-----BEGIN PGP SIGNED MESSAGE----- King Kong Does e$ A while back, when the Microsoft Network hype was at its height, I forwarded a bunch of articles to www-buyinfo entitled "Oh, No, there goes Tokyo", about how Godzilla, in the form of Microsoft's Microsoft Network, was going to stomp all over Tokyo, in the form of the net. This was before I found out that Netscape's codeword for their Navigator HTML Browser was "Mozilla". So, along comes the following on the DCSB list, courtesy of the esteemed Mr. Lethin: Wednesday, December 13, 1995 Refreshments 4:15 PM 4:30 - 6:00 PM Bartos Theater, E15-070 MIT Media Lab Ames Street, Cambridge, MA, USA Electronic Money Made Easy Dan Simon (dansimon@microsoft.com) Mac in hand, I went to a presentation at the Media Lab's basement auditorium, and proceed to transcribe his copious overheads, sans graphics, as fast as my fingers could fly... Which I present here in it's raw form, with a few corrections, his cryptic (ahem...) source citations, viz: (cf., MN 93), and the ocassional note I could cram in when he stopped flipping charts... Oh, no, there goes Zurich... - -------------------- Dan Simon Microsoft, now, Quantum Computing/crypto in Canada before... Characteristics of cash Portable Anonymous Confidential Easily and instantly transferrable Owned by those posessing it Hard to counterfiet Has multiple interchangeable forms. What isn't cash: e-credit Non-anonymous Severely limited confidentiality Complicated payment protocols two parties and both their respective banks involved Ownership determined by record-keeping credit/debit authority What isn't e-cash: offline e-cash One desirable feature of cash: transfer without an intermediary Problem: doublespending: restore prespending state, respend the same cash Tracing re-spent cash useless if it was stolen (cf. (cfnII 0091, Yac94, etc) Tamperproof hardware isn't (cf EGY83) Can broadcast small amounts over the net at a huge payoff. Result cash as secure cellphone codes (better not be worth much...) Offline Electronic "bills" don't work. Too big for offline Offline Electronic coins do work. Easy to counterfiet, not much profit. (He is interested in electronic banknotes, on-line bearer securities, in other words) What can we assume: Customers, merchants, banks may wish to transact via many different possible media: telephone, Internet, cable network, smartcard/terminal, etc. etc. If a customer wants a transaction to be anonymous, then an anonymous medium must be used (no callerid on the telephone, anonymous remailer, etc) A Modest Proposal On-line cash issued by banks owned by a possessor, Cryptographically hard to counterfiet Cash ownership transferred with bank's help aAnonymity of payer, payee based on anonymity of communication medium No individual account or prior trust relationship necessary to use a bank's cash (cf., MN 93) Formal Security E-cash schemes generally described as a set of individual protocols (payer-payee, payer-bank, etc.( with intuitively described goals) But formal proof of security really requires a model of a full network, to consider general attacks, and explicit, formal security goals Previous examples: (BEA91/MR/91) for computation (RS93) for anonymity, (BR93) for authentication What does it look like A bank issued banknote has a serial number,amount, and treasuer's signature It also has an associated "secret" which can be checked against the serial number, but can't be gotten from the serial number Its owner is whoever has the secret... ...but when the issuing bank gets the secret, the banknote is no longer valid. Publish the bank notes as broken, banknotes no longer valid. (spiffy graphic elided) Customer chooses secret s, serial number n, n=h(s) and amount. Customer pays bank (by any method) the amount of the desired banknote bank signs serial number and amount Transaction may be anonymous (physical cash purchase at bank) pick serial number, bank uses private signature to sign the serial number. (May's train-locker applies) How is change made: Customer generates secrets, serial numbers and amounts for smaller banknotes Customer gives bank the secret for the large banknote, and serial numbers and amounts for smaller ones Bank signs serial numbers and amounts, stores the secret for large now-invalid banknote Transaction can be completely anonymous. How is it used for payment Payer simply gives bank note to payee by handing over the secret payee immediately exchanges the banknote for new ones anonymously, if desired Old banknote is now invalid, since bank has its secret; hence payer can't respend it If the bank note is already invalid, bank rejcts exchange and informs payee. "In effect, these are disposable banknotes" "the simple way to get around exploding banknote numbers, issue it with an expiry date." Estimated outstanding certificates numbers would then be within the storage capacity of a bank. (per a crony in the Bank of Japan) Public key from the bank is needed to keep the bank honest. "miracle of one-way function" (shuts down clueless question...) Model Synchronous network of parties, some dishonest, one of them is the bank Parties can exchange messages anonymously Responder can always broadcast Parties and bank keep track of balances each round, a party receives instruction to deposit/withwdraw/ a unit value coin to/from the bank or pay a coin to another party (missed one point..) Security goals