Comment by barbazoo

It's like, I send the server an encrypted math problem. The server has no idea what the math problem is, but homomorphic encryption allows it to compute an (encrypted) result and send that back to me. I get the result and decrypt it for the correct answer. It's novel because you don't have to trust the server with your math problems.

A fairly laymans explanation was posted elsewhere in the thread: https://news.ycombinator.com/item?id=31671914

Quoting it here for convenience:

> With homomorphic encryption, the client sends a series of encrypted numbers. Nobody can decrypt them except the client. The server can do arithmetic with them, making new secret numbers that nobody can decrypt except the client.

> There is no usable information to retain.

> So the question becomes: what can you calculate using arithmetic on secret numbers?

> Well, for this demo, treat every article as a number. Then multiply all the articles you don't want by 0, and the article you want by 1, and add them all together.

> The server just sees that it's multiplying every article by a secret number. It can't tell what the number is. It can't tell if the output is "encrypted article" or "encrypted 000000..."

> Then the server adds them all up. If the client asked for no articles, the result will be "encrypted 000000..." If the client asked for one article, the result will be that article, encrypted. If the client asked for multiple, the result will be a garbled mush of overlapping articles, encrypted. The server can't tell the difference. It just knows it has an encrypted number.

If you found the explanation useful, you can upvote the original comment linked above

For homomorphic encryption in general I wrote this blog post to make the idea accessible:

http://blog.tyrannyofthemouse.com/2013/05/i-added-your-numbe...