Chrptopgyray Caphptogryry old and new Join Jonni Bidwell on a journey of code making and breaking, mystery and intrigue… or as long as there have been so that lovers can “conceal the details of their ROT13 and Atbash are essentially single-key stories there have been secrets liaisons”. An even older substitution system is systems. The Kama Sutra cipher has a fairly – words unspoken for tactical Atbash, originally found in old (circa 500 BC) large keyspace – there are about 8 trillion (8 F advantage or for fear of reprisal. Hebrew texts. Here the first letter of the followed by 12 zeroes) unique ways of pairing Secrets often need to be sent afar, and alphabet, aleph, is replaced by the last, tav; the the alphabet. The general MSC has an their remaining secret en route is of second, beth, by the second to last, shin, and astounding number of possible combinations paramount importance. So it was when so on, effectively reversing the alphabet. The (26 factorial – about 4 followed by 26 zeroes Xerxes’ attack on Sparta was thwarted by latinic equivalent is interchanging A and Z, B – or a little more than 88-bits in modern binary Demaratus (a Greek exile living in Persia, and Y, and so forth. The ROT13 system (a terms), but size isn’t everything... The Arab whose warning message was sent to Sparta Caesar cipher with a shift of 13) is still used on polymath Al-Kindi, in a ninth-century hidden on an apparently blank wax tablet). some websites and newsgroups to obfuscate manuscript titled On Deciphering And so it is when you send your credit card plot spoilers, punchlines or naughty words. Cryptographic Messages, gave the first details across the ether to pay for gadgets, These monoalphabetic substitution ciphers description of breaking MSCs by frequency snacks or socks. (MSCs) are not in any way cryptographically analysis – exploiting the fact that in an Most people will likely be ‘average’ message, some familiar with a substitution letters will occur more cipher, in which one letter is “ The Kama Sutra describes, frequently than others. replaced by another. The best- among other more interesting For example, in English the known of these is the Caesar letter ‘e’ occurs with a relative cipher, in which each letter is tricks, the art of secret writing. ” frequency of about 13%, replaced by one a fixed distance followed by ‘t’ with 9%, and so further down the alphabet, wrapping around secure by today’s standards, but in their time on. This is why Scrabble scoring is the way it when one runs out of letters. It is said that they were likely effective enough – the highway is – the more common the letter, the less it Julius Caesar used this method, replacing A bandits of Caesar’s time being likely illiterate, scores. Other languages have different letters with D, B with E, and so on, wrapping around unlike the masterful wordsmiths of the and frequencies, but the principle remains the with A replacing X, whereas his nephew modern internet. These ciphers do contain a same: replace the most frequently occurring Augustus favoured a shift of just one letter, in germ of the idea of the modern cryptographic letter in the ciphertext with the most which A is replaced by B, B by C etc, but with key, though. Whether it’s the length of the shift frequently occurring letter in the language, no wraparound, so that Z is replaced by the in a Caesar cipher, the dimensions of the then repeat for the next most frequent letter, symbol AA. Scytale, or the pairings used in the Kama Sutra and continue until you are able to fill in the The Kama Sutra also describes, among (no, not those pairings), knowledge of the blanks. The original message might not have other rather more interesting tricks, the art of method of encryption, together with the key, exactly the same letter frequencies as the mlecchita-vikalpa (secret writing). It details a allows one to decipher the message. language, but provided it’s long enough it will substitution cipher in which letters are paired We have 26 possible keys (including the at least be close enough that decryption will and interchanged by a fixed random scheme, trivial zero-shift) for a Caesar cipher, whereas be possible with a little tweaking. 