In cryptography, a Caesar cipher known as Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, so on; the method is named after Julius Caesar. The encryption step performed by a Caesar cipher is incorporated as part of more complex schemes, such as the Vigenère cipher, still has modern application in the ROT13 system; as with all single-alphabet substitution ciphers, the Caesar cipher is broken and in modern practice offers no communications security. The transformation can be represented by aligning two alphabets. For instance, here is a Caesar cipher using a left rotation of three places, equivalent to a right shift of 23: Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ Cipher: XYZABCDEFGHIJKLMNOPQRSTUVW When encrypting, a person looks up each letter of the message in the "plain" line and writes down the corresponding letter in the "cipher" line.
Plaintext: THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG Ciphertext: QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD Deciphering is done in reverse, with a right shift of 3. The encryption can be represented using modular arithmetic by first transforming the letters into numbers, according to the scheme, A → 0, B → 1... Z → 25. Encryption of a letter x by a shift n can be described mathematically as, E n = mod 26. Decryption is performed D n = mod 26; the replacement remains the same throughout the message, so the cipher is classed as a type of monoalphabetic substitution, as opposed to polyalphabetic substitution. The Caesar cipher is named after Julius Caesar, according to Suetonius, used it with a shift of three to protect messages of military significance. While Caesar's was the first recorded use of this scheme, other substitution ciphers are known to have been used earlier. "If he had anything confidential to say, he wrote it in cipher, that is, by so changing the order of the letters of the alphabet, that not a word could be made out.
If anyone wishes to decipher these, get at their meaning, he must substitute the fourth letter of the alphabet, namely D, for A, so with the others." His nephew, Augustus used the cipher, but with a right shift of one, it did not wrap around to the beginning of the alphabet: "Whenever he wrote in cipher, he wrote B for A, C for B, the rest of the letters on the same principle, using AA for Z." Evidence exists that Julius Caesar used more complicated systems, one writer, Aulus Gellius, refers to a treatise on his ciphers: "There is a rather ingeniously written treatise by the grammarian Probus concerning the secret meaning of letters in the composition of Caesar's epistles." It is unknown how effective the Caesar cipher was at the time, but it is to have been reasonably secure, not least because most of Caesar's enemies would have been illiterate and others would have assumed that the messages were written in an unknown foreign language. There is no record at that time of any techniques for the solution of simple substitution ciphers.
The earliest surviving records date to the 9th century works of Al-Kindi in the Arab world with the discovery of frequency analysis. A Caesar cipher with a shift of one is used on the back of the mezuzah to encrypt the names of God; this may be a holdover from an earlier time. The letters of the cryptogram themselves comprise a religiously significant "divine name" which Orthodox belief holds keeps the forces of evil in check. In the 19th century, the personal advertisements section in newspapers would sometimes be used to exchange messages encrypted using simple cipher schemes. Kahn describes instances of lovers engaging in secret communications enciphered using the Caesar cipher in The Times; as late as 1915, the Caesar cipher was in use: the Russian army employed it as a replacement for more complicated ciphers which had proved to be too difficult for their troops to master. Caesar ciphers can be found today in children's toys such as secret decoder rings. A Caesar shift of thirteen is performed in the ROT13 algorithm, a simple method of obfuscating text found on Usenet and used to obscure text, but not used as a method of encryption.
The Vigenère cipher uses a Caesar cipher with a different shift at each position in the text. If the keyword is as long as the message, chosen random, never becomes known to anyone else, is never reused, this is the one-time pad cipher, proven unbreakable; the conditions are so difficult. Keywords shorter than the message, introduce a cy
World War II
World War II known as the Second World War, was a global war that lasted from 1939 to 1945. The vast majority of the world's countries—including all the great powers—eventually formed two opposing military alliances: the Allies and the Axis. A state of total war emerged, directly involving more than 100 million people from over 30 countries; the major participants threw their entire economic and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. World War II was the deadliest conflict in human history, marked by 50 to 85 million fatalities, most of whom were civilians in the Soviet Union and China, it included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease, the only use of nuclear weapons in war. Japan, which aimed to dominate Asia and the Pacific, was at war with China by 1937, though neither side had declared war on the other. World War II is said to have begun on 1 September 1939, with the invasion of Poland by Germany and subsequent declarations of war on Germany by France and the United Kingdom.
