GNU Privacy Guard (GPG) is a tool to provide encryption and signing services for transferring data securely. This post is about generating a public-private key pair for encrypting and decrypting a message. GPG generates a public key of the size preferred by you for a given email id, and a pass-phrase which is the private key.
Generating the Key pair
It is very simple to generate a key pair in GPG. Follow the instructions
$ gpg --gen-key
Then you get the following
gpg (GnuPG) 1.4.11; Copyright (C) 2010 Free Software Foundation, Inc. This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Please select what kind of key you want: (1) RSA and RSA (default) (2) DSA and Elgamal (3) DSA (sign only) (4) RSA (sign only) Your selection? 1 [Select any option] RSA keys may be between 1024 and 4096 bits long. What keysize do you want? (2048) 2048 Requested keysize is 2048 bits Please specify how long the key should be valid. 0 = key does not expire = key expires in n days w = key expires in n weeks m = key expires in n months y = key expires in n years Key is valid for? (0) 2w [You can select any based on the time limit you want] Key expires at Wed 16 Jan 2013 05:22:13 PM IST Is this correct? (y/N) y You need a user ID to identify your key; the software constructs the user ID from the Real Name, Comment and Email Address in this form: "Heinrich Heine (Der Dichter) " Real name: Priya Email address: email@example.com Comment: (Trial Key Pair) Invalid character in comment Comment: Trial Key Pair You selected this USER-ID: "Priya (Trial Key Pair) " Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o You need a Passphrase to protect your secret key. gpg: problem with the agent - disabling agent use We need to generate a lot of random bytes. It is a good idea to perform some other action (type on the keyboard, move the mouse, utilize the disks) during the prime generation; this gives the random number generator a better chance to gain enough entropy. .....+++++ ....+++++ gpg: key 68A4DDFE marked as ultimately trusted public and secret key created and signed. gpg: checking the trustdb gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model gpg: depth: 0 valid: 3 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 3u gpg: next trustdb check due at 2013-01-04 pub 2048R/68A4DDFE 2013-01-02 [expires: 2013-01-16] Key fingerprint = E2C2 22B4 F94E 5569 F5FE 48D1 94E5 A4F4 68A4 DDFE uid Priya (Trial Key Pair) sub 2048R/370B0DB3 2013-01-02 [expires: 2013-01-16]
The pass-phrase you entered is your private key.
Exporting the public key
gpg --export firstname.lastname@example.org > priya1111-pub.gpg
This file will contain the public key. If you want to get the Public key in ascii, give
gpg --armor --export email@example.com > priya1111-pub-asc.gpg
Importing Public Key
gpg --import firstname.lastname@example.org <filename>
Encrypting a file
Consider you want to encrypt the file sun.txt and send it to Priya
Give the command as follows
$ gpg --recipient priya_key --armor --encrypt sun.txt
Here priya-key contains the public key of email@example.com. The above commands generates a file
sun.txt.asc which is the encrypted file. You can send this file through gmail securely as it
is in the encrypted format
Decrypting a file
You can decrypt the file with your private key.
gpg --decrypt sun.txt.asc > sun.txt
You will get the decrypted file.
There are ways to add signature to your file using the public key. You can even add photo ID as a part of the signature. You can refer the following link if interested.