How to sign an APK with more than one certificate?

Question

How do I sign an APK with more than one certificate, so that I can do this when I publish to the Android Market:

Version 1.0 => Signed with Certificate A
Version 2.0 => Signed with Certificate A & B
Version 3.0 => Signed with Certificate B

The Android Market does give me some hope but I am not sure what to make of it, when I upload with a different sign certificate it gives me this message:

"The apk must be signed with at least one certificate in common with the previous version."

Motive:
I have published an app on the market using the Android Signing Tool as part of MotoDev Studio for android. The problem is that it handles the keys itself and there is no way (documented) to get them. I want to change it to use a keystore and certificate that I have more control of and can use even if I don't use MotoDev Studio anymore. Also it seems that MotoDev Studio will lock you out of new features such as shared libraries.

Answer 1

If you want to sign an APK more than once, just do so.

eg. You can sign an APK from the command line using jarsigner like so:
jarsigner -keystore original-keystore my-app-unsigned.apk key-alias alias

Then just repeat this with the second key:
jarsigner -keystore new-signing-keystore my-app-unsigned.apk key-alias

Don't forget to run zipalign afterwards:
zipalign -v 4 my-app-unsigned.apk my-app.apk

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Just re-read the part about MotoDev studio. In that case you can just sign the APK first using MotoDev as usual, then sign with your new key on the command line as above.