From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, formed the Axis alliance with Italy and Japan. Under the Molotov–Ribbentrop Pact of August 1939, Germany and the Soviet Union partitioned and annexed territories of their European neighbours, Finland and the Baltic states. Following the onset of campaigns in North Africa and East Africa, the fall of France in mid 1940, the war continued between the European Axis powers and the British Empire. War in the Balkans, the aerial Battle of Britain, the Blitz, the long Battle of the Atlantic followed. On 22 June 1941, the European Axis powers launched an invasion of the Soviet Union, opening the largest land theatre of war in history; this Eastern Front trapped most crucially the German Wehrmacht, into a war of attrition. In December 1941, Japan launched a surprise attack on the United States as well as European colonies in the Pacific. Following an immediate U. S. declaration of war against Japan, supported by one from Great Britain, the European Axis powers declared war on the U.
S. in solidarity with their Japanese ally. Rapid Japanese conquests over much of the Western Pacific ensued, perceived by many in Asia as liberation from Western dominance and resulting in the support of several armies from defeated territories; the Axis advance in the Pacific halted in 1942. Key setbacks in 1943, which included a series of German defeats on the Eastern Front, the Allied invasions of Sicily and Italy, Allied victories in the Pacific, cost the Axis its initiative and forced it into strategic retreat on all fronts. In 1944, the Western Allies invaded German-occupied France, while the Soviet Union regained its territorial losses and turned toward Germany and its allies. During 1944 and 1945 the Japanese suffered major reverses in mainland Asia in Central China, South China and Burma, while the Allies crippled the Japanese Navy and captured key Western Pacific islands; the war in Europe concluded with an invasion of Germany by the Western Allies and the Soviet Union, culminating in the capture of Berlin by Soviet troops, the suicide of Adolf Hitler and the German unconditional surrender on 8 May 1945.
Following the Potsdam Declaration by the Allies on 26 July 1945 and the refusal of Japan to surrender under its terms, the United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki on 6 and 9 August respectively. With an invasion of the Japanese archipelago imminent, the possibility of additional atomic bombings, the Soviet entry into the war against Japan and its invasion of Manchuria, Japan announced its intention to surrender on 15 August 1945, cementing total victory in Asia for the Allies. Tribunals were set up by fiat by the Allies and war crimes trials were conducted in the wake of the war both against the Germans and the Japanese. World War II changed the political social structure of the globe; the United Nations was established to foster international co-operation and prevent future conflicts. The Soviet Union and United States emerged as rival superpowers, setting the stage for the nearly half-century long Cold War. In the wake of European devastation, the influence of its great powers waned, triggering the decolonisation of Africa and Asia.
Most countries whose industries had been damaged moved towards economic expansion. Political integration in Europe, emerged as an effort to end pre-war enmities and create a common identity; the start of the war in Europe is held to be 1 September 1939, beginning with the German invasion of Poland. The dates for the beginning of war in the Pacific include the start of the Second Sino-Japanese War on 7 July 1937, or the Japanese invasion of Manchuria on 19 September 1931. Others follow the British historian A. J. P. Taylor, who held that the Sino-Japanese War and war in Europe and its colonies occurred and the two wars merged in 1941; this article uses the conventional dating. Other starting dates sometimes used for World War II include the Italian invasion of Abyssinia on 3 October 1935; the British historian Antony Beevor views the beginning of World War II as the Battles of Khalkhin Gol fought between Japan and the fo
Midway Atoll is a 2.4-square-mile atoll in the North Pacific Ocean at 28°12′N 177°21′W. Midway is equidistant between North America and Asia. Midway Atoll is an unincorporated territory of the United States. Midway continues to be the only island in the Hawaiian archipelago, not part of the state of Hawaii. Unlike the other Hawaiian islands, Midway observes Samoa Time, one hour behind the time in the state of Hawaii. For statistical purposes, Midway is grouped as one of the United States Minor Outlying Islands; the Midway Atoll National Wildlife Refuge, encompassing 590,991.50 acres of land and water in the surrounding area, is administered by the United States Fish and Wildlife Service. The refuge and most of its surrounding area are part of the larger Papahānaumokuākea Marine National Monument; until 1993, the atoll was the home of the Naval Air Facility Midway Island. The Battle of Midway, fought between June 4 and 6, 1942, was a critical Allied victory of the Pacific campaign of World War II.
