$Id$ &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 making and verifying signatures. It does not explain in detail the concepts behind public-key cryptography, encryption, and digital signatures. This is covered in Chapter . It also does not explain how to use &gnupg; wisely. This is covered in Chapters and . &Gnupg; uses public-key cryptography so that users may communicate securely. In a public-key system, each user has a public/private keypair. 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? &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 4Option 3 is to generate an ElGamal keypair that is not usable for making signatures. 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) <heinrichh@duesseldorf.de>" 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, ⪚, 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;. 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. To communicate with others you must exchange public keys. To list the keys on your public keyring use the command-line option --list-keys. alice% gpg --list-keys /users/alice/.gnupg/pubring.gpg --------------------------------------- pub 1024D/BB7576AC 1999-06-04 Alice (Judge) <alice@cyb.org> sub 1024g/78E9A8FA 1999-06-04 To send your public key to a correspondent you must first export it. The command-line option --export is used to do this. It takes an additional argument identifying the public key to export. As with the --gen-revoke option, either the key ID or any part of the user ID may be used to identify the key to export. alice% gpg --output alice.gpg --export alice@cyb.org The key is exported in a binary format, but this can be inconvenient when the key is to be sent though email or published on a web page. &Gnupg; therefore supports a command-line option --armor Many command-line options that are frequently used can also be set in a configuration file. that that causes output to be generated in an ASCII-armored format similar to uuencoded documents. In general, any output from &gnupg;, ⪚, keys, encrypted documents, and signatures, can be ASCII-armored by adding the --armor option. alice% gpg --armor --export alice@cyb.org -----BEGIN PGP PUBLIC KEY BLOCK----- Version: GnuPG v0.9.7 (GNU/Linux) Comment: For info see http://www.gnupg.org [...] -----END PGP PUBLIC KEY BLOCK----- A public key may be added to your public keyring with the --import option. alice% gpg --import blake.gpg gpg: key 9E98BC16: public key imported gpg: Total number processed: 1 gpg: imported: 1 alice% gpg --list-keys /users/alice/.gnupg/pubring.gpg --------------------------------------- pub 1024D/BB7576AC 1999-06-04 Alice (Judge) <alice@cyb.org> sub 1024g/78E9A8FA 1999-06-04 pub 1024D/9E98BC16 1999-06-04 Blake (Executioner) <blake@cyb.org> sub 1024g/5C8CBD41 1999-06-04 Once a key is imported it should be validated. &Gnupg; uses a powerful and flexible trust model that does not require you to personally validate each key you import. Some keys may need to be personally validated, however. A key is validated by verifying the key's fingerprint and then signing the key to certify it as a valid key. A key's fingerprint can be quickly viewed with the --fingerprint command-line option, but in order to certify the key you must edit it. alice% gpg --edit-key blake@cyb.org pub 1024D/9E98BC16 created: 1999-06-04 expires: never trust: -/q sub 1024g/5C8CBD41 created: 1999-06-04 expires: never (1) Blake (Executioner) <blake@cyb.org> Command> fpr pub 1024D/9E98BC16 1999-06-04 Blake (Executioner) <blake@cyb.org> Fingerprint: 268F 448F CCD7 AF34 183E 52D8 9BDE 1A08 9E98 BC16 Key verification is a weak point in public-key cryptography, so you must be sure that the fingerprint is correct. The fingerprint displayed should be checked with the key's owner. This may be done in person or over the phone or through any other means as long as you can guarantee that you are communicating with the key's true owner. Once verified you may sign the key to validate it. Command> sign pub 1024D/9E98BC16 created: 1999-06-04 expires: never trust: -/q Fingerprint: 268F 448F CCD7 AF34 183E 52D8 9BDE 1A08 9E98 BC16 Blake (Executioner) <blake@cyb.org> Are you really sure that you want to sign this key with your key: "Alice (Judge) <alice@cyb.org>" Really sign? Once signed you can check the key to list the signatures on it and see the signature that you have added. Every user ID on the key will have one or more self-signatures as well as a signature for each user that has validated the key. Command> check uid Blake (Executioner) <blake@cyb.org> sig! 9E98BC16 1999-06-04 [self-signature] sig! BB7576AC 1999-06-04 Alice (Judge) <alice@cyb.org> To encrypt a document the option --encrypt is used. You must have the public keys of the intended recipients. The software expects the name of the document to encrypt as input or, if omitted, on standard input. The encrypted result is placed on standard output or as specified using the option --output. The