Retrieve keystore from app or change certificate
android
Solution
This is not possible. The keystore contains a certificate which is used to digitally sign your apk. Each certificate is completely unique, and cannot be regenerated or recovered from older apks.
Google relies on this because it is extremely secure, and allows them to really reduce the chances that someone can hack your developer account details and upload a malicious apk as an update to your existing app.
Even if Google allowed you to upload an apk signed with a different key as an update, the Android devices would not allow the user to update the app as the apk installed would have a different signature than the one being installed. This is the same problem you'll encounter if you instal your app from Google Play and then try to install a debug version from Eclipse.
For now, you'll have to reupload the app under a different package name with a different key. In the future, please remember to backup your keystore to multiple locations (but do it securely. Don't accidentally commit it into a public version control system. The keystore should be kept private at all times).
Digital signatures are based upon public-key cryptography. You cannot recover a private key given a public key -- that's the whole point of public-key crypto. Such algorithms are based on one-way functions: things that are easy to do but hard to reverse.
Problem
I made a mess: when I formatted my PC I lost the key I used to sign my android app. Google complains that an app update has to be signed with the same certificate but I don't have it anymore...is there a way to solve this problem? I can access to older signed apks, I can't believe google just relies on a single file...