The GNU Privacy Handbook
========================

  Copyright © 1999 by The Free Software Foundation

  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.1 or
  any later version published by the Free Software Foundation; with no
  Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
  copy of the license is included in the section entitled "GNU Free
  Documentation License".

  Please direct questions, bug reports, or suggestions concerning this
  manual to the maintainer, Mike Ashley (<[email protected]>). When
  referring to the manual please specify which version of the manual you
  have by using this version string: $Name: v1_1 $.

  Contributors to this manual include Matthew Copeland, Joergen Grahn,
  and David A. Wheeler. J Horacio MG has translated the manual to
  Spanish.

  From: <https://www.gnupg.org/gph/en/manual.html>

  Table of Contents
  =================

  1. Getting Started
       Generating a new keypair
             Generating a revocation certificate
       Exchanging keys
             Exporting a public key
             Importing a public key
       Encrypting and decrypting documents
       Making and verifying signatures
             Clearsigned documents
             Detached signatures
  2. Concepts
       Symmetric ciphers
       Public-key ciphers
       Hybrid ciphers
       Digital signatures
  3. Key Management
       Managing your own keypair
             Key integrity
             Adding and deleting key components
             Revoking key components
             Updating a key's expiration time
       Validating other keys on your public keyring
             Trust in a key's owner
             Using trust to validate keys
       Distributing keys
  4. Daily use of GnuPG
       Defining your security needs
             Choosing a key size
             Protecting your private key
             Selecting expiration dates and using subkeys
             Managing your web of trust
       Building your web of trust
       Using GnuPG legally
  5. Topics
       Writing user interfaces
  A. GNU Free Documentation License
       0. PREAMBLE
       1. APPLICABILITY AND DEFINITIONS
       2. VERBATIM COPYING
       3. COPYING IN QUANTITY
       4. MODIFICATIONS
       5. COMBINING DOCUMENTS
       6. COLLECTIONS OF DOCUMENTS
       7. AGGREGATION WITH INDEPENDENT WORKS
       8. TRANSLATION
       9. TERMINATION
       10. FUTURE REVISIONS OF THIS LICENSE
       How to use this License for your documents

  List of Figures
  3-1. A hypothetical web of trust

Chapter 1. Getting Started
==========================

  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 [A] 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) <[email protected]>"

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.

Exchanging keys
===============

  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) <[email protected]>
sub  1024g/78E9A8FA 1999-06-04

Exporting a public key
======================

  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 [email protected]

  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 [B] that
  causes output to be generated in an ASCII-armored format similar to
  uuencoded documents. In general, any output from GnuPG, e.g., keys,
  encrypted documents, and signatures, can be ASCII-armored by adding the
  --armor option.

alice% gpg --armor --export [email protected]
-----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-----

Importing a public key
======================

  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) <[email protected]>
sub  1024g/78E9A8FA 1999-06-04

pub  1024D/9E98BC16 1999-06-04 Blake (Executioner) <[email protected]>
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 [email protected]

pub  1024D/9E98BC16  created: 1999-06-04 expires: never      trust: -/q
sub  1024g/5C8CBD41  created: 1999-06-04 expires: never
(1)  Blake (Executioner) <[email protected]>

Command> fpr
pub  1024D/9E98BC16 1999-06-04 Blake (Executioner) <[email protected]>
            Fingerprint: 268F 448F CCD7 AF34 183E  52D8 9BDE 1A08 9E98 BC16

  A key's fingerprint is verified 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. If
  the fingerprint you get is the same as the fingerprint the key's owner
  gets, then you can be sure that you have a correct copy of the key.

  After checking the fingerprint, you may sign the key to validate it.
  Since key verification is a weak point in public-key cryptography, you
  should be extremely careful and always check a key's fingerprint with
  the owner before signing the key.

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) <[email protected]>

Are you really sure that you want to sign this key
with your key: "Alice (Judge) <[email protected]>"

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) <[email protected]>
sig!       9E98BC16 1999-06-04   [self-signature]
sig!       BB7576AC 1999-06-04   Alice (Judge) <[email protected]>

Encrypting and decrypting documents
===================================

  A public and private key each have a specific role when encrypting and
  decrypting documents. A public key may be thought of as an open safe.
  When a correspondent encrypts a document using a public key, that
  document is put in the safe, the safe shut, and the combination lock
  spun several times. The corresponding private key is the combination
  that can reopen the safe and retrieve the document. In other words,
  only the person who holds the private key can recover a document
  encrypted using the associated public key.

  The procedure for encrypting and decrypting documents is
  straightforward with this mental model. If you want to encrypt a
  message to Alice, you encrypt it using Alice's public key, and she
  decrypts it with her private key. If Alice wants to send you a message,
  she encrypts it using your public key, and you decrypt it with your
  private key.

  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; if omitted, it reads 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 [email protected] 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) <[email protected]>"
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, you use a symmetric cipher 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:

Making and verifying signatures
===============================

  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. For example, Alice would use her
  own private key to digitally sign her latest submission to the Journal
  of Inorganic Chemistry. The associate editor handling her submission
  would use Alice's public key to check the signature to verify that the
  submission indeed came from Alice and that it had not been modified
  since Alice sent it. A consequence of using digital signatures 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) <[email protected]>"
1024-bit DSA key, ID BB7576AC, created 1999-06-04

Enter passphrase:

  The document is compressed before being 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) <[email protected]>"

Clearsigned documents
=====================

  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) <[email protected]>"
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-----

Detached signatures
===================

  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, which is a separate file. 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) <[email protected]>"
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) <[email protected]>"

Chapter 2. Concepts
===================

  GnuPG makes uses of several cryptographic concepts including symmetric
  ciphers, public-key ciphers, and one-way hashing. You can make basic
  use GnuPG without fully understanding these concepts, but in order to
  use it wisely some understanding of them is necessary.

  This chapter introduces the basic cryptographic concepts used in GnuPG.
  Other books cover these topics in much more detail. A good book with
  which to pursue further study is [56]Bruce Schneier's [57] "Applied
  Cryptography".

Symmetric ciphers
=================

  A symmetric cipher is a cipher that uses the same key for both
  encryption and decryption. Two parties communicating using a symmetric
  cipher must agree on the key beforehand. Once they agree, the sender
  encrypts a message using the key, sends it to the receiver, and the
  receiver decrypts the message using the key. As an example, the German
  Enigma is a symmetric cipher, and daily keys were distributed as code
  books. Each day, a sending or receiving radio operator would consult
  his copy of the code book to find the day's key. Radio traffic for that
  day was then encrypted and decrypted using the day's key. Modern
  examples of symmetric ciphers include 3DES, Blowfish, and IDEA.

  A good cipher puts all the security in the key and none in the
  algorithm. In other words, it should be no help to an attacker if he
  knows which cipher is being used. Only if he obtains the key would
  knowledge of the algorithm be needed. The ciphers used in GnuPG have
  this property.

  Since all the security is in the key, then it is important that it be
  very difficult to guess the key. In other words, the set of possible
  keys, i.e., the key space, needs to be large. While at Los Alamos,
  Richard Feynman was famous for his ability to crack safes. To encourage
  the mystique he even carried around a set of tools including an old
  stethoscope. In reality, he used a variety of tricks to reduce the
  number of combinations he had to try to a small number and then simply
  guessed until he found the right combination. In other words, he
  reduced the size of the key space.

