Computer Network Security

In World War II, the German’s usually used 10 cables on the stecker, only five different rotors were in general use, one reflector was in common use, and the reflector and five rotors were known to the Allies.

a. Under these restrictions, show that there are only about 277  possible Enigma keys.

b. Show that if we ignore the stecker, under these restrictions there are fewer than 230   settings.

According to the text, random numbers used in cryptography must be unpredictable.

a. Why are statistically random numbers (which are often used in simulations) not sufficient for cryptographic applications?

b. Suppose that the keystream generated by a stream cipher is predictable in the sense that if you are given n keystream bits, you can determine all subsequent keystream bits. Is this a practical security concern? Why or why not?

Solution:

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