Digital signatures allow an entity to sign a digital message so that it is easy to verify the signature, but it is hard to forge a valid signature on a given message.
It is sometimes useful to be able to delegate the signing to some proxy. The proxy signature schemes allow proxy signers to sign messages on behalf of an original signer, a company or an organization. Bruce Schneier describes this as follows:
Alice, for instance, needs to go on a business trip to someplace which doesn’t have very good computer network access—to the jungles of Africa, for example. Or maybe she is incapacitated after major surgery. She expects to receive some important e-mail, and has instructed her secretary Bob to respond accordingly. How can Alice give Bob the power to sign messages for her, without giving him her private key?
Proxy signatures is a solution. Alice can give Bob a proxy, such that the following properties hold:
- Distinguishability. Proxy signatures are distinguishable from normal signatures by anyone.
- Unforgeability. Only the original signer and the designated proxy signer can create a valid proxy signature.
- Proxy signer’s deviation. A proxy signer cannot create a valid proxy signature not detected as a proxy signature.
- Verifiability. From a proxy signature, a verifier can be convinced of the original signer’s agreement on the signed message.
- Identifiability. An original signer can determine the proxy signer’s identity from a proxy signature.
- Undeniability. A proxy signer cannot disavow an accepted proxy signature he created. In some cases, a stronger form of identifiability is required—that anyone can determine the proxy signer’s identity from the proxy signature.
But what exactly is proxy re-signatures? How and why is it performed? Does it use the same warrant generating and proxy signing mechanisms?
Context: Thinking on a new model for signatures. Was trying to combine the Sanitizable with Proxy Re-Signatures. Will it make a meaningful model and have any useful application?