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

Outline Introduction Proposed Scheme Efficiency Security Analysis Conclusion

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

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

Proposed Scheme Notations:

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

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

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

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

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

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

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

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.

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.