In the past decade, a number of chaos-based watermarking schemes have been proposed. Generally, it is widely accepted that the introduction of chaos effectively enhances the security of digital watermarking. Yet, taking a representative algorithm as an example, this paper reveals vulnerabilities of the chaos-based watermarking schemes against various attacks under Kerckhoff's principle, and the watermark can be successfully removed and decrypted. To achieve higher security, a sub key generation mechanism based on the feature codes of images (SKGM-FCI) is designed. The obtained watermarking key is related to the original images, such that individual sub keys are obtained for different host images. Benefiting from the SKGM-FCI, a blind watermarking scheme based on Discrete Cosine Transform domain of the scrambled image is further proposed and investigated. The theoretical analysis and simulation results validate the effectiveness and the high security level of the proposed scheme.