The United States Navy defended the atoll from a Japanese invasion, defeating a Japanese battle group, marking a turning point in the war in the Pacific Theater. USAAF aircraft based at the original Henderson Field on Eastern Island joined the attack against the Japanese fleet, which suffered losses of four carriers and one heavy cruiser. 40 to 60 people live on the atoll, which includes staff of the U. S. Fish and Wildlife Service and contract workers. Visitation to the atoll is possible only for business reasons as the tourism program has been suspended due to budget cutbacks. In 2012, the last year that the visitor program was in operation, 332 people made the trip to Midway. Tours focused on both the unique ecology of Midway as well as its military history; the economy is derived from governmental sources and tourist fees. Nearly all supplies must be brought to the island by ship or plane, though a hydroponic greenhouse and garden supply some fresh fruits and vegetables; as its name suggests, Midway is equidistant between North America and Asia, lies halfway around the world longitudinally from Greenwich, UK.
It is near the northwestern end of the Hawaiian archipelago, about one-third of the way from Honolulu, Hawaii, to Tokyo, Japan. Midway island is not considered part of the State of Hawaii due to the passage of the Hawaii Organic Act, which formally annexed Hawaii to the United States as a territory, only defined Hawaii as "the islands acquired by the United States of America under an Act of Congress entitled'Joint resolution to provide for annexing the Hawaiian Islands to the United States,' approved July seventh, eighteen hundred and ninety-eight." Although it could be argued that Midway became part of Hawaii when Middlebrooks discovered it in 1859, it was assumed at the time that Midway was independently acquired by the U. S. when Reynolds visited in 1867, so was not considered part of the Territory. In defining which islands the State of Hawaii would inherit from the Territory, the Hawaii Admissions Act clarified the question excluding Midway from the jurisdiction of the state. Midway Atoll is 140 nautical miles east of the International Date Line, about 2,800 nautical miles west of San Francisco, 2,200 nautical miles east of Tokyo.
Midway Atoll is part of a chain of volcanic islands and seamounts extending from Hawaii up to the tip of the Aleutian Islands and known as the Hawaiian–Emperor seamount chain. It consists of a ring-shaped barrier reef nearly five miles in diameter and several sand islets; the two significant pieces of land, Sand Island and Eastern Island, provide a habitat for millions of seabirds. The island sizes are shown in the table above; the atoll, which has a small population, is designated an insular area under the authority of the United States Department of the Interior. Midway was formed 28 million years ago when the seabed underneath it was over the same hotspot from which the Island of Hawaii is now being formed. In fact, Midway was once a shield volcano as large as the island of Lana'i; as the volcano piled up lava flows building the island, its weight depressed the crust and the island subsided over a period of millions of years, a process known as isostatic adjustment. As the island subsided, a coral reef around the former volcanic island was able to maintain itself near sea level by growing upwards.
That reef is now over 516 feet thick. What remains today is a shallow water atoll about 6 miles across. Following Kure Atoll, Midway is the 2nd most northerly atoll in the world; the atoll has some 20 miles of roads, 4.8 miles of pipelines, one port on Sand Island, an airfield. As of 2004, Henderson Field airfield at Midway Atoll, with its one active runway has been designated as an emergency diversion airport for aircraft flying under ETOPS rules. Although the FWS closed all airport operations on November 22, 2004, public access to the island was restored from March 2008. Eastern Island Airstrip is a disused airfield, in use by U. S. forces during the
Sir Francis Harry Hinsley OBE was an English historian and cryptanalyst. He worked at Bletchley Park during the Second World War and wrote on the history of international relations and British Intelligence during the Second World War, he was known as Harry Hinsley. Hinsley's father worked in the coal department of the Walsall Co-Op, his mother Emma Hinsley was a school caretaker, they lived in Birchills, in the parish of St Andrew's, Walsall. Harry was educated at Queen Mary's Grammar School, in 1937 won a scholarship to read history at St. John's College, Cambridge, he went on to be awarded a First in part one of the Historical Tripos. In August 1939 Hinsley visited his girlfriend in the German city of Koblenz. Police required him to report to the police station daily. However, this requirement was waived following the signing of the German-Soviet Pact. A week Hinsley was advised by police via his girlfriend's parents to get out of Germany by "tomorrow at the latest"; this enabled him to cross the Franco-German border.
