Re: file encription/decriptoin iOS
Re: file encription/decriptoin iOS
- Subject: Re: file encription/decriptoin iOS
- From: Jens Alfke <email@hidden>
- Date: Thu, 29 Jun 2017 09:45:50 -0700
> On Jun 29, 2017, at 5:33 AM, Dave Fernandes <email@hidden>
> wrote:
>
> Ah. Thinking about the new device use case helps. Everything must be
> accessible and decryptable using only the iCloud passphrase. But if the same
> passphrase is used both to authorize access to the data and to decrypt it,
> then Apple has the passphrase to decrypt each time the user logs in, do they
> not? So encryption prevents against third parties seeing the data, but not
> Apple itself. Or perhaps the passphrase is used to generate two independent
> secrets and the passphrase itself is never sent over the wire?
Any competent* online service does not store your password. They only store a
hashed version of it.
With web-based logins the password does usually get sent to the server, but it
only keeps it long enough to verify the hash, never storing it persistently.
A login system like Apple’s is probably sending a random challenge value to the
device, which then gets somehow transformed using the password in a way that
can be verified using the hash (I’m hand-waving here, but there are many
algorithms for this.) Or alternatively, the password defines an asymmetric
key-pair**, of which Apple stores the public key, and the login then consists
of signing the challenge value and sending it back, letting Apple verify the
signature.
—Jens
* Unfortunately there are still incompetent services still out there. Match.com
<http://match.com/> is one, for example, or was a few years ago when I briefly
tried using it. If you tell it you forgot your password, it *emails your
password to you*. My jaw hit the floor and I deleted the account immediately.
** Deriving a key-pair from a password isn’t feasible with RSA, but it can be
done with e.g. Curve25519.
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