The increasing reliance on digital signatures for secure authentication and verification necessitates advanced watermarking techniques to protect signature integrity. Transform-domain methods, including the discrete cosine transform (DCT) and the discrete wavelet transform (DWT), are proposed for their potential to balance robustness, imperceptibility, and recognition accuracy in online signature biometrics. This study explores multi-bit watermarking approaches applied to online handwritten signatures using the MCYT signature database. We investigate the effects of embedding multiple bits per sample with adjustable watermark strength ( \(\alpha \) ) in the DCT and DWT domains. The trade-offs between signal distortion, watermark extraction accuracy, and biometric recognition rates are systematically evaluated. Experimental results reveal that while increasing \(\alpha \) enhances watermark robustness, it also leads to perceptible distortions in signature samples. We identify the minimum \(\alpha \) thresholds required for error-free watermark extraction and analyze their impact on identification and verification performance. The proposed multi-bit embedding strategy in the DCT domain demonstrates a viable compromise between robustness and imperceptibility, maintaining acceptable biometric recognition rates. Transform-domain watermarking techniques provide a promising solution for secure and robust online signature biometrics. This study highlights the feasibility of incorporating multi-bit watermarking schemes with adjustable strength into online signature systems, enhancing security while preserving recognition accuracy.
