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Public-Key Cryptography

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  • Computing Shared Secrets
    30 pts · 5151 Solves · 10 Solutions
    Now it's time to calculate a shared secret using data received from your friend Alice. Like before, we will be using the NIST parameters:

    g: 2
    p: 2410312426921032588552076022197566074856950548502459942654116941958108831682612228890093858261341614673227141477904012196503648957050582631942730706805009223062734745341073406696246014589361659774041027169249453200378729434170325843778659198143763193776859869524088940195577346119843545301547043747207749969763750084308926339295559968882457872412993810129130294592999947926365264059284647209730384947211681434464714438488520940127459844288859336526896320919633919


    You have received the following integer from Alice:

    A: 70249943217595468278554541264975482909289174351516133994495821400710625291840101960595720462672604202133493023241393916394629829526272643847352371534839862030410331485087487331809285533195024369287293217083414424096866925845838641840923193480821332056735592483730921055532222505605661664236182285229504265881752580410194731633895345823963910901731715743835775619780738974844840425579683385344491015955892106904647602049559477279345982530488299847663103078045601

    You generate your secret integer $b$ and calculate your public value $B = g^b \mod p$, which you send to Alice.

    b: 12019233252903990344598522535774963020395770409445296724034378433497976840167805970589960962221948290951873387728102115996831454482299243226839490999713763440412177965861508773420532266484619126710566414914227560103715336696193210379850575047730388378348266180934946139100479831339835896583443691529372703954589071507717917136906770122077739814262298488662138085608736103418601750861698417340264213867753834679359191427098195887112064503104510489610448294420720
    B: 518386956790041579928056815914221837599234551655144585133414727838977145777213383018096662516814302583841858901021822273505120728451788412967971809038854090670743265187138208169355155411883063541881209288967735684152473260687799664130956969450297407027926009182761627800181901721840557870828019840218548188487260441829333603432714023447029942863076979487889569452186257333512355724725941390498966546682790608125613166744820307691068563387354936732643569654017172


    Try and compute $B$ yourself and verify things are working using the values above.

    You and Alice are now able to calculate a shared secret by using your secret values $a, b$ with each others public values $B, A$. Note that computing this shared value is infeasible knowing only $\{g,p,A,B\}$, it's the knowledge of $a, b$ which allows the generation of the shared value.

    What is your shared secret?

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