doc: Add documentation for gpgkey2ssh.

* doc/tools.texi: Add documentation for gpgkey2ssh.

--
Signed-off-by: Neal H. Walfield <neal@g10code.com>
Co-authored-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
GnuPG-bug-id: 1067
Debian-bug-id 380241

doc/tools.texi

@@ -21,6 +21,7 @@ GnuPG comes with a couple of smaller tools:
 * gpgparsemail::          Parse a mail message into an annotated format
 * symcryptrun::           Call a simple symmetric encryption tool.
 * gpg-zip::               Encrypt or sign files into an archive.
+* gpgkey2ssh::            Emit GPG public keys in OpenSSH format.
 @end menu
 
 @c
@@ -1893,3 +1894,73 @@ gpg-zip --list-archive test1
 @command{tar}(1),
 @end ifset
 @include see-also-note.texi
+
+
+@c
+@c  GPGKEY2SSH
+@c
+@manpage gpgkey2ssh.1
+@node gpgkey2ssh
+@section Emit GPG public keys in OpenSSH format
+@ifset manverb
+.B gpgkey2ssh \- Emit GPG public keys in OpenSSH format
+@end ifset
+
+@mansect synopsis
+@ifset manverb
+.B  gpgkey2ssh
+.I keyid
+@end ifset
+
+@mansect description
+@command{gpgkey2ssh} emits the public key of an OpenPGP RSA or DSA key
+in a format readable by OpenSSH clients and servers.
+
+It takes only a single argument, a key ID, which designates the
+primary key or subkey whose public key should be converted.
+
+The key ID should use upper-case (A-F, not a-f) for all hex digits
+greater than 9, and the key in question must be present in
+@code{gpg}'s public keyring.
+
+The output of a successful run can be used verbatim as an entry in an
+@code{authorized_keys} file for @code{sshd}, or can be prefixed with a
+host name and appended to a @code{known_hosts} file for @code{ssh}.
+
+@mansect return value
+
+The program returns 0 if the key was successfully converted and
+non-zero if there was an error (e.g., if the key ID was malformed, the
+key was not present in the public keyring, or if the key is not an RSA
+or DSA key).
+
+@mansect environment
+@subsection Environment
+
+@table @asis
+
+@item HOME
+Used to locate the default home directory.
+
+@item GNUPGHOME
+If set directory used instead of "~/.gnupg".
+
+@end table
+
+@mansect files
+@subsection FILES
+
+@table @asis
+
+@item gpg2
+The command used to search the user's keyring.
+
+@end table
+
+@mansect see also
+@ifset isman
+@command{gpg2}(1),
+@command{sshd}(8),
+@command{ssh}(1)
+@end ifset
+@include see-also-note.texi