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CRYPTOGRAPHYBy :- Arjun Bala

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INTRODUCTION

Data that can be read and understood without anyspecial measures is called plaintext or cleartext.

The method of converting plaintext in such a wayas to hide its substance is called encryption.

Encrypting plaintext results in unreadable is calledciphertext.

You use encryption to make sure that information ishidden from anyone for whom it is not intended,

even those who can see the encrypted data. The process of reverting ciphertext to its original

plaintext is called decryption.

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INTRODUCTION(CONT.)

PlainTextEncryption

CipherTextDecryption

PlainText

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CRYPTOGRAPHY

Cryptography is the science of using mathematics to encryptand decrypt data.

Cryptography enables you to store sensitive information ortransmit it across insecure networks (like the Internet) so that itcannot be read by anyone except the intended recipient.

While cryptography is the science of securing data,cryptanalysis is the science of analyzing and breaking securecommunication.

Classical cryptanalysis involves an interesting combination of

analytical reasoning, application of mathematical tools, patternfinding, patience, determination, and luck.

Cryptanalysts are also called attackers.

Cryptology embraces both cryptography and cryptanalysis.

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CONVENTIONAL CRYPTOGRAPHY

PlainTextEncryption

CipherTextDecryption

PlainText

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CAESARSCIPHER(CONT.)

So starting with

ABCDEFGHIJKLMNOPQRSTUVWXYZ

and sliding everything up by 3, you get

DEFGHIJKLMNOPQRSTUVWXYZABC

where D=A, E=B, F=C, and so on.

Using this scheme, the plaintext, SECRETencrypts as VHFUHW. To allow someone

else to read the ciphertext, you tell them thatthe key is 3.

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PUBLIC-KEY CRYPTOGRAPHY

Public-key cryptography uses a pair of keys: apublic key, which encrypts data, and acorresponding private key, for decryption.

Because it uses two keys, it is sometimes calledasymmetric cryptography.

You publish your public key to the world whilekeeping your private key secret.

Anyone who has a public key can encryptinformation but cannot decrypt it.

Only the person who has the correspondingprivate key can decrypt the information.

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PUBLIC-KEY CRYPTOGRAPHY(CONT.)

Public Key Private Key

PlainText

Encryption

CipherTextDecryption

PlainText

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PGP

PGP combines some of the best features of bothconventional and public-key cryptography.

PGP is a hybrid cryptosystem.

PGP creates a session key, which is a one-time-onlysecret key.

This key is a random number generated from therandom movements of your mouse and the keystrokesyou type.

The session key works with a very secure, fastconventional encryption algorithm to encrypt the

plaintext; the result is ciphertext. Once the data is encrypted, the session key is then

encrypted to the recipients public key.

This public key-encrypted session key is transmittedalong with the ciphertext to the recipient.

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PGP(CONT.)

plaintext is encryptedwith session key

session key is encryptedwith public key

ciphertext +

encrypted session key

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PGP(CONT.)

encrypted message

Encryptedsession key

ciphertext

recipients private key used to

decrypt session key

session key used

to decrypt ciphertextPlain Text

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plaintext

hash function

message digest

digest signedwith private key

private keyused for signing

plaintext+

signature

HASH FUNCTION