EC_DRBG backdoor simply proves audit feature
First, some crypto background on the backdoor in question. Dual_EC_DRBG or Dual Elliptic Curve Deterministic Random Bit Generator is an algorithm designed and published by the NSA, standardized by NIST in Special Publication 800-90. After the publication, research was performed on the generator by Dan Shumow and Niels Ferguson, apparently crypto guys who are with Microsoft, who discovered that this dual_EC output could be influenced under certain circumstances, rendering such output suspect with regards to key generation, as once the key is determined, it follows that plaintext can be derived more easily than if output of the algorithm was truly and uniformly random.
Backdooring munitions and things is not new, recall stories of CIA intentionally skewing ingredients in The Anarchist Cookbook so that mixing up a kinetic cocktail could be problematic for the budding mixologist, and also recall stories of mass producing a hyper-flammable US flag that burns with an incredible intensity so as to maybe injure the igniting party, or start an unintentional collateral conflagration.
Possession of this master key when applied to the distorted output of this algorithm can be used to argue that it serves as part of an audit mechanism, where Snowden claims that the algorithm indeed contains a backdoor. If the NSA can break all web encryption by emplacement of this particular backdoor, one can also potentially conclude that if NIST says they would not deliberately weaken a cryptographic standard, it is altogether possible that there could be a breakthrough that doesn’t weaken the algorithm, but allows usage of a master key that when applied to this distortion, can decrypt the text.