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Moving the private key seems unnecessary. Wouldn’t it be better to keep the key locked in the secure enclave on each device, and allow multiple keys for each us

Moving the private key seems unnecessary. Wouldn’t it be better to keep the key locked in the secure enclave on each device, and allow multiple keys for each user? Then devices can also be removed from the account.

6ak74rfy 8y ago
I was thinking the same. I'd double-check any architecture that proposes moving private keys around.
- ptoomey3 8y ago
  I’m not advocating for this homegrown solution, but moving private keys around isn’t necessarily the worst idea ever. A unique key per device requires explicit registration on each device. That is “better security”, but also potentially far worse security user experience. Security user experience isn’t to be undervalued. 2FA today is miserable precisely because of the tragic security user experience. Folks don’t understand how fragile things are if they drop their single device with google authenticator in the lake. I think a securely synced shared key could be a huge usability win with only nominal security downside. For example, I think Apple could pull this off pretty darn well using their iCloud Keychain system.
snarf21 8y ago
Isn't the whole point to keep the private keys in some kind of secure element that is tamper resistant and never exposed? On iOS you can't even export private keys generated on the secure enclave, which is exactly how I expect it to work. There are other ways to securely transfer accounts without creating vulnerabilities.
- ptoomey3 8y ago
  As noted elsewhere in this thread, I’m not advocating this homegrown solution. But, I will play devil’s advocate a bit. Yeah, sure, a Secure Enclave key per device is more secure than not. But, what is your threat model where this is the priority? The only realistic threat model where the enclave (or hardware security key) would come into play is malware running on the device. But, in that situation, you are probably hosed anyway. The malware can read browser session cookies. Or, it can interact with the enclave or external dongle in ways that would grant them access to whatever site they want by sending bogus site challenge requests for site X to the enclave/dongle when you meant to approve a challenge for site Y. I personally think there is potentially a big security usability win by using a securely (key part here) synced private key (via iCloud Keychain for example).
blincoln 8y ago
This is my thought as well. Keep the "broadcast a notification to all other devices registered to the user" part to verify that the new one is authorized, but instead of copying the private key to the new device, tell the server to authorize the newly-generated private key.

6ak74rfy 8y ago

I was thinking the same. I'd double-check any architecture that proposes moving private keys around.

ptoomey3 8y ago
I’m not advocating for this homegrown solution, but moving private keys around isn’t necessarily the worst idea ever. A unique key per device requires explicit registration on each device. That is “better security”, but also potentially far worse security user experience. Security user experience isn’t to be undervalued. 2FA today is miserable precisely because of the tragic security user experience. Folks don’t understand how fragile things are if they drop their single device with google authenticator in the lake. I think a securely synced shared key could be a huge usability win with only nominal security downside. For example, I think Apple could pull this off pretty darn well using their iCloud Keychain system.

snarf21 8y ago

Isn't the whole point to keep the private keys in some kind of secure element that is tamper resistant and never exposed? On iOS you can't even export private keys generated on the secure enclave, which is exactly how I expect it to work. There are other ways to securely transfer accounts without creating vulnerabilities.

ptoomey3 8y ago
As noted elsewhere in this thread, I’m not advocating this homegrown solution. But, I will play devil’s advocate a bit. Yeah, sure, a Secure Enclave key per device is more secure than not. But, what is your threat model where this is the priority? The only realistic threat model where the enclave (or hardware security key) would come into play is malware running on the device. But, in that situation, you are probably hosed anyway. The malware can read browser session cookies. Or, it can interact with the enclave or external dongle in ways that would grant them access to whatever site they want by sending bogus site challenge requests for site X to the enclave/dongle when you meant to approve a challenge for site Y. I personally think there is potentially a big security usability win by using a securely (key part here) synced private key (via iCloud Keychain for example).

blincoln 8y ago

This is my thought as well. Keep the "broadcast a notification to all other devices registered to the user" part to verify that the new one is authorized, but instead of copying the private key to the new device, tell the server to authorize the newly-generated private key.