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Introduction to Cryptography
Cryptography
Cryptography or cryptology (from
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Ancient Greek: “hidden, secret”; and “to write”, or “study”, respectively [1] ) is the practice and study of techniques for
https://en.wikipedia.org/wiki/Cryptography#cite_note-1	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
https://en.wikipedia.org/wiki/Secure_communication	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
secure communicationin the presence of third parties called
https://en.wikipedia.org/wiki/Adversary_(cryptography)	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
adversaries. [2] More generally, cryptography is about constructing and analyzing
https://en.wikipedia.org/wiki/Cryptography#cite_note-rivest90-2	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
https://en.wikipedia.org/wiki/Communications_protocol	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography
protocolsthat prevent third parties or the public from reading private messages; [3] various aspects in
https://en.wikipedia.org/wiki/Cryptography#cite_note-modern-crypto-3	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
https://en.wikipedia.org/wiki/Information_security	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
information securitysuch as data
https://en.wikipedia.org/wiki/Confidentiality	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
confidentiality,
https://en.wikipedia.org/wiki/Data_integrity	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
data integrity,
https://en.wikipedia.org/wiki/Authentication	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame https://en.wikipedia.org/wiki/Cryptography
authentication, and
https://en.wikipedia.org/wiki/Non-repudiation	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
non-repudiation [4] are central to modern cryptography.
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Modern cryptography exists at the intersection of the disciplines of
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mathematics,
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computer science,
https://en.wikipedia.org/wiki/Electrical_engineering	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame https://en.wikipedia.org/wiki/Cryptography
electrical engineering,
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communication science, and
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physics.
Applications of cryptography include
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electronic commerce,
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chip-based payment cards,
https://en.wikipedia.org/wiki/Digital_currencies	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
digital currencies,
https://en.wikipedia.org/wiki/Password	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
computer passwords, and
https://en.wikipedia.org/wiki/Military_communications	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography
military communications.
Cryptography prior to the modern age was effectively synonymous with
https://en.wikipedia.org/wiki/Encryption	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
encryption , the conversion of information from a readable state to apparent
https://en.wikipedia.org/wiki/Nonsense	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
nonsense.
The originator of an encrypted message shares the decoding technique only with intended recipients to preclude access from adversaries.
The cryptography literature
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often uses the namesAlice (“A”) for the sender, Bob (“B”) for the intended recipient, and Eve (“eavesdropper“) for the adversary. [5] Since the development of
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https://en.wikipedia.org/wiki/Cryptography#cite_note-codesintro-5	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
https://en.wikipedia.org/wiki/Rotor_machine	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
rotor cipher machinesin
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World WarIand the advent of
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computersin
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World WarII, the methods used to carry out cryptology have become increasingly complex and its application more widespread.
Modern cryptography is heavily based on mathematical theory and computer science practice; cryptographic
https://en.wikipedia.org/wiki/Algorithm	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
algorithmsare designed around
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computational hardness assumptions, making such algorithms hard to break in practice by any adversary.
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; theoretical advances, e.g., improvements in
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integer factorizationalgorithms, and faster computing technology require these solutions to be continually adapted.
There exist
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information-theoretically secureschemes that provably cannot be broken even with unlimited computing power—an example is the
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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
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espionageand
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seditionhas led many governments to classify it as a weapon and to limit or even prohibit its use and export. [6] In some jurisdictions where the use of cryptography is legal, laws permit investigators to
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https://en.wikipedia.org/wiki/Key_disclosure_law	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
compel the disclosureof encryption keys for documents relevant to an investigation. [7] [8] Cryptography also plays a major role in
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https://en.wikipedia.org/wiki/Cryptography#cite_note-RangerSteve1-8	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
https://en.wikipedia.org/wiki/Digital_rights_management	https://en.wikipedia.org/wiki/Avengers:_Endgame https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography
digital rights managementand
https://en.wikipedia.org/wiki/Copyright_infringement	https://en.wikipedia.org/wiki/Avengers:_Endgame https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
copyright infringementof digital media. [9]
https://en.wikipedia.org/wiki/Cryptography#cite_note-AACS-9	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
Terminology
The first use of the termcryptograph(as opposed tocryptogram) dates back to the 19th century—originating from The Gold-Bug , a novel byEdgar Allan Poe.
