found on the man page of the same name:

$ man...