How to verify downloaded file with .sig file?

You need to import public key: C3C45C06

Can be done in three steps.

1. find public key ID:

   $ gpg gcc-4.7.2.tar.gz.siggpg: Signature made Čt 20. září 2012, 12:30:44 CEST using DSA key ID C3C45C06gpg: Can't check signature: No public key

2. import the public key from key server. It's usually not needed to choose key server, but it can be done with --keyserver <server>. Keyserver examples.

   $ gpg --recv-key C3C45C06gpg: requesting key C3C45C06 from hkp server keys.gnupg.netgpg: key C3C45C06: public key "Jakub Jelinek jakub@redhat.com" importedgpg: no ultimately trusted keys foundgpg: Total number processed: 1gpg: imported: 1

If the command error's out with a timeout, you may be behind a firewall that is blocking the default gpg port. Try using the `--keyserver' option with port 80 (almost all firewalls allow port 80 b/c of web browsing):

$ gpg --keyserver hkp://${HOSTNAME}:80 --recv-keys ${KEY_ID}

3. verify signature:

   $ gpg gcc-4.7.2.tar.gz.siggpg: Signature made Čt 20. září 2012, 12:30:44 CEST using DSA key ID C3C45C06gpg: Good signature from "Jakub Jelinek jakub@redhat.com" [unknown]gpg: WARNING: This key is not certified with a trusted signature!gpg: There is no indication that the signature belongs to the owner.Primary key fingerprint: 33C2 35A3 4C46 AA3F FB29 3709 A328 C3A2 C3C4 5C06

The output should say "Good signature".

gpg: WARNING: This key is not certified with a trusted signature!

Is for another question ;)

This other avenue is particularly useful for verifying GNU projects (e.g. Octave) since the key requested by their signature may not be found in any key server.

From http://ftp.gnu.org/README

There are also .sig files, which contain detached GPG signatures of the above files, automatically signed by the same script that generates them.

You can verify the signatures for gnu project files with the keyring file from:

https://ftp.gnu.org/gnu/gnu-keyring.gpg

In a directory with the keyring file, the source file to verify and the signature file, the command to use is:

$ gpg --verify --keyring ./gnu-keyring.gpg foo.tar.xz.sig

You have to search the public keyservers for the given key id: in your case ID C3C45C06Import the found key in your local keystore and after this the verification should be OK.I use Ubuntu 12.04 and it comes with Seahorse key management software. Before the key import I was seeing this:

~/Downloads$ gpg --verify --keyring ./gnu-keyring.gpg icecat-31.5.0.en-US.linux-x86_64.tar.bz2.sig icecat-31.5.0.en-US.linux-x86_64.tar.bz2gpg: Signature made  9.03.2015 (пн) 22,35,52 EET using RSA key ID D7E04784gpg: Can't check signature: public key not found

After the key import I was seeing this:

~/Downloads$ gpg --verify --keyring ./gnu-keyring.gpg icecat-31.5.0.en-US.linux-x86_64.tar.bz2.sig icecat-31.5.0.en-US.linux-x86_64.tar.bz2gpg: Signature made  9.03.2015 (пн) 22,35,52 EET using RSA key ID D7E04784gpg: Good signature from "Ruben Rodriguez (GNU IceCat releases key) <ruben@gnu.org>"gpg: WARNING: This key is not certified with a trusted signature!gpg:          There is no indication that the signature belongs to the owner.Primary key fingerprint: A573 69A8 BABC 2542 B5A0  368C 3C76 EED7 D7E0 4784