  Britain used machines to guess keys during World War 2. The German
  Enigma had a very large key space, but the British built specialized
  computing engines, the Bombes, to mechanically try keys until the day's
  key was found. This meant that sometimes they found the day's key
  within hours of the new key's use, but it also meant that on some days
  they never did find the right key. The Bombes were not general-purpose
  computers but were precursors to modern-day computers.

  Today, computers can guess keys very quickly, and this is why key size
  is important in modern cryptosystems. The cipher DES uses a 56-bit key,
  which means that there are 2^56 possible keys. 2^56 is
  72,057,594,037,927,936 keys. This is a lot of keys, but a
  general-purpose computer can check the entire key space in a matter of
  days. A specialized computer can check it in hours. On the other hand,
  more recently designed ciphers such as 3DES, Blowfish, and IDEA all use
  128-bit keys, which means there are 2^128 possible keys. This is many,
  many more keys, and even if all the computers on the planet cooperated,
  it could still take more time than the age of the universe to find the
  key.

Public-key ciphers
==================

  The primary problem with symmetric ciphers is not their security but
  with key exchange. Once the sender and receiver have exchanged keys,
  that key can be used to securely communicate, but what secure
  communication channel was used to communicate the key itself? In
  particular, it would probably be much easier for an attacker to work to
  intercept the key than it is to try all the keys in the key space.
  Another problem is the number of keys needed. If there are n people who
  need to communicate, then n(n-1)/2 keys are needed for each pair of
  people to communicate privately. This may be OK for a small number of
  people but quickly becomes unwieldy for large groups of people.

  Public-key ciphers were invented to avoid the key-exchange problem
  entirely. A public-key cipher uses a pair of keys for sending messages.
  The two keys belong to the person receiving the message. One key is a
  public key and may be given to anybody. The other key is a private key
  and is kept secret by the owner. A sender encrypts a message using the
  public key and once encrypted, only the private key may be used to
  decrypt it.

  This protocol solves the key-exchange problem inherent with symmetric
  ciphers. There is no need for the sender and receiver to agree upon a
  key. All that is required is that some time before secret communication
  the sender gets a copy of the receiver's public key. Furthermore, the
  one public key can be used by anybody wishing to communicate with the
  receiver. So only n keypairs are needed for n people to communicate
  secretly with one another.

  Public-key ciphers are based on one-way trapdoor functions. A one-way
  function is a function that is easy to compute, but the inverse is hard
  to compute. For example, it is easy to multiply two prime numbers
  together to get a composite, but it is difficult to factor a composite
  into its prime components. A one-way trapdoor function is similar, but
  it has a trapdoor. That is, if some piece of information is known, it
  becomes easy to compute the inverse. For example, if you have a number
  made of two prime factors, then knowing one of the factors makes it
  easy to compute the second. Given a public-key cipher based on prime
  factorization, the public key contains a composite number made from two
  large prime factors, and the encryption algorithm uses that composite
  to encrypt the message. The algorithm to decrypt the message requires
  knowing the prime factors, so decryption is easy if you have the
  private key containing one of the factors but extremely difficult if
  you do not have it.

  As with good symmetric ciphers, with a good public-key cipher all of
  the security rests with the key. Therefore, key size is a measure of
  the system's security, but one cannot compare the size of a symmetric
  cipher key and a public-key cipher key as a measure of their relative
  security. In a brute-force attack on a symmetric cipher with a key size
  of 80 bits, the attacker must enumerate up to 2^80 keys to find the
  right key. In a brute-force attack on a public-key cipher with a key
  size of 512 bits, the attacker must factor a composite number encoded
  in 512 bits (up to 155 decimal digits). The workload for the attacker
  is fundamentally different depending on the cipher he is attacking.
  While 128 bits is sufficient for symmetric ciphers, given today's
  factoring technology public keys with 1024 bits are recommended for
  most purposes.

Hybrid ciphers
==============

  Public-key ciphers are no panacea. Many symmetric ciphers are stronger
  from a security standpoint, and public-key encryption and decryption
  are more expensive than the corresponding operations in symmetric
  systems. Public-key ciphers are nevertheless an effective tool for
  distributing symmetric cipher keys, and that is how they are used in
  hybrid cipher systems.

  A hybrid cipher uses both a symmetric cipher and a public-key cipher.
  It works by using a public-key cipher to share a key for the symmetric
  cipher. The actual message being sent is then encrypted using the key
  and sent to the recipient. Since symmetric key sharing is secure, the
  symmetric key used is different for each message sent. Hence it is
  sometimes called a session key.

  Both PGP and GnuPG use hybrid ciphers. The session key, encrypted using
  the public-key cipher, and the message being sent, encrypted with the
  symmetric cipher, are automatically combined in one package. The
  recipient uses his private-key to decrypt the session key and the
  session key is then used to decrypt the message.

  A hybrid cipher is no stronger than the public-key cipher or symmetric
  cipher it uses, whichever is weaker. In PGP and GnuPG, the public-key
  cipher is probably the weaker of the pair. Fortunately, however, if an
  attacker could decrypt a session key it would only be useful for
  reading the one message encrypted with that session key. The attacker
  would have to start over and decrypt another session key in order to
  read any other message.

Digital signatures
==================

  A hash function is a many-to-one function that maps its input to a
  value in a finite set. Typically this set is a range of natural
  numbers. A simple hash function is f(x) = 0 for all integers x. A more
  interesting hash function is f(x) = x mod 37, which maps x to the
  remainder of dividing x by 37.

  A document's digital signature is the result of applying a hash
  function to the document. To be useful, however, the hash function
  needs to satisfy two important properties. First, it should be hard to
  find two documents that hash to the same value. Second, given a hash
  value it should be hard to recover the document that produced that
  value.

  Some public-key ciphers [C] could be used to sign documents. The
  signer encrypts the document with his private key. Anybody wishing to
  check the signature and see the document simply uses the signer's
  public key to decrypt the document. This algorithm does satisfy the two
  properties needed from a good hash function, but in practice, this
  algorithm is too slow to be useful.

  An alternative is to use hash functions designed to satisfy these two
  important properties. SHA and MD5 are examples of such algorithms.
  Using such an algorithm, a document is signed by hashing it, and the
  hash value is the signature. Another person can check the signature by
  also hashing their copy of the document and comparing the hash value
  they get with the hash value of the original document. If they match,
  it is almost certain that the documents are identical.

  Of course, the problem now is using a hash function for digital
  signatures without permitting an attacker to interfere with signature
  checking. If the document and signature are sent unencrypted, an
  attacker could modify the document and generate a corresponding
  signature without the recipient's knowledge. If only the document is
  encrypted, an attacker could tamper with the signature and cause a
  signature check to fail. A third option is to use a hybrid public-key
  encryption to encrypt both the signature and document. The signer uses
  his private key, and anybody can use his public key to check the
  signature and document. This sounds good but is actually nonsense. If
  this algorithm truly secured the document it would also secure it from
  tampering and there would be no need for the signature. The more
  serious problem, however, is that this does not protect either the
  signature or document from tampering. With this algorithm, only the
  session key for the symmetric cipher is encrypted using the signer's
  private key. Anybody can use the public key to recover the session key.
  Therefore, it is straightforward for an attacker to recover the session
  key and use it to encrypt substitute documents and signatures to send
  to others in the sender's name.

