Dear experts,

I am a college student and I was asked a question on

Cipher (network security). I am frustrated as I could not

solve this problem myself. If possible, please help.

Scenario:

Consider the subsitution cipher for English text which

consists of A,B,...,Y,Z only (a total of 26 letters). The

encryption rule is to substitute a letter by another

letter which is different from itself. For instance,

subsitute A by W (but not A), B by H (but not B),...etc.

The actual subsitution rule is governed by a key. Once

the key is chosen, the subsitution rule is fixed and can

represented as follows:

A-W; B-H; C-J; D-K; E-Y; .... ;X-B; Y-U; Z-L

Questions:

1) Based on the above cipher system, determin the total

number of different keys

2) If an attacker uses a brute force attack to decrypt a

particular message, and he try 1,000,000 keys in a

second. WHat is the average time that he can decrypt the

message?

3) Is it possible to decrypt the ciphertext "WXEUV" by

this brute force attack? Why?

4) Is it possible to decrypt a ciphertext which consists

of 100,000 letters? WHy?

Please give me some ideas if possible.

Thanks a lot for your help.