apt-get update 失敗 - Linux

請問一下 我 apt-get update 會失敗

說我沒有有效的 public key

該怎麼解決這個問題？

用 apt-get update 也沒用

Hit http://security.debian.org etch/updates/contrib Packages
Hit http://security.debian.org etch/updates/main Sources
Hit http://security.debian.org etch/updates/contrib Sources
Fetched 38.5kB in 3s (12.4kB/s)
Reading package lists... Done
W: There is no public key available for the following key IDs:
9AA38DCD55BE302B
W: GPG error: http://security.debian.org etch/updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9AA38DCD55BE302B
W: You may want to run apt-get update to correct these problems



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