global env
    variable openPgpFileNameOnly
    variable openPgpInstalledDirectory

    if {[catch {openPgpMustBeInstalled}] == 0} then {
      return true
    }

    if {![info exists openPgpInstalledDirectory]} then {
      return false
    }

    if {[isWindows]} then {
      if {[info exists env(ProgramFiles(x86))]} then {
        set programFiles $env(ProgramFiles\(x86\))
      } elseif {[info exists env(ProgramFiles)]} then {
        set programFiles $env(ProgramFiles)
      } else {
        return false
      }

      set directory [file join $programFiles $openPgpInstalledDirectory]
    } else {
      set directory $openPgpInstalledDirectory
    }

    if {![file isdirectory $directory]} then {
      return false
    }

    if {![info exists openPgpFileNameOnly]} then {
      return false
    }

    set fileName [file join $directory $openPgpFileNameOnly]

    if {[file exists $fileName] && [file isfile $fileName]} then {
      return [addToPath $directory]
    }

    return false
  }

  #
  # NOTE: This procedure attempts to verify that a configured implementation
  #       of OpenPGP is installed locally.  There are no arguments.  Script
  #       errors are raised if any problems are found.  The return value is
  #       undefined.
  #
  # <public>
  proc openPgpMustBeInstalled {} {
    variable openPgpFileNameOnly
    variable openPgpFileNamesOnly
    variable openPgpInstalledCommand
    variable openPgpInstalledPattern

    set message {
      Cannot use OpenPGP: it does not appear to be installed.

      GNU Privacy Guard (GPG) may be downloaded from "https://www.gnupg.org/"
      and then installed.  Signed binaries for Windows may be available from
      "https://www.gpg4win.org/".

      Alternatively, it may be possible to install GNU Privacy Guard (GPG) via
      the package management subsystem included with your operating system.
    }

    set found false

    foreach fileName $openPgpFileNamesOnly {
      if {[isEagle]} then {
        if {[catch {
          eval exec -success Success [subst $openPgpInstalledCommand]
        } result] == 0} then {

          set found true; break
        }
      } else {
        if {[catch {
          eval exec [subst $openPgpInstalledCommand]
        } result] == 0} then {
          set found true; break
        }
      }
    }

    if {$found} then {
      #
      # NOTE: Was this procedure already run -AND- did it actually find a
      #       viable OpenPGP file name?
      #
      if {[info exists openPgpFileNameOnly]} then {
        #
        # NOTE: If the OpenPGP file name that we found before (?) does not
        #       match what we already have, issue a log message.
        #
        if {$fileName ne $openPgpFileNameOnly} then {
          pkgLog [appendArgs \
              "the OpenPGP file name is being changed from \"" \
              $openPgpFileNameOnly "\" to \"" $fileName \"]

          set openPgpFileNameOnly $fileName
        }
      } else {
        #
        # NOTE: Configure the OpenPGP file name to the one that was just
        #       found.
        #
        set openPgpFileNameOnly $fileName
      }
    } else {
      #
      # NOTE: If no viable OpenPGP file name was found, raise the error
      #       message.
      #
      error $message
    }

    if {![info exists result] || \
        ![regexp -- $openPgpInstalledPattern $result]} then {
      error "cannot use OpenPGP: unknown or unsupported version"
    }

      if {[isWindows]} then {
        set openPgpInstalledDirectory [file join GNU GnuPG]
      } else {
        set openPgpInstalledDirectory [file join / usr bin]
      }
    }

    #
    # NOTE: These are the candidate names of the executable file used to
    #       invoke the OpenPGP implementation, possibly without a file
    #       extension.
    #
    variable openPgpFileNamesOnly; # DEFAULT: [list gpg2 gpg]

    if {![info exists openPgpFileNamesOnly]} then {
      if {[isWindows]} then {
        set openPgpFileNamesOnly [list gpg2.exe gpg.exe]
      } else {
        set openPgpFileNamesOnly [list gpg2 gpg]
      }
    }

    #
    # NOTE: This is the name of the executable file used to invoke the
    #       OpenPGP implementation, possibly without a file extension.
    #
    variable openPgpFileNameOnly; # DEFAULT: <unset>









    #
    # NOTE: The command to use when attempting to import an OpenPGP key
    #       file.  This must be configured according to the implementation
    #       of OpenPGP in use.
    #
    variable openPgpImportCommand; # DEFAULT: gpg2 --import

    # NOTE: The command to use when attempting to verify that OpenPGP is
    #       installed locally.  This must be configured according to the
    #       implementation of OpenPGP in use.
    #
    variable openPgpInstalledCommand; # DEFAULT: gpg2 --version

    if {![info exists openPgpInstalledCommand]} then {
      set openPgpInstalledCommand {{${fileName}} --version}

    }

    #
    # NOTE: The regular expression pattern used when attempting to verify
    #       that OpenPGP is installed locally.  This must be configured
    #       according to the implementation of OpenPGP in use.
    #