Where are your kids?

Question

You are a mom (or dad) with two boys, Stephen and Philip, aged 8 and 10. While you have been out in the garage, they have been in their separate rooms doing their homework before being allowed to play.

When you come in to check on them, you go into Stephen's room and find no one there, but notice a note left prominently on his desk. It reads:

Mom and Dad,

I NVIQIPTW IG NBNRNBSC CPS OSFY MHLC VJ SGNANL'D KYSKO.

-Stephen

Ps. Philip can decode this for you.

For a moment, you regret letting them watch that documentary on cryptography, even though it did keep them quiet all afternoon yesterday while they were "inventing" their own secret code. You sigh, and walk over to Philip's room. You are surprised to be greeted only by another note left on his desk. It reads:

Mom and Dad,

I EIVO CXBS LZ SGTR'J QEYQU. GE LZLPLZQA IA FNAIAHLO.

-Philip

Ps. Stephen can decode this for you.

Ok, so now you're panicking a little. Where did they go? What have they done? Did they run away? Go to a friend's house? Are they just in the back yard? Your first instinct is to call all their friends, all your friends, the police, the hospitals, the military, the president... but wait. You spot something else on Philip's desk... and remember seeing something like it on Stephen's as well. It's a paper with a bunch of letters on it, and it's labeled "Key."

Ok, so, maybe you're overreacting. They're obviously using the code they invented, and they're only in 4th and 6th grade... how hard could this be to decipher?

Using the information in this puzzle, can you tell me where your kids are? And can you do it without computer assistance?

Hints in the form of what you know about your kids:

• They are pretty responsible and would never leave the house without letting you know where they're going.
• They've been punished enough in the past for playing before their homework was done, that you're sure they wouldn't have left without finishing their homework.
• They can be mischevious, and think they're smarter than they are, but they wouldn't leave you a cipher like this if they knew their brother wouldn't be around to decode it for you.

And an even bigger hint that relates to the cipher's method:

Yesterday, Stephen was bragging about how easy it is to solve ciphers where each letter just represents another letter every time, and that if he invented one, it wouldn't be that simple, but would change based on what you wrote before.

Answer 1

Stephen said

I FINISHED MY HOMEWORK AND WENT OVER TO JOHNNY'S HOUSE.

Philip said

I WENT OVER TO TONY'S HOUSE. MY HOMEWORK IS FINISHED.

This Vigenère cipher didn't have what I would consider a traditional key. Rather, the previous letter written down was used in combination with the ciphered letter to discover the proper letter. For instance, I was used first, and was actually I. N was the next letter. Go to the I column and follow down until you reach N, then go back to the start to find the letter should be F. Next, V is the encrypted letter. Go to column N and follow it down to V. Follow it back to the start to get the letter I. In a nutshell the key to the cipher is the cipher itself shifted one position to the right.

Here is my chicken scratch solution where you can see the column to look at above the cipher: