what is hmac and master file in aem
hmac and master files are used by cyptoSupport bundle in AEM which provides a simple API to encrypt and decrypt binary and string data.
You can also use these files to replicate crypto keys from one instance to another. Check  for more details.
Check  for more details on the hmac cryptography
 CryptoSupport ("The Adobe AEM Quickstart and Web Application.")
 Encapsulated Token Support
 HMACSHA256 Class (System.Security.Cryptography)