{"id":54924,"date":"2020-01-08T00:00:00","date_gmt":"2020-01-07T21:00:00","guid":{"rendered":"https:\/\/prohoster.info\/blog\/blog_prohoster\/predlozhen-metod-opredeleniya-kollizij-v-sha-1-prigodnyj-dlya-ataki-na-pgp"},"modified":"2020-02-18T14:02:59","modified_gmt":"2020-02-18T11:02:59","slug":"predlozhen-metod-opredeleniya-kollizij-v-sha-1-prigodnyj-dlya-ataki-na-pgp","status":"publish","type":"post","link":"https:\/\/prohoster.info\/da\/blog\/news\/predlozhen-metod-opredeleniya-kollizij-v-sha-1-prigodnyj-dlya-ataki-na-pgp","title":{"rendered":"En metode til bestemmelse af kollisioner i SHA-1, der er anvendelig til angreb p\u00e5 PGP, er blevet foresl\u00e5et.","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"

Forskere fra det franske nationale institutt for forskning i datalogi og automatisering (INRIA) og Nanyang Tekniske Universitet (Singapore) har pr\u00e6senteret en angrebsmetode Shambles<\/a><\/noindex> (PDF<\/a><\/noindex>), der pr\u00e6senteres som den f\u00f8rste praktiske implementering af et angreb p\u00e5 SHA-1 algoritmen, som kan bruges til at skabe falske digitale signaturer for PGP og GnuPG. Forskere mener, at alle praktiske angreb p\u00e5 MD5 nu kan anvendes p\u00e5 SHA-1, selvom de stadig kr\u00e6ver betydelige ressourcer for at blive udf\u00f8rt.<\/p>\n

Metoden er baseret p\u00e5 gennemf\u00f8relsen af en kollisionsangreb med en givet pr\u00e6fiks<\/a><\/noindex>, der g\u00f8r det muligt at finde supplementer til to vilk\u00e5rlige datas\u00e6t, som efter tilf\u00f8jelse vil skabe sammenfaldende s\u00e6t, hvor anvendelse af SHA-1-algoritmen vil resultere i den samme hashv\u00e6rdi. Med andre ord kan der for to eksisterende dokumenter beregnes to supplementer, og hvis det ene tilf\u00f8jes til det f\u00f8rste dokument, og det andet til det andet \u2014 vil de resulterende SHA-1-hashes for disse filer v\u00e6re identiske.<\/p>\n

Den nye metode adskiller sig fra tidligere foresl\u00e5ede lignende teknikker ved at \u00f8ge effektiviteten af s\u00f8gningen efter kollisioner og demonstrere praktisk anvendelse i et angreb p\u00e5 PGP. Specifikt lykkedes det forskerne at forberede to \u00e5bne PGP-n\u00f8gler af forskellige st\u00f8rrelser (RSA-8192 og RSA-6144) med forskellige bruger-id'er og certifikater, der for\u00e5rsager SHA-1 kollisioner. Den f\u00f8rste n\u00f8gle<\/a><\/noindex> inkluderede identifikatoren for offeret, og den anden n\u00f8gle<\/a><\/noindex> inkluderede angriberens navn og billede. Takket v\u00e6re kollisionen havde certifikatet med den identificerende n\u00f8gle, der indeholdt angriberens n\u00f8gle og billede, den samme SHA-1 hash som identifikationscertifikatet, der indeholdt n\u00f8gle og offerets navn. <\/p>\n

Angriberen kunne anmode om en digital signatur for sin n\u00f8gle og billede fra et eksternt certifikatcenter, hvorefter den digitale signatur kunne overf\u00f8res til offerets n\u00f8gle. Den digitale signatur forbliver gyldig p\u00e5 grund af kollisionen og valideringen af angriberens n\u00f8gle af certifikatcentret, hvilket giver angriberen kontrol over n\u00f8gle med offerets navn (da SHA-1 hashen for begge n\u00f8gler er identisk). I sidste ende kan angriberen udgive sig for at v\u00e6re offeret og underskrive ethvert dokument p\u00e5 dens vegne.<\/p>\n

