Secure untraceable off-line electronic cash system Sharif University of Technology Scientia Iranica Volume 20, Issue 3, Pp. 637–646, June 2013 Baseri, Takhtaei, Mohajeri Reporter ： yu-yang xiao 1

Outline Introduction Eslami and Talebi’s scheme and its failures Weaknesses of the scheme The proposed scheme Security discussion Conclusion 2

Introduction(1/3) Introduction(1/3)  Nowadays, thanks to the progressing technology of computer networks and the Internet, information technology is used in many aspects of human life.  Since the appearance of electronic commerce, people have been able to carry out their commercial activities by the use of electronic money in their payment transactions.  The first electronic money has been proposed by the David Chaum to have similar properties to paper cash. There are a number of features considered for an electronic cash system. 3

Introduction(2/3) Introduction(2/3) 4  Some of them are listed 1. Anonymity 2. Unforgeability 3. Double spender perceptibility 4. Date attachability 5. Divisibility 6. Transferability 7. Anonymity revocation 8. Portability

Introduction(3/3) Introduction(3/3) 5  In online category, while paying the coin to the merchant, the bank should attend the transaction, validate the coin and check its double spending.  In contrast, in off-line payments, the validation of the coin is done partially by the merchant while paying. After connecting to the bank in the next phase, the validation will be completed. While efficiency is improved, double spending can only be detected after connecting with the bank.

Eslami and Talebi’s scheme and its failures(1/8) 6 Eslami and Talebi’s scheme Central Authority (CA)Bank (B) Merchant (M)Spender (S) Initialization Withdrawal Payment Deposit

7 Initialization Eslami and Talebi’s scheme and its failures(2/8) CAB

8 Initialization Eslami and Talebi’s scheme and its failures(3/8) BS

9 Initialization Eslami and Talebi’s scheme and its failures(4/8) MB

10 Withdrawal Eslami and Talebi’s scheme and its failures(5/8) B S

11 Withdrawal Eslami and Talebi’s scheme and its failures(6/8) S

12 Payment Eslami and Talebi’s scheme and its failures(7/8) S M

13 Deposit Eslami and Talebi’s scheme and its failures(8/8) MB

14 attacking double spender detection Weaknesses of the scheme(1/4) B S

15 attacking double spender detection Weaknesses of the scheme(2/4) S

16 forging the expiration date Weaknesses of the scheme(3/4) S Withdrawal

17 forging the expiration date Weaknesses of the scheme(4/4) Payment S M

The proposed scheme(1/9) 18 Notation g 1,g 2 Two publicly known elements lThe large prime in H A one-way hash function

The proposed scheme(1/5) 19 Opening an account CustomerB

The proposed scheme(2/5) 20 Withdrawal BS

The proposed scheme(3/5) 21 Withdrawal S

22 Payment The proposed scheme(4/5) S M

23 Deposit The proposed scheme(5/5) MB

24 Anonymity Security discussion(1/4) Payment S M

25 Double spender detection Security discussion(2/4) Payment S M

26 Unforgeability Security discussion(3/4) Withdrawal BS

27 Security properties comparison Security discussion(4/4) JuangMartinez-Pelaez et al.Eslami and TalebiOur scheme Anonymity ✓✓✓✓ Unforgeability ✓✓✓✓ Double spender detection ✓✓ × ✓ Date attachability× ✓ × ✓

Conclusion In this paper, author considered one of the latest untraceable electronic cash protocols and showed its weaknesses. Furthermore, author contributed an electronic cash scheme which is immune to the weaknesses of Eslami and Talebi’s scheme. 28