The Purple files were Londo Mollari's secret family files that Adira Tyree briefly stole from him in 2259. They contained "dirt" on most of the other noble Houses of the Centauri Republic, information that Adira's slave master, Trakis, hoped to exploit for profit, even though if the information got out it could lead to the toppling of the whole Republic.
The purple files were encrypted with a triple cipher, which is reminiscent of 3DES, which is a triple (tri-layer) DES encryption algorithm. The access code for Londo's Purple Files was: wine women song. To be more specific, the decryption key for the first layer of encryption was "wine", the decryption key for the second layer was "women", and the decryption key for the third layer of encryption was "song".