中文摘要：

首先提出一个高效的基于身份的密码系统EpA，并且证明该方案是明文感知的。然后将EpA改造为基于身份的门限密码系统ThEpA，该门限系统具有不可重构用户私钥的特性。利用这个特性，构造了强壮的私钥门限托管方案，即恶意托管人数大于或等于门限值时仍然无法获取被托管的用户私钥。

英文摘要：

An efficient ID-based encryption scheme called EpA was constructed and proved in the sense of plaintext awareness. Then EpA was modified to be a threshold ID-based threshold decryption scheme named ThEpA, which was featured by a property called non-reconstruction of the private key with it, a robust key escrow scheme based on ThEpA was proposed. In ThEpA, the decryption key is distributed among n decryption servers, but a single private key could not be obtained even if securities of all these distributed servers are compromised.