  An algorithm that does work is to use a public key algorithm to encrypt
  only the signature. In particular, the hash value is encrypted using
  the signer's private key, and anybody can check the signature using the
  public key. The signed document can be sent using any other encryption
  algorithm including none if it is a public document. If the document is
  modified the signature check will fail, but this is precisely what the
  signature check is supposed to catch. The Digital Signature Standard
  (DSA) is a public key signature algorithm that works as just described.
  DSA is the primary signing algorithm used in GnuPG.

Chapter 3. Key Management
=========================

  Key tampering is a major security weakness with public-key
  cryptography. An eavesdropper may tamper with a user's keyrings or
  forge a user's public key and post it for others to download and use.
  For example, suppose Chloe wants to monitor the messages that Alice
  sends to Blake. She could mount what is called a man in the middle
  attack. In this attack, Chloe creates a new public/private keypair. She
  replaces Alice's copy of Blake's public key with the new public key.
  She then intercepts the messages that Alice sends to Blake. For each
  intercept, she decrypts it using the new private key, reencrypts it
  using Blake's true public key, and forwards the reencrypted message to
  Blake. All messages sent from Alice to Blake can now be read by Chloe.

  Good key management is crucial in order to ensure not just the
  integrity of your keyrings but the integrity of other users' keyrings
  as well. The core of key management in GnuPG is the notion of signing
  keys. Key signing has two main purposes: it permits you to detect
  tampering on your keyring, and it allows you to certify that a key
  truly belongs to the person named by a user ID on the key. Key
  signatures are also used in a scheme known as the web of trust to
  extend certification to keys not directly signed by you but signed by
  others you trust. Responsible users who practice good key management
  can defeat key tampering as a practical attack on secure communication
  with GnuPG.

Managing your own keypair
=========================

  A keypair has a public key and a private key. A public key consists of
  the public portion of the master signing key, the public portions of
  the subordinate signing and encryption subkeys, and a set of user IDs
  used to associate the public key with a real person. Each piece has
  data about itself. For a key, this data includes its ID, when it was
  created, when it will expire, etc. For a user ID, this data includes
  the name of the real person it identifies, an optional comment, and an
  email address. The structure of the private key is similar, except that
  it contains only the private portions of the keys, and there is no user
  ID information.

  The command-line option --edit-key may be used to view a keypair. For
  example,

chloe% gpg --edit-key [email protected]
Secret key is available.

pub  1024D/26B6AAE1  created: 1999-06-15 expires: never      trust: -/u
sub  2048g/0CF8CB7A  created: 1999-06-15 expires: never
sub  1792G/08224617  created: 1999-06-15 expires: 2002-06-14
sub   960D/B1F423E7  created: 1999-06-15 expires: 2002-06-14
(1)  Chloe (Jester) <[email protected]>
(2)  Chloe (Plebian) <[email protected]>
Command>

  The public key is displayed along with an indication of whether or not
  the private key is available. Information about each component of the
  public key is then listed. The first column indicates the type of the
  key. The keyword pub identifies the public master signing key, and the
  keyword sub identifies a public subordinate key. The second column
  indicates the key's bit length, type, and ID. The type is D for a DSA
  key, g for an encryption-only ElGamal key, and G for an ElGamal key
  that may be used for both encryption and signing. The creation date and
  expiration date are given in columns three and four. The user IDs are
  listed following the keys.

  More information about the key can be obtained with interactive
  commands. The command toggle switches between the public and private
  components of a keypair if indeed both components are available.

Command> toggle

sec  1024D/26B6AAE1  created: 1999-06-15 expires: never
sbb  2048g/0CF8CB7A  created: 1999-06-15 expires: never
sbb  1792G/08224617  created: 1999-06-15 expires: 2002-06-14
sbb   960D/B1F423E7  created: 1999-06-15 expires: 2002-06-14
(1)  Chloe (Jester) <[email protected]>
(2)  Chloe (Plebian) <[email protected]>

  The information provided is similar to the listing for the public-key
  component. The keyword sec identifies the private master signing key,
  and the keyword sbb identifies the private subordinates keys. The user
  IDs from the public key are also listed for convenience.

Key integrity
=============

  When you distribute your public key, you are distributing the public
  components of your master and subordinate keys as well as the user IDs.
  Distributing this material alone, however, is a security risk since it
  is possible for an attacker to tamper with the key. The public key can
  be modified by adding or substituting keys, or by adding or changing
  user IDs. By tampering with a user ID, the attacker could change the
  user ID's email address to have email redirected to himself. By
  changing one of the encryption keys, the attacker would also be able to
  decrypt the messages redirected to him.

  Using digital signatures is a solution to this problem. When data is
  signed by a private key, the corresponding public key is bound to the
  signed data. In other words, only the corresponding public key can be
  used to verify the signature and ensure that the data has not been
  modified. A public key can be protected from tampering by using its
  corresponding private master key to sign the public key components and
  user IDs, thus binding the components to the public master key. Signing
  public key components with the corresponding private master signing key
  is called self-signing, and a public key that has self-signed user IDs
  bound to it is called a certificate.

  As an example, Chloe has two user IDs and three subkeys. The signatures
  on the user IDs can be checked with the command check from the key edit
  menu.

chloe% gpg --edit-key chloe
Secret key is available.

pub  1024D/26B6AAE1  created: 1999-06-15 expires: never      trust: -/u
sub  2048g/0CF8CB7A  created: 1999-06-15 expires: never
sub  1792G/08224617  created: 1999-06-15 expires: 2002-06-14
sub   960D/B1F423E7  created: 1999-06-15 expires: 2002-06-14
(1)  Chloe (Jester) <[email protected]>
(2)  Chloe (Plebian) <[email protected]>

Command> check
uid  Chloe (Jester) <[email protected]>
sig!       26B6AAE1 1999-06-15   [self-signature]
uid  Chloe (Plebian) <[email protected]>
sig!       26B6AAE1 1999-06-15   [self-signature]

  As expected, the signing key for each signature is the master signing
  key with key ID 0x26B6AAE1. The self-signatures on the subkeys are
  present in the public key, but they are not shown by the GnuPG
  interface.

Adding and deleting key components
==================================

  Both new subkeys and new user IDs may be added to your keypair after it
  has been created. A user ID is added using the command adduid. You are
  prompted for a real name, email address, and comment just as when you
  create an initial keypair. A subkey is added using the command addkey.
  The interface is similar to the interface used when creating an initial
  keypair. The subkey may be a DSA signing key, and encrypt-only ElGamal
  key, or a sign-and-encrypt ElGamal key. When a subkey or user ID is
  generated it is self-signed using your master signing key, which is why
  you must supply your passphrase when the key is generated.

  Additional user IDs are useful when you need multiple identities. For
  example, you may have an identity for your job and an identity for your
  work as a political activist. Coworkers will know you by your work user
  ID. Coactivists will know you by your activist user ID. Since those
  groups of people may not overlap, though, each group may not trust the
  other user ID. Both user IDs are therefore necessary.

  Additional subkeys are also useful. The user IDs associated with your
  public master key are validated by the people with whom you
  communicate, and changing the master key therefore requires
  recertification. This may be difficult and time consuming if you
  communicate with many people. On the other hand, it is good to
  periodically change encryption subkeys. If a key is broken, all the
  data encrypted with that key will be vulnerable. By changing keys,
  however, only the data encrypted with the one broken key will be
  revealed.