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https://en.wikipedia.org/wiki/Edgar_Allan_Poe	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame https://en.wikipedia.org/wiki/Cryptography
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Until modern times, cryptography referred almost exclusively to
encryption, which is the process of converting ordinary information (called
https://en.wikipedia.org/wiki/Plaintext	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
plaintext) into unintelligible form (called
https://en.wikipedia.org/wiki/Ciphertext	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
ciphertext).
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Decryption is the reverse, in other words, moving from the unintelligible ciphertext back to plaintext.
A
https://en.wikipedia.org/wiki/Cipher	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
cipher (or
cypher) is a pair of algorithms that create the encryption and the reversing decryption.
The detailed operation of a cipher is controlled both by the algorithm and in each instance by a “key“.
https://en.wikipedia.org/wiki/Key_(cryptography)	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
The key is a secret (ideally known only to the communicants), usually a short string of characters, which is needed to decrypt the ciphertext.
Formally, a “cryptosystem” is the ordered list of elements of finite possible plaintexts, finite possible cyphertexts, finite possible keys, and the encryption and decryption algorithms which correspond to each key.
https://en.wikipedia.org/wiki/Cryptosystem	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
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 (or even counter-productive) for most purposes.
Historically, ciphers were often used directly for encryption or decryption without additional procedures such as
https://en.wikipedia.org/wiki/Authentication	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame https://en.wikipedia.org/wiki/Cryptography
authenticationor integrity checks.
There are two kinds of cryptosystems:
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symmetricand
https://en.wikipedia.org/wiki/Public-key_cryptography	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
asymmetric.
In symmetric systems the same key (the secret key) is used to encrypt and decrypt a message.
Data manipulation in symmetric systems is faster than asymmetric systems as they generally use shorter key lengths.
Asymmetric systems use a public key to encrypt a message and a private key to decrypt it.
Use of asymmetric systems enhances the security of communication.
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Examples of asymmetric systems include RSA (Rivest-Shamir-Adleman), and ECC (Elliptic Curve Cryptography).
https://en.wikipedia.org/wiki/Rivest-Shamir-Adleman	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame https://en.wikipedia.org/wiki/Cryptography
https://en.wikipedia.org/wiki/Elliptic_Curve_Cryptography	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
Symmetric models include the commonly used AES (Advanced Encryption Standard) which replaced the older DES (Data Encryption Standard).
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https://en.wikipedia.org/wiki/Data_Encryption_Standard	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
https://en.wikipedia.org/wiki/Cryptography#cite_note-13	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
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In
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colloquialuse, the term “code” is often used to mean any method of encryption or concealment of meaning.
https://en.wikipedia.org/wiki/Code_(cryptography)	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
However, in cryptography,
codehas a more specific meaning.
It means the replacement of a unit of plaintext (i.e., a meaningful word or phrase) with a
https://en.wikipedia.org/wiki/Code_word	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
code word(for example, “wallaby” replaces “attack at dawn”).
https://en.wikipedia.org/wiki/Cryptanalysis	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
Cryptanalysisis the term used for the study of methods for obtaining the meaning of encrypted information without access to the key normally required to do so; i.e., it is the study of how to crack encryption algorithms or their implementations.
Some use the terms
cryptographyand
cryptologyinterchangeably in English, while others (including US military practice generally) use
cryptographyto refer specifically to the use and practice of cryptographic techniques and
cryptologyto refer to the combined study of cryptography and cryptanalysis.
