I've read quite a bit of the StackExchange and HackerNews debates on the user of "peppers" in password hash security. There are a number of different implementations of the idea of a pepper, ranging from an additional hardcoded salt in the code
hash(password, pepper, salt), to encrypting each password hash separately, in which case the secret key is the pepper.
In the case of one of the middle ground approaches, a shared and secret pepper is included via
hash(hmac(password, pepper),salt). This is necessary primarily because of many hashing algorithm's reliance on the Merkle–Damgård construction which is vulnerable to length extension attacks.
However, Argon2 is not based on a Merkle–Damgård construction and therefore this attack would not apply. In using Argon2, can the naive approach of
argon2(password, pepper, salt) be used?
Additionally, the Argon2 specification Introduction seems to indicate that using HMACs, peppers, or secret keys at all is unnecessary. Is Argon2ID with strong memory, threads, and time costs enough on its own?