Chapter 1. Getting Started

Table of Contents
Generating a new keypair
Exchanging keys
Encrypting and decrypting documents
Making and verifying signatures

GnuPG is a tool for secure communication. This chapter is a quick-start guide that covers the core functionality of GnuPG. This includes keypair creation, exchanging and verifying keys, encrypting and decrypting documents, and authenticating documents with digital signatures. It does not explain in detail the concepts behind public-key cryptography, encryption, and digital signatures. This is covered in Chapter 2. It also does not explain how to use GnuPG wisely. This is covered in Chapters 3 and 4.

GnuPG uses public-key cryptography so that users may communicate securely. In a public-key system, each user has a pair of keys consisting of a private key and a public key. A user's private key is kept secret; it need never be revealed. The public key may be given to anyone with whom the user wants to communicate. GnuPG uses a somewhat more sophisticated scheme in which a user has a primary keypair and then zero or more additional subordinate keypairs. The primary and subordinate keypairs are bundled to facilitate key management and the bundle can often be considered simply as one keypair.

Generating a new keypair

The command-line option --gen-key is used to create a new primary keypair.

alice% gpg --gen-key
gpg (GnuPG) 0.9.4; Copyright (C) 1999 Free Software Foundation, Inc.
This program comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to redistribute it
under certain conditions. See the file COPYING for details.

Please select what kind of key you want:
   (1) DSA and ElGamal (default)
   (2) DSA (sign only)
   (4) ElGamal (sign and encrypt)
Your selection?
GnuPG is able to create several different types of keypairs, but a primary key must be capable of making signatures. There are therefore only three options. Option 1 actually creates two keypairs. A DSA keypair is the primary keypair usable only for making signatures. An ElGamal subordinate keypair is also created for encryption. Option 2 is similar but creates only a DSA keypair. Option 4[1] creates a single ElGamal keypair usable for both making signatures and performing encryption. In all cases it is possible to later add additional subkeys for encryption and signing. For most users the default option is fine.

You must also choose a key size. The size of a DSA key must be between 512 and 1024 bits, and an ElGamal key may be of any size. GnuPG, however, requires that keys be no smaller than 768 bits. Therefore, if Option 1 was chosen and you choose a keysize larger than 1024 bits, the ElGamal key will have the requested size, but the DSA key will be 1024 bits.

About to generate a new ELG-E keypair.
              minimum keysize is  768 bits
              default keysize is 1024 bits
    highest suggested keysize is 2048 bits
What keysize do you want? (1024)
The longer the key the more secure it is against brute-force attacks, but for almost all purposes the default keysize is adequate since it would be cheaper to circumvent the encryption than try to break it. Also, encryption and decryption will be slower as the key size is increased, and a larger keysize may affect signature length. Once selected, the keysize can never be changed.

Finally, you must choose an expiration date. If Option 1 was chosen, the expiration date will be used for both the ElGamal and DSA keypairs.

Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0) 
For most users a key that does not expire is adequate. The expiration time should be chosen with care, however, since although it is possible to change the expiration date after the key is created, it may be difficult to communicate a change to users who have your public key.

You must provide a user ID in addition to the key parameters. The user ID is used to associate the key being created with a real person.

You need a User-ID to identify your key; the software constructs the user id
from Real Name, Comment and Email Address in this form:
    "Heinrich Heine (Der Dichter) <>"

Real name: 
Only one user ID is created when a key is created, but it is possible to create additional user IDs if you want to use the key in two or more contexts, e.g., as an employee at work and a political activist on the side. A user ID should be created carefully since it cannot be edited after it is created.

GnuPG needs a passphrase to protect the primary and subordinate private keys that you keep in your possession.

You need a Passphrase to protect your private key.    

Enter passphrase: 
There is no limit on the length of a passphrase, and it should be carefully chosen. From the perspective of security, the passphrase to unlock the private key is one of the weakest points in GnuPG (and other public-key encryption systems as well) since it is the only protection you have if another individual gets your private key. Ideally, the passphrase should not use words from a dictionary and should mix the case of alphabetic characters as well as use non-alphabetic characters. A good passphrase is crucial to the secure use of GnuPG.

Generating a revocation certificate

After your keypair is created you should immediately generate a revocation certificate for the primary public key using the option --gen-revoke. If you forget your passphrase or if your private key is compromised or lost, this revocation certificate may be published to notify others that the public key should no longer be used. A revoked public key can still be used to verify signatures made by you in the past, but it cannot be used to encrypt future messages to you. It also does not affect your ability to decrypt messages sent to you in the past if you still do have access to the private key.

alice% gpg --output revoke.asc --gen-revoke mykey
The argument mykey must be a key specifier, either the key ID of your primary keypair or any part of a user ID that identifies your keypair. The generated certificate will be left in the file revoke.asc. If the --output option is omitted, the result will be placed on standard output. Since the certificate is short, you may wish to print a hardcopy of the certificate to store somewhere safe such as your safe deposit box. The certificate should not be stored where others can access it since anybody can publish the revocation certificate and render the corresponding public key useless.



Option 3 is to generate an ElGamal keypair that is not usable for making signatures.