  Subkeys and user IDs may also be deleted. To delete a subkey or user ID
  you must first select it using the key or uid commands respectively.
  These commands are toggles. For example, the command key 2 selects the
  second subkey, and invoking key 2 again deselects it. If no extra
  argument is given, all subkeys or user IDs are deselected. Once the
  user IDs to be deleted are selected, the command deluid actually
  deletes the user IDs from your key. Similarly, the command delkey
  deletes all selected subkeys from both your public and private keys.

  For local keyring management, deleting key components is a good way to
  trim other people's public keys of unnecessary material. Deleting user
  IDs and subkeys on your own key, however, is not always wise since it
  complicates key distribution. By default, when a user imports your
  updated public key it will be merged with the old copy of your public
  key on his ring if it exists. The components from both keys are
  combined in the merge, and this effectively restores any components you
  deleted. To properly update the key, the user must first delete the old
  version of your key and then import the new version. This puts an extra
  burden on the people with whom you communicate. Furthermore, if you
  send your key to a keyserver, the merge will happen regardless, and
  anybody who downloads your key from a keyserver will never see your key
  with components deleted. Consequently, for updating your own key it is
  better to revoke key components instead of deleting them.

Revoking key components
=======================

  To revoke a subkey it must be selected. Once selected it may be revoked
  with the revkey command. The key is revoked by adding a revocation
  self-signature to the key. Unlike the command-line option --gen-revoke,
  the effect of revoking a subkey is immediate.

Command> revkey
Do you really want to revoke this key? y

You need a passphrase to unlock the secret key for
user: "Chloe (Jester) <[email protected]>"
1024-bit DSA key, ID B87DBA93, created 1999-06-28


pub  1024D/B87DBA93  created: 1999-06-28 expires: never      trust: -/u
sub  2048g/B7934539  created: 1999-06-28 expires: never
sub  1792G/4E3160AD  created: 1999-06-29 expires: 2000-06-28
rev! subkey has been revoked: 1999-06-29
sub   960D/E1F56448  created: 1999-06-29 expires: 2000-06-28
(1)  Chloe (Jester) <[email protected]>
(2)  Chloe (Plebian) <[email protected]>

  A user ID is revoked differently. Normally, a user ID collects
  signatures that attest that the user ID describes the person who
  actually owns the associated key. In theory, a user ID describes a
  person forever, since that person will never change. In practice,
  though, elements of the user ID such as the email address and comment
  may change over time, thus invalidating the user ID.

  The OpenPGP specification does not support user ID revocation, but a
  user ID can effectively be revoked by revoking the self-signature on
  the user ID. For the security reasons described previously,
  correspondents will not trust a user ID with no valid self-signature.

  A signature is revoked by using the command revsig. Since you may have
  signed any number of user IDs, the user interface prompts you to decide
  for each signature whether or not to revoke it.

Command> revsig
You have signed these user IDs:
    Chloe (Jester) <[email protected]>
  signed by B87DBA93 at 1999-06-28
    Chloe (Plebian) <[email protected]>
  signed by B87DBA93 at 1999-06-28
user ID: "Chloe (Jester) <[email protected]>"
signed with your key B87DBA93 at 1999-06-28
Create a revocation certificate for this signature? (y/N)n
user ID: "Chloe (Plebian) <[email protected]>"
signed with your key B87DBA93 at 1999-06-28
Create a revocation certificate for this signature? (y/N)y
You are about to revoke these signatures:
    Chloe (Plebian) <[email protected]>
  signed by B87DBA93 at 1999-06-28
Really create the revocation certificates? (y/N)y

You need a passphrase to unlock the secret key for
user: "Chloe (Jester) <[email protected]>"
1024-bit DSA key, ID B87DBA93, created 1999-06-28


pub  1024D/B87DBA93  created: 1999-06-28 expires: never      trust: -/u
sub  2048g/B7934539  created: 1999-06-28 expires: never
sub  1792G/4E3160AD  created: 1999-06-29 expires: 2000-06-28
rev! subkey has been revoked: 1999-06-29
sub   960D/E1F56448  created: 1999-06-29 expires: 2000-06-28
(1)  Chloe (Jester) <[email protected]>
(2)  Chloe (Plebian) <[email protected]>

  A revoked user ID is indicated by the revocation signature on the ID
  when the signatures on the key's user IDs are listed.
Command> check
uid  Chloe (Jester) <[email protected]>
sig!       B87DBA93 1999-06-28   [self-signature]
uid  Chloe (Plebian) <[email protected]>
rev!       B87DBA93 1999-06-29   [revocation]
sig!       B87DBA93 1999-06-28   [self-signature]

  Revoking both subkeys and self-signatures on user IDs adds revocation
  self-signatures to the key. Since signatures are being added and no
  material is deleted, a revocation will always be visible to others when
  your updated public key is distributed and merged with older copies of
  it. Revocation therefore guarantees that everybody has a consistent
  copy of your public key.

Updating a key's expiration time
================================

  The expiration time of a key may be updated with the command expire
  from the key edit menu. If no key is selected the expiration time of
  the primary key is updated. Otherwise the expiration time of the
  selected subordinate key is updated.

  A key's expiration time is associated with the key's self-signature.
  The expiration time is updated by deleting the old self-signature and
  adding a new self-signature. Since correspondents will not have deleted
  the old self-signature, they will see an additional self-signature on
  the key when they update their copy of your key. The latest
  self-signature takes precedence, however, so all correspondents will
  unambiguously know the expiration times of your keys.

Validating other keys on your public keyring
============================================

  In Chapter 1 a procedure was given to validate your correspondents'
  public keys: a correspondent's key is validated by personally checking
  his key's fingerprint and then signing his public key with your private
  key. By personally checking the fingerprint you can be sure that the
  key really does belong to him, and since you have signed they key, you
  can be sure to detect any tampering with it in the future.
  Unfortunately, this procedure is awkward when either you must validate
  a large number of keys or communicate with people whom you do not know
  personally.

  GnuPG addresses this problem with a mechanism popularly known as the
  web of trust. In the web of trust model, responsibility for validating
  public keys is delegated to people you trust. For example, suppose

    * Alice has signed Blake's key, and
    * Blake has signed Chloe's key and Dharma's key.

  If Alice trusts Blake to properly validate keys that he signs, then
  Alice can infer that Chloe's and Dharma's keys are valid without having
  to personally check them. She simply uses her validated copy of Blake's
  public key to check that Blake's signatures on Chloe's and Dharma's are
  good. In general, assuming that Alice fully trusts everybody to
  properly validate keys they sign, then any key signed by a valid key is
  also considered valid. The root is Alice's key, which is axiomatically
  assumed to be valid.

Trust in a key's owner
======================

  In practice trust is subjective. For example, Blake's key is valid to
  Alice since she signed it, but she may not trust Blake to properly
  validate keys that he signs. In that case, she would not take Chloe's
  and Dharma's key as valid based on Blake's signatures alone. The web of
  trust model accounts for this by associating with each public key on
  your keyring an indication of how much you trust the key's owner. There
  are four trust levels.

  unknown
         Nothing is known about the owner's judgment in key signing. Keys
         on your public keyring that you do not own initially have this
         trust level.

  none
         The owner is known to improperly sign other keys.

  marginal
         The owner understands the implications of key signing and
         properly validates keys before signing them.

  full
         The owner has an excellent understanding of key signing, and his
         signature on a key would be as good as your own.

  A key's trust level is something that you alone assign to the key, and
  it is considered private information. It is not packaged with the key
  when it is exported; it is even stored separately from your keyrings in
  a separate database.

