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In watermarking, the exploitation of synchronization marks embedded within the cover signal has been widely used to provide robustness against geometrical distortions. However, in most schemes, synchronization marks are easily detectable which makes them vulnerable to malicious removal or tampering. Relying on embedded signals which exhibit strong reference features to recover synchronization can therefore lead to security flaws in the watermarking system. In this paper, we present and discuss the synchronization recovery process through detection of the periodicity of the embedded signal. We then propose a scheme to hide synchronization marks through the modulation of the embedded signal by a content-based pseudo-random signal. We study the robustness of this pseudo-random masking signal and illustrate the efficiency of this approach on watermark patterns with a periodic structure.