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
Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not occur until the 18th century; the 19th century saw modest progress in the field after weather observation networks were formed across broad regions. Prior attempts at prediction of weather depended on historical data, it was not until after the elucidation of the laws of physics and more the development of the computer, allowing for the automated solution of a great many equations that model the weather, in the latter half of the 20th century that significant breakthroughs in weather forecasting were achieved. An important domain of weather forecasting is marine weather forecasting as it relates to maritime and coastal safety, in which weather effects include atmospheric interactions with large bodies of water. Meteorological phenomena are observable weather events that are explained by the science of meteorology.
Meteorological phenomena are described and quantified by the variables of Earth's atmosphere: temperature, air pressure, water vapour, mass flow, the variations and interactions of those variables, how they change over time. Different spatial scales are used to describe and predict weather on local and global levels. Meteorology, atmospheric physics, atmospheric chemistry are sub-disciplines of the atmospheric sciences. Meteorology and hydrology compose the interdisciplinary field of hydrometeorology; the interactions between Earth's atmosphere and its oceans are part of a coupled ocean-atmosphere system. Meteorology has application in many diverse fields such as the military, energy production, transport and construction; the word meteorology is from the Ancient Greek μετέωρος metéōros and -λογία -logia, meaning "the study of things high in the air". The ability to predict rains and floods based on annual cycles was evidently used by humans at least from the time of agricultural settlement if not earlier.
Early approaches to predicting weather were practiced by priests. Cuneiform inscriptions on Babylonian tablets included associations between rain; the Chaldeans differentiated 46 ° halos. Ancient Indian Upanishads contain mentions of seasons; the Samaveda mentions sacrifices to be performed. Varāhamihira's classical work Brihatsamhita, written about 500 AD, provides evidence of weather observation. In 350 BC, Aristotle wrote Meteorology. Aristotle is considered the founder of meteorology. One of the most impressive achievements described in the Meteorology is the description of what is now known as the hydrologic cycle; the book De Mundo noted If the flashing body is set on fire and rushes violently to the Earth it is called a thunderbolt. They are all called ` swooping bolts'. Lightning is sometimes smoky, is called'smoldering lightning". At other times, it travels in crooked lines, is called forked lightning; when it swoops down upon some object it is called'swooping lightning'. The Greek scientist Theophrastus compiled a book on weather forecasting, called the Book of Signs.
The work of Theophrastus remained a dominant influence in the study of weather and in weather forecasting for nearly 2,000 years. In 25 AD, Pomponius Mela, a geographer for the Roman Empire, formalized the climatic zone system. According to Toufic Fahd, around the 9th century, Al-Dinawari wrote the Kitab al-Nabat, in which he deals with the application of meteorology to agriculture during the Muslim Agricultural Revolution, he describes the meteorological character of the sky, the planets and constellations, the sun and moon, the lunar phases indicating seasons and rain, the anwa, atmospheric phenomena such as winds, lightning, floods, rivers, lakes. Early attempts at predicting weather were related to prophecy and divining, were sometimes based on astrological ideas. Admiral FitzRoy tried to separate scientific approaches from prophetic ones. Ptolemy wrote on the atmospheric refraction of light in the context of astronomical observations. In 1021, Alhazen showed that atmospheric refraction is responsible for twilight.
St. Albert the Great was the first to propose that each drop of falling rain had the form of a small sphere, that this form meant that the rainbow was produced by light interacting with each raindrop. Roger Bacon was the first to calculate the angular size of the rainbow, he stated. In the late 13th century and early 14th century, Kamāl al-Dīn al-Fārisī and Theodoric of Freiberg were the first to give the correct explanations for the primary rainbow phenomenon. Theoderic went further and explained the secondary rainbow. In 1716, Edmund Halley suggested that aurorae are caused by "magnetic effluvia" moving along the Earth's magnetic field lines. In 1441, King Sejong's son, Prince Munjong of Korea, invented the first standardized rain gauge; these were sent throughout the Joseon dynasty of Korea as an official tool to assess land taxes based
Cryptanalysis of the Enigma
Cryptanalysis of the Enigma ciphering system enabled the western Allies in World War II to read substantial amounts of Morse-coded radio communications of the Axis powers, enciphered using Enigma machines. This yielded military intelligence which, along with that from other decrypted Axis radio and teleprinter transmissions, was given the codename Ultra; this was considered by western Supreme Allied Commander Dwight D. Eisenhower to have been "decisive" to the Allied victory; the Enigma machines were a family of portable cipher machines with rotor scramblers. Good operating procedures, properly enforced, would have made the plugboard Enigma machine unbreakable. However, most of the German military forces, secret services and civilian agencies that used Enigma employed poor operating procedures, it was these poor procedures that allowed the Enigma machines to be reverse-engineered and the ciphers to be read; the German plugboard-equipped Enigma became Nazi Germany's principal crypto-system. It was broken by the Polish General Staff's Cipher Bureau in December 1932, with the aid of French-supplied intelligence material obtained from a German spy.
