The Vigenère cipher is a method of encrypting alphabetic text by using a series of interwoven Caesar ciphers, based on the letters of a keyword. It is a form of polyalphabetic substitution. First described in 1553, the cipher is easy to understand and implement, but it resisted all attempts to break it for three centuries until 1863; this earned it the description le chiffre indéchiffrable. Many people have tried to implement encryption schemes that are Vigenère ciphers. In 1863, Friedrich Kasiski was the first to publish a general method of deciphering Vigenère ciphers; the Vigenère cipher was described by Giovan Battista Bellaso in his 1553 book La cifra del. Sig. Giovan Battista Bellaso, but the scheme was misattributed to Blaise de Vigenère in the 19th century and so acquired its present name; the first well-documented description of a polyalphabetic cipher was formulated by Leon Battista Alberti around 1467 and used a metal cipher disc to switch between cipher alphabets. Alberti's system only switched alphabets after several words, switches were indicated by writing the letter of the corresponding alphabet in the ciphertext.
Johannes Trithemius, in his work Polygraphiae, invented the tabula recta, a critical component of the Vigenère cipher. The Trithemius cipher, provided a progressive, rather rigid and predictable system for switching between cipher alphabets. What is now known as the Vigenère cipher was described by Giovan Battista Bellaso in his 1553 book La cifra del Sig. Giovan Battista Bellaso, he built upon the tabula recta of Trithemius but added a repeating "countersign" to switch cipher alphabets every letter. Whereas Alberti and Trithemius used a fixed pattern of substitutions, Bellaso's scheme meant the pattern of substitutions could be changed by selecting a new key. Keys were single words or short phrases, known to both parties in advance, or transmitted "out of band" along with the message. Bellaso's method thus required strong security for only the key; as it is easy to secure a short key phrase, such as by a previous private conversation, Bellaso's system was more secure. Blaise de Vigenère published his description of a similar but stronger autokey cipher before the court of Henry III of France, in 1586.
In the 19th century, the invention of Bellaso's cipher was misattributed to Vigenère. David Kahn, in his book, The Codebreakers lamented the misattribution by saying that history had "ignored this important contribution and instead named a regressive and elementary cipher for him though he had nothing to do with it"; the Vigenère cipher gained a reputation for being exceptionally strong. Noted author and mathematician Charles Lutwidge Dodgson called the Vigenère cipher unbreakable in his 1868 piece "The Alphabet Cipher" in a children's magazine. In 1917, Scientific American described the Vigenère cipher as "impossible of translation"; that reputation was not deserved. Charles Babbage is known to have broken a variant of the cipher as early as 1854 but failed to publish his work. Kasiski broke the cipher and published the technique in the 19th century, but earlier, some skilled cryptanalysts could break the cipher in the 16th century; the Vigenère cipher is simple enough to be a field cipher if it is used in conjunction with cipher disks.
The Confederate States of America, for example, used a brass cipher disk to implement the Vigenère cipher during the American Civil War. The Confederacy's messages were far from secret, the Union cracked its messages. Throughout the war, the Confederate leadership relied upon three key phrases: "Manchester Bluff", "Complete Victory" and, as the war came to a close, "Come Retribution". Gilbert Vernam tried to repair the broken cipher, but no matter what he did, the cipher was still vulnerable to cryptanalysis. Vernam's work, however led to the one-time pad, a theoretically unbreakable cipher. In a Caesar cipher, each letter of the alphabet is shifted along some number of places. For example, in a Caesar cipher of shift 3, A would become D, B would become E, Y would become B and so on; the Vigenère cipher has several Caesar ciphers in sequence with different shift values. To encrypt, a table of alphabets can be used, termed a tabula recta, Vigenère square or Vigenère table, it has the alphabet written out 26 times in different rows, each alphabet shifted cyclically to the left compared to the previous alphabet, corresponding to the 26 possible Caesar ciphers.
