Several people have emailed or commented on Hacker News about this ‘experiment’, so let me spell out exactly what I did and what it indicates to me.
- Backed up my iPhone to iCloud using the “Backup Now” feature.
- Shut down my phone.
- Set a new password using only a recovery email address.
- Purchased a brand new iPhone from the Apple store.
- Visited an AT&T store to obtain a brand new SIM.
- Entered my new iCloud password into the device and told it to recover the latest backup.
Ben Franklin flying kites in a thunderstorm this is not.
Note that from step (3) onward I had no access to my device or my original iCloud password. This information was completely gone. At the end of the ‘test’ I had access to all of the iMessages that had been logged in my old phone — some of them actually going back months.
To resolve what this means: following step (2) I had no further access to my phone. Any cryptographic keys it may contain were unavailable and offline. Similarly I had no further access to my original iCloud password — although I remembered it, I simply never entered it again. I can’t see how this could function as an encryption key.
And one more note: I sent myself an iMessage from a different device after step (2) and it didn’t arrive. This indicates to me that stored messages are on iCloud servers, not the iMessage servers themselves.
4 thoughts on “A note on the iCloud ‘experiment’”
1. The iMessages are encrypted with a key derived from the iCloud password.
2. Every time the iCloud password is changed, a new encryption key is derived, and all iMessages are decrypted (with old key) and newly encrypted (with new key).
This could explain why iMessages can be recovered with a new device and a new password.
This is a simple asumption (no proof whatsoever that this is correct).
A precision: in the description above, the encryption would be performed by the user's device (not by iCloud which wouldn't need to know the user's password — but only a hash of it or something).
But still, if one is able to change the iCloud password at her convenience, the net result is that she is able to get the iMessages in clear.
But who can do the decryption, if the old password (and device) is lost?
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