A month before the outbreak of World War II, at a conference held near Warsaw, the Polish Cipher Bureau shared its Enigma-breaking techniques and technology with the French and British. During the German invasion of Poland, core Polish Cipher Bureau personnel were evacuated, via Romania, to France where they established the PC Bruno signals intelligence station with French facilities support. Successful cooperation among the Poles, the French, the British at Bletchley Park continued until June 1940, when France surrendered to the Germans. From this beginning, the British Government Code and Cypher School at Bletchley Park built up an extensive cryptanalytic capability; the decryption was of Luftwaffe and a few Heer messages, as the Kriegsmarine employed much more secure procedures for using Enigma. Alan Turing, a Cambridge University mathematician and logician, provided much of the original thinking that led to the design of the cryptanalytical bombe machines that were instrumental in breaking the naval Enigma.
However, the Kriegsmarine introduced an Enigma version with a fourth rotor for its U-boats, resulting in a prolonged period when these messages could not be decrypted. With the capture of relevant cipher keys and the use of much faster US Navy bombes, rapid reading of U-boat messages resumed; the Enigma machines produced a polyalphabetic substitution cipher. During World War I, inventors in several countries realized that a purely random key sequence, containing no repetitive pattern, would, in principle, make a polyalphabetic substitution cipher unbreakable; this led to the development of rotor cipher machines which alter each character in the plaintext to produce the ciphertext, by means of a scrambler comprising a set of rotors that alter the electrical path from character to character, between the input device and the output device. This constant altering of the electrical pathway produces a long period before the pattern—the key sequence or substitution alphabet—repeats. Decrypting enciphered messages involves three stages, defined somewhat differently in that era than in modern cryptography.
First, there is the identification of the system in use, in this case Enigma. Today, it is assumed that an attacker knows how the encipherment process works and breaking is used for solving a key. Enigma machines, had so many potential internal wiring states that reconstructing the machine, independent of particular settings, was a difficult task; the Enigma rotor cipher machine was an excellent system. It generated a polyalphabetic substitution cipher, with a period before repetition of the substitution alphabet, much longer than any message, or set of messages, sent with the same key. A major weakness of the system, was that no letter could be enciphered to itself; this meant that some possible solutions could be eliminated because of the same letter appearing in the same place in both the ciphertext and the putative piece of plaintext. Comparing the possible plaintext Keine besonderen Ereignisse, with a section of ciphertext, might produce the following: The mechanism of the Enigma consisted of a keyboard connected to a battery and a current entry plate or wheel, at the right hand end of the scrambler.
This contained a set of 26 contacts that made electrical connection with the set of 26 spring-loaded pins on the right hand rotor. The internal wiring of the core of each rotor provided an electrical pathway from the pins on one side to different connection points on the other; the left hand side of each rotor made electrical connection with the rotor to its left. The leftmost rotor made contact with the reflector; the reflector provided a set of thirteen paired connections to return the current back through the scrambler rotors, to the lampboard where a lamp under a letter was illuminated. Whenever a key on the keyboard was pressed, the stepping motion was actuated, advancing the rightmost rotor one position; because it moved with each key pressed it is sometimes called the fast rotor. When a notch on that rotor engaged with a pawl on the middle rotor, that too moved. There are a huge n
Bletchley Park is a nineteenth-century mansion and estate near Milton Keynes in Buckinghamshire, constructed during the years following 1883 for the English financier and politician Sir Herbert Samuel Leon in the Victorian Gothic and Dutch Baroque styles, on the site of older buildings of the same name. It has received latter-day fame as the central site for British codebreakers during World War II, although at the time of their operation this fact was a guarded secret. During the Second World War, the estate housed the British Government Code and Cypher School, which penetrated the secret communications of the Axis Powers – most the German Enigma and Lorenz ciphers. According to the official historian of British Intelligence, the "Ultra" intelligence produced at Bletchley shortened the war by two to four years, without it the outcome of the war would have been uncertain; the team at Bletchley Park devised automatic machinery to help with decryption, culminating in the development of Colossus, the world's first programmable digital electronic computer.
