Add basic usage information and fix references

* doc/misc/epa.texi (Top): Add menu entry for new node GnuPG Pinentry.
(Quick Start): Add information on and reference to basic GnuPG
configuration.
(Encrypting/decrypting gpg files): Add usage information.
(GnuPG version compatibility): Update version information.
(GnuPG Pinentry): Add new node.
(Caching Passphrases): Describe mandatory gpg-agent usage for GnuPG
2.0 and later.
(Overview, Encrypting/decrypting gpg files, GnuPG version compatibility)
(Caching Passphrases, Bug Reports): Fix references, terminology,
mark-up, and index entries.  (Bug#64154)

doc/misc/epa.texi

@@ -70,6 +70,7 @@ called EasyPG Library.
 * Quick start::
 * Commands::
 * GnuPG version compatibility::
+* GnuPG Pinentry::
 * Caching Passphrases::
 * Bug Reports::
 * GNU Free Documentation License::  The license for this documentation.
@@ -83,7 +84,8 @@ called EasyPG Library.
 @chapter Overview
 @cindex features of easypg assistant
 
-EasyPG Assistant provides the following features.
+EasyPG Assistant is an Emacs frontend application to @acronym{GnuPG,
+GNU Privacy Guard} that provides the following features:
 
 @itemize @bullet
 @item Key management.
@@ -97,6 +99,22 @@ EasyPG Assistant provides the following features.
 @node Quick start
 @chapter Quick Start
 @cindex introduction to easypg assistant
+@cindex gnupg documentation
+@cindex documentation on gnupg
+@cindex configuration of gnupg
+@cindex introduction to gnupg
+
+You can use EasyPG Assistant without any Emacs or GnuPG configuration
+whatsoever, for example to encrypt and decrypt files automatically
+with symmetric encryption, see @ref{Encrypting/decrypting gpg files}.
+However, to use the full set of EasyPG Assistant's functions you
+should have at least some minimum GnuPG configuration in place.
+
+John Michael Ashley's GNU Privacy Handbook, available online as part
+of @uref{https://gnupg.org/documentation/guides.html, the GnuPG user
+guides}, provides an introduction to GnuPG use and configuration.  In
+contrast to that, the GnuPG manual (@pxref{Top, , Top, gnupg, Using
+the GNU Privacy Guard}) is more of a reference manual.
 
 EasyPG Assistant commands are prefixed by @samp{epa-}.  For example,
 
@@ -410,6 +428,44 @@ decrypted text is inserted in the buffer rather than encrypted one.
 Similarly, when you save the buffer to a @file{foo.gpg} file,
 encrypted data is written.
 
+When you save a buffer to an encrypted file for the first time, EasyPG
+Assistant presents you a list of keys in a buffer @file{*Keys*} where
+you can select recipients for encryption.  @xref{Key management}, for
+a description of the format of that buffer.  You can streamline this
+recipient selection step by customizing variables
+@code{epa-file-encrypt-to} and @code{epa-file-select-keys} described
+further below in this section.
+
+@cindex symmetric encryption, passphrase entry for
+If you do not select any recipient during this step, EasyPG Assistant
+uses symmetric encryption.  As a consequence, you have to enter the
+passphrase twice for every buffer save and every so often for file
+reads, since the GnuPG Agent caches your passphrase for file reads at
+least for some time, but not for buffer saves.  @xref{Caching
+Passphrases}, for more information.
+
+@cindex public key encryption, passphrase entry for
+If you have created your own keypair@footnote{For encryption and
+decryption of files you do not intend to share, you do not have to use
+an email address as recipient during creation of the keypair.  You can
+also use some free-form string that gives information on the use of
+the keypair, like @code{backup} or @code{account database}.}, you can
+select that as recipient, and EasyPG Assistant will use public key
+encryption for that file.  Since GnuPG performs encryption with your
+public key, it does not prompt for a passphrase for the buffer save,
+but it will prompt for your passphrase for file reads every now and
+then, depending on the GnuPG Agent cache configuration.
+
+@cindex tempory files created by easypg assistant
+To encrypt and decrypt files as described above EasyPG Assistant under
+certain circumstances uses intermediate tempory files that contain the
+plain-text contents of the files it processes.  EasyPG Assistant
+creates them below the directory returned by function
+@code{temporary-file-directory} (@pxref{Unique File Names, ,
+Generating Unique File Names, elisp, GNU Emacs Lisp Reference
+Manual}).  If you want to be sure not to leave any plain-text traces,
+use an encrypted file systems at least for that directory.
+
 The file name pattern for encrypted files can be controlled by
 @code{epa-file-name-regexp}.
 