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English is more flexible than several other languages in which
cryptology(done by cryptologists) is always used in the second sense above.
https://en.wikipedia.org/wiki/Request_for_Comments_(identifier)
RFC2828advises thatsteganographyis sometimes included in cryptology.
https://tools.ietf.org/html/rfc2828
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The study of characteristics of languages that have some application in cryptography or cryptology (e.g. frequency data, letter combinations, universal patterns, etc.) is called cryptolinguistics.
History of Cryptography
Before the modern era, cryptography focused on message confidentiality (i.e., encryption)—conversion of
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messagesfrom a comprehensible form into an incomprehensible one and back again at the other end, rendering it unreadable by interceptors or eavesdroppers without secret knowledge (namely the key needed for decryption of that message).
Encryption attempted to ensure
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secrecyin
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communications, such as those of
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spies, military leaders, and
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diplomats.
In recent decades, the field has expanded beyond confidentiality concerns to include techniques for message integrity checking, sender/receiver identity
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authentication,
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digital signatures,
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interactive proofsand
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secure computation, among others.
Computer era
Prior to the early 20th century, cryptography was mainly concerned with
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linguisticand
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lexicographicpatterns.
Since then the emphasis has shifted, and cryptography now makes extensive use of mathematics, including aspects of
https://en.wikipedia.org/wiki/Information_theory	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
information theory,
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computational complexity,
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statistics,
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combinatorics,
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abstract algebra,
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number theory, and finite mathematics generally.
Cryptography is also a branch of
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engineering, but an unusual one since it deals with active, intelligent, and malevolent opposition (see cryptographic engineering and
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security engineering); other kinds of engineering (e.g., civil or chemical engineering) need deal only with neutral natural forces.
There is also active research examining the relationship between cryptographic problems and
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quantum physics(see
https://en.wikipedia.org/wiki/Quantum_cryptography	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
quantum cryptographyand
https://en.wikipedia.org/wiki/Quantum_computer	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
quantum computer).
Just as the development of digital computers and electronics helped in cryptanalysis, it made possible much more complex ciphers.
Furthermore, computers allowed for the encryption of any kind of data representable in any binary format, unlike classical ciphers which only encrypted written language texts; this was new and significant.
Computer use has thus supplanted linguistic cryptography, both for cipher design and cryptanalysis.
Many computer ciphers can be characterized by their operation on
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binary
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bitsequences (sometimes in groups or blocks), unlike classical and mechanical schemes, which generally manipulate traditional characters (i.e., letters and digits) directly.
However, computers have also assisted cryptanalysis, which has compensated to some extent for increased cipher complexity.
Nonetheless, good modern ciphers have stayed ahead of cryptanalysis; it is typically the case that use of a quality cipher is very efficient (i.e., fast and requiring few resources, such as memory or CPU capability), while breaking it requires an effort many orders of magnitude larger, and vastly larger than that required for any classical cipher, making cryptanalysis so inefficient and impractical as to be effectively impossible.
Advent of modern cryptography
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Cryptanalysisof the new mechanical devices proved to be both difficult and laborious.
In the United Kingdom, cryptanalytic efforts at
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Bletchley Parkduring WWII spurred the development of more efficient means for carrying out repetitious tasks.
This culminated in the development of the
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Colossus, the world’s first fully electronic, digital,
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programmablecomputer, which assisted in the decryption of ciphers generated by the German Army’s
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Lorenz SZ40/42machine.
Extensive open academic research into cryptography is relatively recent; it began only in the mid-1970’s.
In recent times, IBM personnel designed the algorithm that became the Federal (i.e., US)
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Data Encryption Standard;
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Whitfield Diffieand
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Martin Hellmanpublished
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their key agreement algorithm;
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and the
https://en.wikipedia.org/wiki/RSA_(algorithm)	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
RSAalgorithm was published in
https://en.wikipedia.org/wiki/Martin_Gardner	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
Martin Gardner‘s
https://en.wikipedia.org/wiki/Scientific_American	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame https://en.wikipedia.org/wiki/Cryptography
Scientific American column.
