Refactor mml-smime.el, mml1991.el, mml2015.el

(Maybe this is the last merge from Gnus git to Emacs git)

Cf. discussion on ding mailing list, messages in
<http://thread.gmane.org/gmane.emacs.gnus.general/86228>.
Common code from the three files mml-smime.el, mml1991.el, and
mml2015.el is moved to mml-sec.el.  Auxiliary functions are added
to gnus-util.el.

The code is supported by test cases with necessary test keys.

Documentation in message.texi is updated.

* doc/misc/message.texi (Security, Using S/MIME):
Update for refactoring mml-smime.el, mml1991.el, mml2015.el.
(Using OpenPGP): Rename from "Using PGP/MIME"; update contents.
(Passphrase caching, Encrypt-to-self, Bcc Warning): New sections.

* lisp/gnus/gnus-util.el (gnus-test-list, gnus-subsetp, gnus-setdiff):
New functions.

* lisp/gnus/mml-sec.el: Require gnus-util and epg.
(epa--select-keys): Autoload.
(mml-signencrypt-style-alist, mml-secure-cache-passphrase): Doc fix.
(mml-secure-openpgp-signers): New user option;
make mml1991-signers and mml2015-signers obsolete aliases to it.
(mml-secure-smime-signers): New user option;
make mml-smime-signers an obsolete alias to it.
(mml-secure-openpgp-encrypt-to-self): New user option;
make mml1991-encrypt-to-self and mml2015-encrypt-to-self obsolete
aliases to it.
(mml-secure-smime-encrypt-to-self): New user option;
make mml-smime-encrypt-to-self an obsolete alias to it.
(mml-secure-openpgp-sign-with-sender): New user option;
make mml2015-sign-with-sender an obsolete alias to it.
(mml-secure-smime-sign-with-sender): New user option;
make mml-smime-sign-with-sender an obsolete alias to it.
(mml-secure-openpgp-always-trust): New user option;
make mml2015-always-trust an obsolete alias to it.
(mml-secure-fail-when-key-problem, mml-secure-key-preferences):
New user options.
(mml-secure-cust-usage-lookup, mml-secure-cust-fpr-lookup)
(mml-secure-cust-record-keys, mml-secure-cust-remove-keys)
(mml-secure-add-secret-key-id, mml-secure-clear-secret-key-id-list)
(mml-secure-cache-passphrase-p, mml-secure-cache-expiry-interval)
(mml-secure-passphrase-callback, mml-secure-check-user-id)
(mml-secure-secret-key-exists-p, mml-secure-check-sub-key)
(mml-secure-find-usable-keys, mml-secure-select-preferred-keys)
(mml-secure-fingerprint, mml-secure-filter-keys)
(mml-secure-normalize-cust-name, mml-secure-select-keys)
(mml-secure-select-keys-1, mml-secure-signer-names, mml-secure-signers)
(mml-secure-self-recipients, mml-secure-recipients)
(mml-secure-epg-encrypt, mml-secure-epg-sign): New functions.

* lisp/gnus/mml-smime.el: Require epg;
refactor declaration and autoloading of epg functions.
(mml-smime-use): Doc fix.
(mml-smime-cache-passphrase, mml-smime-passphrase-cache-expiry):
Obsolete.
(mml-smime-get-dns-cert, mml-smime-get-ldap-cert):
Use format instead of gnus-format-message.
(mml-smime-epg-secret-key-id-list): Remove variable.
(mml-smime-epg-passphrase-callback, mml-smime-epg-find-usable-key)
(mml-smime-epg-find-usable-secret-key): Remove functions.
(mml-smime-epg-sign, mml-smime-epg-encrypt): Refactor.

* lisp/gnus/mml1991.el (mml1991-cache-passphrase)
(mml1991-passphrase-cache-expiry): Obsolete.
(mml1991-epg-secret-key-id-list): Remove variable.
(mml1991-epg-passphrase-callback, mml1991-epg-find-usable-key)
(mml1991-epg-find-usable-secret-key): Remove functions.
(mml1991-epg-sign, mml1991-epg-encrypt): Refactor.

* lisp/gnus/mml2015.el (mml2015-cache-passphrase)
(mml2015-passphrase-cache-expiry): Obsolete.
(mml2015-epg-secret-key-id-list): Remove variable.
(mml2015-epg-passphrase-callback, mml2015-epg-check-user-id)
(mml2015-epg-check-sub-key, mml2015-epg-find-usable-key)
(mml2015-epg-find-usable-secret-key): Remove functions.
(mml2015-epg-decrypt, mml2015-epg-clear-decrypt, mml2015-epg-sign)
(mml2015-epg-encrypt): Refactor.

6 files changed, 897 insertions, 684 deletions

diff --git a/doc/misc/message.texi b/doc/misc/message.texi
index dbc77592a03..761fb772f46 100644
--- a/doc/misc/message.texi
+++ b/doc/misc/message.texi
@@ -938,16 +938,82 @@ Libidn} installed in order to use this functionality.
 @cindex encrypt
 @cindex secure
 
-Using the @acronym{MML} language, Message is able to create digitally
-signed and digitally encrypted messages.  Message (or rather
-@acronym{MML}) currently support @acronym{PGP} (RFC 1991),
-@acronym{PGP/MIME} (RFC 2015/3156) and @acronym{S/MIME}.
+By default, e-mails are transmitted without any protection around the
+Internet, which implies that they can be read and changed by lots of
+different parties.  In particular, they are analyzed under bulk
+surveillance, which violates basic human rights.  To defend those
+rights, digital self-defense is necessary (in addition to legal
+changes), and encryption and digital signatures are powerful
+techniques for self-defense.  In essence, encryption ensures that
+only the intended recipient will be able to read a message, while
+digital signatures make sure that modifications to messages can be
+detected by the recipient.
+
+Nowadays, there are two major incompatible e-mail encryption
+standards, namely @acronym{OpenPGP} and @acronym{S/MIME}.  Both of
+these standards are implemented by the @uref{https://www.gnupg.org/,
+GNU Privacy Guard (GnuPG)}, which needs to be installed as external
+software in addition to GNU Emacs.  Before you can start to encrypt,
+decrypt, and sign messages, you need to create a so-called key-pair,
+which consists of a private key and a public key.  Your @emph{public} key
+(also known as @emph{certificate}, in particular with @acronym{S/MIME}), is
+used by others (a) to encrypt messages intended for you and (b) to verify
+digital signatures created by you.  In contrast, you use your @emph{private}
+key (a) to decrypt messages and (b) to sign messages.  (You may want to
+think of your public key as an open safe that you offer to others such
+that they can deposit messages and lock the door, while your private
+key corresponds to the opening combination for the safe.)
+
+Thus, you need to perform the following steps for e-mail encryption,
+typically outside Emacs.  See, for example, the
+@uref{https://www.gnupg.org/gph/en/manual.html, The GNU Privacy
+Handbook} for details covering the standard @acronym{OpenPGP} with
+@acronym{GnuPG}.
+@enumerate
+@item
+Install GnuPG.
+@item
+Create a key-pair for your own e-mail address.
+@item
+Distribute your public key, e.g., via upload to key servers.
+@item
+Import the public keys for the recipients to which you want to send
+encrypted e-mails.
+@end enumerate
+
+Whether to use the standard @acronym{OpenPGP} or @acronym{S/MIME} is
+beyond the scope of this documentation.  Actually, you can use one
+standard for one set of recipients and the other standard for
+different recipients (depending their preferences or capabilities).
+
+In case you are not familiar with all those acronyms: The standard
+@acronym{OpenPGP} is also called @acronym{PGP} (Pretty Good Privacy).
+The command line tools offered by @acronym{GnuPG} for
+@acronym{OpenPGP} are called @command{gpg} and @command{gpg2}, while
+the one for @acronym{S/MIME} is called @command{gpgsm}.  An
+alternative, but discouraged, tool for @acronym{S/MIME} is
+@command{openssl}.  To make matters worse, e-mail messages can be
+formed in two different ways with @acronym{OpenPGP}, namely
+@acronym{PGP} (RFC 1991/4880) and @acronym{PGP/MIME} (RFC 2015/3156).
+
+The good news, however, is the following: In GNU Emacs, Message
+supports all those variants, comes with reasonable defaults that can
+be customized according to your needs, and invokes the proper command
+line tools behind the scenes for encryption, decryption, as well as
+creation and verification of digital signatures.
+
+Message uses the @acronym{MML} language for the creation of signed
+and/or encrypted messages as explained in the following.
+
 
 @menu
 * Signing and encryption::      Signing and encrypting commands.
 * Using S/MIME::                Using S/MIME
-* Using PGP/MIME::              Using PGP/MIME
+* Using OpenPGP::               Using OpenPGP
+* Passphrase caching::          How to cache passphrases
 * PGP Compatibility::           Compatibility with older implementations
+* Encrypt-to-self::             Reading your own encrypted messages
+* Bcc Warning::                 Do not use encryption with Bcc headers
 @end menu
 
 @node Signing and encryption
@@ -1041,11 +1107,45 @@ programs are required to make things work, and some small general hints.
 @node Using S/MIME
 @subsection Using S/MIME
 
-@emph{Note!}  This section assume you have a basic familiarity with
-modern cryptography, @acronym{S/MIME}, various PKCS standards, OpenSSL and
-so on.
+@acronym{S/MIME} requires an external implementation, such as
+@uref{https://www.gnupg.org/, GNU Privacy Guard} or
+@uref{https://www.openssl.org/, OpenSSL}.  The default Emacs interface
+to the S/MIME implementation is EasyPG (@pxref{Top,,EasyPG Assistant
+User's Manual, epa, EasyPG Assistant User's Manual}), which has been
+included in Emacs since version 23 and which relies on the command
+line tool @command{gpgsm} provided by @acronym{GnuPG}.  That tool
+implements certificate management, including certificate revocation
+and expiry, while such tasks need to be performed manually, if OpenSSL
+is used.
+
+The choice between EasyPG and OpenSSL is controlled by the variable
+@code{mml-smime-use}, which needs to be set to the value @code{epg}
+for EasyPG.  Depending on your version of Emacs that value may be the
+default; if not, you can either customize that variable or place the
+following line in your @file{.emacs} file (that line needs to be
+placed above other code related to message/gnus/encryption):
+
+@lisp
+(require 'epg)
+@end lisp
+
+Moreover, you may want to customize the variables
+@code{mml-default-encrypt-method} and
+@code{mml-default-sign-method} to the string @code{"smime"}.
+
+That's all if you want to use S/MIME with EasyPG, and that's the
+recommended way of using S/MIME with Message.
+
+If you think about using OpenSSL instead of EasyPG, please read the
+BUGS section in the manual for the @command{smime} command coming with
+OpenSSL first.  If you still want to use OpenSSL, the following
+applies.
+
+@emph{Note!}  The remainder of this section assumes you have a basic
+familiarity with modern cryptography, @acronym{S/MIME}, various PKCS
+standards, OpenSSL and so on.
 
-The @acronym{S/MIME} support in Message (and @acronym{MML}) require
+The @acronym{S/MIME} support in Message (and @acronym{MML}) can use
 OpenSSL@.  OpenSSL performs the actual @acronym{S/MIME} sign/encrypt
 operations.  OpenSSL can be found at @uref{http://www.openssl.org/}.
 OpenSSL 0.9.6 and later should work.  Version 0.9.5a cannot extract mail
@@ -1101,26 +1201,44 @@ you use unencrypted keys (e.g., if they are on a secure storage, or if
 you are on a secure single user machine) simply press @code{RET} at
 the passphrase prompt.
 
-@node Using PGP/MIME
-@subsection Using PGP/MIME
+@node Using OpenPGP
+@subsection Using OpenPGP
 
-@acronym{PGP/MIME} requires an external OpenPGP implementation, such
-as @uref{http://www.gnupg.org/, GNU Privacy Guard}. Pre-OpenPGP
+Use of OpenPGP requires an external software, such
+as @uref{https://www.gnupg.org/, GNU Privacy Guard}.  Pre-OpenPGP
 implementations such as PGP 2.x and PGP 5.x are also supported.  The
 default Emacs interface to the PGP implementation is EasyPG
 (@pxref{Top,,EasyPG Assistant User's Manual, epa, EasyPG Assistant
 User's Manual}), but PGG (@pxref{Top, ,PGG, pgg, PGG Manual}) and
 Mailcrypt are also supported.  @xref{PGP Compatibility}.
 
+As stated earlier, messages encrypted with OpenPGP can be formatted
+according to two different standards, namely @acronym{PGP} or
+@acronym{PGP/MIME}.  The variables
+@code{mml-default-encrypt-method} and
+@code{mml-default-sign-method} determine which variant to prefer,
+@acronym{PGP/MIME} by default.
+
+@node Passphrase caching
+@subsection Passphrase caching
+
 @cindex gpg-agent
-Message internally calls GnuPG (the @command{gpg} command) to perform
+Message with EasyPG internally calls GnuPG (the @command{gpg} or
+@command{gpgsm} command) to perform
 data encryption, and in certain cases (decrypting or signing for
-example), @command{gpg} requires user's passphrase.  Currently the
-recommended way to supply your passphrase to @command{gpg} is to use the
+example), @command{gpg}/@command{gpgsm} requires user's passphrase.
+Currently the recommended way to supply your passphrase is to use the
 @command{gpg-agent} program.
 
-To use @command{gpg-agent} in Emacs, you need to run the following
-command from the shell before starting Emacs.
+In particular, the @command{gpg-agent} program supports passphrase
+caching so that you do not need to enter your passphrase for every
+decryption/sign operation.  @xref{Agent Options, , , gnupg, Using the
+GNU Privacy Guard}.
+
+How to use @command{gpg-agent} in Emacs depends on your version of
+GnuPG.  With GnuPG version 2.1, @command{gpg-agent} is started
+automatically if necessary.  With older versions you may need to run
+the following command from the shell before starting Emacs.
 
 @example
 eval `gpg-agent --daemon`
@@ -1135,11 +1253,10 @@ GNU Privacy Guard}.
 Once your @command{gpg-agent} is set up, it will ask you for a
 passphrase as needed for @command{gpg}.  Under the X Window System,
 you will see a new passphrase input dialog appear.  The dialog is
-provided by PIN Entry (the @command{pinentry} command), and as of
-version 0.7.2, @command{pinentry} cannot cooperate with Emacs on a
-single tty.  So, if you are using a text console, you may need to put
-a passphrase into gpg-agent's cache beforehand.  The following command
-does the trick.
+provided by PIN Entry (the @command{pinentry} command), reasonably
+recent versions of which can also cooperate with Emacs on a text
+console.  If that does not work, you may need to put a passphrase into
+gpg-agent's cache beforehand.  The following command does the trick.
 
 @example
 gpg --use-agent --sign < /dev/null > /dev/null
@@ -1181,6 +1298,38 @@ message that can be understood by PGP version 2.
 (Refer to @uref{http://www.gnupg.org/gph/en/pgp2x.html} for more
 information about the problem.)
 
