comp250_cryptography_notes

comp250_cryptography_notes - CRYPTOGRAPHY Secrecy Ciphers...

Info icon This preview shows pages 1–6. Sign up to view the full content.

View Full Document Right Arrow Icon
1 Cryptography C RYPTOGRAPHY • Secrecy • Ciphers • Secret Key Cryptography • Key Exchange • Public Key Cryptography • Digital Signatures • Internet applications
Image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
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
Image of page 2
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 al-Kindi
Image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
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
Image of page 4
5 Cryptography Frequency Analysis Identyfying comon letters, digrams and trigrams...
Image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 6
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern