4 ms·

Are you saying that some random ISP(s), some third party or even their hosting provider can't modify the PDFs and the sums on the fly to inject malware? Do you

by jgome 11y ago

Are you saying that some random ISP(s), some third party or even their hosting provider can't modify the PDFs and the sums on the fly to inject malware? Do you really think most people checks the sums?

ghshephard 11y ago
Here is the way this works. The person who posts PDFs on websites with MD5/SHA1/SHA256 hashes adds a watchdog to verify that those hashes aren't changing - Once you get the framework together, adding a new page with hashes to the watchdog takes just a few seconds. That way, if the random ISP(s) or third parties are modifying those sums on the fly, will trigger the watchdog. As to whether most people check the sum - I have no idea, but at least anybody who wants to take 90 seconds to authenticate the document can just go: x=Project_CAMERASHY_ThreatConnect_Copyright_2015.pdf ; md5 $x; shasum -a 1 $x; shasum -a 256 $x; Keep in mind - I totally agree with you that this isn't a great mechanism, but I would argue it's better than nothing at all. (as long as someone has a watchdog to confirm the hashes aren't being modified in flight - they could probably help their case a little by at least serving those pages with HTTPS). A much better mechanism would be to use OpenBSDs signify (http://www.openbsd.org/papers/bsdcan-signify.html http://www.openbsd.org/papers/bsdcan-signify.html) which solves this whole problem of trying to sign documents with something simple that doesn't involve byzantine chains of trust in a very elegant way. They could just create a key pair: signify -G -p threatpub -s sec And make their public key, which is short, and easy to copy/distribute everywhere - looks like this: untrusted comment: signify public key RWQw2u3UPjm6spK9OYJxylK2jSKz2agskG2EKPsxwFN4IjHVw66dYPhT And then, with each document they create, they just sign the PDF: signify -S -s sec -m Project_CAMERASHY_ThreatConnect_Copyright_2015.pdf Which provides a signature file, signed with their private key: untrusted comment: signature from signify secret key RWQw2u3UPjm6svkWhs4fgy1Qi0P72hp+uDuTxX8bDSvd/qr/7vc55v+PndgDdWOWj0JiLco/CCfOzw6Alau9RTi5gBiHSzuRHAs= Now, those two documents, the PDF and the Signature file - can be distributed everywhere - and are not subject to a malware attack because everyone has ThreatConnect's public key, which they can use to verify any threatconnect file and signature, with the simple command: signify -V -p threatpub -m Project_CAMERASHY_ThreatConnect_Copyright_2015.pdf I'm presuming that's the better mechanism you have in mind for this sort of thing? I think I'll forward our thread over to the threatconnect team, see if they are willing to upgrade their procedures.

by jgome 11y ago

ghshephard 11y ago

Here is the way this works. The person who posts PDFs on websites with MD5/SHA1/SHA256 hashes adds a watchdog to verify that those hashes aren't changing - Once you get the framework together, adding a new page with hashes to the watchdog takes just a few seconds. That way, if the random ISP(s) or third parties are modifying those sums on the fly, will trigger the watchdog. As to whether most people check the sum - I have no idea, but at least anybody who wants to take 90 seconds to authenticate the document can just go: x=Project_CAMERASHY_ThreatConnect_Copyright_2015.pdf ; md5 $x; shasum -a 1 $x; shasum -a 256 $x; Keep in mind - I totally agree with you that this isn't a great mechanism, but I would argue it's better than nothing at all. (as long as someone has a watchdog to confirm the hashes aren't being modified in flight - they could probably help their case a little by at least serving those pages with HTTPS). A much better mechanism would be to use OpenBSDs signify (http://www.openbsd.org/papers/bsdcan-signify.html http://www.openbsd.org/papers/bsdcan-signify.html) which solves this whole problem of trying to sign documents with something simple that doesn't involve byzantine chains of trust in a very elegant way. They could just create a key pair: signify -G -p threatpub -s sec And make their public key, which is short, and easy to copy/distribute everywhere - looks like this: untrusted comment: signify public key RWQw2u3UPjm6spK9OYJxylK2jSKz2agskG2EKPsxwFN4IjHVw66dYPhT And then, with each document they create, they just sign the PDF: signify -S -s sec -m Project_CAMERASHY_ThreatConnect_Copyright_2015.pdf Which provides a signature file, signed with their private key: untrusted comment: signature from signify secret key RWQw2u3UPjm6svkWhs4fgy1Qi0P72hp+uDuTxX8bDSvd/qr/7vc55v+PndgDdWOWj0JiLco/CCfOzw6Alau9RTi5gBiHSzuRHAs= Now, those two documents, the PDF and the Signature file - can be distributed everywhere - and are not subject to a malware attack because everyone has ThreatConnect's public key, which they can use to verify any threatconnect file and signature, with the simple command: signify -V -p threatpub -m Project_CAMERASHY_ThreatConnect_Copyright_2015.pdf I'm presuming that's the better mechanism you have in mind for this sort of thing? I think I'll forward our thread over to the threatconnect team, see if they are willing to upgrade their procedures.