<html><head><meta http-equiv="Content-Type" content="text/html charset=iso-8859-1"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>On 12 Apr 2013, at 00:01, Ben Liddicott <<a href="mailto:ben@liddicott.com">ben@liddicott.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><p dir="ltr">That isn't possible, up to a limit. Proof is that any such transformation can carry only a limited number of bits of data. Therefore any steganographic message can be destroyed by a transformation using the same stego technique</p><div><br></div></blockquote>That would rely on Facebook knowing the stego technique and any associated keys. If it's keyed (ie, Alice and Bob share a key from which they can derive a small subset of the pixels in the image which contain the message), then how can the attacker overwrite that message? The key would denote some small number of bits, drawn from potentially all the bits in the image. The attacker can choose some random key and insert a message using that, but if a key identifies some fraction F of the image, adding another message with an independent key would overwrite F of the first message. As F will typically be small, simple error correction will suffice.</div><div><br></div><div>Facebook could attack this technique by dithering the whole image. But I suspect that you can perturb a small number of pixels more than all the bits, so the degradation caused by dithering all the bits sufficiently to extinguish information encoded in any subset of those bits would be visually unacceptable. </div><div><br></div><div>ian</div></body></html>