🚀 go-pugleaf

   So what are some of the methods used to attack an encrypted text, when the
   encryption method is unknown?

One method used in WW2 was to send divers to recover encoding equipment,
codebooks, etc. from sunken enemy ships (perhaps sometimes ships
that were attacked specifically for the purpose of getting the
crypto stuff).  Another method is to capture the sender or receiver
of the messages and demand the encryption keys etc. at gunpoint.

There are also cryptanalytic methods, but I mention the WW2 story to
underscore the fact that sci.crypt readers responding to "challenges"
are fortunately NOT likely to use ALL of the methods that a real world
codebreaker would use.  Any purely cryptanalytic challenge to
sci.crypt, in order to be fair, must provide as much info as an enemy
spy agency would be able to obtain by hook or by crook---this includes
a complete description of the algorithm, plus some known plaintext of
the same nature as the challenge messages.

Basically, you do whatever you can.  For both cryptanalytic and "war
stories" answers to your question, see "The Codebreakers" by David
Kahn.