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1

The remark of Conrado showed me that I could look at the XOR as a simple key wrapping operation, which is basically just another encryption. In that case it is clear that the scheme is secure for confidentiality; it just adds another layer of encryption. Of course, encryption by itself doesn't offer integrity / authenticity. ECIES cannot offer authenticity ...


3

As written, it makes an additional security assumption on the symmetric cipher; that the attacker can't flip bits in the key (and modify the ciphertext) to gain some advantage. That is likely true for most ciphers we would use in practice, but is nevertheless an additional assumption. This is not difficult to fix; we don't select a random symmetric key $K_d$...


2

By definition, VSS implies broadcast. As such, with $t\geq n/3$, it is not possible to achieve VSS (by the bounds on Byzantine Agreement).


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