As discussed in the "formal spec", user's could be deanonymized with a key tagging attack, so it is suggested that the blind signing public keys be pinned in the extension. And a signed token's time-to-live is effectively however long that token's signing key is considered valid by Cloudflare. So they must eventually be updated and pulled from Cloudflare. Which effectively moves the key tagging attack problem to the extension's key update path.
The extension needs a way to make sure it gets the same keys as everyone else.
Here is my proposed scheme to do this is:
- The extension regularly pulls updates of Cloudflare's public keys anonymously through Tor.
- Cloudflare sends a hash of the current blind signing public key with each captcha challenge.
- The extension compare's the hash of its local public key with the one sent with the captcha. Only proceed if they match.
Since the update path is through Tor, Cloudflare cannot associate a tagged malicious public key with the future captcha's it presents.
As discussed in the "formal spec", user's could be deanonymized with a key tagging attack, so it is suggested that the blind signing public keys be pinned in the extension. And a signed token's time-to-live is effectively however long that token's signing key is considered valid by Cloudflare. So they must eventually be updated and pulled from Cloudflare. Which effectively moves the key tagging attack problem to the extension's key update path.
The extension needs a way to make sure it gets the same keys as everyone else.
Here is my proposed scheme to do this is:
Since the update path is through Tor, Cloudflare cannot associate a tagged malicious public key with the future captcha's it presents.