the calculus of encrypting non-textual data (was Re: concrete steps for improving apt downloading security and privacy)
On Tue, Jul 8, 2014 at 5:13 AM, Andrea Zwirner and...@linkspirit.org wrote: On 07/07/2014 13:09, Joel Rees wrote: Sorry Joel, I almost totally disagree with your vision on privacy and security, but I really i don't want to go into the merit of it, because I think Lou is representing my vision already; I only have a question: Did you know that encrypting a picture sometimes results in a picture that looks like it has been through a random color-permuting filter? Can you proof it? Memory of coursework in encryption. The professor did some simple encryption on uncompressed images and showed how the results tended not to hide the things one would want hidden. Then he pointed out that the parts of an image with the most information are the parts that are least likely to compress. And he pointed out that standard encryption methods tend to be byte-oriented, for speed. He did not require us to do any homework on it, so I don't have any special tools in my notebooks. 30 years ago. Heh. The technology has changed somewhat since then, but I recently read about some standard encrypted sound files that were playable, noisy, but recognizable. Same principle. Or maybe, you can tell the list what the attached image - that is encrypted with Moritz Muehlenhoff's and Florian Weimer's public keys - represent? You'll note that I never said it could be done on every encrypted image. I assume that, now the math has been pointed out to you, you won't mind if I decline the challenge? Cheers (and thanks Mr. Moritz and Mr. Florian - who were the only I had in my keyring - to accept being the judges of the challenge). :-) Give them my regards, and I hope they are not disappointed. I've got other things to do. Andrea Zwirner Sent from my Sylpheed -- Joel Rees Be careful where you see conspiracy. Look first in your own heart. -- To UNSUBSCRIBE, email to debian-security-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/caar43im8zdukxqmnej8_oqcjhreto0p_qtdhyiq5wilpzye...@mail.gmail.com
Re: the calculus of encrypting non-textual data (was Re: concrete steps for improving apt downloading security and privacy)
Joel Rees dijo [Tue, Jul 08, 2014 at 11:11:09PM +0900]: Did you know that encrypting a picture sometimes results in a picture that looks like it has been through a random color-permuting filter? Can you proof it? Memory of coursework in encryption. The professor did some simple encryption on uncompressed images and showed how the results tended not to hide the things one would want hidden. Then he pointed out that the parts of an image with the most information are the parts that are least likely to compress. And he pointed out that standard encryption methods tend to be byte-oriented, for speed. Well... Yes and no. Yes, that happens when you don't do it right. http://gwolf.org/files/desarrollo_y_cripto.pdf#page=40 If you implement the wrong mode of operation (schemes for the five modes of operation shown in the next page), you end up with the problem you mention. This result was achieved using the Electronic Code Book (ECB). Now, the ECB mode should never be used twice for the same plaintext blocks. Oh, and as a sidenote for my images: They are *a bit* tricked. If you apply ECB to a PNG image, as the PNG image is compressed, you will not find this pattern. I saved the original image as a BMP, then ciphered the image, and then copied over (prepended) the BMP header to the new file; that's the reason for the file to be slightly right-shifted: It contains a bit of unnecessary noise. But the effect *is* correct. He did not require us to do any homework on it, so I don't have any special tools in my notebooks. 30 years ago. Heh. The technology has changed somewhat since then, but I recently read about some standard encrypted sound files that were playable, noisy, but recognizable. Same principle. It is very simple. The implementation is one page earlier in the document I sent you. If you replace i.e. AES-128-ECB with AES-128-OFB (and supply an initialization vector, which does not have such secrecy requirements as the key). -- To UNSUBSCRIBE, email to debian-security-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/20140709014205.gj114...@gwolf.org
