It gives you the ability to modify the app without changing its cryptographic signature. If such problem would exist in standard PC world, it would essentially give you the ability to modify the binary without changing its hash.
This is a major blow to essential system of Android security system, the core functionality is broken the consequences can be massive not to mention it will never be fixed on old devices.
It has been discussed over and over on hackernews recently so just tldr version.
Let’s consider two scenarios.
1) NSA forces CA to issues a certificate for google.com and decide to man-in-the-middle you.
In that case there is a mechanism call certificate pinning. To put it simply certificates of Google, Facebook, Twitter etc. are hard coded into Firefox and Google Chrome. (Microsoft provides this ability in IE using latest EMET 4.0). So if someone tries to send you cert for google, which doesn’t match the one hardcoded your browser would get crazy and issue a big red warning :)
2) NSA records your encrypted communication with Google and later obtains Google private key (either by factoring Google public key or using some secret court order or whatever). In this case they CAN’T decrypt your communication with Google because Google uses version of Diffi- Hellman protocol with so called ephemeral keys. More here http://googleonlinesecurity.blogspot.com/2011/11/protecting-.... Ephermal DH is not implement by many sites (hackernews does it, facebook doesn’t)
SSL can be broken in myriad of different ways but at least in these two scenarios you are to certain degree safe
I am not sure if such system existed in the past or if I read about it in some sci-fi book, but it worked as follows:
You generated your key pair. In (almost) every country in almost every city there were “key signers” (basically trusted members of the PGP community). You met with them and they verified your identity and signed your public key. You needed to visit couple of them to get enough signatures to obtain certain level of trust in the PGP community. Once your level of trust was high enough you could start signing keys of other people. Too good to be true I guess…..
Probably not gonna happened, but it would solve so many problems with public key crypto. Key distribution? No problem, tie your public key to your gmail account. Need to communicate with someone? Just send them your public key. Goole would verify that key X belongs to mail Y, another problem solved. Mix it together with some javascript library (source code available by design) and you have almost perfect and simple to use public key crypto for masses. Oh well time to wake up….