+@node Encrypt-to-self
+@subsection Encrypt-to-self
+
+By default, messages are encrypted to all recipients (@code{To},
+@code{Cc}, @code{Bcc} headers).  Thus, you will not be able to decrypt
+your own messages.  To make sure that messages are also encrypted to
+your own key(s), several alternative solutions exist:
+@enumerate
+@item
+Use the @code{encrypt-to} option in the file @file{gpg.conf} (for
+OpenPGP) or @file{gpgsm.conf} (for @acronym{S/MIME} with EasyPG).
+@xref{Invoking GPG, , , gnupg, Using the GNU Privacy Guard}, or
+@xref{Invoking GPGSM, , , gnupg, Using the GNU Privacy Guard}.
+@item
+Include your own e-mail address (for which you created a key-pair)
+among the recipients.
+@item
+Customize the variable @code{mml-secure-openpgp-encrypt-to-self} (for
+OpenPGP) or @code{mml-secure-smime-encrypt-to-self} (for
+@acronym{S/MIME} with EasyPG).
+@end enumerate
+
+@node Bcc Warning
+@subsection Bcc Warning
+
+The @code{Bcc} header is meant to hide recipients of messages.
+However, when encrypted messages are used, the e-mail addresses of all
+@code{Bcc}-headers are given away to all recipients without
+warning, which is a bug, see
+@uref{https://debbugs.gnu.org/cgi/bugreport.cgi?bug=18718}.
+
+
 @node Various Commands
 @section Various Commands
 
diff --git a/lisp/gnus/gnus-util.el b/lisp/gnus/gnus-util.el
index 82a267c9e11..31645fcd315 100644
--- a/lisp/gnus/gnus-util.el
+++ b/lisp/gnus/gnus-util.el
@@ -1996,6 +1996,31 @@ to case differences."
   (defun gnus-timer--function (timer)
     (elt timer 5)))
 
+(defun gnus-test-list (list predicate)
+  "To each element of LIST apply PREDICATE.
+Return nil if LIST is no list or is empty or some test returns nil;
+otherwise, return t."
+  (when (and list (listp list))
+    (let ((result (mapcar predicate list)))
+      (not (memq nil result)))))
+
+(defun gnus-subsetp (list1 list2)
+  "Return t if LIST1 is a subset of LIST2.
+Similar to `subsetp' but use member for element test so that this works for
+lists of strings."
+  (when (and (listp list1) (listp list2))
+    (if list1
+        (and (member (car list1) list2)
+             (gnus-subsetp (cdr list1) list2))
+      t)))
+
+(defun gnus-setdiff (list1 list2)
+  "Return member-based set difference of LIST1 and LIST2."
+  (when (and list1 (listp list1) (listp list2))
+    (if (member (car list1) list2)
+        (gnus-setdiff (cdr list1) list2)
+      (cons (car list1) (gnus-setdiff (cdr list1) list2)))))
+
 (provide 'gnus-util)
 
 ;;; gnus-util.el ends here
diff --git a/lisp/gnus/mml-sec.el b/lisp/gnus/mml-sec.el
index e4c90956788..0a5f472079d 100644
--- a/lisp/gnus/mml-sec.el
+++ b/lisp/gnus/mml-sec.el
@@ -25,7 +25,9 @@
 
 (eval-when-compile (require 'cl))
 
-(autoload 'gnus-subsetp "gnus-util")
+(require 'gnus-util)
+(require 'epg)
+
 (autoload 'mail-strip-quoted-names "mail-utils")
 (autoload 'mml2015-sign "mml2015")
 (autoload 'mml2015-encrypt "mml2015")
@@ -40,6 +42,7 @@
 (autoload 'mml-smime-encrypt-query "mml-smime")
 (autoload 'mml-smime-verify "mml-smime")
 (autoload 'mml-smime-verify-test "mml-smime")
+(autoload 'epa--select-keys "epa")
 
 (defvar mml-sign-alist
   '(("smime"     mml-smime-sign-buffer     mml-smime-sign-query)
@@ -91,7 +94,7 @@ signs and encrypt the message in one step.
 
 Note that the output generated by using a `combined' mode is NOT
 understood by all PGP implementations, in particular PGP version
-2 does not support it!  See Info node `(message)Security' for
+2 does not support it!  See Info node `(message) Security' for
 details."
   :version "22.1"
   :group 'message
@@ -111,7 +114,9 @@ details."
   (if (boundp 'password-cache)
       password-cache
     t)
-  "If t, cache passphrase."
+  "If t, cache OpenPGP or S/MIME passphrases inside Emacs.
+Passphrase caching in Emacs is NOT recommended.  Use gpg-agent instead.
+See Info node `(message) Security'."
   :group 'message
   :type 'boolean)
 
@@ -125,6 +130,21 @@ Whether the passphrase is cached at all is controlled by
   :group 'message
   :type 'integer)
 
+(defcustom mml-secure-safe-bcc-list nil
+  "List of e-mail addresses that are safe to use in Bcc headers.
+EasyPG encrypts e-mails to Bcc addresses, and the encrypted e-mail
+by default identifies the used encryption keys, giving away the
+Bcc'ed identities.  Clearly, this contradicts the original goal of
+*blind* copies.
+For an academic paper explaining the problem, see URL
+`http://crypto.stanford.edu/portia/papers/bb-bcc.pdf'.
+Use this variable to specify e-mail addresses whose owners do not
+mind if they are identifiable as recipients.  This may be useful if
+you use Bcc headers to encrypt e-mails to yourself."
+  :version "25.1"
+  :group 'message
+  :type '(repeat string))
+
 ;;; Configuration/helper functions
 
 (defun mml-signencrypt-style (method &optional style)
@@ -275,6 +295,36 @@ Use METHOD if given.  Else use `mml-secure-method' or
   (interactive)
   (mml-secure-part "smime"))
 
+(defun mml-secure-is-encrypted-p ()
+  "Check whether secure encrypt tag is present."
+  (save-excursion
+    (goto-char (point-min))
+    (re-search-forward
+     (concat "^" (regexp-quote mail-header-separator) "\n"
+             "<#secure[^>]+encrypt")
+     nil t)))
+
+(defun mml-secure-bcc-is-safe ()
+  "Check whether usage of Bcc is safe (or absent).
+Bcc usage is safe in two cases: first, if the current message does
+not contain an MML secure encrypt tag;
+second, if the Bcc addresses are a subset of `mml-secure-safe-bcc-list'.
+In all other cases, ask the user whether Bcc usage is safe.
+Raise error if user answers no.
+Note that this function does not produce a meaningful return value:
+either an error is raised or not."
+  (when (mml-secure-is-encrypted-p)
+    (let ((bcc (mail-strip-quoted-names (message-fetch-field "bcc"))))
+      (when bcc
+        (let ((bcc-list (mapcar #'cadr
+                                (mail-extract-address-components bcc t))))
+          (unless (gnus-subsetp bcc-list mml-secure-safe-bcc-list)
+            (unless (yes-or-no-p "Message for encryption contains Bcc header.\
+  This may give away all Bcc'ed identities to all recipients.\
+  Are you sure that this is safe?\
+  (Customize `mml-secure-safe-bcc-list' to avoid this warning.) ")
+              (error "Aborted"))))))))
+
 ;; defuns that add the proper <#secure ...> tag to the top of the message body
 (defun mml-secure-message (method &optional modesym)
   (let ((mode (prin1-to-string modesym))
@@ -380,6 +430,529 @@ If called with a prefix argument, only encrypt (do NOT sign)."
   (interactive "P")
   (mml-secure-message "pgpauto" (if dontsign 'encrypt 'signencrypt)))
 
