1. What Is Wrong With This Simple Code?
In a simple encryption scheme, letters are input and then encrypted in the following way:
Each letter is assigned a numerical value. “A” is assigned a value of 1, “B” is assigned a value of 2, C = 3, and so on for all the letters of the alphabet up through Z = 26.
Each value is increased by 2.
The values are mapped back into letters. So a value of 3 would be mapped to the letter C.
Example: The letters of the word CAT are mapped to the numbers 3,1,20. These values are each increased by 2 to result in 5,3,22. The values 5,3,22 are then mapped back to the letters FCV.
What Is Wrong With This Scheme?

2. ANSWER: Any letters of Y or Z can not be mapped to any other letter since the values 27 and 28 do not correspond to any letter.
For example, the word YAM is mapped to the values Y=25, A=1, and M=13. These values are increased to 27,3,15 so
YAM is encrypted as ?CO
Furthermore, since we do not know what the first letter of ?CO is, we can not completely decode this word to recover the original word.
To solve this problem, we can use what is called modular arithmetic where we subtract 26 from our new numerical values. So the value of 27 would become = 1. Also, the value of 28 would become = 2. With this system, the word YAM would be encrypted as ACO.
For More Info, See A Very Simple Encryption Scheme