Angrebet er stadig ret kostbart, men det er nu inden for r\u00e6kkevidde for efterretningsbureauer og store virksomheder. Omkostningerne ved at finde en kollisionshash ved brug af det billigere NVIDIA GTX 970 GPU bel\u00f8ber sig til 11.000 dollars, mens det koster 45.000 dollars at finde en kollisionshash med en given pr\u00e6fiks (til sammenligning blev omkostningerne til at finde en SHA-1-kollisionshash i 2012 vurderet til 2 millioner dollars, og i 2015 til 700.000 dollars). For at udf\u00f8re et praktisk angreb p\u00e5 PGP kr\u00e6vede det to m\u00e5neders beregninger med 900 NVIDIA GTX 1060 GPU'er, hvis leje kostede forskerne 75.000 dollars.<\/p>\n

Den metode til opdagelse af kollisoner, som forskerne har foresl\u00e5et, er ca. 10 gange mere effektiv end tidligere resultater \u2014 sv\u00e6rhedsgraden ved beregningen af kollisoner er blevet reduceret til 261,2 operationer, i stedet for 264,7, mens kollisoner med et givet pr\u00e6fiks kr\u00e6ver 263,4 operationer i stedet for 267,1. Forskerne anbefaler at overg\u00e5 fra SHA-1 til brugen af SHA-256 eller SHA-3 s\u00e5 hurtigt som muligt, da de forudser, at omkostningerne ved at udf\u00f8re et angreb vil falde til 10.000 dollars i 2025.<\/p>\n

Udviklerne af GnuPG blev informeret om problemet den 1. oktober (CVE-2019-14855), og den 25. november i udgivelsen af GnuPG 2.2.18 tr\u00e5dte der foranstaltninger i kraft for at blokere de problematiske certifikater \u2014 alle digitale identificerende SHA-1-signaturer, oprettet efter den 19. januar sidste \u00e5r, er nu anerkendt som ugyldige. I CAcert, et af de vigtigste certifikationscentre for PGP-n\u00f8gler, planl\u00e6gger man at overg\u00e5 til mere sikre hashfunktioner til certificering af n\u00f8gler. Udviklerne af OpenSSL har som svar p\u00e5 informationen om den nye angrebsmetode besluttet at deaktivere SHA-1 p\u00e5 det anbefalede standard sikkerhedsniveau (SHA-1 vil ikke kunne anvendes til certifikater og digitale signaturer under forhandlingen af forbindelser).<\/p>\n

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

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

\u0418\u0441\u0441\u043b\u0435\u0434\u043e\u0432\u0430\u0442\u0435\u043b\u0438 \u0438\u0437 \u0444\u0440\u0430\u043d\u0446\u0443\u0437\u0441\u043a\u043e\u0433\u043e \u0433\u043e\u0441\u0443\u0434\u0430\u0440\u0441\u0442\u0432\u0435\u043d\u043d\u043e\u0433\u043e \u0438\u043d\u0441\u0442\u0438\u0442\u0443\u0442\u0430 \u0438\u0441\u0441\u043b\u0435\u0434\u043e\u0432\u0430\u043d\u0438\u0439 \u0432 \u0438\u043d\u0444\u043e\u0440\u043c\u0430\u0442\u0438\u043a\u0435 \u0438 \u0430\u0432\u0442\u043e\u043c\u0430\u0442\u0438\u043a\u0435 (INRIA) \u0438 \u041d\u0430\u043d\u044c\u044f\u043d\u0441\u043a\u043e\u0433\u043e \u0442\u0435\u0445\u043d\u043e\u043b\u043e\u0433\u0438\u0447\u0435\u0441\u043a\u043e\u0433\u043e \u0443\u043d\u0438\u0432\u0435\u0440\u0441\u0438\u0442\u0435\u0442\u0430 (\u0421\u0438\u043d\u0433\u0430\u043f\u0443\u0440) \u043f\u0440\u0435\u0434\u0441\u0442\u0430\u0432\u0438\u043b\u0438 \u043c\u0435\u0442\u043e\u0434 \u0430\u0442\u0430\u043a\u0438 Shambles (PDF), 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