+;;; Common functionality for mml1991.el, mml2015.el, mml-smime.el
+
+(define-obsolete-variable-alias 'mml1991-signers 'mml-secure-openpgp-signers)
+(define-obsolete-variable-alias 'mml2015-signers 'mml-secure-openpgp-signers)
+(defcustom mml-secure-openpgp-signers nil
+  "A list of your own key ID(s) which will be used to sign OpenPGP messages.
+If set, it is added to the setting of `mml-secure-openpgp-sign-with-sender'."
+  :group 'mime-security
+  :type '(repeat (string :tag "Key ID")))
+
+(define-obsolete-variable-alias 'mml-smime-signers 'mml-secure-smime-signers)
+(defcustom mml-secure-smime-signers nil
+  "A list of your own key ID(s) which will be used to sign S/MIME messages.
+If set, it is added to the setting of `mml-secure-smime-sign-with-sender'."
+  :group 'mime-security
+  :type '(repeat (string :tag "Key ID")))
+
+(define-obsolete-variable-alias
+  'mml1991-encrypt-to-self 'mml-secure-openpgp-encrypt-to-self)
+(define-obsolete-variable-alias
+  'mml2015-encrypt-to-self 'mml-secure-openpgp-encrypt-to-self)
+(defcustom mml-secure-openpgp-encrypt-to-self nil
+  "List of own key ID(s) or t; determines additional recipients with OpenPGP.
+If t, also encrypt to key for message sender; if list, encrypt to those keys.
+With this variable, you can ensure that you can decrypt your own messages.
+Alternatives to this variable include Bcc'ing the message to yourself or
+using the encrypt-to or hidden-encrypt-to option in gpg.conf (see man gpg(1)).
+Note that this variable and the encrypt-to option give away your identity
+for *every* encryption without warning, which is not what you want if you are
+using, e.g., remailers.
+Also, use of Bcc gives away your identity for *every* encryption without
+warning, which is a bug, see:
+https://debbugs.gnu.org/cgi/bugreport.cgi?bug=18718"
+  :group 'mime-security
+  :type '(choice (const :tag "None" nil)
+                 (const :tag "From address" t)
+                 (repeat (string :tag "Key ID"))))
+
+(define-obsolete-variable-alias
+  'mml-smime-encrypt-to-self 'mml-secure-smime-encrypt-to-self)
+(defcustom mml-secure-smime-encrypt-to-self nil
+  "List of own key ID(s) or t; determines additional recipients with S/MIME.
+If t, also encrypt to key for message sender; if list, encrypt to those keys.
+With this variable, you can ensure that you can decrypt your own messages.
+Alternatives to this variable include Bcc'ing the message to yourself or
+using the encrypt-to option in gpgsm.conf (see man gpgsm(1)).
+Note that this variable and the encrypt-to option give away your identity
+for *every* encryption without warning, which is not what you want if you are
+using, e.g., remailers.
+Also, use of Bcc gives away your identity for *every* encryption without
+warning, which is a bug, see:
+https://debbugs.gnu.org/cgi/bugreport.cgi?bug=18718"
+  :group 'mime-security
+  :type '(choice (const :tag "None" nil)
+                 (const :tag "From address" t)
+                 (repeat (string :tag "Key ID"))))
+
+(define-obsolete-variable-alias
+  'mml2015-sign-with-sender 'mml-secure-openpgp-sign-with-sender)
+;mml1991-sign-with-sender did never exist.
+(defcustom mml-secure-openpgp-sign-with-sender nil
+  "If t, use message sender to find an OpenPGP key to sign with."
+  :group 'mime-security
+  :type 'boolean)
+
+(define-obsolete-variable-alias
+  'mml-smime-sign-with-sender 'mml-secure-smime-sign-with-sender)
+(defcustom mml-secure-smime-sign-with-sender nil
+  "If t, use message sender to find an S/MIME key to sign with."
+  :group 'mime-security
+  :type 'boolean)
+
+(define-obsolete-variable-alias
+  'mml2015-always-trust 'mml-secure-openpgp-always-trust)
+;mml1991-always-trust did never exist.
+(defcustom mml-secure-openpgp-always-trust t
+  "If t, skip key validation of GnuPG on encryption."
+  :group 'mime-security
+  :type 'boolean)
+
+(defcustom mml-secure-fail-when-key-problem nil
+  "If t, raise an error if some key is missing or several keys exist.
+Otherwise, ask the user."
+  :group 'mime-security
+  :type 'boolean)
+
+(defcustom mml-secure-key-preferences
+  '((OpenPGP (sign) (encrypt)) (CMS (sign) (encrypt)))
+  "Protocol- and usage-specific fingerprints of preferred keys.
+This variable is only relevant if a recipient owns multiple key pairs (for
+encryption) or you own multiple key pairs (for signing).  In such cases,
+you will be asked which key(s) should be used, and your choice can be
+customized in this variable."
+  :group 'mime-security
+  :type '(alist :key-type (symbol :tag "Protocol") :value-type
+                (alist :key-type (symbol :tag "Usage") :value-type
+                       (alist :key-type (string :tag "Name") :value-type
+                              (repeat (string :tag "Fingerprint"))))))
+
+(defun mml-secure-cust-usage-lookup (context usage)
+  "Return preferences for CONTEXT and USAGE."
+  (let* ((protocol (epg-context-protocol context))
+         (protocol-prefs (cdr (assoc protocol mml-secure-key-preferences))))
+    (assoc usage protocol-prefs)))
+
+(defun mml-secure-cust-fpr-lookup (context usage name)
+  "Return fingerprints of preferred keys for CONTEXT, USAGE, and NAME."
+  (let* ((usage-prefs (mml-secure-cust-usage-lookup context usage))
+         (fprs (assoc name (cdr usage-prefs))))
+    (when fprs
+      (cdr fprs))))
+
+(defun mml-secure-cust-record-keys (context usage name keys &optional save)
+  "For CONTEXT, USAGE, and NAME record fingerprint(s) of KEYS.
+If optional SAVE is not nil, save customized fingerprints.
+Return keys."
+  (assert keys)
+  (let* ((usage-prefs (mml-secure-cust-usage-lookup context usage))
+         (curr-fprs (cdr (assoc name (cdr usage-prefs))))
+         (key-fprs (mapcar 'mml-secure-fingerprint keys))
+         (new-fprs (gnus-union curr-fprs key-fprs :test 'equal)))
+    (if curr-fprs
+        (setcdr (assoc name (cdr usage-prefs)) new-fprs)
+      (setcdr usage-prefs (cons (cons name new-fprs) (cdr usage-prefs))))
+    (when save
+        (customize-save-variable
+         'mml-secure-key-preferences mml-secure-key-preferences))
+    keys))
+
+(defun mml-secure-cust-remove-keys (context usage name)
+  "Remove keys for CONTEXT, USAGE, and NAME.
+Return t if a customization for NAME was present (and has been removed)."
+  (let* ((usage-prefs (mml-secure-cust-usage-lookup context usage))
+         (current (assoc name usage-prefs)))
+    (when current
+      (setcdr usage-prefs (remove current (cdr usage-prefs)))
+      t)))
+
+(defvar mml-secure-secret-key-id-list nil)
+
+(defun mml-secure-add-secret-key-id (key-id)
+  "Record KEY-ID in list of secret keys."
+  (add-to-list 'mml-secure-secret-key-id-list key-id))
+
+(defun mml-secure-clear-secret-key-id-list ()
+  "Remove passwords from cache and clear list of secret keys."
+  ;; Loosely based on code inside mml2015-epg-encrypt,
+  ;; mml2015-epg-clear-decrypt, and mml2015-epg-decrypt
+  (dolist (key-id mml-secure-secret-key-id-list nil)
+    (password-cache-remove key-id))
+  (setq mml-secure-secret-key-id-list nil))
+
+(defvar mml1991-cache-passphrase)
+(defvar mml1991-passphrase-cache-expiry)
+
+(defun mml-secure-cache-passphrase-p (protocol)
+  "Return t if OpenPGP or S/MIME passphrases should be cached for PROTOCOL.
+Passphrase caching in Emacs is NOT recommended.  Use gpg-agent instead."
+  (or (and (eq 'OpenPGP protocol)
+           (or mml-secure-cache-passphrase
+               (and (boundp 'mml2015-cache-passphrase)
+                    mml2015-cache-passphrase)
+               (and (boundp 'mml1991-cache-passphrase)
+                    mml1991-cache-passphrase)))
+      (and (eq 'CMS protocol)
+           (or mml-secure-cache-passphrase
+               (and (boundp 'mml-smime-cache-passphrase)
+                    mml-smime-cache-passphrase)))))
+
+(defun mml-secure-cache-expiry-interval (protocol)
+  "Return time in seconds to cache passphrases for PROTOCOL.
+Passphrase caching in Emacs is NOT recommended.  Use gpg-agent instead."
+  (or (and (eq 'OpenPGP protocol)
+           (or (and (boundp 'mml2015-passphrase-cache-expiry)
+                    mml2015-passphrase-cache-expiry)
+               (and (boundp 'mml1991-passphrase-cache-expiry)
+                    mml1991-passphrase-cache-expiry)
+               mml-secure-passphrase-cache-expiry))
+      (and (eq 'CMS protocol)
+           (or (and (boundp 'mml-smime-passphrase-cache-expiry)
+                    mml-smime-passphrase-cache-expiry)
+               mml-secure-passphrase-cache-expiry))))
+
+(defun mml-secure-passphrase-callback (context key-id standard)
+  "Ask for passphrase in CONTEXT for KEY-ID for STANDARD.
+The passphrase is read and cached."
+  ;; Based on mml2015-epg-passphrase-callback.
+  (if (eq key-id 'SYM)
+      (epg-passphrase-callback-function context key-id nil)
+    (let* ((password-cache-key-id
+            (if (eq key-id 'PIN)
+                "PIN"
+               key-id))
+           (entry (assoc key-id epg-user-id-alist))
+           (passphrase
+            (password-read
+             (if (eq key-id 'PIN)
+                 "Passphrase for PIN: "
+               (if entry
+                   (format "Passphrase for %s %s: " key-id (cdr entry))
+                 (format "Passphrase for %s: " key-id)))
+             ;; TODO: With mml-smime.el, password-cache-key-id is not passed
+             ;; as argument to password-read.
+             ;; Is that on purpose?  If so, the following needs to be placed
+             ;; inside an if statement.
+             password-cache-key-id)))
+      (when passphrase
+        (let ((password-cache-expiry (mml-secure-cache-expiry-interval
+                                      (epg-context-protocol context))))
+          (password-cache-add password-cache-key-id passphrase))
+        (mml-secure-add-secret-key-id password-cache-key-id)
+        (copy-sequence passphrase)))))
+
+(defun mml-secure-check-user-id (key recipient)
+  "Check whether KEY has a non-revoked, non-expired UID for RECIPIENT."
+  ;; Based on mml2015-epg-check-user-id.
+  (let ((uids (epg-key-user-id-list key)))
+    (catch 'break
+      (dolist (uid uids nil)
+        (if (and (stringp (epg-user-id-string uid))
+                 (equal (car (mail-header-parse-address
+                              (epg-user-id-string uid)))
+                        (car (mail-header-parse-address
+                              recipient)))
+                 (not (memq (epg-user-id-validity uid)
+                            '(revoked expired))))
+            (throw 'break t))))))
+
+(defun mml-secure-secret-key-exists-p (context subkey)
+  "Return t if keyring for CONTEXT contains secret key for public SUBKEY."
+  (let* ((fpr (epg-sub-key-fingerprint subkey))
+         (candidates (epg-list-keys context fpr 'secret))
+         (candno (length candidates)))
+    ;; If two or more subkeys with the same fingerprint exist, something is
+    ;; terribly wrong.
+    (when (>= candno 2)
+      (error "Found %d secret keys with same fingerprint %s" candno fpr))
+    (= 1 candno)))
+
+(defun mml-secure-check-sub-key (context key usage &optional fingerprint)
+  "Check whether in CONTEXT the public KEY has a usable subkey for USAGE.
+This is the case if KEY is not disabled, and there is a subkey for
+USAGE that is neither revoked nor expired.  Additionally, if optional
+FINGERPRINT is present and if it is not the primary key's fingerprint, then
+the returned subkey must have that FINGERPRINT.  FINGERPRINT must consist of
+hexadecimal digits only (no leading \"0x\" allowed).
+If USAGE is not `encrypt', then additionally an appropriate secret key must
+be present in the keyring."
+  ;; Based on mml2015-epg-check-sub-key, extended by
+  ;; - check for secret keys if usage is not 'encrypt and
+  ;; - check for new argument FINGERPRINT.
+  (let* ((subkeys (epg-key-sub-key-list key))
+         (primary (car subkeys))
+         (fpr (epg-sub-key-fingerprint primary)))
+    ;; The primary key will be marked as disabled, when the entire
+    ;; key is disabled (see 12 Field, Format of colon listings, in
