In public-key infrastructures it is frequently useful to be able to reconstruct private keys. For example, if a user has lost his smartcard that contains his private decryption key, then he cannot decrypt any encrypted file on his computer anymore. So those encrypted files are then inaccessible for the user unless it is possible to reconstruct the decryption key. However, for security reasons it may be important that the key cannot be reconstructed by a single person. That person could abuse the knowledge of the private key. It is more secure if a group of people has to be involved in the reconstruction. In this chapter we describe secret sharing, a protocol that can be used to solve this problem.
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