The Code Book: The Secret History of Codes and Code-breaking. Simon Singh
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Название: The Code Book: The Secret History of Codes and Code-breaking
Автор: Simon Singh
Издательство: HarperCollins
Жанр: Прочая образовательная литература
isbn: 9780007378302
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Meanwhile, countries that were alert to the weakness of the straightforward monoalphabetic substitution cipher were anxious to develop a better cipher, something that would protect their own nation’s messages from being unscrambled by enemy cryptanalysts. One of the simplest improvements to the security of the monoalphabetic substitution cipher was the introduction of nulls, symbols or letters that were not substitutes for actual letters, merely blanks that represented nothing. For example, one could substitute each plain letter with a number between 1 and 99, which would leave 73 numbers that represent nothing, and these could be randomly sprinkled throughout the ciphertext with varying frequencies. The nulls would pose no problem to the intended recipient, who would know that they were to be ignored. However, the nulls would baffle an enemy interceptor because they would confuse an attack by frequency analysis. An equally simple development was that cryptographers would sometimes deliberately misspell words before encrypting the message. Thys haz thi ifekkt off diztaughting thi ballans off frikwenseas – making it harder for the cryptanalyst to apply frequency analysis. However, the intended recipient, who knows the key, can unscramble the message and then deal with the bad, but not unintelligible, spelling.
Another attempt to shore up the monoalphabetic substitution cipher involved the introduction of codewords. The term code has a very broad meaning in everyday language, and it is often used to describe any method for communicating in secret. However, as mentioned in the Introduction, it actually has a very specific meaning, and applies only to a certain form of substitution. So far we have concentrated on the idea of a substitution cipher, whereby each letter is replaced by a different letter, number or symbol. However, it is also possible to have substitution at a much higher level, whereby each word is represented by another word or symbol – this would be a code. For example,
Technically, a code is defined as substitution at the level of words or phrases, whereas a cipher is defined as substitution at the level of letters. Hence the term encipher means to scramble a message using a cipher, while encode means to scramble a message using a code. Similarly, the term decipher applies to unscrambling an enciphered message, and decode to unscrambling an encoded message. The terms encrypt and decrypt are more general, and cover scrambling and unscrambling with respect to both codes and ciphers. Figure 7 presents a brief summary of these definitions. In general, I shall keep to these definitions, but when the sense is clear, I might use a term such as ‘codebreaking’ to describe a process that is really ‘cipher breaking’ – the latter phrase might be technically accurate, but the former phrase is widely accepted.
Figure 7 The science of secret writing and its main branches.
At first sight, codes seem to offer more security than ciphers, because words are much less vulnerable to frequency analysis than letters. To decipher a monoalphabetic cipher you need only identify the true value of each of the 26 characters, whereas to decipher a code you need to identify the true value of hundreds or even thousands of codewords. However, if we examine codes in more detail, we see that they suffer from two major practical failings when compared with ciphers. First, once the sender and receiver have agreed upon the 26 letters in the cipher alphabet (the key), they can encipher any message, but to achieve the same level of flexibility using a code they would need to go through the painstaking task of defining a codeword for every one of the thousands of possible plaintext words. The codebook would consist of hundreds of pages, and would look something like a dictionary. In other words, compiling a codebook is a major task, and carrying it around is a major inconvenience.
Second, the consequences of having a codebook captured by the enemy are devastating. Immediately, all the encoded communications would become transparent to the enemy. The senders and receivers would have to go through the painstaking process of having to compile an entirely new codebook, and then this hefty new tome would have to be distributed to everyone in the communications network, which might mean securely transporting it to every ambassador in every state. In comparison, if the enemy succeeds in capturing a cipher key, then it is relatively easy to compile a new cipher alphabet of 26 letters, which can be memorised and easily distributed.
Even in the sixteenth century, cryptographers appreciated the inherent weaknesses of codes, and instead relied largely on ciphers, or sometimes nomenclators. A nomenclator is a system of encryption that relies on a cipher alphabet, which is used to encrypt the majority of a message, and a limited list of codewords. For example, a nomenclator book might consist of a front page containing the cipher alphabet, and then a second page containing a list of codewords. Despite the addition of codewords, a nomenclator is not much more secure than a straightforward cipher, because the bulk of a message can be deciphered using frequency analysis, and the remaining encoded words can be guessed from the context.
As well as coping with the introduction of the nomenclator, the best cryptanalysts were also capable of dealing with badly spelt messages and the presence of nulls. In short, they were able to break the majority of encrypted messages. Their skills provided a steady flow of uncovered secrets, which influenced the decisions of their masters and mistresses, thereby affecting Europe’s history at critical moments.
Nowhere is the impact of cryptanalysis more dramatically illustrated than in the case of Mary Queen of Scots. The outcome of her trial depended wholly on the battle between her codemakers and Queen Elizabeth’s codebreakers. Mary was one of the most significant figures of the sixteenth century – Queen of Scotland, Queen of France, pretender to the English throne – yet her fate would be decided by a slip of paper, the message it bore, and whether or not that message could be deciphered.
The Babington Plot
On 24 November 1542, the English forces of Henry VIII demolished the Scottish army at the Battle of Solway Moss. It appeared that Henry was on the verge of conquering Scotland and stealing the crown of King James V. After the battle, the distraught Scottish king suffered a complete mental and physical breakdown, and withdrew to the palace at Falkland. Even the birth of a daughter, Mary, just two weeks later could not revive the ailing king. It was as if he had been waiting for news of an heir so that he could die in peace, safe in the knowledge that he had done his duty. Just a week after Mary’s birth, King James V, still only thirty years old, died. The baby princess had become Mary Queen of Scots.
Mary was born prematurely, and initially there was considerable concern that she would not survive. Rumours in England suggested that the baby had died, but this was merely wishful thinking at the English court, which was keen to hear any news that might destabilise Scotland. In fact, Mary soon grew strong and healthy, and at the age of nine months, on 9 September 1543, she was crowned in the chapel of Stirling Castle, surrounded by three earls, bearing on her behalf the royal crown, sceptre and sword.
The fact that Queen Mary was so young offered Scotland a respite from English incursions. It would have been deemed unchivalrous had Henry VIII attempted to invade the country of a recently dead king, now under the rule of an infant queen. Instead, the English king decided on a policy of wooing Mary in the hope of arranging a marriage between her and his son Edward, thereby uniting the two nations under a Tudor sovereign. He began his manoeuvring by releasing the Scottish nobles captured at Solway Moss, on the condition that they campaign in favour of a union with England.
However, after considering Henry’s offer, the Scottish court rejected it in favour of a marriage to Francis, the dauphin of France. Scotland was choosing СКАЧАТЬ