He made the crossing at the bridge between Strasbourg. Stripped of his Reichsmarks by German border guards without French Francs or British Pounds in exchange, Hinsley was left penniless; this led to his sleeping on a park bench in France. Hinsley hitch-hiked to Switzerland from where he returned to the United Kingdom, he made his return. In October 1939, while still at St. John's, he was summoned to an interview with Alastair Denniston, head of the Government Code and Cypher School, was thereby recruited to Bletchley Park's naval section in Hut 4. At Bletchley Park, Hinsley studied the external characteristics of intercepted German messages, a process sometimes termed "traffic analysis": from call signs, times of interception and so forth, he was able to deduce a great deal of information about the structure of Nazi Germany's Kriegsmarine's communication networks, about the structure of the German navy itself. Hinsley helped initiate a programme of seizing Enigma machines and keys from German weather ships, such as the Lauenburg, thereby facilitating Bletchley Park's resumption of interrupted breaking of German Naval Enigma.
In late 1943, Hinsley was sent to liaise with the US Navy in Washington, with the result that an agreement was reached in January 1944 to co-operate in exchanging results on Japanese Naval signals. Towards the end of the war, Hinsley, by a key aide to Bletchley Park chief Edward Travis, was part of a committee which argued for a post-war intelligence agency that would combine both signals intelligence and human intelligence in a single organisation. In the event, the opposite occurred, with GC&CS becoming GCHQ. On 6 April 1946, Hinsley married Hilary Brett-Smith, a graduate from Somerville College, who had worked at Bletchley Park, in Hut 8, they moved to Cambridge after the war. Hinsley was awarded the OBE in 1946, was knighted in 1985. On his death, Sir Harry Hinsley was cremated, his family buried the ashes in Cambridge. After the war, Hinsley returned to St John's College and lectured in history, being in 1969 appointed Professor of the History of International Relations. From 1979 to 1989 he was Master of St John's College and from 1981 to 1983 he was vice-chancellor of the University of Cambridge.
In 1962, Hinsley published Power and the Pursuit of Peace, important as a study of early idealist thought about international relations. Hinsley edited the multi-volume official history British Intelligence in the Second World War, argued that Enigma decryption had speeded Allied victory by 1–4 years while not fundamentally altering the war's outcome, he was criticised by Marian Rejewski and Gordon Welchman, who took exception to inaccuracies in Hinsley's accounts of the history of Enigma decryption in the early volumes of his official history, including crucial errors in chronology. Subsequently, a revised account of the Polish and British contribution was included in volume 3, part 2; the volumes of British Intelligence in the Second World War edited by Hinsley and published by Her Majesty's Stationery Office London are: Volume 1: Its Influence on Strategy and Operations, F. H. Hinsley with E. E. Thomas, C. F. G. Ransome and R. C. Knight, ISBN 0-11-630933-4 Volume 2: Its Influence on Strategy and Operations, F. H. Hinsley with E. E. Thomas, C. F. G. Ransome and R. C.