+    ;; gnupg/doc/DETAILS)
+    (unless (memq 'disabled (epg-sub-key-capability primary))
+      (catch 'break
+        (dolist (subkey subkeys nil)
+          (if (and (memq usage (epg-sub-key-capability subkey))
+                   (not (memq (epg-sub-key-validity subkey)
+                              '(revoked expired)))
+                   (or (eq 'encrypt usage) ; Encryption works with public key.
+                       ;; In contrast, signing requires secret key.
+                       (mml-secure-secret-key-exists-p context subkey))
+                   (or (not fingerprint)
+                       (gnus-string-match-p (concat fingerprint "$") fpr)
+                       (gnus-string-match-p (concat fingerprint "$")
+                                            (epg-sub-key-fingerprint subkey))))
+              (throw 'break t)))))))
+
+(defun mml-secure-find-usable-keys (context name usage &optional justone)
+  "In CONTEXT return a list of keys for NAME and USAGE.
+If USAGE is `encrypt' public keys are returned, otherwise secret ones.
+Only non-revoked and non-expired keys are returned whose primary key is
+not disabled.
+NAME can be an e-mail address or a key ID.
+If NAME just consists of hexadecimal digits (possibly prefixed by \"0x\"), it
+is treated as key ID for which at most one key must exist in the keyring.
+Otherwise, NAME is treated as user ID, for which no keys are returned if it
+is expired or revoked.
+If optional JUSTONE is not nil, return the first key instead of a list."
+  (let* ((keys (epg-list-keys context name))
+         (iskeyid (string-match "\\(0x\\)?\\([0-9a-fA-F]\\{8,\\}\\)" name))
+         (fingerprint (match-string 2 name))
+         result)
+    (when (and iskeyid (>= (length keys) 2))
+      (error
+       "Name %s (for %s) looks like a key ID but multiple keys found"
+       name usage))
+    (catch 'break
+      (dolist (key keys result)
+        (if (and (or iskeyid
+                     (mml-secure-check-user-id key name))
+                 (mml-secure-check-sub-key context key usage fingerprint))
+            (if justone
+                (throw 'break key)
+              (push key result)))))))
+
+(defun mml-secure-select-preferred-keys (context names usage)
+  "Return list of preferred keys in CONTEXT for NAMES and USAGE.
+This inspects the keyrings to find keys for each name in NAMES.  If several
+keys are found for a name, `mml-secure-select-keys' is used to look for
+customized preferences or have the user select preferable ones.
+When `mml-secure-fail-when-key-problem' is t, fail with an error in
+case of missing, outdated, or multiple keys."
+  ;; Loosely based on code appearing inside mml2015-epg-sign and
+  ;; mml2015-epg-encrypt.
+  (apply
+   #'nconc
+   (mapcar
+    (lambda (name)
+      (let* ((keys (mml-secure-find-usable-keys context name usage))
+             (keyno (length keys)))
+        (cond ((= 0 keyno)
+               (when (or mml-secure-fail-when-key-problem
+                         (not (y-or-n-p
+                               (format "No %s key for %s; skip it? "
+                                       usage name))))
+                 (error "No %s key for %s" usage name)))
+              ((= 1 keyno) keys)
+              (t (mml-secure-select-keys context name keys usage)))))
+    names)))
+
+(defun mml-secure-fingerprint (key)
+  "Return fingerprint for public KEY."
+  (epg-sub-key-fingerprint (car (epg-key-sub-key-list key))))
+
+(defun mml-secure-filter-keys (keys fprs)
+  "Filter KEYS to subset with fingerprints in FPRS."
+  (when keys
+    (if (member (mml-secure-fingerprint (car keys)) fprs)
+        (cons (car keys) (mml-secure-filter-keys (cdr keys) fprs))
+      (mml-secure-filter-keys (cdr keys) fprs))))
+
+(defun mml-secure-normalize-cust-name (name)
+  "Normalize NAME to be used for customization.
+Currently, remove ankle brackets."
+  (if (string-match "^<\\(.*\\)>$" name)
+      (match-string 1 name)
+    name))
+
+(defun mml-secure-select-keys (context name keys usage)
+  "In CONTEXT for NAME select among KEYS for USAGE.
+KEYS should be a list with multiple entries.
+NAME is normalized first as customized keys are inspected.
+When `mml-secure-fail-when-key-problem' is t, fail with an error in case of
+outdated or multiple keys."
+  (let* ((nname (mml-secure-normalize-cust-name name))
+         (fprs (mml-secure-cust-fpr-lookup context usage nname))
+         (usable-fprs (mapcar 'mml-secure-fingerprint keys)))
+    (if fprs
+        (if (gnus-subsetp fprs usable-fprs)
+            (mml-secure-filter-keys keys fprs)
+          (mml-secure-cust-remove-keys context usage nname)
+          (let ((diff (gnus-setdiff fprs usable-fprs)))
+            (if mml-secure-fail-when-key-problem
+                (error "Customization of %s keys for %s outdated" usage nname)
+              (mml-secure-select-keys-1
+               context nname keys usage (format "\
+Customized keys
+ (%s)
+for %s not available any more.
+Select anew.  "
+                                               diff nname)))))
+      (if mml-secure-fail-when-key-problem
+          (error "Multiple %s keys for %s" usage nname)
+        (mml-secure-select-keys-1
+         context nname keys usage (format "\
+Multiple %s keys for:
+ %s
+Select preferred one(s).  "
+                                         usage nname))))))
+
+(defun mml-secure-select-keys-1 (context name keys usage message)
+  "In CONTEXT for NAME let user select among KEYS for USAGE, showing MESSAGE.
+Return selected keys."
+  (let* ((selected (epa--select-keys message keys))
+         (selno (length selected))
+         ;; TODO: y-or-n-p does not always resize the echo area but may
+         ;; truncate the message.  Why?  The following does not help.
+         ;; yes-or-no-p shows full message, though.
+         (message-truncate-lines nil))
+    (if selected
+        (if (y-or-n-p
+             (format "%d %s key(s) selected.  Store for %s? "
+                     selno usage name))
+            (mml-secure-cust-record-keys context usage name selected 'save)
+          selected)
+      (unless (y-or-n-p
+               (format "No %s key for %s; skip it? " usage name))
+        (error "No %s key for %s" usage name)))))
+
+(defun mml-secure-signer-names (protocol sender)
+  "Determine signer names for PROTOCOL and message from SENDER.
+Returned names may be e-mail addresses or key IDs and are determined based
+on `mml-secure-openpgp-signers' and `mml-secure-openpgp-sign-with-sender' with
+OpenPGP or `mml-secure-smime-signers' and `mml-secure-smime-sign-with-sender'
+with S/MIME."
+  (if (eq 'OpenPGP protocol)
+      (append mml-secure-openpgp-signers
+              (if (and mml-secure-openpgp-sign-with-sender sender)
+                  (list (concat "<" sender ">"))))
+    (append mml-secure-smime-signers
+            (if (and mml-secure-smime-sign-with-sender sender)
+                (list (concat "<" sender ">"))))))
+
+(defun mml-secure-signers (context signer-names)
+  "Determine signing keys in CONTEXT from SIGNER-NAMES.
+If `mm-sign-option' is `guided', the user is asked to choose.
+Otherwise, `mml-secure-select-preferred-keys' is used."
+  ;; Based on code appearing inside mml2015-epg-sign and
+  ;; mml2015-epg-encrypt.
+  (if (eq mm-sign-option 'guided)
+      (epa-select-keys context "\
+Select keys for signing.
+If no one is selected, default secret key is used.  "
+                       signer-names t)
+    (mml-secure-select-preferred-keys context signer-names 'sign)))
+
+(defun mml-secure-self-recipients (protocol sender)
+  "Determine additional recipients based on encrypt-to-self variables.
+PROTOCOL specifies OpenPGP or S/MIME for a message from SENDER."
+  (let ((encrypt-to-self
+         (if (eq 'OpenPGP protocol)
+             mml-secure-openpgp-encrypt-to-self
+           mml-secure-smime-encrypt-to-self)))
+    (when encrypt-to-self
+      (if (listp encrypt-to-self)
+          encrypt-to-self
+        (list sender)))))
+
+(defun mml-secure-recipients (protocol context config sender)
+  "Determine encryption recipients.
+PROTOCOL specifies OpenPGP or S/MIME with matching CONTEXT and CONFIG
+for a message from SENDER."
+  ;; Based on code appearing inside mml2015-epg-encrypt.
+  (let ((recipients
+         (apply #'nconc
+                (mapcar
+                 (lambda (recipient)
+                   (or (epg-expand-group config recipient)
+                       (list (concat "<" recipient ">"))))
+                 (split-string
+                  (or (message-options-get 'message-recipients)
+                      (message-options-set 'message-recipients
+                                           (read-string "Recipients: ")))
+                  "[ \f\t\n\r\v,]+")))))
+    (nconc recipients (mml-secure-self-recipients protocol sender))
+    (if (eq mm-encrypt-option 'guided)
+        (setq recipients
+              (epa-select-keys context "\
+Select recipients for encryption.
+If no one is selected, symmetric encryption will be performed.  "
+                               recipients))
+      (setq recipients
+            (mml-secure-select-preferred-keys context recipients 'encrypt))
+      (unless recipients
+        (error "No recipient specified")))
+    recipients))
+
+(defun mml-secure-epg-encrypt (protocol cont &optional sign)
+  ;; Based on code appearing inside mml2015-epg-encrypt.
+  (let* ((context (epg-make-context protocol))
+         (config (epg-configuration))
+         (sender (message-options-get 'message-sender))
+         (recipients (mml-secure-recipients protocol context config sender))
+         (signer-names (mml-secure-signer-names protocol sender))
+         cipher signers)
+    (when sign
+      (setq signers (mml-secure-signers context signer-names))
+      (epg-context-set-signers context signers))
+    (when (eq 'OpenPGP protocol)
+      (epg-context-set-armor context t)
+      (epg-context-set-textmode context t))
+    (when (mml-secure-cache-passphrase-p protocol)
+      (epg-context-set-passphrase-callback
+       context
+       (cons 'mml-secure-passphrase-callback protocol)))
+    (condition-case error
+        (setq cipher
+              (if (eq 'OpenPGP protocol)
+                  (epg-encrypt-string context (buffer-string) recipients sign
+                                      mml-secure-openpgp-always-trust)
+                (epg-encrypt-string context (buffer-string) recipients))
+              mml-secure-secret-key-id-list nil)
+      (error
+       (mml-secure-clear-secret-key-id-list)
+       (signal (car error) (cdr error))))
+    cipher))
+
+(defun mml-secure-epg-sign (protocol mode)
+  ;; Based on code appearing inside mml2015-epg-sign.
+  (let* ((context (epg-make-context protocol))
+         (sender (message-options-get 'message-sender))
+         (signer-names (mml-secure-signer-names protocol sender))
+         (signers (mml-secure-signers context signer-names))
+         signature micalg)
+    (when (eq 'OpenPGP protocol)
+      (epg-context-set-armor context t)
+      (epg-context-set-textmode context t))
+    (epg-context-set-signers context signers)
+    (when (mml-secure-cache-passphrase-p protocol)
+      (epg-context-set-passphrase-callback
+       context
+       (cons 'mml-secure-passphrase-callback protocol)))
+    (condition-case error
+        (setq signature
+              (if (eq 'OpenPGP protocol)
+                  (epg-sign-string context (buffer-string) mode)
+                (epg-sign-string context
+                                 (mm-replace-in-string (buffer-string)
+                                                       "\n" "\r\n") t))
+              mml-secure-secret-key-id-list nil)
+      (error
+       (mml-secure-clear-secret-key-id-list)
+       (signal (car error) (cdr error))))
+    (if (epg-context-result-for context 'sign)
+        (setq micalg (epg-new-signature-digest-algorithm
+                      (car (epg-context-result-for context 'sign)))))
+    (cons signature micalg)))
+
 (provide 'mml-sec)
 
