When publishing an application or a custom rom you need to sign the .apk or .zip files with a certificate using a private key. The Android system uses the certificate to identify the author of an application and establish trust relationship between applications. The classic way of doing this was to use keytool then sign it with jarsigner. But This Is Easy Way

XDA-Developers member andyharney has posted his simple easy to use tool.

It's very simple, it will take a signed or unsigned apk and re-sign.
It will also batch sign all apk's that are in the INPUT folder.

It also includes the tools needed to create your own signature to sign the apk's

Changlog v0.3

1. Private Key now generated with 2048bit encryption (was 1024)
2. Private Key now valid for 10,000 days (was 999)
3. ZipAlign added to the script, will prompt upon signing completion.

Download ApkSigner APK to easily sign unsign games and applications.