50 LXF189 October 2014 www.linuxformat.com LXF189.feat_crypto.indd 50 15/08/2014 17:35 Chrptopgyray Caphptogryry Don’t panic, Colonel This triptych shows another WWI example: a bigram, so that our message ‘kernel panic’ have been used, and both keys would be the ADFGX cipher (these letters were chosen encodes to XF GA DA GF GA AG DX GD GF FD changed according to a daily code book. We because they’re different in Morse code). The FA (the space is ignored). In the second plate, rearrange the columns by putting the second first plate is the fractionating key: it encodes we fit this message onto a grid below a second key in alphabetical order, and then read off the each letter of our alphabet (sans the letter z keyword, ‘LINUS’, which is our transposition key. ciphertext column-wise. Thus our encoded because the LXF style guide doesn’t like it) into In practice, a longer transposition key would message is FGGGA XAADF GFDF DAGD AGXF. The discovery of the 1586 Babington Plot Later, this cipher was augmented with failed to receive this, necessitating that (which sought to assassinate Queen Elizabeth the letter V to make the imaginatively-titled messages be retransmitted using old keys. I) led to Mary Queen of Scots and her ADFGVX cipher. In 1918, in a phenomenal tour- This exchange was sometimes intercepted, co-conspirators being executed after their de-force, the French cryptanalyst Georges providing clues as to the new key. During correspondence was decrypted by renowned Painvin managed to decrypt an ADFGVX- World War I, the decrypting of the Zimmerman codebreaker Thomas Phelippes. Letters encrypted message which revealed where the telegram (which invited Mexico to ally with between Mary and Babington had been German forces were planning to attack Paris. Germany) was instrumental to American encrypted by substitution using symbols Painvin lost 15kg of body weight over the involvement in the war. mostly from the Greek alphabet, and course of this crypto-toil. By World War II the Germans had Phelippes was able to forge an addendum to One may wonder if anyone can make a upgraded the Enigma series of machines to one of Mary’s letters requesting the identities truly unbreakable cipher, and one may be present a sufficient cryptographic challenge to of the co-conspirators. Once they were thus shocked to learn that such a thing already Bletchley Park. Polish researches had broken incriminated, heads were off’d. exists. That it has been patented since 1917 the original design as early as 1932, and just A milestone in the history of cryptography may leave one so utterly aghast as to impinge prior to the outbreak of war they shared their was the invention of the so-called Vigenère permanently on one’s health, but this is fact intelligence with the British. Alan Turing cipher in 1553. This was actually the work nonetheless. The chap responsible (for the designed the Bombe machine, which by 1940 of cryptologist Giovan Battista Bellaso, who patent at least) was Gilbert Vernam, and his was doing a fine job of breaking Jerry comms. built on the ideas of Trithemius and Alberti. invention is known as the One Time Pad. The The Enigma machine, despite having a Vigenère did in fact publish a stronger trick is to ensure that there is as much key huge number of rotor, plugboard and stecker autokeying cipher in 1586, but history has material as there is plaintext, that the key settings, had a weakness in that a letter was misattributed this earlier cipher to him. The material is entirely random and perfectly never encrypted to itself. This vastly reduced cipher is a polyalphabetic substitution cipher secret, and no part of the key material is the amount of work that the Bombe and the which uses a keyword to switch cipher used more than once. In practical terms, computers (usually women with a good eye alphabets after each letter. Each letter is though, Vernam’s system is largely useless. for detail and skill at crossword puzzles) had encrypted by a Caesar cipher with shift Generating truly random material is difficult, to do. After a letter was typed on the Enigma, determined by the corresponding letter of the as is distributing a huge amount of it in secret the cipher alphabet was changed by the rotor keyword. This (providing the keyword has and ensuring its destruction post-use. mechanism, in a manner not dissimilar from more than one unique letter) thwarts the Vigenère cipher. There were other layers traditional frequency analysis. The cipher was Enigmatic mathematics of encryption too, but a lot of these were considered so strong that it was dubbed le Wartime cryptography relied heavily on constant settings made redundant when chiffre indéchiffrable, and indecipherable it codebooks which contained daily keys, and Enigma machines were captured. By the end remained until work by Babbage and Kasiski these had a bad habit of falling into enemy of the war there were around 200 Bombes in the mid-19th century. Their efforts centred hands. Once such a breach occurred and in use throughout England. The Americans, on isolating the length of the key: once that is news of it reached HQ, generals were faced being in a much better position for obtaining known then the ciphertext can be separated with the tremendous logistical problem of supplies, were able to build and design 125 into as many chunks; each chunk will be alerting relevant personnel as to the breach much faster Bombes, and the Allies were able encrypted by a different Caesar shift, which and then manufacturing and distributing new to farm out work to these remote behemoths is easily dealt to by frequency analysis.