Problem: several members of a given group need to sign a document. For example, a corporation requires signatures of three senior officers for any large transaction.
Traditionally, handwritten signatures provide for:
- Flexibility: any subgroup can sign the document.
- Accountability: anyone who signs the document can be individually identified and held responsible.
No satisfactory digital solution provides both.
- Using several single-signer signatures is inefficient:
- "Multi-signatures" and "Group Signatures" lack flexibility.
- "Threshold signatures" lack accountability.
Recent Work : "Provable-Subgroup SignaturesÓ, by Micali, Ohta and Reyzin
- An efficient scheme providing complete flexibility and individual accountability
- A general, formal model in which to analyze the security of such schemes.