A secret-key generation strategy is introduced for fading wireless channels. In the model considered, Alice wants to share a key with Bob while keeping the key secret from a passive eavesdropper, Eve. Both Alice-Bob and Alice-Eve channels are assumed to undergo block fading, and perfect channel state information (CSI) is assumed to be known only at the receivers during the transmission. The scheme is based on layered broadcast coding, which facilitates adapting the reliably decoded rate at Bob to the actual channel state without CSI available at Alice. The index of a reliably decoded layer is sent back to Alice via a public and error-free channel, which is exploited by Alice and Bob to generate the secret key. The secrecy key rate is derived. In addition, the optimal power distribution over coded layers is characterized. It is shown that layered coding can increase the secrecy key rate significantly compared with single-level coding.