This paper firstly defines the concept of security classes for spread-spectrum watermarking schemes in a Known Message Attack (KMA) framework. In particular, we define three security classes, namely, by order of increasing security: insecurity, key-security and subspace-security. Then, we present three new spread spectrum watermarking schemes, namely, independent natural watermarking (INW), robust independent natural watermarking (Robust-INW) and independent circular watermarking (ICW). All these new watermarking schemes build on the uniformly distributed random orthogonal matrix which can be estimated by the decoder. And all these schemes (i.e., INW, Robust-INW and ICW) are secure against carriers estimation in the KMA framework.
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