Processes and Protocols

  We first enumerate the basic processes taking place in VarietyCash, and the parties involved in them. Later we describe in detail the requirements and protocols that realize them for the two more relevant processes: Coin Purchase and Payment.

• Registration - A party chooses an identity, and has his/her public key registered at the issuer.
• Enrollment (${\em PY},{\em I}$ or ${\em PA},{\em I}$) - An already registered participant enrolls for a role such as coin purchaser or redeemer.
• Coin Purchase (${\em CP},{\em I},{\em B}$) - The withdrawer specifies what kinds of coins s/he wants, and a corresponding set of coins is then issued and sent to the withdrawer. The coins are paid for by funds of value equal to the total dollar value of the issued coins, which the withdrawer authorizes the issuer to get from his/her bank account.
• Payment (${\em PA},{\em PY},{\em I}$) - The payer pays to the payee a requested sum, using one or more coins totalling to the requested value. The payee immediately (i.e., on-line) validates the coins with the issuer, and may either obtain new coins in return, redeem the coins, or aggregate them for later redemption.
• Change (${\em CH},{\em I}$) - The coin-holder gives the issuer a set of coins, and also specifies what kinds of coins he wants in change, the total dollar value of the requested coins being the same as that of the provided coins. Coins corresponding to the request are then issued to the coin-holder.
• Redeem (${\em RD},{\em I},{\em B}$) - The redeemer gives some set of coins to the issuer, and the latter turns them into real money in the redeemer's bank account.
• Refresh (${\em Rf},{\em I}$) - The refresher turns in old (expired) coins for an equivalent value in new coins.
• Refund - A non-anonymous coin holder can request the issuer to resend him his coins in case of a failure (e.g., disk crash).

Again, we note the distinction between the Registration and Enrollment processes. Intuitively, Registration is a simpler, on-line process that will let a user participate in the transactions. It basically consists of chooshing and ID (possibly a pseudonym) and a secret/public key pair, and making sure that requirements such as availability and security (e.g., minimizing spoiler attacks) are met. On the other hand, Enrollment is accomplished by a combination of on-line and out-of-band steps. It comprises steps such as providing DDA/credit card information, validation of this information, and issuance of initial amount. Being enrolled allows a user to play an ``active'' role in the system, in the sense of generating money conversions from regular to electronic, and viceversa. We leave the details of these two important processes for the full version of the paper, and will assume in the following description that they have already taken place.