@@ -446,11 +502,11 @@ You can also change the default behavior with the variable
 Control whether or not to pop up the key selection dialog.
 @end defvar
 
-For frequently visited files, it might be a good idea to tell Emacs
-which encryption method should be used through @xref{File Variables, ,
-, emacs, the Emacs Manual}.  Use the @code{epa-file-encrypt-to} local
-variable for this.
 @vindex epa-file-encrypt-to
+For frequently visited files, it might be a good idea to tell Emacs
+which encryption method should be used through file variables
+(@pxref{File Variables, , Local Variables in Files, emacs, The Emacs
+Editor}).  Use the @code{epa-file-encrypt-to} local variable for this.
 
 For example, if you want an Elisp file to be encrypted with a
 public key associated with an email address @samp{ueno@@unixuser.org},
@@ -478,6 +534,11 @@ behavior are below.
 @defvar epa-file-cache-passphrase-for-symmetric-encryption
 If non-@code{nil}, cache passphrase for symmetric encryption.  The
 default value is @code{nil}.
+
+For security reasons, this option is turned off by default and not
+recommended to be used.  Instead, consider using the GnuPG Agent, which
+in many cases can do the same job, and does it in a safer way.
+@xref{Caching Passphrases}, for more information.
 @end defvar
 
 @defvar epa-file-inhibit-auto-save
@@ -507,10 +568,17 @@ The @code{epa-keyserver} variable says which server to query.
 @cindex version compatibility with gnupg
 @cindex compatibility with gnupg
 
-As of February 2016, there are three active branches of GnuPG: 2.1,
+As of June 2023, there are three active branches of GnuPG: 2.4, 2.2,
-2.0, and 1.4.  All those branches should work flawlessly with Emacs
+and 1.4.  GnuPG versions 2.4.1 and later suffer from
-with basic use-cases.  They have, however, some incompatible
+@uref{https://dev.gnupg.org/T6481, GnuPG bug T6481} and are hardly
-characteristics, which might be visible when used from Emacs.
+usable with Emacs.  There is a patch for that bug available at least
+for GnuPG version 2.4.1, which your operating system or distribution
+might provide already.  GnuPG 1.4 is considered a legacy version.
+
+Besides that, all of those branches mentioned above should work
+flawlessly with Emacs with basic use-cases.  They have, however, some
+incompatible characteristics, which might be visible when used from
+Emacs.
 
 @itemize
 @item
@@ -519,23 +587,91 @@ means, a key created with GnuPG 2.1 is not visible with 1.4.
 
 @item
 GnuPG 2.1 uses a fixed address for the Unix domain socket used to
-communicate with gpg-agent.  The @code{GPG_AGENT_INFO} environment
+communicate with @command{gpg-agent}.  The @code{GPG_AGENT_INFO}
-variable, which is used by GnuPG 2.0 and 1.4, is ignored.  That means,
+environment variable, which is used by GnuPG 2.0 and 1.4, is ignored.
-if your system has both GnuPG 2.1 and 1.4, the gpg command from GnuPG
+That means, if your system has both GnuPG 2.1 and 1.4, the gpg command
-1.4 is not able to use gpg-agent provided by 2.1 (at least out of box).
+from GnuPG 1.4 is not able to use @command{gpg-agent} provided by 2.1
+(at least out of box).
 