  The GnuPG key editor may be used to adjust your trust in a key's owner.
  The command is trust. In this example Alice edits her trust in Blake
  and then updates the trust database to recompute which keys are valid
  based on her new trust in Blake.

alice% gpg --edit-key blake

pub  1024D/8B927C8A  created: 1999-07-02 expires: never      trust: q/f
sub  1024g/C19EA233  created: 1999-07-02 expires: never
(1)  Blake (Executioner) <[email protected]>

Command> trust
pub  1024D/8B927C8A  created: 1999-07-02 expires: never      trust: q/f
sub  1024g/C19EA233  created: 1999-07-02 expires: never
(1)  Blake (Executioner) <[email protected]>

Please decide how far you trust this user to correctly
verify other users' keys (by looking at passports,
checking fingerprints from different sources...)?

1 = Don't know
2 = I do NOT trust
3 = I trust marginally
4 = I trust fully
s = please show me more information
m = back to the main menu

Your decision? 3

pub  1024D/8B927C8A  created: 1999-07-02 expires: never      trust: m/f
sub  1024g/C19EA233  created: 1999-07-02 expires: never
(1)  Blake (Executioner) <[email protected]>

Command> quit
[...]

  Trust in the key's owner and the key's validity are indicated to the
  right when the key is displayed. Trust in the owner is displayed first
  and the key's validity is second [D]. The four trust/validity levels
  are abbreviated: unknown (q), none (n), marginal (m), and full (f). In
  this case, Blake's key is fully valid since Alice signed it herself.
  She initially has an unknown trust in Blake to properly sign other keys
  but decides to trust him marginally.

Using trust to validate keys
============================

  The web of trust allows a more elaborate algorithm to be used to
  validate a key. Formerly, a key was considered valid only if you signed
  it personally. A more flexible algorithm can now be used: a key K is
  considered valid if it meets two conditions:

   1. it is signed by enough valid keys, meaning
         + you have signed it personally,
         + it has been signed by one fully trusted key, or
         + it has been signed by three marginally trusted keys; and
   2. the path of signed keys leading from K back to your own key is five
      steps or shorter.

  The path length, number of marginally trusted keys required, and number
  of fully trusted keys required may be adjusted. The numbers given above
  are the default values used by GnuPG.

  Figure 3-1 shows a web of trust rooted at Alice. The graph
  illustrates who has signed who's keys. The table shows which keys Alice
  considers valid based on her trust in the other members of the web.
  This example assumes that two marginally-trusted keys or one
  fully-trusted key is needed to validate another key. The maximum path
  length is three.

  When computing valid keys in the example, Blake and Dharma's are always
  considered fully valid since they were signed directly by Alice. The
  validity of the other keys depends on trust. In the first case, Dharma
  is trusted fully, which implies that Chloe's and Francis's keys will be
  considered valid. In the second example, Blake and Dharma are trusted
  marginally. Since two marginally trusted keys are needed to fully
  validate a key, Chloe's key will be considered fully valid, but
  Francis's key will be considered only marginally valid. In the case
  where Chloe and Dharma are marginally trusted, Chloe's key will be
  marginally valid since Dharma's key is fully valid. Francis's key,
  however, will also be considered marginally valid since only a fully
  valid key can be used to validate other keys, and Dharma's key is the
  only fully valid key that has been used to sign Francis's key. When
  marginal trust in Blake is added, Chloe's key becomes fully valid and
  can then be used to fully validate Francis's key and marginally
  validate Elena's key. Lastly, when Blake, Chloe, and Elena are fully
  trusted, this is still insufficient to validate Geoff's key since the
  maximum certification path is three, but the path length from Geoff
  back to Alice is four.

  The web of trust model is a flexible approach to the problem of safe
  public key exchange. It permits you to tune GnuPG to reflect how you
  use it. At one extreme you may insist on multiple, short paths from
  your key to another key K in order to trust it. On the other hand, you
  may be satisfied with longer paths and perhaps as little as one path
  from your key to the other key K. Requiring multiple, short paths is a
  strong guarantee that K belongs to whom your think it does. The price,
  of course, is that it is more difficult to validate keys since you must
  personally sign more keys than if you accepted fewer and longer paths.

  Figure 3-1. A hypothetical web of trust
  =======================================

  [63]A graph indicating who has signed who's key
  <https://www.gnupg.org/gph/en/signatures.jpg>

  Row:               1
  Marginal Trust:    None
  Full Trust:        Dharma
  Marginal Validity: None
  Full Validity:     Blake, Chloe, Dharma, Francis

  Row:               2
  Marginal Trust:    Blake, Dharma
  Full Trust:        None
  Marginal Validity: Francis
  Full Validity:     Blake, Chloe, Dharma

  Row:               3
  Marginal Trust:    Chloe, Dharma
  Full Trust:        None
  Marginal Validity: Chloe, Francis
  Full Validity:     Blake, Dharma

  Row:               4
  Marginal Trust:    Blake, Chloe, Dharma
  Full Trust:        None
  Marginal Validity: Elena
  Full Validity:     Blake, Chloe, Dharma, Francis

  Row:               5
  Marginal Trust:    None
  Full Trust:        Blake, Chloe, Elena
  Marginal Validity: None
  Full Validity:     Blake, Chloe, Elena, Francis

Distributing keys
=================

  Ideally, you distribute your key by personally giving it to your
  correspondents. In practice, however, keys are often distributed by
  email or some other electronic communication medium. Distribution by
  email is good practice when you have only a few correspondents, and
  even if you have many correspondents, you can use an alternative means
  such as posting your public key on your World Wide Web homepage. This
  is unacceptable, however, if people who need your public key do not
  know where to find it on the Web.

  To solve this problem public key servers are used to collect and
  distribute public keys. A public key received by the server is either
  added to the server's database or merged with the existing key if
  already present. When a key request comes to the server, the server
  consults its database and returns the requested public key if found.

  A keyserver is also valuable when many people are frequently signing
  other people's keys. Without a keyserver, when Blake sign's Alice's key
  then Blake would send Alice a copy of her public key signed by him so
  that Alice could add the updated key to her ring as well as distribute
  it to all of her correspondents. Going through this effort fulfills
  Alice's and Blake's responsibility to the community at large in
  building tight webs of trust and thus improving the security of PGP. It
  is nevertheless a nuisance if key signing is frequent.

  Using a keyserver makes the process somewhat easier. When Blake signs
  Alice's key he sends the signed key to the key server. The key server
  adds Blake's signature to its copy of Alice's key. Individuals
  interested in updating their copy of Alice's key then consult the
  keyserver on their own initiative to retrieve the updated key. Alice
  need never be involved with distribution and can retrieve signatures on
  her key simply by querying a keyserver.

  One or more keys may be sent to a keyserver using the command-line
  option --send-keys. The option takes one or more key specifiers and
  sends the specified keys to the key server. The key server to which to
  send the keys is specified with the command-line option --keyserver.
  Similarly, the option --recv-keys is used to retrieve keys from a
  keyserver, but the option --recv-keys requires a key ID be used to
  specify the key. In the following example Alice updates her public key
  with new signatures from the keyserver certserver.pgp.com and then
  sends her copy of Blake's public key to the same keyserver to
  contribute any new signatures she may have added.

alice% gpg --keyserver certserver.pgp.com --recv-key 0xBB7576AC
gpg: requesting key BB7576AC from certserver.pgp.com ...
gpg: key BB7576AC: 1 new signature

gpg: Total number processed: 1
gpg:         new signatures: 1
alice% gpg --keyserver certserver.pgp.com --send-key [email protected]
gpg: success sending to 'certserver.pgp.com' (status=200)

  There are several popular keyservers in use around the world. The major
  keyservers synchronize themselves, so it is fine to pick a keyserver
  close to you on the Internet and then use it regularly for sending and
  receiving keys.