Correctness: if all parties are honest, then instructions result in correct new balances Integrity: parties cannot be ripped off Honest bank never accepts more coins than have been withdrawn Party can detect dishonest bank Anonymity: information available to any coalition should be indistinquishable for any sequence of instructions consistent with the starting and ending points of distinguised coins entering/leaving coalition Weak anonymity, contends that that's what we have with serial numbered physical notes anyway Can make stronger with subsequent anonymous change operations at a bank... (can also buy with cash at a desk, also on the net with another form of digital cash ;-) Efficiency: cost of security should grow slowly (polylog at worst) with size of network. Convenience Features Only payer, payee, issuing bank involved in transaction Each validation and transfer immediate Portable Easy to handle Expiry dates can be added. If anonymous withdrawl, then can get lost money back on expiry date, if coins weren't cashed. Security Digital signature makes cash unforgeable Online validation prevents double spending Secrecy of secret preserved even if serial number publically available Bank's *only* secret is its private signature key Digital signature publicly commits bank to redeeming its e-cash; hence customers need not have special trust relationship with bank. Audience Q: Borenstein's scenario. A: bad problem. someone can split bank private keys, for instance (No problem, strawman, what happens when someone breaks Ft. Knox?... --RAH ;-) Extra features Payee can have the e-cash non-anonymously deposited in bank account for easier handling and better security (bank as back up) Payer can include transaction details with payment package or hide them in the secret (to create transaction record to be archived by, for example, by the bank) helpful in dispute. "Secure payments" and E-cash "Don't send cash in the mail" even if it gets there, receiver need not admit it One solution: secure payment protocols (Microsoft/Visa STT) Non-anonymous: transaction details attached to payment mediated by payer's credit card issuer Payee can delay response and batch process transactions "Secure Payments" Protocol's basic idea: payer sends encrypted package to payee giving credit card number and transaction details, "payment instructions". Only acquirer can read it Valid credit card number plus payer's digital signature assure acquirer that payment is legitimate (missed one point) Secure ecash payments Payer substitutes ecash secrets for credit card number in payment instruction package payment desiring ecash settlement. Includes ecash serial numbers amounts when sending package to acquirer E-cash issuing bank bank checks validity of ecash payment and exchanges it for merchant as apprpriate by signing serial numbers, amounts Instant payment, no bad debt Questions Where is the communications cost/ security tradeoff? Solutions to dine and dash problem with dishonest bank (anonymity must broken to nark on the bank) (cf Cle85)? Efficient offline anonymous micropayments solutions? Implimentation pitfalls? - ------------------- So, there are my notes, such as they are... I walked away from this thinking that on-line cash covers a multitude of sins: You can stick any "secret" into digital cash as long as it's unique and you trust the bank to keep it. If bank signs the secret they've shared with you and reveal it before some expiry date, they can be put out of business, because they've blown their reputation. If they claim that you've not shared a secret, you expose the secret and their signature, same happens. If you put an expiry date on Simon's digital cash, you don't have to keep track of it all. If you have an expiry date on Simon's cash, if you loose it, you can tell the bank, and if it's not cashed by the expiry date they can give you your money back. There's a distinction between on-line "banknotes", which are valuable enough to forge, and off-line "coins", which are not. Simon has a system which has just enough anonymity to be economically useful, but not perfect enough to keep the truly paranoid happy. Must be something in the water in Redmond. Essentially, the bank becomes a quasi-anonymous "line" in what's normally a peer-to-peer transaction. Takes getting used to. He's got a web page with all of this in gory detail, I bet, and his e-mail address is at the top of this post. Speaking of micropayments, Mark Manasse, Ron Rivest, Adi Shamir, and Silvio Micali did another talk on Friday which touched on several cool micropayment schemes, and a much more cost-effective way to do key revocation. Though I got there exactly 6 minutes late, I missed one presentation, and didn't take any notes for the rest. Hope someone else here did. I saw lots of DCSB/Cypherpunks there... Any takers? Cheers, Bob Hettinga -----BEGIN PGP SIGNATURE----- Version: 2.6.2 iQCVAwUBMNdbfvgyLN8bw6ZVAQH7qwP9GKSx7BegCr5lAOmG6lBo4OYgCPUO9j8N bqf89b46rTkYrTkSB2DDX7BG/wu4nv/pnf7rA7UBHJjydZuiAVG9KplzevUWFoaI xVeW8AoVPgKvkZsELK4VrrOayAXNxX9FBX7vGwsedl1SjQgyA3Qo62799GOIDrpQ xuXYtTtaTfw= =LQk+ -----END PGP SIGNATURE----- ----------------- Robert Hettinga (rah@shipwright.com) e$, 44 Farquhar Street, Boston, MA 02131 USA (617) 958-3971 "Reality is not optional." --Thomas Sowell The NEW(!) e$ Home Page: http://thumper.vmeng.com/pub/rah/
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