On the Security of Encrypted Secret Sharing

Sensitive electronic data must often be kept confidential over very long periods of time. Known solutions such as conventional encryption, cascaded encryption, and information theoretic schemes suffer from several weaknesses or serious disadvantages that we shortly discuss. We present a method for combining arbitrary encryption algorithms. The scheme has the following properties: (1) It is a (k, n)-threshold scheme, i.e. only k=n of then applied algorithms are needed for decryption. (2) The scheme´s effective bit security is the sum of the lengths of the k shortest keys. (3) Under adaptive chosen plaintext attacks, this security level remains intact until at least k algorithms are compromised. (4) Under adaptive chosen chipertext attacks, the security level decreases with each compromised algorithm at most by the corresponding key length. (5) The scheme increases the effective key lengths of repeatedly applied encryption algorithms.