Chapter 4. Daily use of GnuPG
=============================

  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.

Defining your security needs
============================

  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 four issues:

    * choosing the key size of your public/private keypair,
    * protecting your private key,
    * selecting expiration dates and using subkeys, 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 masquerading 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.

Choosing a key size
===================

  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
===========================

  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,
  e.g., 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.

Selecting expiration dates and using subkeys
============================================

  By default, a DSA master signing key and an ElGamal encryption subkey
  are generated when you create a new keypair. This is convenient,
  because the roles of the two keys are different, and you may therefore
  want the keys to have different lifetimes. The master signing key is
  used to make digital signatures, and it also collects the signatures of
  others who have confirmed your identity. The encryption key is used
  only for decrypting encrypted documents sent to you. Typically, a
  digital signature has a long lifetime, e.g., forever, and you also do
  not want to lose the signatures on your key that you worked hard to
  collect. On the other hand, the encryption subkey may be changed
  periodically for extra security, since if an encryption key is broken,
  the attacker can read all documents encrypted to that key both in the
  future and from the past.

  It is almost always the case that you will not want the master key to
  expire. There are two reasons why you may choose an expiration date.
  First, you may intend for the key to have a limited lifetime. For
  example, it is being used for an event such as a political campaign and
  will no longer be useful after the campaign is over. Another reason is
  that if you lose control of the key and do not have a revocation
  certificate with which to revoke the key, having an expiration date on
  the master key ensures that the key will eventually fall into disuse.

  Changing encryption subkeys is straightforward but can be inconvenient.
  If you generate a new keypair with an expiration date on the subkey,
  that subkey will eventually expire. Shortly before the expiration you
  will add a new subkey and publish your updated public key. Once the
  subkey expires, those who wish to correspond with you must find your
  updated key since they will no longer be able to encrypt to the expired
  key. This may be inconvenient depending on how you distribute the key.
  Fortunately, however, no extra signatures are necessary since the new
  subkey will have been signed with your master signing key, which
  presumably has already been validated by your correspondents.

  The inconvenience may or may not be worth the extra security. Just as
  you can, an attacker can still read all documents encrypted to an
  expired subkey. Changing subkeys only protects future documents. In
  order to read documents encrypted to the new subkey, the attacker would
  need to mount a new attack using whatever techniques he used against
  you the first time.

  Finally, it only makes sense to have one valid encryption subkey on a
  keyring. There is no additional security gained by having two or more
  active subkeys. There may of course be any number of expired keys on a
  keyring so that documents encrypted in the past may still be decrypted,
  but only one subkey needs to be active at any given time.

Managing your web of trust
==========================

  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, though, 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.

Building your web of trust
==========================

  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 [E], 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
  initiative and get your key signed by others outside of your initial
  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 [66]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 publicly 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.

Using GnuPG legally
===================

  The legal status of encryption software varies from country to country,
  and law regarding encryption software is rapidly evolving.
  [67]Bert-Japp Koops has an excellent [68]Crypto Law Survey to which you
  should refer for the legal status of encryption software in your
  country.

Chapter 5. Topics
=================

  This chapter covers miscellaneous topics that do not fit elsewhere in
  the user manual. As topics are added, they may be collected and
  factored into chapters that stand on their own. If you would like to
  see a particular topic covered, please suggest it. Even better,
  volunteer to write a first draft covering your suggested topic!

Writing user interfaces
=======================

  [69]Alma Whitten and [70]Doug Tygar have done a [71]study on NAI's PGP
  5.0 user interface and came to the conclusion that novice users find
  PGP confusing and frustrating. In their human factors study, only four
  out of twelve test subjects managed to correctly send encrypted email
  to their team members, and three out of twelve emailed the secret
  without encryption. Furthermore, half of the test subjects had a
  technical background.

  These results are not surprising. PGP 5.0 has a nice user interface
  that is excellent if you already understand how public-key encryption
  works and are familiar with the web-of-trust key management model
  specified by OpenPGP. Unfortunately, novice users understand neither
  public-key encryption nor key management, and the user interface does
  little to help.

  You should certainly read Whitten and Tygar's report if you are writing
  a user interface. It gives specific comments from each of the test
  subjects, and those details are enlightening. For example, it would
  appear that many of subjects believed that a message being sent to
  other people should be encrypted to the test subject's own public key.
  Consider it for a minute, and you will see that it is an easy mistake
  to make. In general, novice users have difficulty understanding the
  different roles of the public key and private key when using GnuPG. As
  a user interface designer, you should try to make it clear at all times
  when one of the two keys is being used. You could also use wizards or
  other common GUI techniques for guiding the user through common tasks,
  such as key generation, where extra steps, such as generating a key
  revocation certification and making a backup, are all but essential for
  using GnuPG correctly. Other comments from the paper include the
  following.

    * Security is usually a secondary goal; people want to send email,
      browse, and so on. Do not assume users will be motivated to read
      manuals or go looking for security controls.
    * The security of a networked computer is only as strong as its
      weakest component. Users need to be guided to attend to all aspects
      of their security, not left to proceed through random exploration
      as they might with a word processor or a spreadsheet.
    * Consistently use the same terms for the same actions. Do not
      alternate between synonyms like "encrypt" and "encipher".
    * For inexperienced users, simplify the display. Too much information
      hides the important information. An initial display configuration
      could concentrate on giving the user the correct model of the
      relationship between public and private keys and a clear
      understanding of the functions for acquiring and distributing keys.

  Designing an effective user interface for key management is even more
  difficult. The OpenPGP web-of-trust model is unfortunately quite
  obtuse. For example, the specification imposes three arbitrary trust
  levels onto the user: none, marginal, and complete. All degrees of
  trust felt by the user must be fit into one of those three cubbyholes.
  The key validation algorithm is also difficult for non-computer
  scientists to understand, particularly the notions of "marginals
  needed" and "completes needed". Since the web-of-trust model is
  well-specified and cannot be changed, you will have to do your best and
  design a user interface that helps to clarify it for the user. A
  definite improvement, for example, would be to generate a diagram of
  how a key was validated when requested by the user. Relevant comments
  from the paper include the following.

    * Users are likely to be uncertain on how and when to grant accesses.
    * Place a high priority on making sure users understand their
      security well enough to prevent them from making potentially
      high-cost mistakes. Such mistakes include accidentally deleting the
      private key, accidentally publicizing a key, accidentally revoking
      a key, forgetting the pass phrase, and failing to back up the key
      rings.

Appendix A. GNU Free Documentation License
==========================================

  Version 1.1, March 2000

    Copyright (C) 2000 Free Software Foundation, Inc. 59 Temple Place,
    Suite 330, Boston, MA 02111-1307 USA Everyone is permitted to copy
    and distribute verbatim copies of this license document, but
    changing it is not allowed.

0. PREAMBLE

  The purpose of this License is to make a manual, textbook, or other
  written document "free" in the sense of freedom: to assure everyone the
  effective freedom to copy and redistribute it, with or without
  modifying it, either commercially or noncommercially. Secondarily, this
  License preserves for the author and publisher a way to get credit for
  their work, while not being considered responsible for modifications
  made by others.