At different points in the encryption process, the cipher uses a different alphabet from one of the rows. The alphabet used at each point depends on a repeating keyword. For example, suppose that the plaintext to be encrypted is ATTACKATDAWN; the person sending the message chooses a keyword and repeats it until it matches the length of the plaintext, for example, the keyword "LEMON": LEMONLEMONLEEach row starts with a key letter. The rest of the row holds the letters A to Z. Although there are 26 key rows shown, a code will use only as many keys as there are unique letters in the key string, here just 5 keys:. For successive letters of the message, successive letters of the key string will be taken and each message letter enciphered by using its corresponding key row; the next letter of the key is chosen, that row is gone along to find the column heading that matches the message character. The letter at the intersection of is the enciphered letter. For example, the first letter of the pl
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
In mathematics, the binary logarithm is the power to which the number 2 must be raised to obtain the value n. That is, for any real number x = log 2 n ⟺ 2 x = n. For example, the binary logarithm of 1 is 0, the binary logarithm of 2 is 1, the binary logarithm of 4 is 2, the binary logarithm of 32 is 5; the binary logarithm is the logarithm to the base 2. The binary logarithm function is the inverse function of the power of two function; as well as log2, alternative notations for the binary logarithm include lg, ld, lb, log. The first application of binary logarithms was in music theory, by Leonhard Euler: the binary logarithm of a frequency ratio of two musical tones gives the number of octaves by which the tones differ. Binary logarithms can be used to calculate the length of the representation of a number in the binary numeral system, or the number of bits needed to encode a message in information theory. In computer science, they count the number of steps needed for related algorithms. Other areas in which the binary logarithm is used include combinatorics, the design of sports tournaments, photography.
Binary logarithms are included in the standard C mathematical functions and other mathematical software packages. The integer part of a binary logarithm can be found using the find first set operation on an integer value, or by looking up the exponent of a floating point value; the fractional part of the logarithm can be calculated efficiently. The powers of two have been known since antiquity. IX.32 and IX.36. And the binary logarithm of a power of two is just its position in the ordered sequence of powers of two. On this basis, Michael Stifel has been credited with publishing the first known table of binary logarithms in 1544, his book Arithmetica Integra contains several tables that show the integers with their corresponding powers of two. Reversing the rows of these tables allow them to be interpreted as tables of binary logarithms. Earlier than Stifel, the 8th century Jain mathematician Virasena is credited with a precursor to the binary logarithm. Virasena's concept of ardhacheda has been defined as the number of times a given number can be divided evenly by two.
This definition gives rise to a function that coincides with the binary logarithm on the powers of two, but it is different for other integers, giving the 2-adic order rather than the logarithm. The modern form of a binary logarithm, applying to any number was considered explicitly by Leonhard Euler in 1739. Euler established the application of binary logarithms to music theory, long before their applications in information theory and computer science became known; as part of his work in this area, Euler published a table of binary logarithms of the integers from 1 to 8, to seven decimal digits of accuracy. The binary logarithm function may be defined as the inverse function to the power of two function, a increasing function over the positive real numbers and therefore has a unique inverse. Alternatively, it may be defined as ln n/ln 2, where ln is the natural logarithm, defined in any of its standard ways. Using the complex logarithm in this definition allows the binary logarithm to be extended to the complex numbers.
As with other logarithms, the binary logarithm obeys the following equations, which can be used to simplify formulas that combine binary logarithms with multiplication or exponentiation: log 2 x y = log 2 x + log 2 y log 2 x y = log 2 x − log 2 y log 2 x y = y log 2 x. For more, see list of logarithmic identities. In mathematics, the binary logarithm of a number n is written as log2 n. However, several other notations for this function have been used or proposed in application areas; some authors write the binary logarithm as the notation listed in The Chicago Manual of Style. Donald Knuth credits this notation to a suggestion of Edward Reingold, but its use in both information theory and computer science dates to before Reingold was active; the binary logarithm has been written as log n with a prior statement that the default base for the logarithm is 2. Another notation, used for the same function is ld n, from Latin logarithmus dualis aka logarithmus dyadis; the DIN 1302, ISO 31-11 and ISO 80000-2 standards recommend yet another lb n.
