How is ubuntu update secured against man in the middle attack?

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http://askubuntu.com – Originally I want(ed) to know how ubuntu update process is being secured against tampering ofpackages by a man in the middle. This has been this question here: Are Virus/rootkits via ubuntu updates possible? [closed] It has been closed for being assumed to be a duplicate. Still the assumed duplicates cover some quite different topics (like how cd/flashdrive autorun etc effects safety etc.). So I post the question again. I seek an answer to how the updated process is secured against a man in the middle attack. Pardon me for the potential annoyance, but the topic deeply interested me since th (HowTos)