Improving the security of this tag
Please read the article on HMAC. What you are trying is exactly what is described in the chapter design principle:


  The design of the HMAC specification was motivated by the existence of attacks on more trivial mechanisms for combining a key with a hash function. For example, one might assume the same security that HMAC provides could be achieved with $MAC = H(key ∥ message)$. However, this method suffers from a serious flaw: with most hash functions, it is easy to append data to the message without knowing the key and obtain another valid MAC ("length-extension attack"). ...


There are only two differences to your idea:


Reverse the order of message and key/tag
The idea of having a "tag", where I can only assume it is the usual case as with passwords: Low entropy, unsuitable to be used as a cryptographic key.


So basically you should just use HMAC (if that fits your needs), but put your tag into a password-based key derivation function first, e.g. PBKDF2, scrypt or bcrypt. That key can then be used for HMAC.

Digital signatures on the other hand also offer non-repudiation, and HMACs don't because both the sender and the receiver know the same symmetric key and thus could both have created the HMAC.