Departmental Bulletin Paper 複数の秘密を考慮したパスワード認証付き秘密分散法
A Password-Protected Secret Sharing Supporting Multiple Secrets

新井, 貴大

Password-Protected Secret Sharing (PPSS) presented by Bagherzandi et al. is proposed in order to resolve drawback of secret sharing which is unauthorized users can access storages storing partial information can reconstruct a secret. PPSS is a secret sharing that ensures only the owner of the secret who knows correct password to obtain the original secret by applying password authentication to partial information. But, their model requires secure channel between user and servers and independent secret/public key pair at the distribution phase for each secret. When a secret is large, their scheme encrypts the secret with symmetric key encryption (SKE) and the symmetric key with CPA secure public key encryption (PKE).Because of such combination, it seems difficult to prove strong security (i.e., CCA security) of their scheme at least in the standard model. In this paper, we propose a new PPSS model and scheme. Proposed model deals with multiple secrets with using a single secret key/public key pair and does not require secure channel during the distribution phase. Proposed scheme does not use a simple combination of SKE and PKE but use Kurosawa-Desmedt hybrid encryption that is proven to be CCA secure in the standard model, and is constructed by combining public key encryption part of this hybrid encryption with password authentication.The scheme is expected to be more secure than that of Bagherzandi et al.

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