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The Caesar cipher | Journey into cryptography | Computer Science | Khan Academy

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    Prvu poznatu šifru, šifru zamenjivanja,
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    koristio je Julije Cezar oko
    58. god. pre nove ere.
  • 0:12 - 0:15
    Danas je poznata pod
    imenom "Cezarova šifra."
  • 0:15 - 0:18
    Cezar je pomerao svako
    slovo svojih vojnih komandi
  • 0:18 - 0:20
    kako bi one bile nerazumljive
  • 0:20 - 0:22
    ako bi ih protivnik presreo.
  • 0:22 - 0:25
    Zamislite da Alisa i Bob
    odluče da komuniciraju
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    koristeći Cezarovu šifru.
  • 0:27 - 0:29
    Prvo, moraju unapred da se dogovore
  • 0:29 - 0:32
    koji će pomak koristiti, recimo 3.
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    Da bi šifrovala svoju poruku,
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    Alisa bi morala da pomeri za 3
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    svako slovo svoje originalne poruke,
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    tako da A postane D, B postane E,
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    C postane F, i tako dalje.
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    Ova nečitka, to jest šifrovana, poruka
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    se javno šalje Bobu.
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    Zatim Bob jednostavno pomera za
    tri slova unazad svako slovo poruke
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    da bi pročitao originalnu poruku.
  • 0:58 - 1:02
    Zvuči neverovatno, ali ovakva jednostavna
    šifra je korištena od strane
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    vojnih predvodnika stotinama
    godina nakon Cezara.
  • 1:04 - 1:07
    FILMSKI PRIKAZ CEZARA:
    "Borio sam se i pobedio.
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    Ali nisam osvojio ljudski duh,
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    on je nesavladiv."
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    Ipak, katanac je jak onoliko koliko je
    jaka njegova najslabija tačka.
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    Obijač katanca bi mogao da traži
    mehaničke nedostatke,
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    ili ako to ne uspe, da izvuče informacije,
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    kako bi suzio broj mogućih kombinacija.
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    Procesi obijanja katanca
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    i dešifrovanja koda su veoma slični.
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    Slaba tačka Cezarove šifre
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    je objavljena 800 godina kasnije
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    od strane arapskog matematičara
    po imenu Al-Kindi.
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    On je razbio Cezarov kod koristeći
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    jednu važnu karakteristiku jezika
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    na kojem je poruka napisana.
  • 1:43 - 1:45
    Ako pogledate tekst bilo koje knjige,
  • 1:45 - 1:47
    i izračunate učestalost
    svakog slova,
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    naići ćete na dosledan obrazac.
  • 1:50 - 1:54
    Na primer, ovo su učestalosti slova
    u engleskom jeziku.
  • 1:54 - 1:57
    Moglo bi se reći da je to otisak
    engleskog jezika.
  • 1:57 - 1:59
    Ostavljamo ovaj otisak kada komuniciramo,
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    ne primećujući ga.
  • 2:01 - 2:03
    Ova smernica je jedno od
    najdragocenijih oružja
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    za razbijače kodova.
  • 2:05 - 2:08
    Da bi razbili kod, oni broje učestalost
  • 2:08 - 2:11
    svakog slova u šifrovanom tekstu,
  • 2:11 - 2:14
    i proveravaju koliko daleko
    se pomerio otisak.
  • 2:14 - 2:17
    Na primer, ako je H najbrojnije slovo
  • 2:17 - 2:19
    u šifrovanoj poruci, umesto E,
  • 2:19 - 2:22
    onda je pomak najverovatnije 3.
  • 2:22 - 2:24
    Tako pomeraju slova
  • 2:24 - 2:26
    da bi otkrili originalnu poruku.
  • 2:26 - 2:28
    Ovo se nazive "analiza učestalosti".
  • 2:28 - 2:31
    I to je bio kraj sigurnosti
    Cezarove šifre.
Title:
The Caesar cipher | Journey into cryptography | Computer Science | Khan Academy
Description:

Brit explains the Caesar cipher, the first popular substitution cipher, and shows how it was broken with "frequency analysis"

Watch the next lesson: https://www.khanacademy.org/computing/computer-science/cryptography/crypt/v/polyalphabetic-cipher?utm_source=YT&utm_medium=Desc&utm_campaign=computerscience

Missed the previous lesson? https://www.khanacademy.org/computing/computer-science/cryptography/crypt/v/intro-to-cryptography?utm_source=YT&utm_medium=Desc&utm_campaign=computerscience

Computer Science on Khan Academy: Learn select topics from computer science - algorithms (how we solve common problems in computer science and measure the efficiency of our solutions), cryptography (how we protect secret information), and information theory (how we encode and compress information).

About Khan Academy: Khan Academy is a nonprofit with a mission to provide a free, world-class education for anyone, anywhere. We believe learners of all ages should have unlimited access to free educational content they can master at their own pace. We use intelligent software, deep data analytics and intuitive user interfaces to help students and teachers around the world. Our resources cover preschool through early college education, including math, biology, chemistry, physics, economics, finance, history, grammar and more. We offer free personalized SAT test prep in partnership with the test developer, the College Board. Khan Academy has been translated into dozens of languages, and 100 million people use our platform worldwide every year. For more information, visit www.khanacademy.org, join us on Facebook or follow us on Twitter at @khanacademy. And remember, you can learn anything.

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Video Language:
English
Team:
Khan Academy
Duration:
02:36

Serbian subtitles

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