[ubuntu] Where do I download the public key that is used to signed Ubuntu cdimages?

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http://ubuntuforums.org – Hi, I am new to Ubuntu but this much I know: all Ubuntu cdimages are signed by someone before they are uploaded to Ubuntu's servers and mirrors, am I right? If that is the case, could someone tell me where I can download the public key used to sign the ISO images, especially 12.04.2 LTS (Precise Pangolin)? The reason is I use Gpg4win 2.1.1 to verify the authenticity of downloaded ISO images on my Microsoft Windows platform before burning them to DVD or USB. I try not to use MD5 checksums as MD5 has been compromised by hackers. My friends have advised me to trust and use SHA1 hashes instead (Hardware)