) Two new computer codes are being developed to prevent unauthorized access to classified information. The first consists of six digits (each chosen from 0 to 9); the second consists of three digits (from 0 to 9) followed by two letters (A to Z, excluding I and O).
(i) Which code is better at preventing unauthorized access (defined as break ing the code in one attempt)?
[3 marks]
(ii) If both codes are implemented, the first followed by the second, what is probability of gaining access in a single attempt?
Assumptions
We are not told whether digits/letters can be repeated or not. So we will assume repetition is allowed. Similarly, it is possible for the first digit to be zero since a code is needed and not a number. Thus, a code can be one or more zeros.
i. Better code
The fist code has six digits chosen from 10 numbers (0-9). Thus, there are 10 ways of choosing each of the six digits.
The total number of possible codes is "10^6=1,000,000"
The probability of breaking the code in one attempt is "\\frac{1}{10^6}=\\frac{1}{1,000,000}"
The second code has three digits selected from 0-9 and two letters selected from A-Z excluding and O. Assuming repetition is allowed, there are 10 ways of selecting each digit and 24 ways of selecting each letter.
The total number of possible codes is "10^3\\times24^2=576,000"
The probability of breaking the code in one attempt is "\\frac{1}{576,000}"
Since "\\frac{1}{1,000,000}<\\frac{1}{576,000}" the first code is better in preventing unauthorized access.
ii
If both codes are implemented, (independent codes) the probability of gaining an authorized access in a single attempt is "\\frac{1}{1,000,000}\\times\\frac{1}{576,000}=\\frac{1}{576,000,000,000}"
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