There are tow methods naming ***statistical analysis or [Frequency analysis][1]*** and ***pattern matching***.  
Note that in statistical analysis Eve should compute Frequency for $aLetter \oplus aLetter$ using some tool like [this][2]. A real historical example for Frequency analysis is [VENOVA project][3].


  [1]: http://en.wikipedia.org/wiki/Frequency_analysis
  [2]: http://robert-lerner.com/live-letter-frequency-analysis.php
  [3]: http://www.characterfrequencyanalyzer.com/english/index.php