Cryptography

The Enigma machine, used by Germany in World War II, implemented a complex cipher to protect sensitive communications. - 2fa to Ale  - Ale to And  - And to Apr  - Aqa to G5; Cryptography is the field concerned with linguistic and mathematical techniques for securing information, particularly in communications. Historically, cryptography was concerned solely with encryption; that is, means of converting information from its normal, comprehensible form into an incomprehensible format, rendering it unreadable without secret knowledge. Encryption was used primarily to ensure secrecy in important communications, such as those of spies, military leaders, and diplomats. In recent decades, however, the field of cryptography has expanded its remit: modern cryptography provides mechanisms for more than just keeping secrets and has a variety of applications including, for example, authentication, digital signatures, electronic voting and digital cash. Moreover, people without extraordinary needs for secrecy use cryptographic technology, which is often built transparently into much of computing and telecommunications infrastructure. - Bab to Bur  - Gir to Bli  - Blo to Bor  - Bod to Rid Cryptography is an interdisciplinary subject, drawing from several fields. Older forms of cryptography were chiefly concerned with patterns in language. More recently, the emphasis has shifted, and cryptography makes extensive use of mathematics, particularly discrete mathematics, including topics from number theory, information theory, computational complexity, statistics and combinatorics. Cryptography is also considered a branch of engineering, but it is considered to be an unusual one as it deals with active, intelligent and malevolent opposition (see cryptographic engineering and security engineering). Cryptography is a tool used within computer and network security. - Bri to Cal  - Alv to Car  - Car to Cha  - Che to Chr An associated field is steganography — the study of hiding the very existence of a message, and not necessarily the contents of the message itself (for example, microdots, or invisible ink). - Chr to Coc  - Coc to Dan  - Dan to Deb  - See to Dev Terminology The study of how to circumvent the use of cryptography is called cryptanalysis, or codebreaking. Cryptography and cryptanalysis are sometimes grouped together under the umbrella term cryptology, encompassing the entire subject. In practice, the term cryptography (from the Greek kryptós, "hidden", and gráphein, "to write") is often used to refer to the field as a whole; crypto is an informal abbreviation. The first recorded usage of the word 'cryptography' occurs in Sir Thomas Browne's Discourse of 1658 entitled The Garden of Cyrus: "the strange Cryptography of Gaffarel in his Starrie Booke of Heaven". - Dd; to Dog  - Dog to Dow  - Drp to Edd  - Edd to Fet Encryption (or enciphering) is the process of converting ordinary information (plaintext) into an unreadable form, termed ciphertext, or a cryptogram. Decryption, or deciphering, is the reverse process, recovering the plaintext back from the ciphertext. A cipher (sometimes spelt cypher) is an algorithm for encryption and decryption. The exact operation of a cipher is normally controlled by a key — some secret piece of information that customizes how the ciphertext is produced. Cryptographic protocols specify the details of how ciphers (and other cryptographic primitives) are to be used to achieve specific tasks. A suite of protocols, ciphers, key management, user-prescribed actions implemented together as a system constitute a cryptosystem. - Fye to Loc  - Mac to Gre  - Gra to Hal  - Ham to Hib In ordinary parlance, a (secret) "code" is often used synonymously with "cipher". In cryptography, however, the term has a specialised technical meaning: Codes are a historical method which involves substituting larger units of text, typically words or phrases (for example, "apple pie" replaces "attack at dawn"). In contrast, classical ciphers usually substitute or rearrange individual letters (or small groups of letters) — for example, "attack at dawn" becomes "buubdl bu ebxo" by substitution. - Hid to Hud  - Hum to Jap  - Jel to Kar  - Kar to Lab Cryptanalysis Main article: Cryptanalysis The goal of cryptanalysis is to find some weaknesses or insecurity in a cryptographic scheme. Cryptanalysis might be undertaken by a hostile attacker, attempting to subvert a system; or by the system's designer (or others) wishing to evaluate whether a system is secure. - Leg to Mag  - Mag to Mar  - Mar to Men  - Men to Mon There are a wide variety of cryptanalytic attacks, and they can be classified in several ways. One distinction concerns what an attacker can know and do in order to learn secret information. For example, does the cryptanalyst have access only to the ciphertext? Does he also know or can he guess some corresponding plaintexts? Or even: Can he choose arbitrary plaintexts to be encrypted? These scenarios correspond to ciphertext only, known plaintext and chosen plaintext attacks, respectively. - Mom to Mot  - Mot to Nat  - Nat to Nor  - Nor to Pet While pure cryptanalysis uses weaknesses in the algorithms themselves, other attacks are based upon the implementation, known as side-channel