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Unformatted text preview: 1 Cryptography C RYPTOGRAPHY • Secrecy • Ciphers • Secret Key Cryptography • Key Exchange • Public Key Cryptography • Digital Signatures • Internet applications 2 Cryptography Secrecy • Alice wants to send a message ( plaintext p ) to Bob • The communication channel is insecure and can be eavesdropped by Trudy • If Alice and Bob have previously agreed on an encryption scheme ( cipher ), the message can be sent encrypted ( ciphertext c ) • Issues: what is a good cipher? what is the complexity of encrypting/decrypting? what is the size of the ciphertext, relative to the plaintext? if Alice and Bob have never interacted before, how can they agree on a cipher? Alice Bob c encrypt decrypt p c p Trudy 3 Cryptography Traditional Cryptography • Ciphers were already studied in ancient times • Caesar’s cipher : replace a with d replace b with e ... replace z with c • A more general monoalphabetic substitution cipher maps each letter to some other letter • Armed with simple statistcal knowledge, Trudy can easily break a monalphabetic substitution cypher most frequent letters in English: e, t, o, a, n, i, ... most frequent digrams: th, in, er, re, an, ... most frequent trigrams: the, ing, and, ion, ... • The first description of the frequency analysis attack appears in a book written in the 9th century by the Arab philosopher alKindi 4 Cryptography Example (S. Singh, The Code Book, 1999) • Ciphertext PCQ VMJYPD LBYK LYSO KBXBJXWXV BXV ZCJPO EYPD KBXBJYUXJ LBJOO KCPK. CP LBO LBCMKXPV XPV IYJKL PYDBL, QBOP KBO BXV OPVOV LBO LXRO CI SX'XJMI, KBO JCKO XPV EYKKOV LBO DJCMPV ZOICJO BYS, KXUYPD: 'DJOXL EYPD, ICJ X LBCMKXPV XPV CPO PYDBLK Y BXNO ZOOP JOACMPLYPD LC UCM LBO IXZROK CI FXKL XDOK XPV LBO RODOPVK CI XPAYOPL EYPDK. SXU Y SXEO KC ZCRV XK LC AJXNO X IXNCMJ CI UCMJ SXGOKLU?' OFYRCDMO, LXROK IJCS LBO LBCMKXPV XPV CPO PYDBLK • Frequencies 5 Cryptography Frequency Analysis • Identyfying comon letters, digrams and trigrams......
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This note was uploaded on 12/01/2009 for the course COMP COMP 250 taught by Professor Blanchette during the Spring '04 term at McGill.
 Spring '04
 BLANCHETTE

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