String Encryption Problem

We are to create an encryption function. It will implement a substitution cypher. For simplicity, it assumes that the string it is encoding is all lowercase letters or spaces. It takes a source string, a destination string and a key. It also assumes the destination string is long enough.

Inputs

The main() gets a string as input from the user and a key. It passes these to the encryption function and prints the encrypted and clear strings.

Outputs

The initial string is printed and so is the encrypted version.

Notes

A substitution cypher is a very simple code. It is often invented by young children. The key is a small number that is used as an offset into the alphabet. Each characater in the cleartext string is replaced by a letter offset by the key. If the encoded character is not in the range of the alphabet, we wrap around to the beginning of the alphabet. Whitespace is unchanged. A couple of examples:
Key is 3
a --> d
b --> e
c --> f
...
w --> z
x --> a
y --> b
z --> c

Answer

Here is one solution to this problem.