scsi@lemm.ee 2 months ago
To try and bake down the complex answers, if you are basically familiar with PGP or SSH keys the concept of a Passkey is sort of in the same ballpark. But instead of using the same SSH keypair more than once, Passkeys create a new keypair for every use (website) and possibly every device (e.g. 2 phones using 1 website may create 2 sets of keypars, one on each device) - and additionally embeds the username (making it “one-click login”):
- creating a passkey is the client and server establishing a ring of trust (“challenge”) and then generating a public and private pair of keys (think
ssh-keygen …) - embedded in the keypair is the user ID/username and credential ID, which sort of maps to the three fields of a SSH keypair (encryption type, key, userid optional in SSH keys) but not really, think concept not details
- when using a passkey, the server sends the client a “challenge”, the client prompts the user to unlock the private key (device PIN, biometric, Bitwarden master password, etc.)
- the “challenge” (think crypto math puzzle) is signed with the private key and returned to the server along with the username and credential ID
- the server, who has stored the public key, looks it up using the username + credential ID, then verifies the signature somewhat like SSH or PGP does
- like SSH or PGP, this means the private key never leaves the device/etc. being used by the client and is used to only sign the crypto math puzzle challenge
The client private key is stored hopefully in a secure part of the phone/laptop (“enclave” or TPM hardware module) which locks it to that device; using a portable password manager instead such as Bitwarden is attractive since the private keys are stored in BW’s data (so can be synced across devices, backed up, etc.)
They use the phrase “replay” a lot to mean that sending the same password to a website is vulnerable to it being intercepted and used n+1 times (hacker); in the keypair model this doesn’t happen because each “challenge” is a unique crypto math puzzle generated dynamically every use, like TOTP/2FA but “better” because there’s no simple hash seed (TOTP/2FA use a constant seed saved by the client but it’s not as robust crypto).