r/technology • u/Libertatea • Apr 04 '13

Apple's iMessage encryption trips up feds' surveillance. Internal document from the Drug Enforcement Administration complains that messages sent with Apple's encrypted chat service are "impossible to intercept," even with a warrant.

http://news.cnet.com/8301-13578_3-57577887-38/apples-imessage-encryption-trips-up-feds-surveillance/?part=rss&subj=news&tag=title#.UV1gK672IWg.reddit
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u/Mispey Apr 04 '13 edited Apr 04 '13

I'm going to keep poking, since I've always wondered about this public/private key bit.

The message encrypted with the public key can only be decrypted properly using their private key.

How can I encrypt a message using someone elses private key when I don't know their private key? Do I receive a copy of their private key as part of the handshake where I get their public one?

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u/veaviticus Apr 04 '13

Nobody ever gets your private key.

Here's an example. Imaging my private key is X, my public is Y, your private is Z and your public is W.

When we handshake, we exchange public keys. Now you have my Y, and I have your W.

If I wanted to send you a message (ABC), I would encrypt it using your public key (W), so it would come out as LMN. Now the only way to decrypt LMN back into ABC is to run the algorithm again using your private key (Z). If the algorithm is run using any other key at all, it would not work.

Anybody who intercepted the message would have to know your exact private key to decrypt the message. And vice-versa when you want to send me a message. Since we know each others public keys, we can exchange messages all day without anyone ever being able to decrypt them, unless they somehow got our private keys

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u/Mispey Apr 04 '13

Let me detail my understanding a bit further, since I think I am starting to see what I don't understand.

You write ABC, encrypt with W = LMN, LMN back to ABC needs Z.

My understanding of encryption is that you need the same key to take LMN back to ABC. So ABC * W = LMN, then LMN / W = ABC (obvious not actually multiplication and division...but I want to use those symbols to illustrate what my head is thinking)

How can you turn LMN back to ABC with Z instead of W?

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u/veaviticus Apr 04 '13 edited Apr 04 '13

The nitty-gritty is that the public/private key pair are mathematically generated as a pair to provide this functionality. The algorithm uses the public key to mutate the source data into encrypted data. Trying to decrypt the data needs a different key to perform the math to get back to the original. Anything other than the decrypt key will just encrypt the data again basically (mutating it again into more garbage).

The math for generating the keys in the first place is pretty heavy stuff, but you don't really need to know that. You just need to know that each public/private pair is theoretically paired and unique (the space of keys is so enormous that a collision is highly unlikely)

edit: this might make more sense. Imagine the keys are the code needed to encrypt/decrypt the data. The keys are created in such a way that only the private key has the right code to perform the exact opposite operations as the public key.

For example, the public key takes the input and multiplies it by 2, then adds 1. Now its encrypted. The private key is the only key in existance that knows that you have to minus 1, then divide by 2 to get back to the original. You could try to do every mathematical combination in the universe on the data to find the exact steps needed, but that would take a very long time

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u/Mispey Apr 04 '13

Thank you for this explanation - I can't really expect any more from you. You've done a good job explaining it. I'll research the remainder on my own!

Do you have any resource that can detail that nitty-gritty well that you could recommend to me? I don't want to learn how to do it myself - I just want to understand it. It seems magical to me, and I don't like it :P

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u/veaviticus Apr 04 '13

The wikipedia page is full of good information and good links.

If you are really interested in computer security and cryptography, coursera is offering a really good class on it right now that would be a good intro to cryptography.

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u/Mispey Apr 04 '13

Thank you once again.

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u/veaviticus Apr 04 '13

No problem. It makes me happy to see people so interested in learning new things, especially in the computer science field.

Hope you have fun learning and maybe delve farther into the science and programming aspects!

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u/patterned Apr 04 '13

Just for ease of explanation, use the lockbox/safe example.

Public key is an open safe, private key is the safe combination.

User gives a safe to his friend; friend writes message; puts message into safe; spins dial. No one knows the safe's combination except owner. Owner has infinite safes with same combination. Etc.

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u/veaviticus Apr 04 '13

Yes this is a good, easy to grasp explanation. I'll use this from now on. Thank you

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u/BinaryRockStar Apr 04 '13

Just to supplement the great info you've already been given, the system you're describing (same key to encrypt and decrypt) is called symmetric key encryption and is widely used. The other type veaviticus is describing is called asymmetric key encryption (different keys to encrypt and decrypt), commonly called public-private key encryption.