1. Proxy Blind Signature Scheme
Author: Sunder Lai , Amit Kumar Awasthi
Source: Journal of Information Science and Engineering
Date: Dec/3/2003
Speaker: 陳東怡

2. Outline Introduction Proposed Scheme Efficiency Security Analysis
Conclusion

3. Introduction Blind signature: Proxy blind signature:
User C can obtain signature of A on any given message without revealing any information of the message
Proxy blind signature:
The original singer A delegates his signature authority to proxy singer B in such a way B can sign any message on behalf of the original A

4. Introduction Proxy unprotected: Proxy protected:
Proxy and original singer can both generate
a valid proxy signature
Proxy protected:
Only proxy can generate a valid signature
Original A
Signature
Authority
Proxy blind
signature
Proxy B
User C

5. Proposed Scheme
Notations:

6. Proposed Scheme (cont.)
Proxy phase
1. Proxy Generation
Choose and compute
2. Proxy Delivery
The original singer sends (s, r ) to a proxy B in a secure way and makes public

7. Proposed Scheme (cont.)
3. Proxy Verification
Verify if
if (s ,r) satisfies this congruence, B accepts it as a
valid proxy, otherwise rejects it.
Signing phase
1. B choose a random num
computes
and sends it to the receiver C

8. Proposed Scheme (cont.)
2. C choose randomly
and computes
and sends e to B
3. After receiving e, B computes
and sends it to C
4. Now C computes
The tuple (m, Sp, e’ ) is the proxy blind signature

9. Proposed Scheme (cont.)
Verification phase
The verifier of the proxy blind signature computes
Here e’’ = e’ if the tuple (m, Sp, e) is a valid proxy signature

10. Proposed Scheme (cont.)
Proxy protected case
Proxy phase
1. Proxy Generation
Choose and compute
2. Proxy Delivery
The original singer sends (s, r ) to a proxy B in a secure
way and makes public

11. Proposed Scheme (cont.)
3. Proxy Verification and key alternation
After confirming the validity of the pair (σ ,r) B
alters the proxy key as
The Signing phase and the Verification phase are
same as in the proxy unprotected case

12. Efficiency Phase Total Proxy generation Signature generation Signature
verification
4E+2M
3E+3M+2I
2E+M
9E+6M+2I
where E denotes Exponential computation,
M denotes Multiplication load ,
I denotes Inversion

13. Security Analysis
With different congruence, the original signature and the proxy signature can be distinguished.
It is impossible to create a valid proxy signature without knowing xB or s or both.
Original signer cannot deny his agreement because yp is computed from ya.

14. Conclusion
A proxy blind signature scheme with which a proxy user is able to make proxy blind signature and verifier may verify it very similar to proxy signature schemes.