Monthly Archives: December 2008

Folder Actions in OS X

Posted by on December 31, 2008 No comments

Awhile back I started getting annoyed at the process of manually moving files around after downloading, so I decided to investigate one of Apple’s more hidden features…Folder Actions! For those who don’t know, folder actions are configurable scripts that can be attached to folders and run whenever the contents of the folder change. They can do anything from copying mp3s you download from Amazon MP3 automatically into iTunes to converting documents automatically into PDF.

In a fit of boredom I have made a screencast showing how to make a simple folder action. If you’d like to follow along yourself, here is the applescript script (please note that you’ll need to create a folder named JPEGs in your ~/Downloads folder as well). Open Script Editor and paste it in to begin!

on adding folder items to my_folder after receiving the_files
tell application "Finder"
set jpgFolder to folder "JPEGs" in my_folder
repeat with f in the_files
if f's name ends with ".jpg" then
move f to jpgFolder
end if
end repeat
end tell
end adding folder items to

Obviously this represents a profoundly simple use case, but folder actions can have near limitless complexity. Unfortunately there are several issues with the implementation of this feature in OS X. Whenever the folder action actually triggers you will briefly get a flicker in your foreground window as the Finder takes focus, and for reasons I can’t quite nail down actions do not seem to trigger with 100% reliability. Despite these issues, this is a powerful tool that can be exploited in fascinating ways.

SSL Is Not Broken

Posted by on December 30, 2008 No comments

This morning an important and ingenious method of compromising the chain of trust for PKI was published. Naturally the internet is in a tizzy about the implications of this break, but misinformation rules the day.

SSL is in no way broken, but CAs still issuing MD5 certificates simply MUST stop as soon as possible. There are really two components to this (extremely clever) break:

1) a further reduction in the time required to compute a useful MD5 collision which the authors of the paper have not yet detailed.

2) CAs not using randomized serials to increase the entropy provided to the hash function.

If #2 is immediately rectified by the CAs then the difficulty of this attack increases dramatically. Of course, if they have any sanity they will switch to both SHA-1 and randomized serials.

It’s important to note, for those who don’t have the patience to read through the paper, that this describes a method of issuing a new subordinate CA from1 a trusted CA that can, in turn, issue any certificate without proper vetting. These certs would be trusted by every browser2 and OS containing the cert used for the initial issuance. This is not a compromise of MD5 certificates in the wild.

That distinction may seem inconsequential in view of the potential for a completely broken chain of trust, but it is important. Provided that the potentially vulnerable3 CAs (RapidSSL, possibly others) remedy their issues then this attack is completely neutered until such time as SHA1 is compromised to a similar extent. If those CAs take quick action then drastic steps (such as the revocation of whole root CAs and mass reissuance to all affected customers) should not be necessary.

 

Update (1/11/09): For future reference, RapidSSL ceased using MD5 signatures in their certificates within a day of the release of this paper.

  1. The actual subordinate CA is not being signed by the CA, only a valid cert, so saying it’s “from” the CA is misleading. ↩
  2. With the exception of Opera, which requires a valid CRL/OCSP response from every link in the chain. As described in the paper the authors were unable to include a crlDistributionPoint node in their ASN.1 structure so while browsers such as IE, Firefox, and Safari would simply trust the cert, Opera would explicitly deny trust. ↩
  3. Being vulnerable to this attack requires both the active use of MD5 in new certificates, an exactly guessable validity via automated issuance or other means, and sequential or otherwise predictable serial numbers. ↩

I Caved

Posted by on December 28, 2008 No comments

I gave up and added this blog because Twitter was too constraining.  I suspect I will almost never update this, but at least I can dump some useful info here for google users to consume.