Practical-Cryptography-for-Developers-Book | Practical Cryptography for Developers : Hashes , MAC , Key | Cryptography library

I'm having trouble finding some information. Does anyone know if the value returned from cipher.getAuthTag() (--> returns MAC) can be publicly visible?

TL;DR

Can a message authentication code be publicly visible, or does this need to be kept secret like a password?

Some background, I am trying to encrypt a file. I found this stackoverflow question and answer that helped me get started. https://stackoverflow.com/a/27345933/11070228

After doing some research in the nodejs documentation, I found that the answer uses a deprecated function. createCipher. The new function to use should be createCipheriv.

So, to use the new createCipheriv, I used the documentation to write a new encryption and decryption function, similar to the one in the post using the new createCipheriv function. After writing the decryption function, I got an error that was

Error: Unsupported state or unable to authenticate data

After googling that issue, it led me here to this github post. In a nutshell, it said that the authTag generated with the cipher is needed to decrypt the file.

I did not know what this authTag was, and neither did anyone I knew. So I started googling that and it let me to this blogpost. It states

The authTag is the message authentication code (MAC) calculated during the encryption.

And here is a wikipedia article on what a message authentication code is.

So. Here is my question. Can a message authentication code be publicly visible, or does this need to be kept secret like a password?

My code, not as relevant, but might help someone create the encryption and decryption using createCipheriv and createDecipheriv.

Encryption