I learned the other day that FIDO2 keys have a counter so that if an attacker does manage to clone the key, and both keys continue to be used, the counter for them will desync and the server can detect the clone.
Pretty cool, wonder how that plays out in practice.
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Like I'm pretty sure the recommendation is allow for two keys but AWS still doesn't, so I'm a bit doubtful that the average backend is detecting/ handing this, but I'm certainly curious to hear otherwise.
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Should allow for more than 2 keys since adding more keys is the best way to offer recovery.
My Google account has 3 security keys: YubiKey 5, Trezor Model T and Pixel 5 Titan M.
Can restore Trezor by fetching 2/3 Cryptosteel backups and setting the 2FA counter to Unix time.
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So in this case (with Trezor) you're cloning the seed/ counters across devices? I'm not super familiar with that system.
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Trezor generates a high entropy seed as a BIP39 seed phrase (they created the standard) and has you write it down or physically store it via an approach like cryptosteel.com and enter it again to confirm.
You only need the seed phrase to recovery if it's broken or lost.
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You wouldn't normally clone it. You have a way to recover if the hardware gets broken or lost somehow though. U2F/FIDO2 won't start working again with sites enforcing the counter until you set the counter higher than it was previously though. Setting to Unix time is an easy way.
BIP39 seed phrases are really nice. It uses a word list of 2048 carefully chosen words for the use case. Normal seed phrase is 12 words, which is 134 bits of data. It's a 128 bit key with 4-bit checksum. It supports entering a passphrase as a 13th word for hidden wallets.
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First 4 letters of each word are unique. UI will auto-complete it and hardware backup solutions only need to support 4 letters per word.
There's also SLIP39 for splitting up a seed:
github.com/satoshilabs/sl
The seed gets used to derive an infinite number of other keys.
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