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

  • 0:04 - 0:08
    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.
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    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.
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    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,
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    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
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    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
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    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:

Converting fractions to decimals sometimes requires us to brush up on our long division skills. We'll walk you through it.

Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/pre-algebra/decimals-pre-alg/decimal-to-fraction-pre-alg/e/converting_fractions_to_decimals?utm_source=YT&utm_medium=Desc&utm_campaign=PreAlgebra

Watch the next lesson: https://www.khanacademy.org/math/pre-algebra/decimals-pre-alg/decimal-to-fraction-pre-alg/v/decimals-and-fractions?utm_source=YT&utm_medium=Desc&utm_campaign=PreAlgebra

Missed the previous lesson?
https://www.khanacademy.org/math/pre-algebra/decimals-pre-alg/decimal-to-fraction-pre-alg/v/converting-fractions-to-decimals?utm_source=YT&utm_medium=Desc&utm_campaign=PreAlgebra

Pre-Algebra on Khan Academy: No way, this isn't your run of the mill arithmetic. This is Pre-algebra. You're about to play with the professionals. Think of pre-algebra as a runway. You're the airplane and algebra is your sunny vacation destination. Without the runway you're not going anywhere. Seriously, the foundation for all higher mathematics is laid with many of the concepts that we will introduce to you here: negative numbers, absolute value, factors, multiples, decimals, and fractions to name a few. So buckle up and move your seat into the upright position. We're about to take off!

About Khan Academy: Khan Academy offers practice exercises, instructional videos, and a personalized learning dashboard that empower learners to study at their own pace in and outside of the classroom. We tackle math, science, computer programming, history, art history, economics, and more. Our math missions guide learners from kindergarten to calculus using state-of-the-art, adaptive technology that identifies strengths and learning gaps. We've also partnered with institutions like NASA, The Museum of Modern Art, The California Academy of Sciences, and MIT to offer specialized content.

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

Serbian subtitles

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