I read that the secret $IV$ and secret prefix $MAC$ methods are equivalent. Intuitively, this sounds reasonable because the $IV$ and the secret prefix are both the foundation of the inner state at the point of time when the actual message is hashed/compressed.
E.g. in "MDx-MAC and Building Fast MACs from Hash Functions" (link) by Preneel and Oorschot. Section 4.1 The Secret Prefix Method: "If the key consists of a complete block, this corresponds to a hash function with a secret IV."
With secret $IV$ $MAC$ I mean that the key is written into the $IV$ variables / the internal state before the message is hashed.
With secret prefix $MAC$ I consider the function $H(k\ ||\ m)$ where $H$ is a hash function, $k$ is the secret key, $m$ is the message and $||$ denotes concatenation.
I guess the security properties of these two schemes are indeed equivalent but how does this work in practice? For example some attacks against $MD4$ $MAC$s involve the work of offline $MD4$ computations. I can do that on messages with standard $MD4$ in the secret prefix $MAC$ setting (just leaving the unknown key out), but how can one do an offline computation when the secret $IV$ is unknown. How should the hash function be initialized and how (if at all) are the attacks applicable?
Examples of such attacks are:
New Key-Recovery Attacks on HMAC/NMAC-MD4 and NMAC-MD5 (this targets $HMAC$ instead of secret prefix $MAC$, but I guess the attack is adaptable)
Password Recovery Attack on Authentication Protocol MD4(Password || Challenge) (... must leave out the reference here, due to missing repudiation)