Media authentication is important in content delivery via untrusted intermediaries, such as peer-to-peer (P2P) file sharing. Many differently encoded versions of a media file might exist. Our previous work applied distributed source coding not only to distinguish the legitimate diversity of encoded images from tampering but also to localize tampered regions in an image already deemed to be inauthentic. In both cases, authentication requires a Slepian-Wolf encoded image projection that is supplied to the decoder. We extend our scheme to authenticate images that have undergone affine warping. Our approach incorporates an Expectation Maximization algorithm into the Slepian-Wolf decoder. Experimental results demonstrate that the proposed algorithm can distinguish legitimate encodings of authentic images from illegitimately modified versions, despite an arbitrary affine warping, using authentication data of less than 250 bytes per image.