According to these standards, lg n should not be used for the binary logarithm, as it is instead reserved for the common logarithm log10 n. The number of digits in the binary representation of a positive integer n is the integral part of 1 + log2 n, i.e. ⌊ log 2 n ⌋ + 1. In information theory, t
Cryptography or cryptology is the practice and study of techniques for secure communication in the presence of third parties called adversaries. More cryptography is about constructing and analyzing protocols that prevent third parties or the public from reading private messages. Modern cryptography exists at the intersection of the disciplines of mathematics, computer science, electrical engineering, communication science, physics. Applications of cryptography include electronic commerce, chip-based payment cards, digital currencies, computer passwords, military communications. Cryptography prior to the modern age was synonymous with encryption, the conversion of information from a readable state to apparent nonsense; the originator of an encrypted message shares the decoding technique only with intended recipients to preclude access from adversaries. The cryptography literature uses the names Alice for the sender, Bob for the intended recipient, Eve for the adversary. Since the development of rotor cipher machines in World War I and the advent of computers in World War II, the methods used to carry out cryptology have become complex and its application more widespread.
Modern cryptography is based on mathematical theory and computer science practice. It is theoretically possible to break such a system, but it is infeasible to do so by any known practical means; these schemes are therefore termed computationally secure. There exist information-theoretically secure schemes that provably cannot be broken with unlimited computing power—an example is the one-time pad—but these schemes are more difficult to use in practice than the best theoretically breakable but computationally secure mechanisms; the growth of cryptographic technology has raised a number of legal issues in the information age. Cryptography's potential for use as a tool for espionage and sedition has led many governments to classify it as a weapon and to limit or prohibit its use and export. In some jurisdictions where the use of cryptography is legal, laws permit investigators to compel the disclosure of encryption keys for documents relevant to an investigation. Cryptography plays a major role in digital rights management and copyright infringement of digital media.
The first use of the term cryptograph dates back to the 19th century—originating from The Gold-Bug, a novel by Edgar Allan Poe. Until modern times, cryptography referred exclusively to encryption, the process of converting ordinary information into unintelligible form. Decryption is the reverse, in other words, moving from the unintelligible ciphertext back to plaintext. A cipher is a pair of algorithms that create the reversing decryption; the detailed operation of a cipher is controlled both by the algorithm and in each instance by a "key". The key is a secret a short string of characters, needed to decrypt the ciphertext. Formally, a "cryptosystem" is the ordered list of elements of finite possible plaintexts, finite possible cyphertexts, finite possible keys, the encryption and decryption algorithms which correspond to each key. Keys are important both formally and in actual practice, as ciphers without variable keys can be trivially broken with only the knowledge of the cipher used and are therefore useless for most purposes.
Ciphers were used directly for encryption or decryption without additional procedures such as authentication or integrity checks. There are two kinds of cryptosystems: asymmetric. In symmetric systems the same key is used to decrypt a message. Data manipulation in symmetric systems is faster than asymmetric systems as they use shorter key lengths. Asymmetric systems use a public key to encrypt a private key to decrypt it. Use of asymmetric systems enhances the security of communication. Examples of asymmetric systems include RSA, ECC. Symmetric models include the used AES which replaced the older DES. In colloquial use, the term "code" is used to mean any method of encryption or concealment of meaning. However, in cryptography, code has a more specific meaning, it means the replacement of a unit of plaintext with a code word. Cryptanalysis is the term used for the study of methods for obtaining the meaning of encrypted information without access to the key required to do so; some use the terms cryptography and cryptology interchangeably in English, while others use cryptography to refer to the use and practice of cryptographic techniques and cryptology to refer to the combined study of cryptography and cryptanalysis.
English is more flexible than several other languages in which crypto
In cryptography, a substitution cipher is a method of encrypting by which units of plaintext are replaced with ciphertext, according to a fixed system. The receiver deciphers the text by performing the inverse substitution. Substitution ciphers can be compared with transposition ciphers. In a transposition cipher, the units of the plaintext are rearranged in a different and quite complex order, but the units themselves are left unchanged. By contrast, in a substitution cipher, the units of the plaintext are retained in the same sequence in the ciphertext, but the units themselves are altered. There are a number of different types of substitution cipher. If the cipher operates on single letters, it is termed a simple substitution cipher. A monoalphabetic cipher uses fixed substitution over the entire message, whereas a polyalphabetic cipher uses a number of substitutions at different positions in the message, where a unit from the plaintext is mapped to one of several possibilities in the ciphertext and vice versa.
