http://i3xi5qxvbrngh3g6o7czwjfxwjzigook7zxzjmgwg5b7xnjcn5hzciad.onion/proposals/226-bridgedb-database-improvements.html
For the Social BridgeDistributor, hash digests of the following datatypes
SHOULD be stored in the RDBMS, in order to prevent double-spending and
replay attacks:
- Invite Tickets
These are anonymous, unlinkable, unforgeable, and verifiable tokens
which are occasionally handed out to well-behaved Users by the Social
BridgeDistributor to permit new Users to be invited into the system.