Pure brute force is brute force attack that does not use outside probability
information that is not found inherently in the key-space being searched [7].
Letter frequency analysis attack is an attempt to use the frequency of characters
appearing in a training set to increase the effectiveness of a brute force
attacks [16].
Markov models in password cracking is a way to represent the join probability
of different characters appearing together [15].
Targeted brute force attacks can comprise letter frequency analysis and
Markov models, but applies outside logic to these attacks [19]. For example
performing letter frequency analysis attacks, but use a different character
set for each character position.