$Id$ &Gnupg; is a complex tool with technical, social, and legal issues surrounding it. Technically, it has been designed to be used in situations having drastically different security needs. This complicates key management. Socially, using &gnupg; is not strictly a personal decision. To use &gnupg effectively both parties communicating must use it. Finally, as of 1999, laws regarding digital encryption, and in particular whether or not using &gnupg; is legal, vary from country to country and is currently being debated by many national governments. This chapter addresses these issues. It gives practical advice on how to use &gnupg; to meet your security needs. It also suggests ways to promote the use of &gnupg; for secure communication between yourself and your colleagues when your colleagues are not currently using &gnupg;. Finally, the legal status of &gnupg; is outlined given the current status of encryption laws in the world. &Gnupg; is a tool you use to protect your privacy. Your privacy is protected if you can correspond with others without eavesdroppers reading those messages. How you should use &gnupg; depends on the determination and resourcefulness of those who might want to read your encrypted messages. An eavesdropper may be an unscrupulous system administrator casually scanning your mail, it might be an industrial spy trying to collect your company's secrets, or it might be a law enforcement agency trying to prosecute you. Using &gnupg; to protect against casual eavesdropping is going to be different than using &gnupg; to protect against a determined adversary. Your goal, ultimately, is to make it more expensive to recover the unencrypted data than that data is worth. Customizing your use of &gnupg; revolves around three issues: the key size of your public/private keypair, protecting your private key, and managing your web of trust. A well-chosen key size protects you against brute-force attacks on encrypted messages. Protecting your private key prevents an attacker from simply using your private key to decrypt encrypted messages and sign messages in your name. Correctly managing your web of trust prevents attackers from masquarading as people with whom you communicate. Ultimately, addressing these issues with respect to your own security needs is how you balance the extra work required to use &gnupg; with the privacy it gives you. Selecting a key size depends on the key. In OpenPGP, a public/private keypair usually has multiple keys. At the least it has a master signing key, and it probably has one or more additional subkeys for encryption. Using default key generation parameters with &gnupg;, the master key will be a DSA key, and the subkeys will be ElGamal keys. DSA allows a key size up to 1024 bits. This is not especially good given today's factoring technology, but that is what the standard specifies. Without question, you should use 1024 bit DSA keys. ElGamal keys, on the other hand, may be of any size. Since &gnupg; is a hybrid public-key system, the public key is used to encrypt a 128-bit session key, and the private key is used to decrypt it. Key size nevertheless affects encryption and decryption speed since the cost of these algorithms is exponential in the size of the key. Larger keys also take more time to generate and take more space to store. Ultimately, there are diminishing returns on the extra security a large key provides you. After all, if the key is large enough to resist a brute-force attack, an eavesdropper will merely switch to some other method for obtaining your plaintext data. Examples of other methods include robbing your home or office and mugging you. 1024 bits is thus the recommended key size. If you genuinely need a larger key size then you probably already know this and should be consulting an expert in data security. Protecting your private key is the most important job you have to use &gnupg; correctly. If someone obtains your private key, then all data encrypted to the private key can be decrypted and signatures can be made in your name. If you lose your private key, then you will no longer be able to decrypt documents encrypted to you in the future or in the past, and you will not be able to make signatures. Losing sole possession of your private key is catastrophic. Regardless of how you use &gnupg; you should store the public key's revocation certificate and a backup of your private key on write-protected media in a safe place. For example, you could burn them on a CD-ROM and store them in your safe deposit box at the bank in a sealed envelope. Alternatively, you could store them on a floppy and hide it in your house. Whatever you do, they should be put on media that is safe to store for as long as you expect to keep the key, and you should store them more carefully than the copy of your private key you use daily. To help safeguard your key, &Gnupg; does not store your raw private key on disk. Instead it encrypts it using a symmetric encryption algorithm. That is why you need a passphrase to access the key. Thus there are two barriers an attacker must cross to access your private key: (1) he must actually acquire the key, and (2) he must get past the encryption. Safely storing your private key is important, but there is a cost. Ideally, you would keep the private key on a removable, write-protected disk such as a floppy disk, and you would use it on a single-user machine not connected to a network. This may be inconvenient or impossible for you to do. For example, you may not own your own machine and must use a computer at work or school, or it may mean you have to physically disconnect your computer from your cable modem every time you want to use &gnupg; This does not mean you cannot or should not use &gnupg;. It means only that you have decided that the data you are protecting is important enough to encrypt but not so important as to take extra steps to make the first barrier stronger. It is your choice. A good passphrase is absolutely critical when using &gnupg;. Any attacker who gains access to your private key must bypass the encryption on the private key. Instead of brute-force guessing the key, an attacker will almost certainly instead try to guess the passphrase. The motivation for trying passphrases is that most people choose a passphrase that is easier to guess than a random 128-bit key. If the passphrase is a word, it is much cheaper to try all the words in the dictionaries of the world's languages. Even if the word is permuted, &eg, k3wldood, it is still easier to try dictionary words with a catalog