document is compressed for additional security in addition to encrypting it. alice% gpg --output doc.gpg --encrypt --recipient blake@cyb.org doc The --recipient option is used once for each recipient and takes an extra argument specifying the public key to which the document should be encrypted. The encrypted document can only be decrypted by someone with a private key that complements one of the recipients' public keys. In particular, you cannot decrypt a document encrypted by you unless you included your own public key in the recipient list. To decrypt a message the option --decrypt is used. You need the private key to which the message was encrypted. Similar to the encryption process, the document to decrypt is input, and the decrypted result is output. blake% gpg --output doc --decrypt doc.gpg You need a passphrase to unlock the secret key for user: "Blake (Executioner) <blake@cyb.org>" 1024-bit ELG-E key, ID 5C8CBD41, created 1999-06-04 (main key ID 9E98BC16) Enter passphrase: Documents may also be encrypted without using public-key cryptography. Instead, only a symmetric cipher is used to encrypt the document. The key used to drive the symmetric cipher is derived from a passphrase supplied when the document is encrypted, and for good security, it should not be the same passphrase that you use to protect your private key. Symmetric encryption is useful for securing documents when the passphrase does not need to be communicated to others. A document can be encrypted with a symmetric cipher by using the --symmetric option. alice% gpg --output doc.gpg --symmetric doc Enter passphrase: A digital signature certifies and timestamps a document. If the document is subsequently modified in any way, a verification of the signature will fail. A digital signature can serve the same purpose as a hand-written signature with the additional benefit of being tamper-resistant. The &gnupg; source distribution, for example, is signed so that users can verify that the source code has not been modified since it was packaged. Creating and verifying signatures uses the public/private keypair in an operation different from encryption and decryption. A signature is created using the private key of the signer. The signature is verified using the corresponding public key. A consequence is that it is difficult to deny that you made a digital signature since that would imply your private key had been compromised. The command-line option --sign is used to make a digital signature. The document to sign is input, and the signed document is output. alice% gpg --output doc.sig --sign doc You need a passphrase to unlock the private key for user: "Alice (Judge) <alice@cyb.org>" 1024-bit DSA key, ID BB7576AC, created 1999-06-04 Enter passphrase: The document is compressed before signed, and the output is in binary format. Given a signed document, you can either check the signature or check the signature and recover the original document. To check the signature use the --verify option. To verify the signature and extract the document use the --decrypt option. The signed document to verify and recover is input and the recovered document is output. blake% gpg --output doc --decrypt doc.sig gpg: Signature made Fri Jun 4 12:02:38 1999 CDT using DSA key ID BB7576AC gpg: Good signature from "Alice (Judge) <alice@cyb.org>" A common use of digital signatures is to sign usenet postings or email messages. In such situations it is undesirable to compress the document while signing it. The option --clearsign causes the document to be wrapped in an ASCII-armored signature but otherwise does not modify the document. alice% gpg --clearsign doc You need a passphrase to unlock the secret key for user: "Alice (Judge) <alice@cyb.org>" 1024-bit DSA key, ID BB7576AC, created 1999-06-04 -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 [...] -----BEGIN PGP SIGNATURE----- Version: GnuPG v0.9.7 (GNU/Linux) Comment: For info see http://www.gnupg.org iEYEARECAAYFAjdYCQoACgkQJ9S6ULt1dqz6IwCfQ7wP6i/i8HhbcOSKF4ELyQB1 oCoAoOuqpRqEzr4kOkQqHRLE/b8/Rw2k =y6kj -----END PGP SIGNATURE----- A signed document has limited usefulness. Other users must recover the original document from the signed version, and even with clearsigned documents, the signed document must be edited to recover the original. Therefore, there is a third method for signing a document that creates a detached signature. A detached signature is created using the --detach-sig option. alice% gpg --output doc.sig --detach-sig doc You need a passphrase to unlock the secret key for user: "Alice (Judge) <alice@cyb.org>" 1024-bit DSA key, ID BB7576AC, created 1999-06-04 Enter passphrase: Both the document and detached signature are needed to verify the signature. The --verify option can be to check the signature. blake% gpg --verify doc.sig doc gpg: Signature made Fri Jun 4 12:38:46 1999 CDT using DSA key ID BB7576AC gpg: Good signature from "Alice (Judge) <alice@cyb.org>"