Question about Signing Application for Android

I got a question about singing applications in Android.

I worked on an update 开发者_开发知识库for an app dat was already made and put in the Market before. I signed it and tried to upload it as an update but it said that the keystore was different.

I emailed the original developer and he signed it for me. I asked if I had to send it to him each time I wanted to upload an update.

This is what I got back from him:

It's not the first time I've done this , so I know for sure it works . The apk you give me must be unsigned , then I sign it , then you're developers than up to the market . After that you will not need my keystore again . Maybe some things changed since the last time I did this , but in order to know what is happening I need to know all the details .

This is an unsigned apk ? the other one was uploaded on market ? Did you encountered any error .... what was the message . Attached you can find the signed apk .

Is this true? If I upload the signed APK he gave me, can I use a different keystore for the next update?

No, the person you're interacting with is wrong. You need to sign new apk each time you want to upload an update and each time the keystore should be the same, otherwise market treats you as the other person.

As far as I know, you can only upload updates that were signed with the same key.