Knight, ISBN 0-11-630934-2 Volume 3, Part 1: Its Influence on Strategy and Operations, F. H. Hinsley with E. E. Thomas, C. F. G. Ransome and R. C. Knight, ISBN 0-11-630935-0 Volume 3, Part 2: Its Influence on Strategy and Operations, F. H. Hinsley with E. E. Thomas, C. A. G. Simkins, C. F. G. Ransom, ISBN 0-11-630940-7 Includes Bibliography, The Polish and British Contributions to the Breaking of the Enigma. Volume 4: Security and Counter-Intelligence, F. H. Hinsley and C. A. G. Simkins, ISBN 0-11-630952-0 Abridged Version, F. H. Hinsley, ISBN 0-11-630956-3 ISBN 0-521-44304-0Hinsley co-edited and contributed to Codebreakers: The Inside Story of Bletchley Park, which contains personal accounts from those who worked at Bletchley Park; the Hinsley Memorial Lecture, an annual lecture on an international relations topic, is held every year at St John's College in memory of Hinsley. Erskine, Ralph; the Bletchley Park Codebreakers, Biteback Publishing Ltd, ISBN 978-1-84954-078-0 Updated and extended version of Action This Day: From Breaking of the Eni
Battle of Midway
The Battle of Midway was a decisive naval battle in the Pacific Theater of World War II that took place between 4 and 7 June 1942, only six months after Japan's attack on Pearl Harbor and one month after the Battle of the Coral Sea. The United States Navy under Admirals Chester Nimitz, Frank Jack Fletcher, Raymond A. Spruance defeated an attacking fleet of the Imperial Japanese Navy under Admirals Isoroku Yamamoto, Chūichi Nagumo, Nobutake Kondō near Midway Atoll, inflicting devastating damage on the Japanese fleet that proved irreparable. Military historian John Keegan called it "the most stunning and decisive blow in the history of naval warfare"; the Japanese operation, like the earlier attack on Pearl Harbor, sought to eliminate the United States as a strategic power in the Pacific, thereby giving Japan a free hand in establishing its Greater East Asia Co-Prosperity Sphere. The Japanese hoped another demoralizing defeat would force the U. S. to capitulate in the Pacific War and thus ensure Japanese dominance in the Pacific.
Luring the American aircraft carriers into a trap and occupying Midway was part of an overall "barrier" strategy to extend Japan's defensive perimeter, in response to the Doolittle air raid on Tokyo. This operation was considered preparatory for further attacks against Fiji and Hawaii itself; the plan was handicapped by faulty Japanese assumptions of the American reaction and poor initial dispositions. Most American cryptographers were able to determine the date and location of the planned attack, enabling the forewarned U. S. Navy to prepare its own ambush. Four Japanese and three American aircraft carriers participated in the battle; the four Japanese fleet carriers—Akagi, Kaga, Sōryū and Hiryū, part of the six-carrier force that had attacked Pearl Harbor six months earlier—were all sunk, as was the heavy cruiser Mikuma. The U. S. lost a destroyer. After Midway and the exhausting attrition of the Solomon Islands campaign, Japan's capacity to replace its losses in materiel and men became insufficient to cope with mounting casualties, while the United States' massive industrial and training capabilities made losses far easier to replace.
The Battle of Midway, along with the Guadalcanal Campaign, is considered a turning point in the Pacific War. After expanding the war in the Pacific to include Western outposts, the Japanese Empire had attained its initial strategic goals taking the Philippines, Malaya and the Dutch East Indies; because of this, preliminary planning for a second phase of operations commenced as early as January 1942. There were strategic disagreements between the Imperial Army and Imperial Navy, infighting between the Navy's GHQ and Admiral Isoroku Yamamoto's Combined Fleet, a follow-up strategy was not formed until April 1942. Admiral Yamamoto succeeded in winning the bureaucratic struggle with a thinly veiled threat to resign, after which his plan for the Central Pacific was adopted. Yamamoto's primary strategic goal was the elimination of America's carrier forces, which he regarded as the principal threat to the overall Pacific campaign; this concern was acutely heightened by the Doolittle Raid on 18 April 1942, in which 16 U.
S. Army Air Forces B-25 Mitchell bombers launched from USS Hornet bombed targets in Tokyo and several other Japanese cities; the raid, while militarily insignificant, was a shock to the Japanese and showed the existence of a gap in the defenses around the Japanese home islands as well as the accessibility of Japanese territory to American bombers. This, other successful hit-and-run raids by American carriers in the South Pacific, showed that they were still a threat, although reluctant to be drawn into an all-out battle. Yamamoto reasoned that another air attack on the main U. S. Naval base at Pearl Harbor would induce all of the American fleet to sail out to fight, including the carriers. However, considering the increased strength of American land-based air power on the Hawaiian Islands since the 7 December attack the previous year, he judged that it was now too risky to attack Pearl Harbor directly. Instead, Yamamoto selected Midway, a tiny atoll at the extreme northwest end of the Hawaiian Island chain 1,300 miles from Oahu.
