Mcm chapter5

Chapter 5 questions and code.
Nice solution, but I don't think the question intends the key's length to be XOR'ed, or the keys `HiThar` and `secret` will produce equivalent output. I think it wants each `char` of key to XOR the phrase in turn. Single statement lambdas can also be done without braces and a closing `;` if you want. This solution would look like:

```java
int k = 0;
StringBuilder enc = new StringBuilder();
phrase.chars().forEach( ch -> enc.append( (char)(ch ^ key.charAt(k++ % key.length())) ) );
return enc.toString();
```