[cryptography] How are expired code-signing certs revoked?

Peter Gutmann pgut001 at cs.auckland.ac.nz
Wed Dec 7 16:56:29 EST 2011

Steven Bellovin <smb at cs.columbia.edu> writes:

>Assume that there is some benefit to digitally-signed code.

There is at least one very obvious benefit: When malware is signed, it can't
mutate on each generation any more but has to remain static.  This makes it
easier for the anti-malware folks to detect.

You can also use it a second way:

  When malware authors have signed their products (at least until now) with
  fraudulently-obtained certificates (but not stolen ones) the only thing that
  they've signed with that particular certificate is malware. This means that
  once a particular signed binary has been detected as being malware the virus
  scanner can extract the signing certificate and know that anything else that
  contains that particular certificate will also be malware, with the
  certificate providing a convenient fixed signature string for virus scanners
  to look for.  This actually provides a real, effective use for code-signing
  certificates, although it's certainly one that the PKI folks would never
  have dreamed of.

  Unfortunately as with many other arms-race tricks it only works as long as
  the malware authors don't try to counter it, either by buying a new
  certificate for each piece of malware that they release (it's not as if
  they're going to run out of stolen credit cards and identities in a hurry)
  or by siphoning large numbers of benign applications from software-
  distribution sites, signing them, and re-uploading them to other software
  distribution sites so that the signed files that constitute actual malware
  get lost in the noise.

>Let's figure out what we're trying to accomplish; after that, we can try to
>figure out how to do it.

See above, code signatures help increase the detecability of malware, although
in more or less the reverse of the way that it was intended.


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