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I'm building .apk file for my Android application using Android tools - Export... as guided here
My question is what are signed and unsigned .apk file?
How can we use usngined .apk file? (I tried to export an unsigned one but I cannot install it for my Galaxy Nexus)
Signing Android apps is just like signing any other application; it provides the user with some level of assurance that the code hasn't been tampered with since you released it.
Unsigned apps should be usable generally but do require lowering the security level. A quick Google search found a number of articles that explained that all you have to do is check the Unknown Sources box in Settings->Security.
Export Unsigned apk means you will get the unsigned apk that is not signed by any keystore. A keystore is a binary file that contains a set of private keys. The app will need to be signed with a keystore to be used or installed using the Google Play App Store.
Export Signed apk means you get the apk and it will provide the option to make the sign apk with existing keystore.
Android requires that all apps be digitally signed with a certificate before they can be installed. Android uses this certificate to identify the author of an app, and the certificate does not need to be signed by a certificate authority. Android apps often use self-signed certificates. The app developer holds the certificate's private key. More info here.
A signed apk is an android package file that has been digitally singed with a certificate for which the developer holds the private key. When you are doing developing your application a special debug key is created by the dev tools.
http://developer.android.com/tools/publishing/app-signing.html
Devices require that all applications be signed, so that is why you can't install an unsigned apk.
There are sonme instructions here on self signing
I hope this helps.
It is explained in details in this blog: http://www.nowherenearithaca.com/2011/12/difference-between-signed-and-unsigned.html
Basically the difference is:
The signed apk is simply the unsigned apk that has been signed via the JDK jarsigner tool.
FYI, the APK must be signed in order to be installed on an Android device, you can get more details on the signatures in the Android documentation: http://developer.android.com/tools/publishing/app-signing.html
来源：https://stackoverflow.com/questions/18397498/what-is-the-differences-between-export-signed-and-unsigned-apk-file