{"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\/pt\/blog\/news\/predlozhen-metod-opredeleniya-kollizij-v-sha-1-prigodnyj-dlya-ataki-na-pgp","title":{"rendered":"Foi proposto um m\u00e9todo para defini\u00e7\u00e3o de colis\u00f5es no SHA-1, aplic\u00e1vel a ataques no PGP.","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"

Pesquisadores do instituto franc\u00eas de pesquisa em inform\u00e1tica e automa\u00e7\u00e3o (INRIA) e da Universidade Tecnol\u00f3gica de Nanyang (Cingapura) apresentaram um m\u00e9todo de ataque Desastre<\/a><\/noindex> (PDF<\/a><\/noindex>), que \u00e9 apresentado como a primeira implementa\u00e7\u00e3o pr\u00e1tica de um ataque ao algoritmo SHA-1, que pode ser usado para criar assinaturas digitais falsas PGP e GnuPG. Os pesquisadores acreditam que agora todos os ataques pr\u00e1ticos ao MD5 podem ser aplicados ao SHA-1, embora ainda requeiram recursos significativos para serem realizados.<\/p>\n

O m\u00e9todo \u00e9 baseado na realiza\u00e7\u00e3o de um ataque de colis\u00e3o com prefixo definido<\/a><\/noindex>, permitindo que para dois conjuntos de dados arbitr\u00e1rios sejam encontrados complementos, cuja anexa\u00e7\u00e3o resultar\u00e1 em conjuntos que causam colis\u00f5es, utilizando o algoritmo SHA-1, que resultar\u00e1 na forma\u00e7\u00e3o do mesmo hash de sa\u00edda. Em outras palavras, para dois documentos existentes, pode-se calcular dois complementos, e se um for anexado ao primeiro documento e o outro ao segundo \u2014 os hashes SHA-1 resultantes desses arquivos ser\u00e3o iguais.<\/p>\n

O novo m\u00e9todo se diferencia das t\u00e9cnicas semelhantes anteriormente propostas pela melhoria na efici\u00eancia da busca de colis\u00f5es e pela demonstra\u00e7\u00e3o de aplica\u00e7\u00e3o pr\u00e1tica para ataques ao PGP. Em particular, os pesquisadores conseguiram preparar duas chaves p\u00fablicas PGP de tamanhos diferentes (RSA-8192 e RSA-6144) com identificadores de usu\u00e1rio distintos e com certificados que causam colis\u00e3o com SHA-1. A primeira chave<\/a><\/noindex> inclu\u00eda o identificador da v\u00edtima, enquanto a segunda chave<\/a><\/noindex> inclu\u00eda o nome e a imagem do atacante. Assim, devido \u00e0 colis\u00e3o encontrada, o certificado identificador que inclu\u00eda a chave e a imagem do atacante tinha o mesmo hash SHA-1 que o certificado identificador que inclu\u00eda a chave e o nome da v\u00edtima. <\/p>\n

O atacante poderia solicitar uma assinatura digital para sua chave e imagem em uma autoridade certificadora externa, e depois transferir a assinatura digital para a chave da v\u00edtima. A assinatura digital permanece v\u00e1lida devido \u00e0 colis\u00e3o e \u00e0 valida\u00e7\u00e3o da chave do atacante pela autoridade certificadora, permitindo que o atacante tome controle tamb\u00e9m da chave com o nome da v\u00edtima (uma vez que o hash SHA-1 para ambas as chaves \u00e9 id\u00eantico). Assim, o atacante pode se passar pela v\u00edtima e assinar qualquer documento em seu nome.<\/p>\n

O ataque ainda \u00e9 bastante oneroso, mas j\u00e1 se torna acess\u00edvel a ag\u00eancias de intelig\u00eancia e grandes corpora\u00e7\u00f5es. Para a simples busca de colis\u00f5es usando uma GPU NVIDIA GTX 970 mais barata, os custos foram de 11 mil d\u00f3lares, enquanto para encontrar colis\u00f5es com um prefixo determinado \u2014 45 mil d\u00f3lares (para compara\u00e7\u00e3o, em 2012, os custos para encontrar colis\u00f5es no SHA-1 eram estimados em 2 milh\u00f5es de d\u00f3lares, e em 2015 \u2014 700 mil). Para realizar um ataque pr\u00e1tico ao PGP, foram necess\u00e1rios dois meses de computa\u00e7\u00e3o com 900 GPUs NVIDIA GTX 1060, cujo aluguel custou 75 mil d\u00f3lares aos pesquisadores.<\/p>\n

O m\u00e9todo proposto pelos pesquisadores para detectar colis\u00f5es \u00e9 aproximadamente 10 vezes mais eficaz do que as conquistas anteriores \u2014 o n\u00edvel de complexidade dos c\u00e1lculos de colis\u00e3o foi reduzido para 261,2 opera\u00e7\u00f5es, em vez de 264,7, e as colis\u00f5es com um prefixo determinado para 263,4 opera\u00e7\u00f5es em vez de 267,1. Os pesquisadores recomendam a transi\u00e7\u00e3o do SHA-1 para o uso do SHA-256 ou SHA-3 o mais r\u00e1pido poss\u00edvel, pois, de acordo com suas previs\u00f5es, em 2025, o custo de realizar um ataque cair\u00e1 para 10 mil d\u00f3lares.<\/p>\n

Os desenvolvedores do GnuPG foram notificados sobre o problema em 1\u00ba de outubro (CVE-2019-14855) e em 25 de novembro, na vers\u00e3o do GnuPG 2.2.18, tomaram medidas para bloquear os certificados problem\u00e1ticos \u2014 todas as assinaturas digitais identificadoras SHA-1, criadas ap\u00f3s 19 de janeiro do ano passado, agora s\u00e3o reconhecidas como inv\u00e1lidas. Na CAcert, um dos principais centros de certifica\u00e7\u00e3o para chaves PGP, planejam passar a utilizar fun\u00e7\u00f5es de hash mais seguras para a certifica\u00e7\u00e3o de chaves. Os desenvolvedores do OpenSSL, em resposta \u00e0 informa\u00e7\u00e3o sobre o novo m\u00e9todo de ataque, decidiram desativar o SHA-1 no primeiro n\u00edvel de seguran\u00e7a por padr\u00e3o proposto (o SHA-1 n\u00e3o poder\u00e1 ser usado para certificados e assinaturas digitais no processo de negocia\u00e7\u00e3o de conex\u00f5es).<\/p>\n

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

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

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