Well, it is really hard to be categorical in saying that there is NO backdoor in open source AT ALL.
Actually being open can allows an army of programmers posing as contributors attempting to introduce a carefully crafted glitch with plausible deniability in mind that could go undetected for a while.
So it is not really impossible, but way more complicated demanding a lot of planning and effort.
The hard part in an algorithm I guess is to be able to convince people that a bad idea is actually a good one.
So I guess that instead of messing with an open algorithm, it would be easier to be messing up with its implementation.
@ArticMine Very insightful. DRMs could be NSA's secret weapons. It would make a lot of sense actually, I always thought that the RIAA and the MPAA had a ridiculous amount of power for mere creative and commercial rights. Especially considering the shady and aggressive tactics to implement DRMs such as the infamous Sony Rootkit.
There is actually a very good case for DRM as an NSA secret weapon. The following NSA document from the Snowden leaks: http://cryptome.org/2013/09/nsa-sigint-enabling-propublica-13-0905.pdf refers in the first paragraph to
The Sony rootkit of 2005 http://en.wikipedia.org/wiki/Sony_BMG_copy_protection_rootkit_scandal was very significant because it illustrated very clearly the very close relationship between malware and DRM. In reality they are both trying to solve the same problem with the same adversary, so it is hardly surprising that they would use very similar techniques or that one can be a vector for the other.