Codebreaking operations at Bletchley Park came to an end in 1946 and all information about the wartime operations was classified until the mid 1970s. After the war, the Post Office took over the site and used it as a management school, but by 1990 the huts in which the codebreakers worked were being considered for demolition and redevelopment, the Bletchley Park Trust formed in 1991 to save large portions of the site from developers. More Bletchley Park has been open to the public and houses interpretive exhibits and rebuilt huts as they would have appeared during their wartime operations, as well as The National Museum of Computing, established on the site which includes a rebuilt Colossus machine, receives hundreds of thousands of visitors annually; the site appears in the Domesday Book as part of the Manor of Eaton. Browne Willis built a mansion there in 1711, but after Thomas Harrison purchased the property in 1793 this was pulled down, it was first known as Bletchley Park after its purchase by Samuel Lipscomb Seckham in 1877.
The estate of 581 acres was bought in 1883 by Sir Herbert Samuel Leon, who expanded the then-existing farmhouse into what architect Landis Gores called a "maudlin and monstrous pile" combining Victorian Gothic and Dutch Baroque styles. In 1938, the mansion and much of the site was bought by a builder planning a housing estate, but in May 1938 Admiral Sir Hugh Sinclair, head of the Secret Intelligence Service, bought the mansion and 58 acres of land for £6,000, using his own money after the Government said they did not have the budget to do so, for use by GC&CS and SIS in the event of war. A key advantage seen by Sinclair and his colleagues was Bletchley's geographical centrality, it was immediately adjacent to Bletchley railway station, where the "Varsity Line" between Oxford and Cambridge – whose universities were expected to supply many of the code-breakers – met the main West Coast railway line connecting London, Manchester, Liverpool and Edinburgh. Watling Street, the main road linking London to the north-west was close by, high-volume communication links were available at the telegraph and telephone repeater station in nearby Fenny Stratford.
Bletchley Park was known as "B. P." to those who worked there. "Station X", "London Signals Intelligence Centre", "Government Communications Headquarters" were all cover names used during the war. The formal posting of the many "Wrens" – members of the Women's Royal Naval Service – working there, was to HMS Pembroke V. Royal Air Force names of Bletchley Park and its outstations included RAF Eastcote, RAF Lime Grove and RAF Church Green; the postal address that staff had to use was "Room 47, Foreign Office". After the war, the Government Code & Cypher School became the Government Communications Headquarters moving to Eastcote in 1946 and to Cheltenham in the 1950s; the site was used by various government agencies, including the GPO and the Civil Aviation Authority. One large building, block F was demolished in 1987 by which time the site was being run down with tenants leaving. In 1990 the site was at risk of being sold for housing development. However, Milton Keynes Council made it into a conservation area.
Bletchley Park Trust was set up in 1991 by a group of people. The initial trustees included Roger Bristow, Ted Enever, Peter Wescombe, Dr Peter Jarvis of the Bletchley Archaeological & Historical Society, Tony Sale who in 1994 became the first director of the Bletchley Park Museums. Commander Alastair Denniston was operational head of GC&CS from 1919 to 1942, beginning with its formation from the Admiralty's Room 40 and the War Office's MI1b. Key GC&CS cryptanalysts who moved from London to Bletchley Park included John Tiltman, Dillwyn "Dilly" Knox, Josh Cooper, Nigel de Grey; these people had a variety of backgrounds – linguists and chess champions were common, in Knox's case papyrology. The British War Office recruited top solvers of cryptic crossword puzzles, as these individuals had strong lateral thinking skills. On the day Britain declared war on Germany, Denniston wrote to the Foreign Office about recruiting "men of the professor type". Personal networking drove early recruitments of men from the universities of Cambridge and Oxford.