 ;;; mml-sec.el ends here
diff --git a/lisp/gnus/mml-smime.el b/lisp/gnus/mml-smime.el
index b19c9e89ba9..a40595ecbd5 100644
--- a/lisp/gnus/mml-smime.el
+++ b/lisp/gnus/mml-smime.el
@@ -32,9 +32,17 @@
 (autoload 'message-narrow-to-headers "message")
 (autoload 'message-fetch-field "message")
 
+;; Prefer epg over openssl if it is available as epg uses GnuPG's gpgsm,
+;; which features full-fledged certificate management, while openssl requires
+;; major manual efforts for certificate revocation and expiry and has bugs
+;; as documented under man smime(1).
+(ignore-errors (require 'epg))
+
 (defcustom mml-smime-use (if (featurep 'epg) 'epg 'openssl)
-  "Whether to use OpenSSL or EPG to decrypt S/MIME messages.
-Defaults to EPG if it's loaded."
+  "Whether to use OpenSSL or EasyPG (EPG) to handle S/MIME messages.
+Defaults to EPG if it's available.
+If you think about using OpenSSL, please read the BUGS section in the manual
+for the `smime' command coming with OpenSSL first.  EasyPG is recommended."
   :group 'mime-security
   :type '(choice (const :tag "EPG" epg)
                  (const :tag "OpenSSL" openssl)))
@@ -57,6 +65,9 @@ Defaults to EPG if it's loaded."
   "If t, cache passphrase."
   :group 'mime-security
   :type 'boolean)
+(make-obsolete-variable 'mml-smime-cache-passphrase
+                        'mml-secure-cache-passphrase
+                        "25.1")
 
