what is the realtionship between pgp and gpg ?

GNU Privacy Guard (GnuPG or GPG) is a free software alternative to the PGP suite of cryptographic software.

man gpg

It is a tool to provide digital encryption and signing services

Note: check http://en.wikipedia.org/wiki/Asymmetric_key_algorithm to see what is digital encryption and signing

great doc

http://www.dewinter.com/gnupg_howto/english/GPGMiniHowto.html

test

pet@girl:~/xxx$ gpg -e -r peter dd
pet@girl:~/xxx$ gpg -d dd.gpg>ddddd

pet@girl:~$ tar cvf xxx.tar xxx
xxx/
xxx/dd.gpg
xxx/ddddd
pet@girl:~$ ls
dd  Desktop  edocs  github  kdocs  xxx  xxx.tar
pet@girl:~$ gpg  -r peter -e xxx.tar
pet@girl:~$ ls
dd  Desktop  edocs  github  kdocs  xxx  xxx.tar  xxx.tar.gpg
pet@girl:~$ mkdir ddddd
pet@girl:~$ mv xxx.tar.gpg  ddddd/
pet@girl:~$ cd ddddd/
pet@girl:~/ddddd$ ls
xxx.tar.gpg
pet@girl:~/ddddd$ gpg -d xxx.tar.gpg -o d.tar
usage: gpg [options] --decrypt [filename]
pet@girl:~/ddddd$ gpg -d xxx.tar.gpg >d.tar

You need a passphrase to unlock the secret key for
user: "peter king (hi) <g@g.com>"
2048-bit ELG-E key, ID 3EDF51F9, created 2009-11-23 (main key ID 51B133C3)

gpg: encrypted with 2048-bit ELG-E key, ID 3EDF51F9, created 2009-11-23
      "peter king (hi) <g@g.com>"
pet@girl:~/ddddd$ ls
d.tar  xxx.tar.gpg
pet@girl:~/ddddd$ tar xvf d.tar
xxx/
xxx/dd.gpg
xxx/ddddd
pet@girl:~/ddddd$ ls
d.tar  xxx  xxx.tar.gpg
pet@girl:~/ddddd$

seahorse

go with no key

pet@girl:~$ ls
dd  Desktop  edocs  github  kdocs
pet@girl:~$ gpg -c dd
pet@girl:~$ ls
dd  dd.gpg  Desktop  edocs  github  kdocs
pet@girl:~$ gpg -d dd.gpg
gpg: CAST5 encrypted data
gpg: encrypted with 1 passphrase
iii
gpg: WARNING: message was not integrity protected
pet@girl:~$ gpg --version
gpg (GnuPG) 1.4.9

below is to decrypt it in asianux with no key

[root@dhcp28 pet]# gpg -d dd.gpg
gpg: directory `/root/.gnupg' created
gpg: new configuration file `/root/.gnupg/gpg.conf' created
gpg: WARNING: options in `/root/.gnupg/gpg.conf' are not yet active during this run
gpg: keyring `/root/.gnupg/secring.gpg' created
gpg: keyring `/root/.gnupg/pubring.gpg' created
gpg: CAST5 encrypted data
gpg: encrypted with 1 passphrase
ehlool iii
gpg: WARNING: message was not integrity protected
[root@dhcp28 pet]# gpg --version
gpg (GnuPG) 1.4.5