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.
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?
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)
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)
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) <firstname.lastname@example.org>" Real name:
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:
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 [...]
Option 3 is to generate an ElGamal keypair that is not usable for making signatures.