Espaço de chave (criptografia)

Em criptografia, o espaço de chave de um algoritmo é o conjunto de todas as permutações possíveis de uma chave.[1][2]

Descrição

Para evitar que um adversário use um ataque de força bruta para identificar a chave usada para criptografar uma mensagem, o espaço de chave é, normalmente, definido de maneira que seja grande o suficiente para tornar tal ataque inviável. Na média, metade do espaço de chave precisa ser percorrido para encontrar a chave desejada.[3]

Outra característica desejável é que a chave seja definida de uma maneira realmente aleatória dentro de todas as possíveis permutações de chaves. Do contrário, se o adversário usar algum fator que possa ter influenciado a forma como a chave foi escolhida, ele poderá reduzir drasticamente a quantidade de permutações a serem percorridas (e portanto o tempo necessário para fazê-lo). Seres humanos não costumam escolher senhas de uma forma aleatória o que faz com que adversários normalmente tentem ataques de dicionário antes de um ataque de força bruta dado que a primeira opção costuma produzir uma resposta correta em muito menos tempo do que um ataque de força bruta que sistematicamente percorra todo o espaço de chave.[4]

Exemplos

Se uma chave tem 8 bits (um byte), o espaço de chave contém 28 or 256 possibilidades. Advanced Encryption Standard (AES) pode usar uma chave simétrica de 256 bits, o que resulta em um espaço de chave contendo 2256 (ou 1,1579 x 1077) possibilidades.

A cifra DES utiliza uma chave de 56 bits, o que resulta em um espaço de chave relativamente pequeno de 256 (ou 7,2058 x 1016) opções, o que, conforme demonstrado em 1998, pode ser completamente percorrido em 56 horas por um computador de mesa comum.[5]

Referências

  1. ↑ «CISSP Exam Preparation» (em inglês). http://www.flashcardmachine.com/: Flashcard machine. 22 de outubro de 2009. Consultado em 11 de março de 2010. All possible values that can be set to generate a key. 
  2. ↑ «What is a "keyspace"?» (em inglês). http://www.experts123.com/: experts123. 26 de abril de 2017. Consultado em 11 de março de 2011. A "keyspace" is the theoretical set of all possible permutations of a key, given a set key size. 
  3. ↑ «Why do the statistics measure estimate time to having 50% of the keyspace searched?» (em inglês). 26 de abril de 2017. Consultado em 11 de dezembro de 2018. we don't know where in the keyspace of 72,000,000,000,000,000 the right answer is. On the average, only 50% of the keyspace needs to be searched before a solution is found. 
  4. ↑ «Password Cracking» (em inglês). Departamento de Ciência da Computação da Universidade de Denver. Consultado em 11 de dezembro de 2018 
  5. ↑ Congressional Record. Col: 17 (em inglês). 144. [S.l.]: United States Senate. 7 de outubro de 1998. p. 25124 

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