How Spies Communicate
Cryptography also known as cryptology is the practice and study of techniques used for secure communication in the presence of third parties.
Cryptography techniques prior to the modern age was effectively synonymous with encryption – the conversion of information from a readable state to apparent visible nonsense. The person or originator of an encrypted message shared the decoding technique needed to recover the original information only with intended recipients, thereby preventing unwanted persons from doing the same.
Since World War I and the invention of the computer, methods used to carry out cryptology have become extremely complex and its application more widespread. Cryptography today uses the disciplines of mathematics, computer science, and electrical engineering. The application of cryptography is now used for ATM cards, computer passwords, and electronic commerce.
Until modern times cryptography referred almost exclusively to encryption, which is the process of converting plain text into cipher text. Decryption is the reverse, in other words, moving from the cipher text back to plain text.
A cipher (or cypher) is a pair of algorithms that create the encryption and the reversing decryption. The operation of a cipher is controlled both by the algorithm and in each instance by a “key.” The “key” is a secret parameter known only to the communicants for the specific exchange of messages.
Cryptanalysis is the study of how to crack encryption algorithms or their implementations.
History of cryptography and cryptanalysis
Before the modern era, cryptology was concerned solely with encryption. Encryption was used to attempt to ensure secrecy in communications, such as those of spies, military leaders, and diplomats. In recent times, the field has expanded beyond confidentiality concerns and now includes techniques for message integrity checking, sender/receiver identity authentication, digital signatures, interactive proofs and secure computation, among others.
The earliest forms of secret writing required little more than pen and paper, as most people could not read. Those who were more literate and had literate opponents required actually cryptography. The main classical cipher types are transposition ciphers, which rearrange the order of letters in a message (e.g., ‘hello dolly’ becomes ‘ehlol yllod’ in a simple rearrangement scheme, and substitution ciphers, which systematically replace letters or groups of letters with other letters or groups of letters (e.g., ‘fly at once’ becomes ‘gmz bu podf’) by replacing each letter with the one following it in the Latin alphabet). These simple versions however have never offered much confidentiality from enterprising opponents.
An early substitution cipher was the Caesar cipher, in which each letter in the plaintext was replaced by a letter some fixed number of positions further down the alphabet. Suetonius, a Roman historian reports that Julius Caesar used it with a shift of three to communicate with his generals.
Atbash is an example of an early Hebrew cipher. The earliest known use of cryptography is some carved ciphertext on stone in Egypt at around 1900 BCE.
Many mechanical encryption/decryption devices were invented early in the 20th century, and several patented, among them the rotor machines—famously including the Enigma machine used by the German government and military from the late ’20s and during World War II. The ciphers implemented by these machines brought about a substantial increase in cryptanalytic difficulty after WWI.
Cryptanalysis of the new mechanical devices proved to be both difficult and laborious. In Great Britain, cryptanalytic efforts during WWII spurred the development of more efficient means for carrying out repetitious tasks. This culminated in the development of the Colossus, the world’s first fully electronic, digital, programmable computer, which assisted in the decryption of ciphers generated by the German Army’s Lorenz SZ40/42 machine.
Just as the development of digital computers and electronics helped in cryptanalysis, it made possible much more complex ciphers. Furthermore, computers allowed for the encryption of any kind of data unlike classical ciphers which only encrypted written language texts. Computer use has thus supplanted linguistic cryptography, both for cipher design and cryptanalysis.
Decode these substitution ciphers. (J bn dszqubobmztu)
Start communicating with your friends in school through code. Those chits you pass on henceforth will always be safe.
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