security computer cryptography

May 19th, 2015
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learn how to hack computer networks

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Cryptography and CryptanalysisFeryal AlayontUniversity of ArizonaDecember 9, 20031Cryptography: derived from the Greek words kryptos, meaninghidden, and graphos, meaning writing.Cryptography is the art of secret writing; its intend is to providesecure communication over insecure channels.AliceBobinsecure channel6Eve(eavesdropper)2A cryptosystem or cipher is a procedure to render messagesunintelligible except to the authorized or intended recipient.BobAlice- Encrypt ciphertext- Decryptplaintextoriginal6text66Evesecure channel(to send the key)3More precisely:A cryptosystem consists of two sets, a set of possible plaintexts anda set of possible ciphertexts, and an invertible function ek , where kis the key, from the set of plaintexts to the ciphertexts. Theencryption of a plaintext m is the ciphertext ek (m) and thedecryption function is e1k :1e1(ciphertext)=ekk (ek (m)) = m = plaintext4Example: The Caesar cipher:To encrypt a message, each letter in the message is moved forwardby 3. We get the following map for letter substitution:aDbEcFx AThe encryption of the plaintextattack on tuesdayis the ciphertextDWWDEN RQ WXHVGDB5yBzCCryptanalysis is the art of breaking into secure communications.More precisely, a cryptanalyst tries to obtain the plaintext or thedecryption function in a cryptosystem by eavesdropping into theinsecure channel.Kerchkhoff s pr

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