{"id":33517,"date":"2019-10-31T21:53:04","date_gmt":"2019-10-31T18:53:04","guid":{"rendered":"https:\/\/prohoster.info\/blog\/predstavlen-bolee-effektivnyj-metod-opredeleniya-prefiksov-kollizij-dlya-sha-1\/"},"modified":"2019-10-31T21:53:04","modified_gmt":"2019-10-31T18:53:04","slug":"predstavlen-bolee-effektivnyj-metod-opredeleniya-prefiksov-kollizij-dlya-sha-1","status":"publish","type":"post","link":"https:\/\/prohoster.info\/da\/blog\/news\/predstavlen-bolee-effektivnyj-metod-opredeleniya-prefiksov-kollizij-dlya-sha-1","title":{"rendered":"En mere effektiv metode til at bestemme kollisionspr\u00e6fikser for SHA-1 er blevet pr\u00e6senteret.","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"

Forskere fra det franske nationale institut for forskning inden for datalogi og automatisering (INRIA) og Nanyang Technological University (Singapore) har udviklet<\/a><\/noindex> forbedret metode<\/a><\/noindex> angreb<\/a><\/noindex> SHA-1-algoritmen, som v\u00e6sentligt forenkler oprettelsen af to forskellige dokumenter med samme SHA-1-hash. Kernen i metoden er at reducere den komplette kollisionspr\u00f8vningsoperation i SHA-1 til kollisionsangreb med fast prefix<\/a><\/noindex>, hvor en kollision opst\u00e5r ved tilstedev\u00e6relsen af bestemte pr\u00e6fikser, uanset de \u00f8vrige data i s\u00e6ttet. Med andre ord kan man beregne to pr\u00e6definerede pr\u00e6fikser, og hvis den ene tilf\u00f8jes til \u00e9t dokument, og den anden tilf\u00f8jes til et andet - vil de resulterende SHA-1 hashes for disse filer v\u00e6re ens.<\/p>\n

Denne type angreb kr\u00e6ver stadig enorme beregninger, og udv\u00e6lgelse af pr\u00e6fikser er mere kompliceret end almindelig kollisionspr\u00f8vning, men den praktiske effekt af resultatet er ogs\u00e5 v\u00e6sentligt h\u00f8jere. Hvis den hurtigste metode til at finde pr\u00e6fikser af kollision i SHA-1 tidligere kr\u00e6vede 277.1 operationer, s\u00e5 reducerer den nye metode antallet af beregninger til et interval fra 266.9 til 269.4. P\u00e5 dette niveau risikoberegnes omkostningerne ved angrebet til under hundrede tusinde dollars, hvilket er overkommeligt for efterretningstjenester og store virksomheder. Til sammenligning kr\u00e6ves der omkring 264.7 operationer for at finde en almindelig kollision.<\/p>\n

I den forrige<\/a><\/noindex> demonstrationer<\/a><\/noindex> Google's evne til at generere forskellige PDF-filer med samme SHA-1-hash blev der anvendt<\/a><\/noindex> en teknik til at kombinere to dokumenter til \u00e9n fil, skifte det synlige lag og forskyde lagets valgm\u00e6rke ind i omr\u00e5det for kollisionsfrembringelse. Med lignende ressourceomkostninger (Google brugte et \u00e5r p\u00e5 at finde den f\u00f8rste SHA-1-kollision p\u00e5 et cluster best\u00e5ende af 110 GPU'er) g\u00f8r den nye metode det muligt at opn\u00e5 en SHA-1-kollision for to vilk\u00e5rlige datas\u00e6t. Praktisk set kan man forberede TLS-certifikater, hvor forskellige dom\u00e6ner n\u00e6vnes, men hashes SHA-1 er identiske. Denne mulighed g\u00f8r det muligt for et uheldigt certifikatmyndighed at oprette et certifikat til digital signatur, som kan anvendes til autorisering af falske certifikater til vilk\u00e5rlige dom\u00e6ner. Problemet kan ogs\u00e5 udnyttes til at kompromittere protokoller, der er afh\u00e6ngige af frav\u00e6ret af kollisioner, s\u00e5som TLS, SSH og IPsec.<\/p>\n

Den foresl\u00e5ede strategi til at finde pr\u00e6fikser for kollisioner indeb\u00e6rer, at beregningerne opdeles i to trin. I det f\u00f8rste trin udf\u00f8res der en s\u00f8gning efter blokke, der er t\u00e6t p\u00e5 en kollision, ved at indbygge tilf\u00e6ldige variabler i en foruddefineret m\u00e5lgruppe af forskelle. I det andet trin, p\u00e5 niveauet for de enkelte blokke, sammenlignes de opn\u00e5ede forskningsk\u00e6der med de tilknyttede tilstande, der f\u00f8rer til kollisioner, ved hj\u00e6lp af metoder fra traditionelle kollisionstestangreb. <\/p>\n

Selvom den teoretiske mulighed for et angreb p\u00e5 SHA-1 blev bevist allerede i 2005, og den f\u00f8rste kollision i praksis blev fundet<\/a><\/noindex> i 2017, er SHA-1 stadig i brug og d\u00e6kkes af visse standarder og teknologier (TLS 1.2, Git osv.). Hovedm\u00e5let med dette arbejde var \u00f8nsket om at give endnu et v\u00e6gtigt argument for straks at stoppe med at bruge SHA-1, is\u00e6r i certifikater og digitale signaturer.<\/p>\n

Yderligere kan det n\u00e6vnes publicering<\/a><\/noindex> resultaterne<\/noindex> af kryptanalyse af blokke-krypteringer SIMON-32\/64<\/a><\/noindex>, udviklet af NSA i USA og i 2018 godkendt som standard ISO\/IEC 29167-21:2018<\/a><\/noindex>.
\nForskere har form\u00e5et at udvikle en metode til at genoprette den hemmelige n\u00f8gle baseret p\u00e5 to kendte par af klartekst og chiffertekst. Med begr\u00e6nsede beregningsressourcer kr\u00e6ver n\u00f8glepr\u00f8vning mellem nogle f\u00e5 timer og flere dage. Den teoretiske succesrate for angrebet vurderes til 0,25, mens den praktiske for den eksisterende prototype er 0,025.<\/p>\n

<\/a><\/noindex><\/p>\n

Kilde: opennet.ru<\/a><\/p>","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"excerpt":{"rendered":"

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