Pycrypto will not build on Mac OS X 10.6 up with Xcode 4 installed.
There are numerous search results for this but I was unable to find a ticket on this bug tracker for this:
I've seen 100s of work arounds, however, not one of them fixes the actual problem which is that pycrypto tries to use the 10.4 SDK and adds the -ppc compilation flag even if you have set:
ARCHFLAGS="-arch i386 -arch x86_64"
To fix the problem the compile options either need to respect ARCHFLAGS, detect the version of Xcode and install accordingly, or just remove -ppc completely.
Pycrypto doesn't install using easy_install or pip because they try to reinstall pycrypto which has to be built from source in order to work. (The only reason I'm even using pycrypto is to use Fabric.)