As I am reading the Paper: [MDx-MAC and building fast MACs from hash
functions][1]

I am having trouble understanding the Attack on the Envelope Method (4.3). In particular, I am not sure what chaining variables are referring to and what Prerequisites have to be met in order to execute the attack.

Maybe a more detailed/ rephrased explanation could help me to understand the gist of the attack.


  [1]: https://people.scs.carleton.ca/~paulv/papers/Crypto95.pdf