In this paper we present an invisible spatial domain watermarking technique. The technique divides the image into n small blocks and the intensity of some of these blocks are modified depending on the key, which is a secret random binary string of length m < n. Given an attacked image, the recovery process generates a bit pattern of length m and we show that from this pattern it is possible to get back the exact key using either standard correlation measure or error correcting codes. The recovery process employs window matching technique on the attacked image, which has been found to work successfully. Our method can survive common image transformations and intentional attacks both in spatial and frequency domain. Most importantly, it also survives the nonlinear geometric attacks, e.g., Stirmark. To the best of our knowledge, here we experiment with an exhaustive set of attacks which has not been provided elsewhere.