Following their work in 1976, it became popular to consider cryptography systems based on mathematical problems that are easy to state but have been found difficult to solve.
https://en.wikipedia.org/wiki/Cryptography#cite_note-31	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
31
Since then, cryptography has become a widely used tool in communications,
https://en.wikipedia.org/wiki/Computer_network	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
computer networks, and
https://en.wikipedia.org/wiki/Computer_security	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
computer securitygenerally.
Some modern cryptographic techniques can only keep their keys secret if certain mathematical problems are
https://en.wikipedia.org/wiki/Computational_complexity_theory#Intractability
intractable, such as the
https://en.wikipedia.org/wiki/Integer_factorization	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
integer factorizationor the
https://en.wikipedia.org/wiki/Discrete_logarithm	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
discrete logarithmproblems, so there are deep connections with
https://en.wikipedia.org/wiki/Abstract_mathematics	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
abstract mathematics.
There are very few cryptosystems that are proven to be unconditionally secure.
The
https://en.wikipedia.org/wiki/One-time_pad	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
one-time padis one, and was proven to be so by Claude Shannon.
There are a few important algorithms that have been proven secure under certain assumptions.
For example, the infeasibility of factoring extremely large integers is the basis for believing that
https://en.wikipedia.org/wiki/RSA_(cryptosystem)	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
RSAis secure, and some other systems, but even so proof of unbreakability is unavailable since the underlying mathematical problem remains open.
In practice, these are widely used, and are believed unbreakable in practice by most competent observers.
There are systems similar to RSA, such as one by
https://en.wikipedia.org/wiki/Michael_O._Rabin	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
Michael O. Rabinthat are provably secure provided factoring
n = pqis impossible; it is quite unusable in practice.
The
https://en.wikipedia.org/wiki/Discrete_logarithm_problem	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography
discrete logarithm problemis the basis for believing some other cryptosystems are secure, and again, there are related, less practical systems that are provably secure relative to the solvability or insolvability discrete log problem.
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32
As well as being aware of cryptographic history, cryptographic algorithm and system designers must also sensibly consider probable future developments while working on their designs.
For instance, continuous improvements in computer processing power have increased the scope of
https://en.wikipedia.org/wiki/Brute-force_attack	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
brute-force attacks, so when specifying
https://en.wikipedia.org/wiki/Key_length	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame https://en.wikipedia.org/wiki/Cryptography
key lengths, the required key lengths are similarly advancing.
https://en.wikipedia.org/wiki/Cryptography#cite_note-fortify-33	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
33
The potential effects of
https://en.wikipedia.org/wiki/Quantum_computing	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
quantum computingare already being considered by some cryptographic system designers developing
https://en.wikipedia.org/wiki/Post-quantum_cryptography	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
post-quantum cryptography; the announced imminence of small implementations of these machines may be making the need for preemptive caution rather more than merely speculative. [4]
https://en.wikipedia.org/wiki/Cryptography#cite_note-hac-4	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
Symmetric-key cryptography
Main article:Symmetric-key algorithm
https://en.wikipedia.org/wiki/Symmetric-key_algorithm	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
https://en.wikipedia.org/wiki/File:Symmetric_key_encryption.svg	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography
https://upload.wikimedia.org/wikipedia/commons/thumb/2/27/Symmetric_key_encryption.svg/250px-Symmetric_key_encryption.svg.png
diagram showing encrypt with a key and decrypt process
Symmetric-key cryptography, where a single key is used for encryption and decryption
Symmetric-key cryptography refers to encryption methods in which both the sender and receiver share the same key (or, less commonly, in which their keys are different, but related in an easily computable way).
This was the only kind of encryption publicly known until June 1976.
