Crypting and decrypting is part of an encryption project. Some call it cipher and decipher but it’s the same mechanism. When you start encrypting a document for example, you start by choosing the key with which you’ll hide the real message. Nowadays we use programs such as Axcrypt, Kruptos, Folder Lock, etc… which gives us the opportunity to choose between different forms of encryption and without bothering ourselves to find a way to hide a message. But before the 20th century everything was different. There were different methods which were ingenious for their time. A little bit of History:
The earliest known use of cryptography is found in non-standard hieroglyphs carved into the wall of a tomb from the Old Kingdom of Egypt circa 1900 BCE. These are not thought to be serious attempts at secret communications, however, but rather to have been attempts at mystery, intrigue, or even amusement for literate onlookers. These are examples of still other uses of cryptography, or of something that looks (impressively if misleadingly) like it. Some clay tablets from Mesopotamia somewhat later are clearly meant to protect information—one dated near 1500 BCE was found to encrypt a craftsman’s recipe for pottery glaze, presumably commercially valuable.Later still, Hebrew scholars made use of simple monoalphabetic substitution ciphers (such as the Atbash cipher) beginning perhaps around 500 to 600 BCE.
David Kahn notes in The Codebreakers that modern cryptology originated among the Arabs, the first people to systematically document cryptanalytic methods. The invention of the frequency-analysis technique for breaking monoalphabetic substitution ciphers, by Al-Kindi, an Arab mathematician, sometime around AD 800 proved to be the single most significant cryptanalytic advance until World War II. Al-Kindi wrote a book on cryptography entitled Risalah fi Istikhraj al-Mu’amma (Manuscript for the Deciphering Cryptographic Messages), in which he described the first cryptanalytic techniques, including some for polyalphabetic ciphers, cipher classification, Arabic phonetics and syntax, and, most importantly, gave the first descriptions on frequency analysis. He also covered methods of encipherments, cryptanalysis of certain encipherments, and statistical analysis of letters and letter combinations in Arabic.
Ahmad al-Qalqashandi (AD 1355–1418) wrote the Subh al-a ‘sha, a 14-volume encyclopedia which included a section on cryptology. This information was attributed to Ibn al-Durayhim who lived from AD 1312 to 1361, but whose writings on cryptography have been lost. The list of ciphers in this work included both substitution and transposition, and for the first time, a cipher with multiple substitutions for each plaintext letter. Also traced to Ibn al-Durayhim is an exposition on and worked example of cryptanalysis, including the use of tables of letter frequencies and sets of letters which cannot occur together in one word.
The earliest example of the homophonic substitution cipher is the one used by Duke of Mantua in the early 1400s. Homophonic cipher replaces each letter with multiple symbols depending on the letter frequency. The cipher is ahead of the time because it combines monoalphabetic and polyalphabetic features.
Essentially all ciphers remained vulnerable to the cryptanalytic technique of frequency analysis until the development of the polyalphabetic cipher, and many remained so thereafter. The polyalphabetic cipher was most clearly explained by Leon Battista Alberti around the year AD 1467, for which he was called the “father of Western cryptology”.
So, as long as we have the key to decipher a document, everything is ok, but when we don’t, the problem begins.
Let’s take an example:
Let’s say that we have an ancient book with several chapters – written in a common language – among these chapters some of them have curious detached letters in the beginning, some repeat themselves and some don’t. These strange letters don’t form a word and are not an abbreviation, so what are they exactly? We start reading those special chapters, but nothing in them seems to refer to these letters. So I assume that the solution is elsewhere.
Maybe they’re not letters after all, what if they were numbers? But how are we going to find the solution if we don’t know what those numbers refer to? And this goes on till we find the key to unlock the message.
At least nowadays it is much much easier, if we don’t have the password, we may find another way around…