This proposal is making Scrappy(SeCure Rate Assuring Protocol with PrivacY),
a new scheme with strong privacy for Privacy State Token.

Scrappy is a rate-limiter the same as Privacy Pass, but this restants to the timing correlation attack.
Also, Scrappy works as E2E (between users and web service) in the hot paths (i.e., sign and verify),
so that the issuer does not need to receive high access from users.

Note

• Although Scrappy mainly assumes the user has a unique resource in the hardware device (in particular, the TPM),
  we can use other unique resources, the same as the Trust Token API.
• TODO

Reference

Scrappy:
https://www.ndss-symposium.org/ndss-paper/scrappy-secure-rate-assuring-protocol-with-privacy/

Privacy State Token:
https://developers.google.com/privacy-sandbox/protections/private-state-tokens?hl=en