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30
https://en.wikipedia.org/wiki/File:International_Data_Encryption_Algorithm_InfoBox_Diagram.svg
https://upload.wikimedia.org/wikipedia/commons/thumb/a/af/International_Data_Encryption_Algorithm_InfoBox_Diagram.svg/220px-International_Data_Encryption_Algorithm_InfoBox_Diagram.svg.png
logic diagram showing International Data Encryption Algorithm cypher process
One round (out of 8.5) of theIDEAcipher, used in most versions ofPGPand OpenPGP compatible software for time-efficient encryption of messages
https://en.wikipedia.org/wiki/International_Data_Encryption_Algorithm	https://en.wikipedia.org/wiki/Encryption
https://en.wikipedia.org/wiki/Pretty_Good_Privacy	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
Symmetric key ciphers are implemented as either
https://en.wikipedia.org/wiki/Block_ciphers	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
block ciphersor
https://en.wikipedia.org/wiki/Stream_ciphers	https://en.wikipedia.org/wiki/Avengers:_Endgame https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
stream ciphers.
A block cipher enciphers input in blocks of plaintext as opposed to individual characters, the input form used by a stream cipher.
The
https://en.wikipedia.org/wiki/Data_Encryption_Standard	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
Data Encryption Standard(DES) and the
https://en.wikipedia.org/wiki/Advanced_Encryption_Standard	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
Advanced Encryption Standard(AES) are block cipher designs that have been designated
https://en.wikipedia.org/wiki/Cryptography_standards	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
cryptography standardsby the US government (though DES’s designation was finally withdrawn after the AES was adopted).
https://en.wikipedia.org/wiki/Cryptography#cite_note-aes-34	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
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Despite its deprecation as an official standard, DES (especially its still-approved and much more secure
https://en.wikipedia.org/wiki/Triple-DES	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
triple-DESvariant) remains quite popular; it is used across a wide range of applications, from ATM encryption
https://en.wikipedia.org/wiki/Cryptography#cite_note-atm-35	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
35
to
https://en.wikipedia.org/wiki/E-mail_privacy	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
e-mail privacy
https://en.wikipedia.org/wiki/Cryptography#cite_note-opgp-36	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
36
and
https://en.wikipedia.org/wiki/Secure_Shell	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
secure remote access.
https://en.wikipedia.org/wiki/Cryptography#cite_note-ssh-37	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
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Many other block ciphers have been designed and released, with considerable variation in quality.
Many, even some designed by capable practitioners, have been thoroughly broken, such as
https://en.wikipedia.org/wiki/FEAL	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
FEAL. [4]
https://en.wikipedia.org/wiki/Cryptography#cite_note-hac-4	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
https://en.wikipedia.org/wiki/Cryptography#cite_note-schneierbook-38	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
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Stream ciphers, in contrast to the ‘block’ type, create an arbitrarily long stream of key material, which is combined with the plaintext bit-by-bit or character-by-character, somewhat like the
https://en.wikipedia.org/wiki/One-time_pad	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
one-time pad.
In a stream cipher, the output stream is created based on a hidden internal state that changes as the cipher operates.
That internal state is initially set up using the secret key material.
https://en.wikipedia.org/wiki/RC4	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
RC4is a widely used stream cipher; see
https://en.wikipedia.org/wiki/Category:Stream_ciphers	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
Category:Stream ciphers. [4] Block ciphers can be used as stream ciphers; see
https://en.wikipedia.org/wiki/Cryptography#cite_note-hac-4	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
https://en.wikipedia.org/wiki/Block_cipher_modes_of_operation	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography
Block cipher modes of operation.
https://en.wikipedia.org/wiki/Cryptographic_hash_functions	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
Cryptographic hash functionsare a third type of cryptographic algorithm.
They take a message of any length as input, and output a short, fixed length
https://en.wikipedia.org/wiki/Hash_function	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
hash, which can be used in (for example) a digital signature.
