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 free1.38. To see if a number, say , is divisible by , you just add up the digits of its decimalrepresentation, and see if the result is divisible by role="math" localid="1658402816137" .
( , so it is not divisible by ).
To see if the same number is divisible by , you can do this: subdivide the number into pairs ofdigits, from the right-hand end , add these numbers and see if the sum is divisible by (if it's too big, repeat).
How about ? To see if the number is divisible by , subdivide it into triples from the end add these up, and see if the sum is divisible by .
This is true for any prime other than and . That is, for any prime , there is an integer such that in order to see if divides a decimal number , we break into -tuples of decimal digits (starting from the right-hand end), add up these -tuples, and check if the sum is divisible by .
(a) What is the smallest such for ? For ?
(b) Show that is a divisor of .
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.
Prove that the grade-school multiplication algorithm (page 24), when applied to binary numbers, always gives the right answer.
Suppose you want to compute the nth Fibonacci number , modulo an integer . Can you find an efficient way to do this?
Suppose that instead of using a compositein the RSA cryptosystem (Figure 1.9), we simply use a prime modulus p . As in RSA, we would have an encryption exponent e, and the encryption of a message would be Prove that this new cryptosystem is not secure, by giving an efficient algorithm to decrypt: that is, an algorithm that given and as input, computes . Justify the correctness and analyze the running time of your decryption algorithm.
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