 @item
 GnuPG 2.1 (2.1.5 or later) has a mechanism to direct the Pinentry
-password prompt to the Emacs minibuffer@footnote{To enable this
+password prompt to the Emacs minibuffer.  @xref{GnuPG Pinentry}.
-feature, add @samp{allow-emacs-pinentry} to
-@file{~/.gnupg/gpg-agent.conf} and let gpg-agent reload the
-configuration, with: @samp{gpgconf --reload gpg-agent}}, which would
-be useful when you use Emacs remotely or from a text-only terminal.
-That feature is not available in other versions, and more
-specifically, with 2.0 (as of 2.0.29), there is no way to avoid the
-graphical prompt.
 @end itemize
 
+@node GnuPG Pinentry
+@chapter GnuPG Pinentry
+@cindex gnupg pinentry
+@cindex pinentry provided by gnupg
+
+An important component of the GnuPG suite is the Pinentry, which
+allows for secure entry of passphrases requested by GnuPG.  GnuPG
+delivers various different programs as Pinentry, ranging from bland
+TTY-only @command{pinentry-tty} to fancy graphical dialogs for various
+desktop environments, like @command{pinentry-gnome3}.  Your operating
+system usually determines which of these is used by default.
+
+Note that the selection of a concrete Pinentry program determines only
+@emph{how} GnuPG queries for passphrases and not @emph{how often}.
+For the latter question see @ref{Caching Passphrases}.
+
+@cindex pinentry, emacs as
+With some configuration Emacs can also play the role of a Pinentry.
+The most natural choice, available with GnuPG 2.1.5 and later, is to
+use Emacs itself as Pinentry for requests that are triggered by Emacs.
+For example, if you open a file whose name ends with @file{.gpg} using
+automatic decryption, you most likely also want to enter the
+passphrase for that request in Emacs.
+
+@cindex loopback pinentry
+This so called @dfn{loopback Pinentry} has the added benefit that it
+works also when you use Emacs remotely or from a text-only terminal.
+To enable it:
+
+@enumerate
+@item
+@vindex allow-loopback-pinentry
+Ensure that option @code{allow-loopback-pinentry} is configured for
+@command{gpg-agent}, which should be the default.  @xref{Agent
+Options, , Option Summary, gnupg, Using the GNU Privacy Guard}.
+
+@item
+@vindex epg-pinentry-mode
+Customize variable @code{epg-pinentry-mode} to @code{loopback} in
+Emacs.
+@end enumerate
+
+There are other options available to use Emacs as Pinentry, you might
+come across a Pinentry called @command{pinentry-emacs} or
+@command{gpg-agent} option @code{allow-emacs-pinentry}.  However,
+these are considered insecure or semi-obsolete and might not be
+supported by your operating system or distribution.  For example,
+Debian GNU/Linux supports only the loopback Pinentry described above.
+
+@ignore
+In case somebody requests these:
+
+Use Emacs for all GnuPG requests:
+
+Make @command{pinentry-emacs} the default Pinentry by means of your
+operating system.  Install package @file{pinentry.el} from GNU ELPA
+and execute @kbd{M-x pinentry-start} to start the Emacs Pinentry
+service.  @emph{All} GnuPG passphrase requests should then result in a
+minibuffer prompt in the running Emacs.  If Emacs or the Emacs
+Pinentry service are not running, passphrase requests fail.
+
+Use Emacs for all GnuPG requests with other Pinentry as fallback:
+
+Ensure the other Pinentry supports Emacs; @command{pinentry-curses}
+does, for example.  Configure @command{gpg-agent} option
+@code{allow-emacs-pinentry}.  Set environment variable
+@code{INSIDE_EMACS} for the calling process.  Install package
+@file{pinentry.el}.  Now if Emacs is running and @kbd{M-x
+pinentry-start} has been executed, all GnuPG passphrase requests
+should result in a minibuffer prompt in the running Emacs.  If Emacs
+or the Emacs Pinentry service are not running, GnuPG uses the other
+Pinentry instead.
+@end ignore
+
 @node Caching Passphrases
 @chapter Caching Passphrases
 @cindex caching passphrases
@@ -545,35 +681,33 @@ graphical prompt.
 Typing passphrases is a troublesome task if you frequently open and
 close the same file.  GnuPG and EasyPG Assistant provide mechanisms to
 remember your passphrases for a limited time.  Using these, you only
-need to re-enter the passphrase occasionally.
+need to re-enter the passphrase occasionally.  However, the
-However, the configuration is a bit
+configuration is a bit confusing since it depends on your GnuPG
-confusing since it depends on your GnuPG installation@xref{GnuPG
+installation (@pxref{GnuPG version compatibility}), encryption method
-version compatibility}, encryption method (symmetric or public key),
+(symmetric or public key), and whether or not you want to use
-and whether or not you want to use gpg-agent.  Here are some
+GnuPG Agent.  As an additional constraint, use of the GnuPG Agent is
-questions:
+mandatory for GnuPG 2.0 and later.  Here are some questions:
 