This meant that Midway was outside the effective range of all of the American aircraft stationed on the main Hawaiian islands. Midway was not important in the larger scheme of Japan's intentions, but the Japanese felt the Americans would consider Midway a vital outpost of Pearl Harbor and would therefore be compelled to defend it vigorously; the U. S. did consider Midway vital: after the battle, establishment of a U. S. submarine base on Midway allowed submarines operating from Pearl Harbor to refuel and re-provision, extending their radius of operations by 1,200 miles. In addition to serving as a seaplane base, Midway's airstrips served as a forward staging point for bomber attacks on Wake Island. Typical of Japanese naval planning during World War II, Yamamoto's battle plan for taking Midway was exceedingly complex, it required the careful and timely coordination of multiple battle groups over hundreds of miles of open sea. His design was predicated on optimistic intelligence suggesting that USS Enterprise and USS Hornet, forming Task Force 16, were the only carriers available to the U.
S. Pacific Fleet. During the Battle of the Coral Sea one month earlier, USS Lexington had been sunk and USS Yorktown suffered considerable damage such that the Japanese believed she too had
In cryptography, ciphertext or cyphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. Ciphertext is known as encrypted or encoded information because it contains a form of the original plaintext, unreadable by a human or computer without the proper cipher to decrypt it. Decryption, the inverse of encryption, is the process of turning ciphertext into readable plaintext. Ciphertext is not to be confused with codetext because the latter is a result of a code, not a cipher. Let m be the plaintext message that Alice wants to secretly transmit to Bob and let E k be the encryption cipher, where k is a cryptographic key. Alice must first transform the plaintext into ciphertext, c, in order to securely send the message to Bob, as follows: c = E k. In a symmetric-key system, Bob knows Alice's encryption key. Once the message is encrypted, Alice can safely transmit it to Bob. In order to read Alice's message, Bob must decrypt the ciphertext using E k − 1, known as the decryption cipher, D k: D k = D k = m.
Alternatively, in a non-symmetric key system, not just Alice and Bob, knows the encryption key. Only Bob knows the decryption key D k, decryption proceeds as D k = m; the history of cryptography began thousands of years ago. Cryptography uses a variety of different types of encryption. Earlier algorithms were performed by hand and are different from modern algorithms, which are executed by a machine. Historical pen and paper ciphers used in the past are sometimes known as classical ciphers, they include: Substitution cipher: the units of plaintext are replaced with ciphertext Polyalphabetic substitution cipher: a substitution cipher using multiple substitution alphabets Polygraphic substitution cipher: the unit of substitution is a sequence of two or more letters rather than just one Transposition cipher: the ciphertext is a permutation of the plaintext Historical ciphers are not used as a standalone encryption technique because they are quite easy to crack. Many of the classical ciphers, with the exception of the one-time pad, can be cracked using brute force.
Modern ciphers are more secure than classical ciphers and are designed to withstand a wide range of attacks. An attacker should not be able to find the key used in a modern cipher if he knows any amount of plaintext and corresponding ciphertext. Modern encryption methods can be divided into the following categories: Private-key cryptography: the same key is used for encryption and decryption Public-key cryptography: two different keys are used for encryption and decryptionIn a symmetric key algorithm, the sender and receiver must have a shared key set up in advance and kept secret from all other parties. In an asymmetric key algorithm, there are two separate keys: a public key is published and enables any sender to perform encryption, while a private key is kept secret by the receiver and enables only him to perform correct decryption. Symmetric key ciphers can be divided into block ciphers and stream ciphers. Block ciphers operate on fixed-length groups of bits, called blocks, with an unvarying transformation.
Stream ciphers encrypt plaintext digits one at a time on a continuous stream of data and the transformation of successive digits varies during the encryption process. Cryptanalysis is the study of methods for obtaining the meaning of encrypted information, without access to the secret information, required to do so; this involves knowing how the system works and finding a secret key. Cryptanalysis is referred to as codebreaking or cracking the code. Ciphertext is the easiest part of a cryptosystem to obtain and therefore is an important part of cryptanalysis. Depending on what information is available and what type of cipher is being analyzed, crypanalysts can follow one or more attack models to crack a cipher. Ciphertext-only: the cryptanalyst has access only to a collection of ciphertexts or codetexts Known-plaintext: the attacker has a set of ciphertexts to which he knows the corresponding plaintext Chosen-plaintext attack: the attacker can obtain the ciphertexts corresponding to an arbitrary set of plaintexts of his own choosing Batch chosen-plaintext attack: where the cryptanalyst chooses all plaintexts before any of them are encrypted.
