hmac and master files are used by cyptoSupport bundle[1] 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 [2] for more details.
Check [3] for more details on the hmac cryptography
[1] CryptoSupport ("The Adobe AEM Quickstart and Web Application.")
[2] Encapsulated Token Support
[3] HMACSHA256 Class (System.Security.Cryptography)
