Slides do Módulo de Criptografia - Si.lopesgazzani.com.br
Slides do Módulo de Criptografia - Si.lopesgazzani.com.br
Slides do Módulo de Criptografia - Si.lopesgazzani.com.br
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
Cifras Cifras polialfabéticas<<strong>br</strong> />
polialfabéticas<<strong>br</strong> />
polialfabéticas<<strong>br</strong> />
polialfabéticas: polialfabéticas : : : Cifra Cifra <strong>de</strong> <strong>de</strong> <strong>de</strong> <strong>de</strong> <strong>de</strong> Vigenère<<strong>br</strong> />
Vigenère<<strong>br</strong> />
As substituições são feitas utilizan<strong>do</strong><<strong>br</strong> />
utilizan<strong>do</strong>-se se vários alfabetos<<strong>br</strong> />
Exemplo: cifra <strong>de</strong> Vigenère (1585)<<strong>br</strong> />
A chave é formada por letras que i<strong>de</strong>ntificam qual alfabeto<<strong>br</strong> />
está sen<strong>do</strong> utiliza<strong>do</strong> para cifrar <strong>de</strong>terminada letra <strong>do</strong> texto<<strong>br</strong> />
claro<<strong>br</strong> />
Normalmente a chave é a repetição <strong>de</strong> uma palavra-chave que<<strong>br</strong> />
possui tamanho inferior ao <strong>do</strong> texto claro<<strong>br</strong> />
“No <strong>com</strong>eço <strong>do</strong> século 20 20, um <strong>do</strong>s nossos presi<strong>de</strong>ntes (Campos Salles –<<strong>br</strong> />
entre 1898 e 1902) quan<strong>do</strong> assumiu foi a Inglaterra negociar a nossa<<strong>br</strong> />
dívida externa e <strong>de</strong> lá se <strong>com</strong>unicou <strong>com</strong> seus pares no Brasil usan<strong>do</strong> este<<strong>br</strong> />
sistema <strong>de</strong> cifragem, não observan<strong>do</strong> o fato <strong>de</strong> que os ingleses já sabiam<<strong>br</strong> />
<strong>com</strong>o que<strong>br</strong>ar a cifra <strong>de</strong>s<strong>de</strong> 1865 1865, quan<strong>do</strong> um oficial <strong>do</strong> exército prussiano<<strong>br</strong> />
chama<strong>do</strong> Kasiski <strong>de</strong>sco<strong>br</strong>iu <strong>com</strong>o que<strong>br</strong>á-la.”<<strong>br</strong> />
41<<strong>br</strong> />
Cifra Cifra <strong>de</strong> <strong>de</strong> <strong>de</strong> <strong>de</strong> Vernam<<strong>br</strong> />
Vernam<<strong>br</strong> />
43<<strong>br</strong> />
Charles Babbage<<strong>br</strong> />
(1791-1871)<<strong>br</strong> />
Major Friedrich Wilhelm Kasiski<<strong>br</strong> />
(1805-1881) 42<<strong>br</strong> />
Utiliza a reversibilida<strong>de</strong> da função XOR<<strong>br</strong> />
C = P ⊕ K<<strong>br</strong> />
P = C ⊕ K<<strong>br</strong> />
K:10011100111<<strong>br</strong> />
P:01011110001<<strong>br</strong> />
:01011110001<<strong>br</strong> />
C:11000010110<<strong>br</strong> />
K:10011100111<<strong>br</strong> />
P:01011110001<<strong>br</strong> />
01011110001<<strong>br</strong> />
Gilbert Sandford Vernam<<strong>br</strong> />
(1890-1960)<<strong>br</strong> />
Cifra Cifra <strong>de</strong> <strong>de</strong> Vigenère<<strong>br</strong> />
Vigenère<<strong>br</strong> />
Palavra Palavra-chave chave: : “grito”<<strong>br</strong> />
K: GRITOGRITOGRITOGRITOGRITO (chave)<<strong>br</strong> />
P: VAMOSATACARAMANHASEMFALTA (texto claro)<<strong>br</strong> />
C: BRUHGGKIVOXRUTBNRAXALRTMO (texto cifra<strong>do</strong>)<<strong>br</strong> />
Cifra Cifra Cifra Cifra One One-Time One Time Time Pad<<strong>br</strong> />
Pad<<strong>br</strong> />
44<<strong>br</strong> />
Hipótese : Texto cifra<strong>do</strong> <strong>de</strong> 1.000 letras<<strong>br</strong> />
• Chave <strong>de</strong> 5 letras => 5 conjuntos <strong>de</strong> 200 letras<<strong>br</strong> />
(Análise <strong>de</strong> freqüência fácil)<<strong>br</strong> />
• Chave <strong>de</strong> 20 letras => 20 conjuntos <strong>de</strong> 50<<strong>br</strong> />
letras (Análise <strong>de</strong> freqüência mais difícil)<<strong>br</strong> />
• Chave <strong>de</strong> 1.000 letras => 1.000 conjuntos <strong>de</strong> 1<<strong>br</strong> />
letra (Análise <strong>de</strong> freqüência impossível)<<strong>br</strong> />
Blaise <strong>de</strong> Vigenère<<strong>br</strong> />
(1523-196)<<strong>br</strong> />
Se a chave for verda<strong>de</strong>iramente aleatória, nunca reutilizada, e<<strong>br</strong> />
mantida em segre<strong>do</strong>, a one one-time time pad po<strong>de</strong> ser inque<strong>br</strong>ável<<strong>br</strong> />
inque<strong>br</strong>ável.<<strong>br</strong> />
Quan<strong>do</strong> usada corretamente<<strong>br</strong> />
corretamente, , esta cifra provê segurança perfeita<<strong>br</strong> />
para as mensagens (cifra cifra inviolável<<strong>br</strong> />
inviolável). ). O elo fraco é o elemento<<strong>br</strong> />
humano humano.<<strong>br</strong> />
Técnica<<strong>br</strong> />
Escolher <strong>com</strong>o chave um<<strong>br</strong> />
string <strong>de</strong> bits aleatórios.<<strong>br</strong> />
Converter o texto simples em<<strong>br</strong> />
um string <strong>de</strong> bits (utilizan<strong>do</strong><<strong>br</strong> />
o Código ASCII);<<strong>br</strong> />
Calcular o OR exclusivo<<strong>br</strong> />
(XOR) <strong>do</strong>s <strong>do</strong>is strings.<<strong>br</strong> />
Imune a ataques<<strong>br</strong> />
Teoria da Informação – não existe<<strong>br</strong> />
nenhuma informação na<<strong>br</strong> />
mensagem, to<strong>do</strong>s os textos<<strong>br</strong> />
possíveis <strong>com</strong> o tamanho da<strong>do</strong> One One-time time pad Russo,<<strong>br</strong> />
são igualmente prováveis. captura<strong>do</strong> pelo MI5<<strong>br</strong> />
Joseph Oswald Mauborgne<<strong>br</strong> />
(1881-1971)<<strong>br</strong> />
Amostra <strong>de</strong> um one-time one time pad<<strong>br</strong> />
11