Este error me dio en mi version 10.04 LTS cuando en la consola intente hacer un:

sudo aptitude update

El error que me reporto a mi era:

"W: GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A9A06AEF9CB8DB0"

Pero la PUBKEY (codigo binario que ves al final del error) puede cambiar dependiendo del repositorio. Los codigos de error pueden ser:

W: GPG error: http://archive.canonical.com jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5

W: GPG error: http://download.virtualbox.org jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY DCF9F87B6DFBCBAE

W: GPG error: http://packages.medibuntu.org jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2EBC26B60C5A2783

W: GPG error: http://archive.ubuntu.com jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5

W: GPG error: http://archive.ubuntu.com jaunty-updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5

W: GPG error: http://archive.getdeb.net jaunty-getdeb-testing Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A8A515F046D7E7CF

W: GPG error: http://archive.ubuntu.com jaunty-security Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5

W: GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6E871C4A881574DE

W: GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 28A8205077558DD0

W: GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D739676F7613768D

W: GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8C851674F96FD737

W: GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C0B56813051D8B58

W: GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4874D3686E80C6B7

W: GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 60D11217247D1CFF

W: GPG error: http://archive.ubuntu.com jaunty-backports Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5

W: GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A9A06AEF9CB8DB0

W: GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6D975C4791E7EE5E

W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used.GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A9BF3BB4E5E17B5

W: GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7FB8BEE0A1F196A8

W: Failed to fetch http://ppa.launchpad.net/chromium-da...jaunty/Release

W: Some index files failed to download, they have been ignored, or old ones used instead.

Para soluccionar el error hay que volver a descargar las PUBKEYS (llaves o códigos) usando el numero hexadecimal que te da en el error, con el siguiente comando desde la consola, poniendo el código hexadecimal al final del comando:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys CODIGO_HEXADECIMAL

Por ejemplo:

Nos sale el siguiente error:

W: GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C0B56813051D8B58

habria que poner en la cosola:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys C0B56813051D8B58

Una vez ejecutado el comando nos saldria un mensaje de importacion de la PUBKEY parecido a:

Executing: gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /etc/apt/secring.gpg --trustdb-name /etc/apt/trustdb.gpg --keyring /etc/apt/trusted.gpg --primary-keyring /etc/apt/trusted.gpg --keyserver keyserver.ubuntu.com --recv-keys C0B56813051D8B58
gpg: solicitando clave ******* de hkp servidor keyserver.ubuntu.com
gpg: clave ********: clave pública "Launchpad PPA for Ubuntu Wine Team" importada
gpg: Cantidad total procesada: 1
gpg: importadas: 1 (RSA: 1)

Una vez importada la PUBKEY podemos seguir con el upgrade:

sudo aptitude upgrade

Mensaje original desde : http://swiss.ubuntuforums.org/showthread.php?t=1221323
Gracias a: altonbr