  This License is a kind of "copyleft", which means that derivative works
  of the document must themselves be free in the same sense. It
  complements the GNU General Public License, which is a copyleft license
  designed for free software.

  We have designed this License in order to use it for manuals for free
  software, because free software needs free documentation: a free
  program should come with manuals providing the same freedoms that the
  software does. But this License is not limited to software manuals; it
  can be used for any textual work, regardless of subject matter or
  whether it is published as a printed book. We recommend this License
  principally for works whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONS

  This License applies to any manual or other work that contains a notice
  placed by the copyright holder saying it can be distributed under the
  terms of this License. The "Document", below, refers to any such manual
  or work. Any member of the public is a licensee, and is addressed as
  "you".

  A "Modified Version" of the Document means any work containing the
  Document or a portion of it, either copied verbatim, or with
  modifications and/or translated into another language.

  A "Secondary Section" is a named appendix or a front-matter section of
  the Document that deals exclusively with the relationship of the
  publishers or authors of the Document to the Document's overall subject
  (or to related matters) and contains nothing that could fall directly
  within that overall subject. (For example, if the Document is in part a
  textbook of mathematics, a Secondary Section may not explain any
  mathematics.) The relationship could be a matter of historical
  connection with the subject or with related matters, or of legal,
  commercial, philosophical, ethical or political position regarding
  them.

  The "Invariant Sections" are certain Secondary Sections whose titles
  are designated, as being those of Invariant Sections, in the notice
  that says that the Document is released under this License.

  The "Cover Texts" are certain short passages of text that are listed,
  as Front-Cover Texts or Back-Cover Texts, in the notice that says that
  the Document is released under this License.

  A "Transparent" copy of the Document means a machine-readable copy,
  represented in a format whose specification is available to the general
  public, whose contents can be viewed and edited directly and
  straightforwardly with generic text editors or (for images composed of
  pixels) generic paint programs or (for drawings) some widely available
  drawing editor, and that is suitable for input to text formatters or
  for automatic translation to a variety of formats suitable for input to
  text formatters. A copy made in an otherwise Transparent file format
  whose markup has been designed to thwart or discourage subsequent
  modification by readers is not Transparent. A copy that is not
  "Transparent" is called "Opaque".

  Examples of suitable formats for Transparent copies include plain ASCII
  without markup, Texinfo input format, LaTeX input format, SGML or XML
  using a publicly available DTD, and standard-conforming simple HTML
  designed for human modification. Opaque formats include PostScript,
  PDF, proprietary formats that can be read and edited only by
  proprietary word processors, SGML or XML for which the DTD and/or
  processing tools are not generally available, and the machine-generated
  HTML produced by some word processors for output purposes only.

  The "Title Page" means, for a printed book, the title page itself, plus
  such following pages as are needed to hold, legibly, the material this
  License requires to appear in the title page. For works in formats
  which do not have any title page as such, "Title Page" means the text
  near the most prominent appearance of the work's title, preceding the
  beginning of the body of the text.

2. VERBATIM COPYING

  You may copy and distribute the Document in any medium, either
  commercially or noncommercially, provided that this License, the
  copyright notices, and the license notice saying this License applies
  to the Document are reproduced in all copies, and that you add no other
  conditions whatsoever to those of this License. You may not use
  technical measures to obstruct or control the reading or further
  copying of the copies you make or distribute. However, you may accept
  compensation in exchange for copies. If you distribute a large enough
  number of copies you must also follow the conditions in section 3.

  You may also lend copies, under the same conditions stated above, and
  you may publicly display copies.

3. COPYING IN QUANTITY

  If you publish printed copies of the Document numbering more than 100,
  and the Document's license notice requires Cover Texts, you must
  enclose the copies in covers that carry, clearly and legibly, all these
  Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts
  on the back cover. Both covers must also clearly and legibly identify
  you as the publisher of these copies. The front cover must present the
  full title with all words of the title equally prominent and visible.
  You may add other material on the covers in addition. Copying with
  changes limited to the covers, as long as they preserve the title of
  the Document and satisfy these conditions, can be treated as verbatim
  copying in other respects.

  If the required texts for either cover are too voluminous to fit
  legibly, you should put the first ones listed (as many as fit
  reasonably) on the actual cover, and continue the rest onto adjacent
  pages.

  If you publish or distribute Opaque copies of the Document numbering
  more than 100, you must either include a machine-readable Transparent
  copy along with each Opaque copy, or state in or with each Opaque copy
  a publicly-accessible computer-network location containing a complete
  Transparent copy of the Document, free of added material, which the
  general network-using public has access to download anonymously at no
  charge using public-standard network protocols. If you use the latter
  option, you must take reasonably prudent steps, when you begin
  distribution of Opaque copies in quantity, to ensure that this
  Transparent copy will remain thus accessible at the stated location
  until at least one year after the last time you distribute an Opaque
  copy (directly or through your agents or retailers) of that edition to
  the public.

  It is requested, but not required, that you contact the authors of the
  Document well before redistributing any large number of copies, to give
  them a chance to provide you with an updated version of the Document.

4. MODIFICATIONS

  You may copy and distribute a Modified Version of the Document under
  the conditions of sections 2 and 3 above, provided that you release the
  Modified Version under precisely this License, with the Modified
  Version filling the role of the Document, thus licensing distribution
  and modification of the Modified Version to whoever possesses a copy of
  it. In addition, you must do these things in the Modified Version:

   A. Use in the Title Page (and on the covers, if any) a title distinct
      from that of the Document, and from those of previous versions
      (which should, if there were any, be listed in the History section
      of the Document). You may use the same title as a previous version
      if the original publisher of that version gives permission.
   B. List on the Title Page, as authors, one or more persons or entities
      responsible for authorship of the modifications in the Modified
      Version, together with at least five of the principal authors of
      the Document (all of its principal authors, if it has less than
      five).
   C. State on the Title page the name of the publisher of the Modified
      Version, as the publisher.
   D. Preserve all the copyright notices of the Document.
   E. Add an appropriate copyright notice for your modifications adjacent
      to the other copyright notices.
   F. Include, immediately after the copyright notices, a license notice
      giving the public permission to use the Modified Version under the
      terms of this License, in the form shown in the Addendum below.
   G. Preserve in that license notice the full lists of Invariant
      Sections and required Cover Texts given in the Document's license
      notice.
   H. Include an unaltered copy of this License.
   I. Preserve the section entitled "History", and its title, and add to
      it an item stating at least the title, year, new authors, and
      publisher of the Modified Version as given on the Title Page. If
      there is no section entitled "History" in the Document, create one
      stating the title, year, authors, and publisher of the Document as
      given on its Title Page, then add an item describing the Modified
      Version as stated in the previous sentence.
   J. Preserve the network location, if any, given in the Document for
      public access to a Transparent copy of the Document, and likewise
      the network locations given in the Document for previous versions
      it was based on. These may be placed in the "History" section. You
      may omit a network location for a work that was published at least
      four years before the Document itself, or if the original publisher
      of the version it refers to gives permission.
   K. In any section entitled "Acknowledgements" or "Dedications",
      preserve the section's title, and preserve in the section all the
      substance and tone of each of the contributor acknowledgements
      and/or dedications given therein.
   L. Preserve all the Invariant Sections of the Document, unaltered in
      their text and in their titles. Section numbers or the equivalent
      are not considered part of the section titles.
   M. Delete any section entitled "Endorsements". Such a section may not
      be included in the Modified Version.
   N. Do not retitle any existing section as "Endorsements" or to
      conflict in title with any Invariant Section.