This is the meaning of an unqualified use of "chosen-plaintext attack". Adaptive chosen-plaintext attack: where the cryptanalyst makes a series of interactive queries, choosing subsequent plaintexts based on the information from the previous encryptions. Chosen-ciphertext attack: the attacker can obt
Cryptanalysis is the study of analyzing information systems in order to study the hidden aspects of the systems. Cryptanalysis is used to breach cryptographic security systems and gain access to the contents of encrypted messages if the cryptographic key is unknown. In addition to mathematical analysis of cryptographic algorithms, cryptanalysis includes the study of side-channel attacks that do not target weaknesses in the cryptographic algorithms themselves, but instead exploit weaknesses in their implementation. 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 the British Bombes and Colossus computers at Bletchley Park in World War II, to the mathematically advanced computerized schemes of the present. Methods for breaking modern cryptosystems involve solving constructed problems in pure mathematics, the best-known being integer factorization.
Given some encrypted data, the goal of the cryptanalyst is to gain as much information as possible about the original, unencrypted data. It is useful to consider two aspects of achieving this; the first is breaking the system —, discovering how the encipherment process works. The second is solving the key, unique for a particular encrypted message or group of messages. Attacks can be classified based on; as a basic starting point it is assumed that, for the purposes of analysis, the general algorithm is known. This is a reasonable assumption in practice — throughout history, there are countless examples of secret algorithms falling into wider knowledge, variously through espionage and reverse engineering.: Ciphertext-only: the cryptanalyst has access only to a collection of ciphertexts or codetexts. Known-plaintext: the attacker has a set of ciphertexts to which he knows the corresponding plaintext. Chosen-plaintext: the attacker can obtain the ciphertexts corresponding to an arbitrary set of plaintexts of his own choosing.
Adaptive chosen-plaintext: like a chosen-plaintext attack, except the attacker can choose subsequent plaintexts based on information learned from previous encryptions. Adaptive chosen ciphertext attack. Related-key attack: Like a chosen-plaintext attack, except the attacker can obtain ciphertexts encrypted under two different keys; the keys are unknown. Attacks can be characterised by the resources they require; those resources include: Time -- the number of computation steps. Memory — the amount of storage required to perform the attack. Data — the quantity and type of plaintexts and ciphertexts required for a particular approach. It's sometimes difficult to predict these quantities especially when the attack isn't practical to implement for testing, but academic cryptanalysts tend to provide at least the estimated order of magnitude of their attacks' difficulty, for example, "SHA-1 collisions now 252."Bruce Schneier notes that computationally impractical attacks can be considered breaks: "Breaking a cipher means finding a weakness in the cipher that can be exploited with a complexity less than brute force.
Never mind that brute-force might require 2128 encryptions. The results of cryptanalysis can vary in usefulness. For example, cryptographer Lars Knudsen classified various types of attack on block ciphers according to the amount and quality of secret information, discovered: Total break — the attacker deduces the secret key. Global deduction — the attacker discovers a functionally equivalent algorithm for encryption and decryption, but without learning the key. Instance deduction — the attacker discovers additional plaintexts not known. Information deduction — the attacker gains some Shannon information about plaintexts not known. Distinguishing algorithm — the attacker can distinguish the cipher from a random permutation. Academic attacks are against weakened versions of a cryptosystem, such as a block cipher or hash function with some rounds removed. Many, but not all, attacks become exponentially more difficult to execute as rounds are added to a cryptosystem, so it's possible for the full cryptosystem to be strong though reduced-round variants are weak.
Nonetheless, partial breaks that come close to breaking the original cryptosystem may mean that a full break will follow. In academic cryptography, a weakness or a break in a scheme is defined quite conservatively: it might require impractical amounts of time, memory, or known plaintexts, it might require the attacker be able to do things many real-world attackers can't: for example, the attacker may need to choose particular plaintexts to be encrypted or to ask for plaintexts to be encrypted using several keys related to the secret key. Furthermore