For good hash functions, an attacker cannot find two messages that produce the same hash.
https://en.wikipedia.org/wiki/MD4	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
MD4is a long-used hash function that is now broken;
https://en.wikipedia.org/wiki/MD5	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
MD5, a strengthened variant of MD4, is also widely used but broken in practice.
The US
https://en.wikipedia.org/wiki/National_Security_Agency	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame https://en.wikipedia.org/wiki/Cryptography
National Security Agencydeveloped the Secure Hash Algorithm series of MD5-like hash functions: SHA-0 was a flawed algorithm that the agency withdrew;
https://en.wikipedia.org/wiki/SHA-1	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
SHA-1is widely deployed and more secure than MD5, but cryptanalysts have identified attacks against it; the
https://en.wikipedia.org/wiki/SHA-2	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
SHA-2family improves on SHA-1, but is vulnerable to clashes as of 2011; and the US standards authority thought it “prudent” from a security perspective to develop a new standard to “significantly improve the robustness of
https://en.wikipedia.org/wiki/National_Institute_of_Standards_and_Technology
NIST‘s overall hash algorithm toolkit.”
https://en.wikipedia.org/wiki/Cryptography#cite_note-39	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
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Thus, a
https://en.wikipedia.org/wiki/NIST_hash_function_competition	https://en.wikipedia.org/wiki/Encryption
hash function design competitionwas meant to select a new U.S. national standard, to be called
https://en.wikipedia.org/wiki/SHA-3	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
SHA-3, by 2012.
The competition ended on October 2, 2012 when the NIST announced that
https://en.wikipedia.org/wiki/Keccak	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
Keccakwould be the new SHA-3 hash algorithm.
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Unlike block and stream ciphers that are invertible, cryptographic hash functions produce a hashed output that cannot be used to retrieve the original input data.
Cryptographic hash functions are used to verify the authenticity of data retrieved from an untrusted source or to add a layer of security.
https://en.wikipedia.org/wiki/Message_authentication_code	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
Message authentication codes(MACs) are much like cryptographic hash functions, except that a secret key can be used to authenticate the hash value upon receipt; [4] this additional complication blocks an attack scheme against baredigest algorithms, and so has been thought worth the effort.
https://en.wikipedia.org/wiki/Cryptography#cite_note-hac-4	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
https://en.wikipedia.org/wiki/Md5	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Avengers:_Endgame
Modern cryptography
The modern field of cryptography can be divided into several areas of study.
The chief ones are discussed here; see
https://en.wikipedia.org/wiki/Topics_in_Cryptography	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption
Topics in Cryptographyfor more.
Public-key cryptography
https://en.wikipedia.org/wiki/File:Public_key_encryption.svg	https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography
https://upload.wikimedia.org/wikipedia/commons/thumb/f/f9/Public_key_encryption.svg/250px-Public_key_encryption.svg.png
diagram of Public-key cryptography showing public key and private key
Public-key cryptography, where different keys are used for encryption and decryption.
Symmetric-key cryptosystems use the same key for encryption and decryption of a message, although a message or group of messages can have a different key than others.
A significant disadvantage of symmetric ciphers is the
https://en.wikipedia.org/wiki/Key_management	https://en.wikipedia.org/wiki/Avengers:_Endgame https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Cryptography
key managementnecessary to use them securely.
Each distinct pair of communicating parties must, ideally, share a different key, and perhaps for each ciphertext exchanged as well.
The number of keys required increases as the
https://en.wikipedia.org/wiki/Square_(algebra)	https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Encryption https://en.wikipedia.org/wiki/Avengers:_Endgame
squareof the number of network members, which very quickly requires complex key management schemes to keep them all consistent and secret.
https://en.wikipedia.org/wiki/File:Diffie_and_Hellman.jpg	https://en.wikipedia.org/wiki/Encryption
https://upload.wikimedia.org/wikipedia/commons/thumb/8/88/Diffie_and_Hellman.jpg/220px-Diffie_and_Hellman.jpg
headshots of Whitfield Diffie and Martin Hellman