  If the Modified Version includes new front-matter sections or
  appendices that qualify as Secondary Sections and contain no material
  copied from the Document, you may at your option designate some or all
  of these sections as invariant. To do this, add their titles to the
  list of Invariant Sections in the Modified Version's license notice.
  These titles must be distinct from any other section titles.

  You may add a section entitled "Endorsements", provided it contains
  nothing but endorsements of your Modified Version by various
  parties--for example, statements of peer review or that the text has
  been approved by an organization as the authoritative definition of a
  standard.

  You may add a passage of up to five words as a Front-Cover Text, and a
  passage of up to 25 words as a Back-Cover Text, to the end of the list
  of Cover Texts in the Modified Version. Only one passage of Front-Cover
  Text and one of Back-Cover Text may be added by (or through
  arrangements made by) any one entity. If the Document already includes
  a cover text for the same cover, previously added by you or by
  arrangement made by the same entity you are acting on behalf of, you
  may not add another; but you may replace the old one, on explicit
  permission from the previous publisher that added the old one.

  The author(s) and publisher(s) of the Document do not by this License
  give permission to use their names for publicity for or to assert or
  imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

  You may combine the Document with other documents released under this
  License, under the terms defined in section 4 above for modified
  versions, provided that you include in the combination all of the
  Invariant Sections of all of the original documents, unmodified, and
  list them all as Invariant Sections of your combined work in its
  license notice.

  The combined work need only contain one copy of this License, and
  multiple identical Invariant Sections may be replaced with a single
  copy. If there are multiple Invariant Sections with the same name but
  different contents, make the title of each such section unique by
  adding at the end of it, in parentheses, the name of the original
  author or publisher of that section if known, or else a unique number.
  Make the same adjustment to the section titles in the list of Invariant
  Sections in the license notice of the combined work.

  In the combination, you must combine any sections entitled "History" in
  the various original documents, forming one section entitled "History";
  likewise combine any sections entitled "Acknowledgements", and any
  sections entitled "Dedications". You must delete all sections entitled
  "Endorsements."

6. COLLECTIONS OF DOCUMENTS

  You may make a collection consisting of the Document and other
  documents released under this License, and replace the individual
  copies of this License in the various documents with a single copy that
  is included in the collection, provided that you follow the rules of
  this License for verbatim copying of each of the documents in all other
  respects.

  You may extract a single document from such a collection, and
  distribute it individually under this License, provided you insert a
  copy of this License into the extracted document, and follow this
  License in all other respects regarding verbatim copying of that
  document.

7. AGGREGATION WITH INDEPENDENT WORKS

  A compilation of the Document or its derivatives with other separate
  and independent documents or works, in or on a volume of a storage or
  distribution medium, does not as a whole count as a Modified Version of
  the Document, provided no compilation copyright is claimed for the
  compilation. Such a compilation is called an "aggregate", and this
  License does not apply to the other self-contained works thus compiled
  with the Document, on account of their being thus compiled, if they are
  not themselves derivative works of the Document.

  If the Cover Text requirement of section 3 is applicable to these
  copies of the Document, then if the Document is less than one quarter
  of the entire aggregate, the Document's Cover Texts may be placed on
  covers that surround only the Document within the aggregate. Otherwise
  they must appear on covers around the whole aggregate.

8. TRANSLATION

  Translation is considered a kind of modification, so you may distribute
  translations of the Document under the terms of section 4. Replacing
  Invariant Sections with translations requires special permission from
  their copyright holders, but you may include translations of some or
  all Invariant Sections in addition to the original versions of these
  Invariant Sections. You may include a translation of this License
  provided that you also include the original English version of this
  License. In case of a disagreement between the translation and the
  original English version of this License, the original English version
  will prevail.

9. TERMINATION

  You may not copy, modify, sublicense, or distribute the Document except
  as expressly provided for under this License. Any other attempt to
  copy, modify, sublicense or distribute the Document is void, and will
  automatically terminate your rights under this License. However,
  parties who have received copies, or rights, from you under this
  License will not have their licenses terminated so long as such parties
  remain in full compliance.

10. FUTURE REVISIONS OF THIS LICENSE

  The Free Software Foundation may publish new, revised versions of the
  GNU Free Documentation License from time to time. Such new versions
  will be similar in spirit to the present version, but may differ in
  detail to address new problems or concerns. See
  http://www.gnu.org/copyleft/.

  Each version of the License is given a distinguishing version number.
  If the Document specifies that a particular numbered version of this
  License "or any later version" applies to it, you have the option of
  following the terms and conditions either of that specified version or
  of any later version that has been published (not as a draft) by the
  Free Software Foundation. If the Document does not specify a version
  number of this License, you may choose any version ever published (not
  as a draft) by the Free Software Foundation.

How to use this License for your documents
==========================================

  To use this License in a document you have written, include a copy of
  the License in the document and put the following copyright and license
  notices just after the title page:

    Copyright (c) YEAR YOUR NAME. Permission is granted to copy,
    distribute and/or modify this document under the terms of the GNU
    Free Documentation License, Version 1.1 or any later version
    published by the Free Software Foundation; with the Invariant
    Sections being LIST THEIR TITLES, with the Front-Cover Texts being
    LIST, and with the Back-Cover Texts being LIST. A copy of the
    license is included in the section entitled "GNU Free Documentation
    License".

  If you have no Invariant Sections, write "with no Invariant Sections"
  instead of saying which ones are invariant. If you have no Front-Cover
  Texts, write "no Front-Cover Texts" instead of "Front-Cover Texts being
  LIST"; likewise for Back-Cover Texts.

  If your document contains nontrivial examples of program code, we
  recommend releasing these examples in parallel under your choice of
  free software license, such as the GNU General Public License, to
  permit their use in free software.

 Notes
 =====

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

  [B]
  Many command-line options that are frequently used can also be set in a
  configuration file.

  [C]
  The cipher must have the property that the actual public key or private
  key could be used by the encryption algorithm as the public key. RSA is
  an example of such an algorithm while ElGamal is not an example.

  [D]
  GnuPG overloads the word "trust" by using it to mean trust in an
  owner and trust in a key. This can be confusing. Sometimes trust in an
  owner is referred to as owner-trust to distinguish it from trust in a
  key. Throughout this manual, however, "trust" is used to mean trust
  in a key's owner, and "validity" is used to mean trust that a key
  belongs to the human associated with the key ID.

  [E]
  In this section, GnuPG refers to the GnuPG implementation of OpenPGP as
  well as other implementations such as NAI's PGP product.

References
==========

 56. http://www.counterpane.com/schneier.html
 57. http://www.counterpane.com/applied.html
 63. https://www.gnupg.org/gph/en/signatures.jpg
 66. http://www.herrons.com/kb2nsx/keysign.html
 67. http://cwis.kub.nl/~frw/people/koops/bertjaap.htm
 68. http://cwis.kub.nl/~frw/people/koops/lawsurvy.htm
 69. http://www.cs.cmu.edu/~alma
 70. http://www.cs.berkeley.edu/~tygar
 71. http://reports-archive.adm.cs.cmu.edu/anon/1998/abstracts/98-155.html

You are viewing proxied material from tilde.pink. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.