Question 4
The kid-RSA algorithm is a simplified version of the RSA algorithm. Kid-RSA
takes as its input four numbers a, b, a1, b1. The four numbers are
converted into four values M, e, d and n, according to the following equations:
M = a*b-1
e = a1* M + a
d=b1* M + b
n = (e* d) / M
A plaintext message P can be encrypted to an encrypted message C with
the public key (n,e) using:
C = e* P (mod n).
Once encrypted, C can be converted back to P using the private key 'd'
as follows:
P = C* d (mod n)
(a) if a = 949, b = 112, a1 = 524 and b1 = 4266, calculate M, e, d and n. Show
your working.
[2]
(b) State the public key for these values of a,b,a1 and b1.
[2]
(c) Encrypt the message 'Silver' using the public key. Show your working.
[4]
(d) Decrypt the message. Show your working.
[4]
(e) Name a simple shift cipher.
[2]
(f) Explain how the shift cipher you mentioned works, with examples.
[4]
(g) How would you go about cracking a shift cipher.
[4]
(h) Research the enigma encryption that was used in the second world war.
i. What kind of cipher was this?
[2]
ii. Compare it to the shift cipher you mentioned above
[4]
iii. Which cipher is the most secure? Why?
[2]