Chapter 1: Q12E (page 48)
What is ?
Short Answer
The solution is .
Chapter 1: Q12E (page 48)
What is ?
The solution is .
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Get started for freeThe algorithm for computing by repeated squaring does not necessarily lead to the minimum number of multiplications. Give an example of where the exponentiation can be performed using fewer multiplications, by some other method.
In the RSA cryptosystem, Alice’s public key is available to everyone. Suppose that her private key d is compromised and becomes known to Eve. Show that if (a common choice) then Eve can efficiently factor N.
Wilson's theorem says that a numberis prime if and only if
.
(a) If is prime, then we know every number is invertible modulo . Which of thesenumbers is their own inverse?
(b) By pairing up multiplicative inverses, show thatrole="math" localid="1658725109805" for prime p.
(c) Show that if N is not prime, then .(Hint: Consider
(d) Unlike Fermat's Little Theorem, Wilson's theorem is an if-and-only-if condition for primality. Why can't we immediately base a primality test on this rule?
Show that
(Hint: To show an upper bound, compare with . To show a lower bound, compare it with ).
Digital signatures, continued.Consider the signature scheme of Exercise .
(a) Signing involves decryption, and is therefore risky. Show that if Bob agrees to sign anything he is asked to, Eve can take advantage of this and decrypt any message sent by Alice to Bob.
(b) Suppose that Bob is more careful, and refuses to sign messages if their signatures look suspiciously like text. (We assume that a randomly chosen messagethat is, a random number in the range is very unlikely to look like text.) Describe a way in which Eve can nevertheless still decrypt messages from Alice to Bob, by getting Bob to sign messages whose signatures look random.
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