Option fields in bids

The bids sent to the SAM can include optional fields which can be used in the following various scenarios:

• Authorization of bidders are important in many cases such as bidding for government contract and bidding for natural resources. In order to provide authorization of bidders, the bids collector can contain a subcomponent, an authority checker, which checks whether the bidder is authorized. Only authorized bids can then be inserted to the bids table. Several solutions for the authorization mechanism exist. For example, an authorized list can be established in the authority checker in which each entry is a pair (bidder ID, secret). If a bid contains the correct bidder ID and secret, it is authorized. An even simpler solution is to establish a shared group secret for all the authorized bidders. Any bid which contains the correct secret is regarded as authorized. Well-designed protocols can prevent these shared secrets from leaking out without the help of any authorized bidder. Schemes based on a PKI are also possible.
• Digital cash can be included in the optional fields. At the end of the auction, the winner's digital cash can be collected as the payment.

  Some atomic transactions can be used to guarantee that the goods is given to the winner only if the money is transferred to the seller's account and the bidder does not need to pay anything if the goods is not given to him.

• The bidder can include a secret key in the optional fields. At the end of the auction, the goods (if it is electronic) or some secret information about picking up the goods can be encrypted with the secret key and published. Hence, the winner can get the goods without revealing any information about himself and hence to be fully anonymous.
• Non-repudiation of bidders can also be a desired property. To achieve this, either the bidders can sign the bids, or they can include authentication secrets in the bids.

Without the SAM to carry out the market, including sensitive information such as digital cash and bidder-specified secret keys in the bids raises various risks. For example, the auctioneer can then get the bidder-specified secret keys and hence get the goods without paying for anything.