Read The Code Book for Free Online Page B

Focusing on words with only one letter is a standard cryptanalytic trick, and I have included it among a list of cryptanalytic tips in Appendix B . This particular trick works only because this ciphertext still has spaces between the words. Often, a cryptographer will remove all the spaces to make it harder for an enemy interceptor to unscramble the message.
    Although we have spaces between words, the following trick would also work where the ciphertext has been merged into a single string of characters. The trick allows us to spot the letter h, once we have already identified the letter e. In the English language, the letter h frequently goes before the letter e (as in the, then, they, etc.), but rarely after e. The table below shows how frequently the O, which we think represents e, goes before and after all the other letters in the ciphertext. The table suggests that B represents h, because it appears before 0 on 9 occasions, but it never goes after it. No other letter in the table has such an asymmetric relationship with O.

    Each letter in the English language has its own unique personality, which includes its frequency and its relation to other letters. It is this personality that allows us to establish the true identity of a letter, even when it has been disguised by monoalphabetic substitution.
    We have now confidently established four letters, O = e, X = a, Y = i and B = h, and we can begin to replace some of the letters in the ciphertext with their plaintext equivalents. I shall stick to the convention of keeping ciphertext letters in upper case, while putting plaintext letters in lower case. This will help to distinguish between those letters we still have to identify, and those that have already been established.
PCQ VMJiPD LhiK LiSe KhahJaWaV haV ZCJPe EiPD KhahJiUaJ LhJee KCPK. CP Lhe LhCMKaPV aPV IiJKL PiDhL, QheP Khe haV ePVeV Lhe LaRe CI Sa’aJMI, Khe JCKe aPV EiKKev Lhe DJCMPV ZeICJe h i S, KaUiPD: “DJeaL EiPD, ICJ a LhCMKaPV aPV CPe PiDhLK i haNe ZeeP JeACMPLiPD LC UCM Lhe IaZReK CI FaKL aDeK aPV Lhe ReDePVK CI aPAiePL EiPDK. SaU i SaEe KC ZCRV aK LC AJaNe a IaNCMJ CI UCMJ SaGeKLU?”
    eFiRCDMe, LaReK IJCS Lhe LhCMKaPV aPV CPe PiDhLK

    This simple step helps us to identify several other letters, because we can guess some of the words in the ciphertext. For example, the most common three-letter words in English are the and and, and these are relatively easy to spot-Lhe, which appears six times, and aPV, which appears five times. Hence, L probably represents t, P probably represents n, and V probably represents d. We can now replace these letters in the ciphertext with their true values:
nCQ dMJinD thiK tiSe KhahJaWad had ZCJne EinD KhahJiUaJ thJee KCnK. Cn the thCMKand and IiJKt niDht, Qhen Khe had ended the taRe CI Sa’aJMI, Khe JCKe and EiKKed the DJCMnd ZeICJe hiS, KaUinD: “DJeat EinD, ICJ a thCMKand and Cne niDhtK i haNe Zeen JeACMntinD tC UCM the IaZReK CI FaKt aDeK and the ReDendK CI anAient EinDK. SaU i SaEe KC ZCRd aK tC AJaNe a IaNCMJ CI UCMJ SaGeKtU?”
    eFiRCDMe, taReK IJCS the thCMKand and Cne niDhtK

    Once a few letters have been established, cryptanalysis progresses very rapidly. For example, the word at the beginning of the second sentence is Cn. Every word has a vowel in it, so C must be a vowel. There are only two vowels that remain to be identified, u and o; u does not fit, so C must represent o. We also have the word Khe, which implies that K represents either t or s. But we already know that L = t, so it becomes clear that K = s. Having identified these two letters, we insert them into the ciphertext, and there appears the phrase thoMsand and one niDhts. A sensible guess for this would be thousand and one nights, and it seems likely that the final line is telling us that this is a passage from Tales from the Thousand and One Nights . This implies that M = u, I = f, J = r, D = g, R = l, and S = m.
    We could continue trying to establish other letters by guessing other words, but