There have been many attempts to break the remaining cipher(s). Most attempts have tried other historical texts as keys (e.g., Magna Carta, various books of the Bible, the U.S. Constitution, and the Virginia Royal Charter), assuming the ciphertexts were produced with some book cipher, but none have been recognized as successful to date. Breaking the cipher(s) may depend on random chance (as, for instance, stumbling upon a book key if the two remaining ciphertexts are actually book ciphers); so far, even the most skilled cryptanalysts who have attempted them have been defeated. Of course, Beale could have used a document that he had written himself for either or both of the remaining keys or either a document of his own or randomly selected characters for the third source, in either case rendering "any" further attempts to crack the codes useless.