Verifiable partial key escrow

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Abstract
One of the main objections to existing proposals for key escrow is that the individual's privacy relies on too high a level of trust in the law enforcement agencies. In par- ticular, even if the government is trustworthy today, it may be replaced by an un-trustworthy government to- morrow which could immediately and suddenly recover the secret keys of all users. "Partial key escrow" was suggested to address this concern, in the context of DES keys. Only some part of a user key is escrowed, so that the authority must make a computational effort to find the rest. We extend this idea and provide schemes to perform partial key escrow in a verifiable manner in a public-key encryption setting. We uncover some subtle issues which must be ad- dressed for any partial key escrow scheme to be secure, the most important of which is the danger of early recov- ery. We show that other proposals for verifiable partial key escrow suffer from the early recovery problem, and thus do not in fact offer an advantage over standard key-escrow schemes. Our verifiable partial key escrow scheme for the Diffie-Hellman cryptosystem does not suffer from early recovery.

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