There are tow methods naming statistical analysis or Frequency analysis and pattern matching.
Note that in statistical analysis Eve should compute Frequency for $aLetter \oplus aLetter$ using some tool like this. A real historical example for Frequency analysis is VENOVA project.
EDIT: Having statistical analysis of $aLetter \oplus aLetter$ like this says:
If a character has X distribution, The two character($c_1 \oplus c_2) with probability P are c1, c2.