Anonymous Digital Cash  Ashok Reddy  Madhu Tera  Laxminarayan Muktinutalapati (Lux)  Venkat Nagireddy

Overview  What is digital cash?  Need for anonymous digital cash  Concepts in anonymous digital cash  Protocol: Dining Cryptographers’ protocol  Achieving anonymity  Illustration  Practical concerns  Conclusion

What Is Digital Cash?  Digital cash is a digitally signed payment message that serves as a medium of exchange

Need for Anonymous Digital Cash  Increase in electronic surveillance by governments and other institutions  Lack of privacy features associated with ordinary electronic transactions

Concepts in Anonymous Digital Cash Anonymity is chiefly concerned with  Unlinkability  Untraceability

Protocol  Dining cryptographers’ protocol

Dining Cryptographers…(2)  Model

Achieving Anonymity  Blind digital signatures  Blinding factor

Illustration  The Digital Bank would offer electronic bank notes: messages signed using a particular private key  The electronic bank notes could be authenticated using a corresponding public key  The bank would also make public, a key to authenticate electronic documents sent from the bank to its customers

Illustration…(2)  To withdraw a dollar from the bank, Alice generates a note number (each note bears a different number, akin to the serial number on a bill); she chooses a 100-digit number at random  Before sending the note number to the bank for signing, Alice multiplies it by a random (blinding) factor  Now she signs the number with the private key corresponding to her "digital pseudonym"

Illustration…(3)  After receiving the blinded note signed by the bank, Alice divides out the blinding factor and uses the note as before  The blinded note numbers are, therefore, "unconditionally untraceable"

Practical Concerns  Counterfeiting or Double-spending: Fraudulently spending the same money more than once Remedy:  Checking each note against an on-line central list when it is spent  Using tamper-resistant hardware (called an "observer")  Generating blinded notes that require the payer to answer a random numeric query about each note when making a payment

Practical Concerns…(2)  Framing: An attempt by a bank to fraudulently claim that a customer has double-spent the same piece of cash when the customer hasn’t. Remedy:  Similar to those discussed earlier

Practical Concerns…(3) Other concerns:  Theft  Adaptability

Conclusion  Anonymous cash is "unconditionally untraceable" which provides enough privacy to the user  A system (implementing anonymous digital cash) closely resembling our current payment system will be easier for consumers to understand and adapt to

Questions?