 (defcustom mml-smime-passphrase-cache-expiry mml-secure-passphrase-cache-expiry
   "How many seconds the passphrase is cached.
@@ -64,6 +75,9 @@ Whether the passphrase is cached at all is controlled by
 `mml-smime-cache-passphrase'."
   :group 'mime-security
   :type 'integer)
+(make-obsolete-variable 'mml-smime-passphrase-cache-expiry
+                        'mml-secure-passphrase-cache-expiry
+                        "25.1")
 
 (defcustom mml-smime-signers nil
   "A list of your own key ID which will be used to sign a message."
@@ -202,7 +216,7 @@ Whether the passphrase is cached at all is controlled by
                                         "")))))
           (if (setq cert (smime-cert-by-dns who))
               (setq result (list 'certfile (buffer-name cert)))
-            (setq bad (gnus-format-message "`%s' not found. " who))))
+            (setq bad (format "`%s' not found. " who))))
       (quit))
     result))
 
@@ -221,7 +235,7 @@ Whether the passphrase is cached at all is controlled by
                                         "")))))
           (if (setq cert (smime-cert-by-ldap who))
               (setq result (list 'certfile (buffer-name cert)))
-            (setq bad (gnus-format-message "`%s' not found. " who))))
+            (setq bad (format "`%s' not found. " who))))
       (quit))
     result))
 
@@ -317,82 +331,28 @@ Whether the passphrase is cached at all is controlled by
 (defvar inhibit-redisplay)
 (defvar password-cache-expiry)
 
-(autoload 'epg-make-context "epg")
-(autoload 'epg-passphrase-callback-function "epg")
-(declare-function epg-context-set-signers "epg" (context signers))
-(declare-function epg-context-result-for "epg" (context name))
-(declare-function epg-new-signature-digest-algorithm "epg" (cl-x) t)
-(declare-function epg-verify-result-to-string "epg" (verify-result))
-(declare-function epg-list-keys "epg" (context &optional name mode))
-(declare-function epg-verify-string "epg"
-                  (context signature &optional signed-text))
-(declare-function epg-sign-string "epg" (context plain &optional mode))
-(declare-function epg-encrypt-string "epg"
-                  (context plain recipients &optional sign always-trust))
-(declare-function epg-context-set-passphrase-callback "epg"
-                  (context passphrase-callback))
-(declare-function epg-sub-key-fingerprint "epg" (cl-x) t)
-(declare-function epg-configuration "epg-config" ())
-(declare-function epg-expand-group "epg-config" (config group))
-(declare-function epa-select-keys "epa"
-                  (context prompt &optional names secret))
-
-(defvar mml-smime-epg-secret-key-id-list nil)
-
-(defun mml-smime-epg-passphrase-callback (context key-id ignore)
-  (if (eq key-id 'SYM)
-      (epg-passphrase-callback-function context key-id nil)
-    (let* (entry
-           (passphrase
-            (password-read
-             (if (eq key-id 'PIN)
-                 "Passphrase for PIN: "
-               (if (setq entry (assoc key-id epg-user-id-alist))
-                   (format "Passphrase for %s %s: " key-id (cdr entry))
-                 (format "Passphrase for %s: " key-id)))
-             (if (eq key-id 'PIN)
-                 "PIN"
-               key-id))))
-      (when passphrase
-        (let ((password-cache-expiry mml-smime-passphrase-cache-expiry))
-          (password-cache-add key-id passphrase))
-        (setq mml-smime-epg-secret-key-id-list
-              (cons key-id mml-smime-epg-secret-key-id-list))
-        (copy-sequence passphrase)))))
-
-(declare-function epg-key-sub-key-list   "epg" (key) t)
-(declare-function epg-sub-key-capability "epg" (sub-key) t)
-(declare-function epg-sub-key-validity   "epg" (sub-key) t)
-
-(defun mml-smime-epg-find-usable-key (keys usage)
-  (catch 'found
-    (while keys
-      (let ((pointer (epg-key-sub-key-list (car keys))))
-        (while pointer
-          (if (and (memq usage (epg-sub-key-capability (car pointer)))
-                   (not (memq (epg-sub-key-validity (car pointer))
-                              '(revoked expired))))
-              (throw 'found (car keys)))
-          (setq pointer (cdr pointer))))
-      (setq keys (cdr keys)))))
-
-;; XXX: since gpg --list-secret-keys does not return validity of each
-;; key, `mml-smime-epg-find-usable-key' defined above is not enough for
-;; secret keys.  The function `mml-smime-epg-find-usable-secret-key'
-;; below looks at appropriate public keys to check usability.
-(defun mml-smime-epg-find-usable-secret-key (context name usage)
-  (let ((secret-keys (epg-list-keys context name t))
-        secret-key)
-    (while (and (not secret-key) secret-keys)
-      (if (mml-smime-epg-find-usable-key
-           (epg-list-keys context (epg-sub-key-fingerprint
-                                   (car (epg-key-sub-key-list
-                                         (car secret-keys)))))
-           usage)
-          (setq secret-key (car secret-keys)
-                secret-keys nil)
-        (setq secret-keys (cdr secret-keys))))
-    secret-key))
+(eval-when-compile
+  (autoload 'epg-make-context "epg")
+  (autoload 'epg-context-set-armor "epg")
+  (autoload 'epg-context-set-signers "epg")
+  (autoload 'epg-context-result-for "epg")
+  (autoload 'epg-new-signature-digest-algorithm "epg")
+  (autoload 'epg-verify-result-to-string "epg")
+  (autoload 'epg-list-keys "epg")
+  (autoload 'epg-decrypt-string "epg")
+  (autoload 'epg-verify-string "epg")
+  (autoload 'epg-sign-string "epg")
+  (autoload 'epg-encrypt-string "epg")
+  (autoload 'epg-passphrase-callback-function "epg")
+  (autoload 'epg-context-set-passphrase-callback "epg")
+  (autoload 'epg-sub-key-fingerprint "epg")
+  (autoload 'epg-configuration "epg-config")
+  (autoload 'epg-expand-group "epg-config")
+  (autoload 'epa-select-keys "epa"))
+
+(declare-function epg-key-sub-key-list   "ext:epg" (key))
+(declare-function epg-sub-key-capability "ext:epg" (sub-key))
+(declare-function epg-sub-key-validity   "ext:epg" (sub-key))
 
