Cryptanalysis (from the Greek kryptós, "hidden", and analýein, "to loosen" or "to untie") is the study of methods for obtaining the meaning of encrypted information, without access to the secret information that is normally required to do so. Typically, this involves knowing how the system works and finding a secret key. In non-technical language, this is the practice of codebreaking or cracking the code, although these phrases also have a specialised technical meaning .
Even though the goal has been the same, the methods and techniques of cryptanalysis have changed drastically through the history of cryptography, adapting to increasing cryptographic complexity, ranging from the pen-and-paper methods of the past, through machines like Bombes and Colossus computers in World War II, to the computer-based schemes of the present. The results of cryptanalysis have also changed — it is no longer possible to have unlimited success in codebreaking, and there is a hierarchical classification of what constitutes an attack. In the mid-1970s, a new class of cryptography was introduced: asymmetric cryptography. Methods for breaking these cryptosystems are typically radically different from before, and usually involve solving carefully constructed problems in pure mathematics, the best-known being integer factorization.
In practice, frequency analysis relies as much on linguistic knowledge as it does on statistics, but as ciphers became more complex, mathematics became more important in cryptanalysis. This change was particularly evident before and during World War II, where efforts to crack Axis ciphers required new levels of mathematical sophistication. Moreover, automation was first applied to cryptanalysis in that era with the Polish Bomba device, the British Bombe development of it, the use of punched card equipment, and in the Colossus computers — the first electronic digital computers to be controlled by a program.
Successful cryptanalysis has undoubtedly influenced history; the ability to read the presumed-secret thoughts and plans of others can be a decisive advantage. For example, in England in 1587, Mary, Queen of Scots was tried and executed for treason for her involvement in three plots to assassinate Elizabeth I of England which were known about because her coded correspondence with fellow conspirators had been deciphered by Thomas Phelippes; in World War I, the breaking of the Zimmermann Telegram was instrumental in bringing the United States into the war; in World War II, the cryptanalysis of the German ciphers — including the Enigma machine and the Lorenz cipher — has been credited with everything between shortening the end of the European war by a few months to determining the eventual result (see Ultra). The United States also benefited from the cryptanalysis of the Japanese Purple code (see Magic).
Governments have long recognized the potential benefits of cryptanalysis for intelligence, both military and diplomatic, and established dedicated organizations devoted to breaking the codes and ciphers of other nations, for example, GCHQ and the NSA, organizations which are still very active today.
India was one of the earliest issuers of coins (circa 6th century BC). The first "rupee" is believed to have been introduced by Sher Shah Suri (1486--1545), based on a ratio of 40 copper pieces (paisa) per rupee. Among the earliest issues of paper rupees were those by the Bank of Hindustan (1770--1832), the General Bank of Bengal and Bihar (1773--75, established by Warren Hastings) and the Bengal Bank (1784--91), amongst others.
Historically, the rupee, derived from the Sanskrit word raupya, which means silver, was a silver coin. This had severe consequences in the nineteenth century, when the strongest economies in the world were on the gold standard. The discovery of vast quantities of silver in the U.S. and various European colonies resulted in a decline in the relative value of silver to gold. Suddenly the standard currency of India could not buy as much from the outside world. This event was known as "the fall of the rupee".
Currency notes are printed at the Currency Note Press, Nashik, Bank Note Press, Dewas, Bharatiya Note Mudra Nigam (P) Limited presses at Salboni and Mysore and at the Watermark Paper Manufacturing Mill, Hoshangabad.
The current series of banknotes, which began in 1996, is called the Mahatma Gandhi series. At present, banknotes are issued in the denominations of 5, 10, 20, 50, 100, 500 and 1000. The Zero rupee note is not an official government issue but a symbol of protest and it is printed and distributed by an NGO in India.