Whereas the embedding distortion, the payload and the robustness of digital watermarking schemes are well understood, the notion of security is still not completely well defined. The approach proposed in the last five years is too theoretical and solely considers the embedding process, which is half of the watermarking scheme. This paper proposes a new measurement of watermarking security, called the effective key length, which captures the difficulty for the adversary to get access to the watermarking channel. This new methodology is applied to the Distortion Compensated Dither Modulation Quantized Index Modulation (DC-DM QIM) watermarking scheme where the dither vector plays the role of the secret key. This paper presents theoretical and practical computations of the effective key length. It shows that this scheme is not secure as soon as the adversary gets observations in the Known Message Attack context.