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In this paper we study the effects of synchronization errors on the bit error rate performance of spatial domain image watermarking. For that, we introduce the concept of the random jitter attack. This attack is characterized by the displacement of each pixel position by a random amount given by an arbitrary distribution, followed by interpolation over the modified sampling grid. An analogy is made between this attack and the errors introduced by timing jitter during sampling. Under such an analogy, a channel model is proposed to describe the effects of the jitter attack on the watermarked image. This model suggests that the effects of the jitter attack can be analyzed as the addition of signal dependent noise to the watermarked image. We derive expressions that relate the strength of the jitter attack to the bit error rate obtained for watermark decoding. These expressions are compared to experimental results obtained by performing the random jitter attack over a spatial domain spread spectrum watermark.