Lect. 18: Cryptographic Protocols. Contents . Cryptographic Protocols Special Signatures Secret Sharing and Threshold Cryptography Zero-knowledge Proofs Identification, Authentication Protecting your P rivacy . 1. Cryptographic Protocols . Cryptographic Protocols .
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Lect. 18: Cryptographic Protocols
Secret Sharing and Threshold Cryptography
Protecting your Privacy
1. Cryptographic Protocols
Security Requirements in Protocols
according to applications
2. Special Signatures
- Blind Signature
- Proxy Signature
Signing without seeing the message
- We should not reveal the content of the letter to the signer.
- For example, using a carbon-enveloped message
1) Send an carbon-enveloped message
2) Sign on the envelop
3) Take off the envelop and
get the signed message
Motivation of Blind Signature
David Chaum’s Blind Signature
He is also named as
the “father of E-cash”
Blind signature scheme is a protocol that allows the provider to obtain a valid signature for a message m from the signer without him seeing the message and its signature.
RSA-based Blind Signature
Get a signature for a message m.
m’ = H(m) re mod N
σ’ = m’d mod N
σ = σ’ r-1 mod N
σ = σ’ r-1 mod N = (H(m) re)d r-1 mod N = H(m)d mod N
σ isa valid signature of the signer
The signer cannot have any information on m and σ.