 @enumerate
-@item Do you use GnuPG version 2.1 or 2.0 instead of GnuPG version 1.4?
+@item Do you use GnuPG version 2.0 or later instead of GnuPG version 1.4?
 @item Do you use symmetric encryption rather than public key encryption?
-@item Do you want to use gpg-agent?
+@item Do you want to use GnuPG Agent?
 @end enumerate
 
 Here are configurations depending on your answers:
 
 @multitable {111} {222} {333} {configuration configuration configuration}
 @item @b{1} @tab @b{2} @tab @b{3} @tab Configuration
-@item Yes @tab Yes @tab Yes @tab Set up gpg-agent.
+@item Yes   @tab Yes   @tab Must  @tab Set up GnuPG Agent.
-@item Yes @tab Yes @tab No @tab You can't, without gpg-agent.
+@item Yes   @tab No    @tab Must  @tab Set up GnuPG Agent.
-@item Yes @tab No @tab Yes @tab Set up gpg-agent.
+@item No    @tab Yes   @tab Yes   @tab Set up elisp passphrase cache.
-@item Yes @tab No @tab No @tab You can't, without gpg-agent.
+@item No    @tab Yes   @tab No    @tab Set up elisp passphrase cache.
-@item No @tab Yes @tab Yes @tab Set up elisp passphrase cache.
+@item No    @tab No    @tab Yes   @tab Set up GnuPG Agent.
-@item No @tab Yes @tab No @tab Set up elisp passphrase cache.
+@item No    @tab No    @tab No    @tab You can't, without GnuPG Agent.
-@item No @tab No @tab Yes @tab Set up gpg-agent.
-@item No @tab No @tab No @tab You can't, without gpg-agent.
 @end multitable
 
-To set up gpg-agent, follow the instruction in GnuPG manual.
+To set up GnuPG Agent, follow the instruction in @ref{Invoking
-@pxref{Invoking GPG-AGENT, , Invoking GPG-AGENT, gnupg}.
+GPG-AGENT, , , gnupg, Using the GNU Privacy Guard}.
 
 To set up elisp passphrase cache, set
 @code{epa-file-cache-passphrase-for-symmetric-encryption}.
@@ -586,8 +720,8 @@ To set up elisp passphrase cache, set
 
 Bugs and problems with EasyPG Assistant are actively worked on by the
 Emacs development team.  Feature requests and suggestions are also
-more than welcome.  Use @kbd{M-x report-emacs-bug}, @pxref{Bugs, ,
+more than welcome.  Use @kbd{M-x report-emacs-bug}, see @ref{Bugs, ,
-Bugs, emacs, Reporting Bugs}.
+Reporting Bugs, emacs, The Emacs Editor}.
 
 When submitting a bug report, please try to describe in excruciating
 detail the steps required to reproduce the problem.  Also try to