Cryptography

Cryptography

Cryptography is the ancient science of encoding messages so that only the sender and receiver can understand them. 

The art of protecting information by transforming it (encrypting it) into an unreadable format, called cipher text. Only those who possess a secret key can decipher (or decrypt) the message into plain text. Encrypted messages can sometimes be broken by cryptanalysis, also called codebreaking, although modern cryptography techniques are virtually unbreakable.

As the Internet and other forms of electronic communication become more prevalent, electronic security is becoming increasingly important. Cryptography is used to protect e-mail messages, credit card information, and corporate data. One of the most popular cryptography systems used on the Internet is Pretty Good Privacybecause it's effective and free.


Within the context of any application-to-application communication, there are some specific security requirements, including:

• Authentication: The process of proving one's identity. (The primary forms of host-to-host authentication on the Internet today are name-based or address-based, both of which are notoriously weak.)
• Privacy/confidentiality: Ensuring that no one can read the message except the intended receiver.
• Integrity: Assuring the receiver that the received message has not been altered in any way from the original.
• Non-repudiation: A mechanism to prove that the sender really sent this message.

Cryptography, then, not only protects data from theft or alteration, but can also be used for user authentication.
Classified boardly as:

• Secret Key Cryptography (SKC): Uses a single key for both encryption and decryption
• Public Key Cryptography (PKC): Uses one key for encryption and another for decryption
• Hash Functions: Uses a mathematical transformation to irreversibly "encrypt" information

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1.Secret key (or symmetric) cryptography

Symmetric encryption (also called private-key encryption or secret-key encryption) involves using the same key for encryption and decryption.

Encryption involves applying an operation (an algorithm) to the data to be encrypted using the private key to make them unintelligible. The slightest algorithm (such as an exclusive OR) can make the system nearly tamper proof (there being so such thing as absolute security).

However, in the 1940s, Claude Shannon proved that to be completely secure, private-key systems need to use keys that are at least as long as the message to be encrypted. Moreover, symmetric encryption requires that a secure channel be used to exchange the key, which seriously diminishes the usefulness of this kind of encryption system.

The main disadvantage of a secret-key cryptosystem is related to the exchange of keys. Symmetric encryption is based on the exchange of a secret (keys). The problem of key distribution therefore arises:

Moreover, a user wanting to communicate with several people while ensuring separate confidentiality levels has to use as many private keys as there are people. For a group of N people using a secret-key cryptosystem, it is necessary to distribute a number of keys equal to N * (N-1) / 2.

In the 1920s, Gilbert Vernam and Joseph Mauborgne developed the One-Time Pad method (sometimes called "One-Time Password" and abbreviated OTP), based on a randomly generated private key that is used only once and is then destroyed. During the same period, the Kremlin and the White House were connected by the famous red telephone, that is, a telephone where calls were encrypted thanks to a private key according to the one-time pad method. The private key was exchanged thanks to the diplomatic bag (playing the role of secure channel).

2.Public Key Cryptography (PKC)

The principle of public-key encryption

The principle of asymmetric encryption (also called public-key encryption) first appeared in 1976, with the publication of a work about cryptography by Whitfield Diffie and Martin Hellman.

In an asymmetric cryptosystem (or public-key cryptosystem), keys exists in pairs:

• A public key for encryption;
• A secret key for decryption.

In a public-key encryption system, users choose a random key that only they know (this is the private key). From this key, they each automatically deduce an algorithm (this is the public key). Users exchange this public key over an insecure channel.

When a user wants to send a message to another user, he simply needs to encrypt the message to be sent using the recipient's public key (which he can find, for example, in a key server such as an LDAP directory). The latter will be capable of decrypting the message with his private key (that only he knows).

This system is based on a function that is easy to compute in one direction (called a one-way trapdoor function) and is mathematically extremely hard to invert without the private key (called the trapdoor).

To put this in images, this means having a user randomly create a small metal key (the private key) and then produce a large number of padlocks (public keys) he keeps in a locker that can be accessed by anyone (the locker plays the role of an insecure channel). To send him a document, each user can take an (open) padlock, close a portfolio containing the document with this padlock, then send the portfolio to the owner of the public key (the padlock's owner). Only the owner will be capable of opening the portfolio with his private key.

Advantages and disadvantages

The problem of communicating the decryption key no longer exists, in that public keys can be sent freely. Public-key encryption therefore lets people exchange encrypted messages without having a shared secret.

On the other hand, the challenge involves making sure the public key you recover actually belongs to the person you want to send the encrypted information to!

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3.HAsh Function

A cryptographic hash function is a hash function that takes an arbitrary block of data and returns a fixed-size bit string, the cryptographic hash value, such that any (accidental or intentional) change to the data will (with very high probability) change the hash value. The data to be encoded are often called the message, and the hash value is sometimes called the message digest or simply digest.
The ideal cryptographic hash function has four main properties:

• it is easy to compute the hash value for any given message
• it is infeasible to generate a message that has a given hash
• it is infeasible to modify a message without changing the hash
• it is infeasible to find two different messages with the same hash.

Cryptographic hash functions have many information security applications, notably in digital signatures, message authentication codes (MACs), and other forms of authentication. They can also be used as ordinary hash functions, to index data in hash tables, for fingerprinting, to detect duplicate data or uniquely identify files, and as checksums to detect accidental data corruption. Indeed, in information security contexts, cryptographic hash values are sometimes called (digital) fingerprints, checksums, or just hash values, even though all these terms stand for more general functions with rather different properties and purposes.

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ata Encryption Standard

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