of permutations. The same problem applies to quotations. In general, passphrases based on natural-language utterances are poor passphrases since there is little randomness and lots of redundancy in natural language. You should avoid natural language passphrases if you can. A good passphrase is one that you can remember but is hard for someone to guess. It should include characters from the whole range of printable characters on your keyboard. This includes uppercase alphabetics characters, numbers, and special characters such as } and |. Be creative and spend a little time considering your passphrase; a good choice is important to ensure your privacy. As with protecting your private key, managing your web of trust is another aspect of using &gnupg; that requires balancing security against ease of use. If you are using &gnupg; to protect against casual eavesdropping and forgeries then you can afford to be relatively trusting of other people's signatures. On the other hand, if you are concerned that there may be a determined attacker interested in invading your privacy, then you should be much less trusting of other signatures and spend more time personally verifying signatures. Regardless of your own security needs, through, you should always be careful when signing other keys. It is selfish to sign a key with just enough confidence in the key's validity to satisfy your own security needs. Others, with more stringent security needs, may want to depend on your signature. If they cannot depend on you then that weakens the web of trust and makes it more difficult for all &gnupg; users to communicate. Use the same care in signing keys that you would like others to use when you depend on their signatures. In practice, managing your web of trust reduces to assigning trust to others and tuning the options --marginals-needed and --completes-needed. Any key you personally sign will be considered valid, but except for small groups, it will not be practical to personally sign the key of every person with whom you communicate. You will therefore have to assign trust to others. It is probably wise to be accurate when assigning trust and then use the options to tune how careful &gnupg; is with key validation. As a concrete example, you may fully trust a few close friends that you know are careful with key signing and then marginally trust all others on your keyring. From there, you may set --completes-needed to 1 and --marginals-needed to 2. If you are more concerned with security you might choose values of 1 and 3 or 2 and 3 respectively. If you are less concerned with privacy attacks and just want some reasonable confidence about validity, set the values to 1 and 1. In general, higher numbers for these options imply that more people would be needed to conspire against you in order to have a key validated that does not actually belong to the person whom you think it does. Wanting to use &gnupg; yourself is not enough. In order to use to communicate securely with others you must have a web of trust. At first glance, however, building a web of trust is a daunting task. The people with whom you communicate need to use &gnupg;In this section, &gnupg; refers to the &gnupg; implementation of OpenPGP as well as other implementations such as NAI's PGP product., and there needs to be enough key signing so that keys can be considered valid. These are not technical problems; they are social problems. Nevertheless, you must overcome these problems if you want to use &gnupg;. When getting started using &gnupg; it is important to realize that you need not securely communicate with every one of your correspondents. Start with a small circle of people, perhaps just yourself and one or two others who also want to exercise their right to privacy. Generate your keys and sign each other's public keys. This is your initial web of trust. By doing this you will appreciate the value of a small, robust web of trust and will be more cautious as you grow your web in the future. In addition to those in your initial web of trust, you may want to communicate securely with others who are also using &gnupg;. Doing so, however, can be awkward for two reasons: (1) you do not always know when someone uses or is willing to use &gnupg;, and (2) if you do know of someone who uses it, you may still have trouble validating their key. The first reason occurs because people do not always advertise that they use &gnupg;. The way to change this behavior is to set the example and advertise that you use &gnupg;. There are at least three ways to do this: you can sign messages you mail to others or post to message boards, you can put your public key on your web page, or, if you put your key on a keyserver, you can put your key ID in your email signature. If you advertise your key then you make it that much more acceptable for others to advertise their keys. Furthermore, you make it easier for others to start communicating with you securely since you have taken the initiative and made it clear that you use &gnupg;. Key validation is more difficult. If you do not personally know the person whose key you want to sign, then it is not possible to sign the key yourself. You must rely on the signatures of others and hope to find a chain of signatures leading from the key in question back to your own. To have any chance of finding a chain, you must take the intitive and get your key signed by others outside of your intitial web of trust. An effective way to accomplish this is to participate in key signing parties. If you are going to a conference look ahead of time for a key signing party, and if you do not see one being held, offer to hold one. You can also be more passive and carry your fingerprint with you for impromptu key exchanges. In such a situation the person to whom you gave the fingerprint would verify it and sign your public key once he returned home. Keep in mind, though, that this is optional. You have no obligation to either publically advertise your key or sign other people's keys. The power of &gnupg; is that it is flexible enough to adapt to your security needs whatever they may be. The social reality, however, is that you will need to take the initiative if you want to grow your web of trust and use &gnupg; for as much of your communication as possible. The legal status of encryption software varies from country to country, and law regarding encryption software is rapidly evolving. Bert-Japp Koops has an excellent Crypto Law Survey to which you should refer for the legal status of encryption software in your country.