confuzed on chromium update tool(courgette)

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I read google open source project(chromium) recently,and interest in the google update project(courgette),courgette is used for update. In server, the courgette generate patch file by calculate the difference of two version. then the client get the patch file and composite the new version file. when i test the courgette tool, I find that the digital signature information has been broken in the process of generate the patch file. I want know how to solve this problem. u can get more information about courgette in http://www.chromium.org/developers/design-documents/software-updates-courgette[^].