attacks. If a cryptanalyst has access to, say, the amount of time the algorithm took to encrypt a number of plaintexts, he may be able to use a timing attack to break a cipher that is otherwise resistant to analysis. An attacker also might consider studying the pattern and length of messages to derive valuable information; this is known as traffic analysis. - Pet to Pok  - Pol to Pus  - Qui to Ri;  - Rio to Sla If a cryptosystem uses a key derived from a password, it may be at risk of exhaustive search, due to the insufficient size or randomness of passwords. This is a common weak point in cryptographic systems. For network applications, a password-authenticated key agreement protocol can address some of the limitations of passwords. For standalone applications, either a secure means for storing the password-derived data and/or a passphrase is often recommended. - Sam to Sho  - Sho to Spo  - Spo to Sus  - Swi to Tan Linear and differential cryptanalysis are general methods for symmetric key cryptography. When cryptography relies on hard mathematical problems, as is usually the case in asymmetric cryptography, algorithms for tasks such as factoring become potential tools for cryptanalysis. - Too to Tri  - Tri to Und  - Une to Bik  - Ine to Wav History of cryptography The Ancient Greek skytale may have been one of the earliest devices used to implement a cipher. - Wb; to Yug  - Yuy to Cryptography has had a long and colourful history. Generally, the earliest forms of secret writing (now collectively termed classical cryptography) required only pen and paper. The two main categories of classical ciphers are transposition ciphers, which rearrange the order of letters in a message, and substitution ciphers, which systematically replace letters or groups of letters with other letters or groups of letters. One of the earliest and simplest substitution ciphers was the Caesar cipher, used by Julius Caesar. Text encrypted by classical ciphers tends to reveal a certain statistical information about the plaintext. By using this information, classical ciphers are easily broken (e.g. by frequency analysis). Classical ciphers still enjoy popularity today, though mostly as puzzles (see cryptogram). Various devices and aids have been used for encryption. One of the earliest may have been the scytale. Early in the 20th century, several mechanical devices were invented for performing encryption, including rotor machines — most famously the Enigma machine used by Germany in World War II. The ciphers implemented by these machines brought about a significant increase in the complexity of cryptanalysis. With the advent of digital computers and electronics, very complex ciphers could be implemented. A characteristic of computer ciphers is that they operate on binary strings, unlike classical and mechanical schemes, which use an alphabet of around 26 letters (depending on the language). Computer ciphers are also much more resistant to cryptanalysis; few are susceptible to a ciphertext-only attack. Extensive academic research into modern cryptography is relatively recent — it only began in the open community during the 1970s with the specification of the Data Encryption Standard (DES) and the invention of RSA. Since then, cryptography has become a widely-used tool in communications and computer security. As well as noting lessons from its history, cryptographers are also careful to consider the future. Moore's law is routinely taken into account when specifying key lengths, and the potential effects of quantum computing have already been considered. Secure communications See also: Information security Cryptography is commonly used for securing communications. Desired properties can include: Confidentiality, also known as secrecy: only an authorised recipient should be able to extract the contents of the message from its encrypted form. Otherwise, it should not be possible to obtain any significant information about the message contents. Integrity: the recipient should be able to determine if the message has been altered during transmission. Authentication: the recipient should be able to identify the sender, and verify that the purported sender actually did send the message. Non-repudiation: the sender should not be able to deny sending the message. Anti-replay: the message should not be allowed to be sent multiple times to the recipient without the sender knowing. Proof of delivery: the sender should be able to prove the recipient received the message. Cryptography can provide mechanisms to help achieve all of the above. However, some goals are not always necessary, practical or even desirable in some contexts. For example, the sender of a message may wish to remain anonymous; clearly non-repudiation would be inappropriate in that case. Symmetric key cryptography Main article: Symmetric key algorithm Symmetric key ciphers either use the same key for encryption and decryption, or the key used for decryption is easily calculated from the key used for encryption. Other terms include secret-key, private-key, one-key and single-key cryptography. One iteration (out of 6–10) of the block cipher SAFER-K. Modern computer-implemented ciphers can be a lot more complex than those performed by hand or electromechanical machines.