Trustworthy women were recruited for administrative and clerical jobs. In one 1941 recruiting stratagem, The Daily Te
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
The Enigma machines are a series of electro-mechanical rotor cipher machines developed and used in the early- to mid-20th century to protect commercial and military communication. Enigma was invented by the German engineer Arthur Scherbius at the end of World War I. Early models were used commercially from the early 1920s, adopted by military and government services of several countries, most notably Nazi Germany before and during World War II. Several different Enigma models were produced, but the German military models, having a plugboard, were the most complex. Japanese and Italian models were in use. Around December 1932, Marian Rejewski, a Polish mathematician and cryptanalyst, while working at the Polish Cipher Bureau, used the theory of permutations and flaws in the German military message encipherment procedures to break the message keys of the plugboard Enigma machine. Rejewski achieved this result without knowledge of the wiring of the machine, so the result did not allow the Poles to decrypt actual messages.
The French spy Hans-Thilo Schmidt obtained access to German cipher materials that included the daily keys used in September and October 1932. Those keys included the plugboard settings; the French passed the material to the Poles, Rejewski used some of that material and the message traffic in September and October to solve for the unknown rotor wiring. The Polish mathematicians were able to build their own Enigma machines, which were called Enigma doubles. Rejewski was aided by cryptanalysts Jerzy Różycki and Henryk Zygalski, both of whom had been recruited with Rejewski from Poznań University; the Polish Cipher Bureau developed techniques to defeat the plugboard and find all components of the daily key, which enabled the Cipher Bureau to read the German Enigma messages starting from January 1933. Over time, the German cryptographic procedures improved, the Cipher Bureau developed techniques and designed mechanical devices to continue reading the Enigma traffic; as part of that effort, the Poles exploited quirks of the rotors, compiled catalogues, built a cyclometer to help make a catalogue with 100,000 entries, made Zygalski sheets and built the electro-mechanical cryptologic bomb to search for rotor settings.
In 1938, the Germans added complexity to the Enigma machines that became too expensive for the Poles to counter. The Poles had six bomby, but when the Germans added two more rotors, ten times as many bomby were needed, the Poles did not have the resources. On 26 and 27 July 1939, in Pyry near Warsaw, the Poles initiated French and British military intelligence representatives into their Enigma-decryption techniques and equipment, including Zygalski sheets and the cryptologic bomb, promised each delegation a Polish-reconstructed Enigma; the demonstration represented a vital basis for the British continuation and effort. During the war, British cryptologists decrypted a vast number of messages enciphered on Enigma; the intelligence gleaned from this source, codenamed "Ultra" by the British, was a substantial aid to the Allied war effort. Though Enigma had some cryptographic weaknesses, in practice it was German procedural flaws, operator mistakes, failure to systematically introduce changes in encipherment procedures, Allied capture of key tables and hardware that, during the war, enabled Allied cryptologists to succeed and "turned the tide" in the Allies' favour.
The German firm Scherbius & Ritter, co-founded by Arthur Scherbius, patented ideas for a cipher machine in 1918 and began marketing the finished product under the brand name Enigma in 1923 targeted at commercial markets. With its adoption by the German Navy in 1926 and the German Army and Air Force soon after, the name Enigma became known in military circles; the word enigma is a Latin word, derived from the Ancient Greek word enigma used in English, but not native German. Like other rotor machines, the Enigma machine is a combination of mechanical and electrical subsystems; the mechanical subsystem consists of a keyboard. The mechanical parts act in such a way; when a key is pressed, one or more rotors rotate on the spindle. On the sides of the rotors are a series of electrical contacts that, after rotation, line up with contacts on the other rotors or fixed wiring on either end of the spindle; when the rotors are properly aligned, each key on the keyboard is connected to a unique electrical pathway through the series of contacts and internal wiring.
Current from a battery, flows through the pressed key, into the newly configured set of circuits and back out again lighting one display lamp, which shows the output letter. For example, when encrypting a message starting ANX... the operator would first press the A key, the Z lamp might light, so Z would be the first letter of the ciphertext. The operator would next press N, X in the same fashion, so on. Current flowed from the battery through a depressed bi-directional keyboard switch to the plugboard. Next, it passed through the plug "A" via the entry wheel, through the wiring of the three or four installed rotors, entered the reflector; the reflector returned the current, via an different path, back through the rotors and entry wheel, proceeding through plug "S" connected with a cable to plug "D", another bi-directional switch to light the appropriate lamp. The repeated changes of electrical