# Mật mã kryptos

### What is cryptography?

Cryptography is a method of protecting information và communications through the use of codes, so that only those for whom the information is intended can read và process it.

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In computer science, cryptography refers lớn secure information và communication techniques derived from mathematical concepts and a set of rule-based calculations called algorithms, khổng lồ transform messages in ways that are hard to lớn decipher. These deterministic algorithms are used for cryptographic key generation, digital signing, verification to lớn protect data privacy, web browsing on the internet và confidential communications such as credit card transactions và email.

### Cryptography techniques

Cryptography is closely related to lớn the disciplines of cryptology and cryptanalysis. It includes techniques such as microdots, merging words with images & other ways to lớn hide information in storage or transit. However, in today"s computer-centric world, cryptography is most often associated with scrambling plaintext (ordinary text, sometimes referred to as *cleartext*) into ciphertext (a process called encryption), then back again (known as decryption). Individuals who practice this field are known as cryptographers.

Modern cryptography concerns itself with the following four objectives:

**Confidentiality.**The information cannot be understood by anyone for whom it was unintended.

**Integrity.**The information cannot be altered in storage or transit between sender và intended receiver without the alteration being detected.

**Non-repudiation.**The creator/sender of the information cannot deny at a later stage their intentions in the creation or transmission of the information.

**Authentication.**The sender & receiver can confirm each other"s identity and the origin/destination of the information.

Procedures & protocols that meet some or all of the above criteria are known as cryptosystems. Cryptosystems are often thought khổng lồ refer only to lớn mathematical procedures & computer programs; however, they also include the regulation of human behavior, such as choosing hard-to-guess passwords, logging off unused systems và not discussing sensitive procedures with outsiders.

Cryptography is the process of encrypting và decrypting data.### Cryptographic algorithms

Cryptosystems use a phối of procedures known as cryptographic algorithms, or ciphers, to encrypt and decrypt messages to lớn secure communications among computer systems, devices and applications.

A cipher suite uses one algorithm for encryption, another algorithm for message authentication and another for key exchange. This process, embedded in protocols & written in software that runs on operating systems (OSes) và networked computer systems, involves:

public và private key generation for data encryption/decryption key exchange### Types of cryptography

**Single-key or symmetric-key encryption**** algorithms** create a fixed length of bits known as a block cipher with a secret key that the creator/sender uses to lớn encipher data (encryption) & the receiver uses to decipher it. One example of symmetric-key cryptography is the Advanced Encryption Standard (AES). AES is a specification established in November 2001 by the National Institute of Standards và Technology (NIST) as a Federal Information Processing Standard (FIPS 197) to lớn protect sensitive information. The standard is mandated by the U.S. Government & widely used in the private sector.

In June 2003, AES was approved by the U.S. Government for classified information. It is a royalty-free specification implemented in software & hardware worldwide. AES is the successor to the Data Encryption Standard (DES) và DES3. It uses longer key lengths -- 128-bit, 192-bit, 256-bit -- to lớn prevent brute force và other attacks.

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**Public-key or asymmetric-key encryption**** algorithms** use a pair of keys, a public key associated with the creator/sender for encrypting messages và a private key that only the originator knows (unless it is exposed or they decide to nội dung it) for decrypting that information.

Examples of public-key cryptography include:

Elliptic Curve Digital Signature Algorithm (ECDSA) used by BitcoinTo maintain data integrity in cryptography, **hash functions**, which return a deterministic output from an input value, are used to map data to lớn a fixed data size. Types of cryptographic hash functions include SHA-1 (Secure Hash Algorithm 1), SHA-2 and SHA-3.

### Cryptography concerns

Attackers can bypass cryptography, thủ thuật into computers that are responsible for data encryption and decryption, & exploit weak implementations, such as the use of default keys. However, cryptography makes it harder for attackers lớn access messages & data protected by encryption algorithms.

Growing concerns about the processing power of quantum computing lớn break current cryptography encryption standards led NIST khổng lồ put out a hotline for papers among the mathematical and science community in năm 2016 for new public key cryptography standards.

Unlike today"s computer systems, quantum computing uses quantum bits (qubits) that can represent both 0s & 1s, & therefore perform two calculations at once. While a large-scale quantum computer may not be built in the next decade, the existing infrastructure requires standardization of publicly known and understood algorithms that offer a secure approach, according khổng lồ NIST. The deadline for submissions was in November 2017, analysis of the proposals is expected to take three lớn five years.

### History of cryptography

The word "cryptography" is derived from the Greek *kryptos*, meaning hidden.

The prefix "crypt-" means "hidden" or "vault," và the suffix "-graphy" stands for "writing."

The origin of cryptography is usually dated from about 2000 B.C., with the Egyptian practice of hieroglyphics. These consisted of complex pictograms, the full meaning of which was only known lớn an elite few.

The first known use of a modern cipher was by Julius Caesar (100 B.C. Khổng lồ 44 B.C.), who did not trust his messengers when communicating with his governors and officers. For this reason, he created a system in which each character in his messages was replaced by a character three positions ahead of it in the Roman alphabet.

In recent times, cryptography has turned into a battleground of some of the world"s best mathematicians and computer scientists. The ability to securely store & transfer sensitive information has proved a critical factor in success in war and business.

Because governments vì not want certain entities in and out of their countries to lớn have access to lớn ways to lớn receive & send hidden information that may be a threat khổng lồ national interests, cryptography has been subject khổng lồ various restrictions in many countries, ranging from limitations of the usage and export of software to the public dissemination of mathematical concepts that could be used to develop cryptosystems.

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However, the internet has allowed the spread of powerful programs and, more importantly, the underlying techniques of cryptography, so that today many of the most advanced cryptosystems and ideas are now in the public domain.

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