Welcome to the resource topic for
**2001/009**

**Title:**

Robust key-evolving public key encryption schemes

**Authors:**
Wen-Guey Tzeng, Zhi-Jia Tzeng

**Abstract:**

We propose a key-evolving paradigm to deal with the key

exposure problem of public key encryption schemes.

The key evolving paradigm is like the one used for

forward-secure digital signature schemes.

Let time be divided into time periods such that

at time period j, the decryptor holds the secret key

SK_j, while the public key PK is fixed during its

lifetime.

At time period j, a sender encrypts a message m as

\langle j, c\rangle, which can be decrypted only

with the private key SK_j.

When the time makes a transit from period j to j+1, the

decryptor updates its private key from SK_j to SK_{j+1}

and deletes SK_j immediately.

The key-evolving paradigm assures that compromise of the

private key SK_j does not jeopardize the message encrypted

at the other time periods.

\par

We propose two key-evolving public key encryption schemes

with z-resilience such that compromise of z private keys

does not affect confidentiality of messages encrypted in

other time periods.

Assuming that the DDH problem is hard,

we show one scheme semantically secure against passive

adversaries and the other scheme semantically secure against

the adaptive chosen ciphertext attack under the random

oracle.

**ePrint:**
https://eprint.iacr.org/2001/009

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