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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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PUBLICKEY CRYPTOGRAPHY
Publickey 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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PUBLICKEY 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 publickey cryptography.
PGP is a hybrid cryptosystem.
PGP creates a session key, which is a onetimeonlysecret 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 keyencrypted 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