Substitution of single letters separately—simple substitution—can be demonstrated by writing out the alphabet in some order to represent the substitution. This is termed a substitution alphabet; the cipher alphabet may be shifted or reversed or scrambled in a more complex fashion, in which case it is called a mixed alphabet or deranged alphabet. Traditionally, mixed alphabets may be created by first writing out a keyword, removing repeated letters in it writing all the remaining letters in the alphabet in the usual order. Using this system, the keyword "zebras" gives us the following alphabets: A message of flee at once. We are discovered! enciphers to SIAA ZQ LKBA. VA ZOA RFPBLUAOAR! Traditionally, the ciphertext is written out in blocks of fixed length, omitting punctuation and spaces; these blocks are called "groups", sometimes a "group count" is given as an additional check. Five-letter groups are traditional, dating from when messages used to be transmitted by telegraph: SIAAZ QLKBA VAZOA RFPBL UAOAR If the length of the message happens not to be divisible by five, it may be padded at the end with "nulls".
These can be any characters that decrypt to obvious nonsense, so the receiver can spot them and discard them. The ciphertext alphabet is sometimes different from the plaintext alphabet. For example: Such features make little difference to the security of a scheme, however – at the least, any set of strange symbols can be transcribed back into an A-Z alphabet and dealt with as normal. In lists and catalogues for salespeople, a simple encryption is sometimes used to replace numeric digits by letters. Example: MAT would be used to represent 120. Although the traditional keyword method for creating a mixed substitution alphabet is simple, a serious disadvantage is that the last letters of the alphabet tend to stay at the end. A stronger way of constructing a mixed alphabet is to perform a columnar transposition on the ordinary alphabet using the keyword, but this is not done. Although the number of possible keys is large, this cipher is not strong, is broken. Provided the message is of reasonable length, the cryptanalyst can deduce the probable meaning of the most common symbols by analyzing the frequency distribution of the ciphertext.
This allows formation of partial words, which can be tentatively filled in, progressively expanding the solution. In some cases, underlying words can be determined from the pattern of their letters. Many people solve such ciphers for recreation, as with cryptogram puzzles in the newspaper. According to the unicity distance of English, 27.6 letters of ciphertext are required to crack a mixed alphabet simple substitution. In practice about 50 letters are needed, although some messages can be broken with fewer if unusual patterns are found. In other cases, the plaintext can be contrived to have a nearly flat frequency distribution, much longer plaintexts will be required by the cryptanalyst. One once-common variant of the substitution cipher is the nomenclator. Named after the public official who announced the titles of visiting dignitaries, this cipher uses a small code sheet containing letter and word substitution tables, sometimes homophonic, that converted symbols into numbers; the code portion was restricted to the names of important people, hence the name of the cipher.
The symbols for whole words and letters were not distinguished in the ciphertext. The Rossignols' Great Cipher used by Louis XIV of France was one. Nomenclators were the standard fare of diplomatic correspondence and advanced political conspiracy from the early fifteenth century to the late eighteenth century. Although government intelligence cryptanalysts were systematically breaking nomenclators by the mid-sixteenth century, superior systems had been available since 1467, the usual response to cryp
Claude Elwood Shannon was an American mathematician, electrical engineer, cryptographer known as "the father of information theory". Shannon is noted for having founded information theory with a landmark paper, A Mathematical Theory of Communication, that he published in 1948, he is well known for founding digital circuit design theory in 1937, when—as a 21-year-old master's degree student at the Massachusetts Institute of Technology —he wrote his thesis demonstrating that electrical applications of Boolean algebra could construct any logical numerical relationship. Shannon contributed to the field of cryptanalysis for national defense during World War II, including his fundamental work on codebreaking and secure telecommunications. Shannon was born in Petoskey and grew up in Gaylord, Michigan, his father, Claude, Sr. a descendant of early settlers of New Jersey, was a self-made businessman, for a while, a Judge of Probate. Shannon's mother, Mabel Wolf Shannon, was a language teacher, served as the principal of Gaylord High School.
