I met this error while updating.I read questions and answers about this problem , but the point is signing key. It doesn't return signing key!

Get:1 http://archive.canonical.com lucid Release.gpg [198B]

Ign http://archive.canonical.com/ lucid/partner Translation-en_US

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Get:24 http://archive.ubuntu.com lucid-security/main Packages [380kB]          

Get:25 http://archive.ubuntu.com lucid-security/restricted Packages [14B]      

Get:26 http://archive.ubuntu.com lucid-security/multiverse Packages [4,556B]   

Get:27 http://archive.ubuntu.com lucid-security/main Sources [118kB]           

Get:28 http://archive.ubuntu.com lucid-security/restricted Sources [14B]       

Get:29 http://archive.ubuntu.com lucid-security/universe Packages [125kB]      

Get:30 http://archive.ubuntu.com lucid-security/universe Sources [34.5kB]      

Fetched 12.8MB in 10min 33s (20.3kB/s)                                         

Reading package lists... Done

W: GPG error: http://archive.canonical.com lucid Release: Unknown error executing gpgv

W: GPG error: http://archive.ubuntu.com lucid Release: Unknown error executing gpgv

W: GPG error: http://archive.ubuntu.com lucid-updates Release: Unknown error executing gpgv

W: GPG error: http://archive.ubuntu.com lucid-security Release: Unknown error executing gpgv