Please follow the naming conventions, which is UpperCamelCase for property, method and event names (i.e. the first letter should be a capital letter) and lowerCamelCase for parameter names, no underscores .
Also you might want to read this
to understand, that HMAC is not about encryption, it is hash based message authentication code, so your extension is not really an encryption tool… EDIT: I now updated the subject into “Hash Value tool extension” and I hope, you are ok with that…
Hashing vs. Encryption algorithms
There is a fundamental difference between Hashing and Encryption algorithms, see this stackoverflow answer: Hashing is one way. You can not get your data/string from a hash code. Encryption is 2 way - you can decrypt again the encrypted string if you have the key with you.