Most of the first 16 years of Shannon's life were spent in Gaylord, where he attended public school, graduating from Gaylord High School in 1932. Shannon showed an inclination towards electrical things, his best subjects were science and mathematics. At home he constructed such devices as models of planes, a radio-controlled model boat and a barbed-wire telegraph system to a friend's house a half-mile away. While growing up, he worked as a messenger for the Western Union company, his childhood hero was Thomas Edison, who he learned was a distant cousin. Both Shannon and Edison were descendants of John Ogden, a colonial leader and an ancestor of many distinguished people. Shannon was an atheist. In 1932, Shannon entered the University of Michigan, where he was introduced to the work of George Boole, he graduated in 1936 with two bachelor's degrees: one in electrical engineering and the other in mathematics. In 1936, Shannon began his graduate studies in electrical engineering at MIT, where he worked on Vannevar Bush's differential analyzer, an early analog computer.
While studying the complicated ad hoc circuits of this analyzer, Shannon designed switching circuits based on Boole's concepts. In 1937, he wrote A Symbolic Analysis of Relay and Switching Circuits. A paper from this thesis was published in 1938. In this work, Shannon proved that his switching circuits could be used to simplify the arrangement of the electromechanical relays that were used in telephone call routing switches. Next, he expanded this concept, proving that these circuits could solve all problems that Boolean algebra could solve. In the last chapter, he presented diagrams of several circuits, including a 4-bit full adder. Using this property of electrical switches to implement logic is the fundamental concept that underlies all electronic digital computers. Shannon's work became the foundation of digital circuit design, as it became known in the electrical engineering community during and after World War II; the theoretical rigor of Shannon's work superseded the ad hoc methods. Howard Gardner called Shannon's thesis "possibly the most important, the most noted, master's thesis of the century."Shannon received his Ph.
D. degree from MIT in 1940. Vannevar Bush had suggested that Shannon should work on his dissertation at the Cold Spring Harbor Laboratory, in order to develop a mathematical formulation for Mendelian genetics; this research resulted in Shannon's PhD thesis, called An Algebra for Theoretical Genetics. In 1940, Shannon became a National Research Fellow at the Institute for Advanced Study in Princeton, New Jersey. In Princeton, Shannon had the opportunity to discuss his ideas with influential scientists and mathematicians such as Hermann Weyl and John von Neumann, he had occasional encounters with Albert Einstein and Kurt Gödel. Shannon worked across disciplines, this ability may have contributed to his development of mathematical information theory. Shannon joined Bell Labs to work on fire-control systems and cryptography during World War II, under a contract with section D-2 of the National Defense Research Committee. Shannon is credited with the invention of signal-flow graphs, in 1942, he discovered the topological gain formula while investigating the functional operation of an analog computer.
For two months early in 1943, Shannon came into contact with the leading British mathematician Alan Turing. Turing had been posted to Washington to share with the U. S. Navy's cryptanalytic service the methods used by the British Government Code and Cypher School at Bletchley Park to break the ciphers used by the Kriegsmarine U-boats in the north Atlantic Ocean, he was interested in the encipherment of speech and to this end spent time at Bell Labs. Shannon and Turing met at teatime in the cafeteria. Turing showed Shannon his 1936 paper that defined what is now known as the "Universal Turing machine". In 1945, as the war was coming to an end, the NDRC was issuing a summary of technical reports as a last step prior to its eventual closing down. Inside the volume on fire control, a special essay titled Data Smoothing and Prediction in Fire-Control Systems, coauthored by Shannon, Ralph Beebe Blackman, Hendrik Wade Bode, formally treated the problem of smoothing the data in fire-control by analogy with "the problem of separating a signal from interfering noise in communications systems."
In other words, it modeled the problem in terms of data and signal processing and thus heralded the coming of the Information Age. Shannon's w
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