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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


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