 (autoload 'mml-compute-boundary "mml")
 
@@ -401,146 +361,37 @@ Whether the passphrase is cached at all is controlled by
 (declare-function message-options-set "message" (symbol value))
 
 (defun mml-smime-epg-sign (cont)
-  (let* ((inhibit-redisplay t)
-         (context (epg-make-context 'CMS))
-         (boundary (mml-compute-boundary cont))
-         (sender (message-options-get 'message-sender))
-         (signer-names (or mml-smime-signers
-                           (if (and mml-smime-sign-with-sender sender)
-                               (list (concat "<" sender ">")))))
-         signer-key
-         (signers
-          (or (message-options-get 'mml-smime-epg-signers)
-              (message-options-set
-               'mml-smime-epg-signers
-               (if (eq mm-sign-option 'guided)
-                   (epa-select-keys context "\
-Select keys for signing.
-If no one is selected, default secret key is used.  "
-                                    signer-names
-                                    t)
-                 (if (or sender mml-smime-signers)
-                     (delq nil
-                           (mapcar
-                            (lambda (signer)
-                              (setq signer-key
-                                    (mml-smime-epg-find-usable-secret-key
-                                     context signer 'sign))
-                              (unless (or signer-key
-                                          (y-or-n-p
-                                           (format
-                                            "No secret key for %s; skip it? "
-                                            signer)))
-                                (error "No secret key for %s" signer))
-                              signer-key)
-                            signer-names)))))))
-         signature micalg)
-    (epg-context-set-signers context signers)
-    (if mml-smime-cache-passphrase
-        (epg-context-set-passphrase-callback
-         context
-         #'mml-smime-epg-passphrase-callback))
-    (condition-case error
-        (setq signature (epg-sign-string context
-                                         (mm-replace-in-string (buffer-string)
-                                                               "\n" "\r\n")
-                                         t)
-              mml-smime-epg-secret-key-id-list nil)
-      (error
-       (while mml-smime-epg-secret-key-id-list
-         (password-cache-remove (car mml-smime-epg-secret-key-id-list))
-         (setq mml-smime-epg-secret-key-id-list
-               (cdr mml-smime-epg-secret-key-id-list)))
-       (signal (car error) (cdr error))))
-    (if (epg-context-result-for context 'sign)
-        (setq micalg (epg-new-signature-digest-algorithm
-                      (car (epg-context-result-for context 'sign)))))
+  (let ((inhibit-redisplay t)
+        (boundary (mml-compute-boundary cont)))
     (goto-char (point-min))
-    (insert (format "Content-Type: multipart/signed; boundary=\"%s\";\n"
-                    boundary))
-    (if micalg
-        (insert (format "\tmicalg=%s; "
-                        (downcase
-                         (cdr (assq micalg
-                                    epg-digest-algorithm-alist))))))
-    (insert "protocol=\"application/pkcs7-signature\"\n")
-    (insert (format "\n--%s\n" boundary))
-    (goto-char (point-max))
-    (insert (format "\n--%s\n" boundary))
-    (insert "Content-Type: application/pkcs7-signature; name=smime.p7s
+    (let* ((pair (mml-secure-epg-sign 'CMS cont))
+           (signature (car pair))
+           (micalg (cdr pair)))
+      (insert (format "Content-Type: multipart/signed; boundary=\"%s\";\n"
+                      boundary))
+      (if micalg
+          (insert (format "\tmicalg=%s; "
+                          (downcase
+                           (cdr (assq micalg
+                                      epg-digest-algorithm-alist))))))
+      (insert "protocol=\"application/pkcs7-signature\"\n")
+      (insert (format "\n--%s\n" boundary))
+      (goto-char (point-max))
+      (insert (format "\n--%s\n" boundary))
+      (insert "Content-Type: application/pkcs7-signature; name=smime.p7s
 Content-Transfer-Encoding: base64
 Content-Disposition: attachment; filename=smime.p7s
 
 ")
-    (insert (base64-encode-string signature) "\n")
-    (goto-char (point-max))
-    (insert (format "--%s--\n" boundary))
-    (goto-char (point-max))))
+      (insert (base64-encode-string signature) "\n")
+      (goto-char (point-max))
+      (insert (format "--%s--\n" boundary))
+      (goto-char (point-max)))))
 
 (defun mml-smime-epg-encrypt (cont)
   (let* ((inhibit-redisplay t)
-         (context (epg-make-context 'CMS))
-         (config (epg-configuration))
-         (recipients (message-options-get 'mml-smime-epg-recipients))
-         cipher signers
-         (sender (message-options-get 'message-sender))
-         (signer-names (or mml-smime-signers
-                           (if (and mml-smime-sign-with-sender sender)
-                               (list (concat "<" sender ">")))))
          (boundary (mml-compute-boundary cont))
-         recipient-key)
-    (unless recipients
-      (setq recipients
-            (apply #'nconc
-                   (mapcar
-                    (lambda (recipient)
-                      (or (epg-expand-group config recipient)
-                          (list recipient)))
-                    (split-string
-                     (or (message-options-get 'message-recipients)
-                         (message-options-set 'message-recipients
-                                              (read-string "Recipients: ")))
-                     "[ \f\t\n\r\v,]+"))))
-      (when mml-smime-encrypt-to-self
-        (unless signer-names
-          (error "Neither message sender nor mml-smime-signers are set"))
-        (setq recipients (nconc recipients signer-names)))
-      (if (eq mm-encrypt-option 'guided)
-          (setq recipients
-                (epa-select-keys context "\
-Select recipients for encryption.
-If no one is selected, symmetric encryption will be performed.  "
-                                 recipients))
-        (setq recipients
-              (mapcar
-               (lambda (recipient)
-                 (setq recipient-key (mml-smime-epg-find-usable-key
-                                      (epg-list-keys context recipient)
-                                      'encrypt))
-                 (unless (or recipient-key
-                             (y-or-n-p
-                              (format "No public key for %s; skip it? "
-                                      recipient)))
-                   (error "No public key for %s" recipient))
-                 recipient-key)
-               recipients))
-        (unless recipients
-          (error "No recipient specified")))
-      (message-options-set 'mml-smime-epg-recipients recipients))
-    (if mml-smime-cache-passphrase
-        (epg-context-set-passphrase-callback
-         context
-         #'mml-smime-epg-passphrase-callback))
-    (condition-case error
-        (setq cipher
-              (epg-encrypt-string context (buffer-string) recipients)
-              mml-smime-epg-secret-key-id-list nil)
-      (error
-       (while mml-smime-epg-secret-key-id-list
-         (password-cache-remove (car mml-smime-epg-secret-key-id-list))
-         (setq mml-smime-epg-secret-key-id-list
-               (cdr mml-smime-epg-secret-key-id-list)))
-       (signal (car error) (cdr error))))
+         (cipher (mml-secure-epg-encrypt 'CMS cont)))
     (delete-region (point-min) (point-max))
     (goto-char (point-min))
     (insert "\
diff --git a/lisp/gnus/mml1991.el b/lisp/gnus/mml1991.el
index 6469636451f..bb5c940f173 100644
--- a/lisp/gnus/mml1991.el
+++ b/lisp/gnus/mml1991.el
@@ -63,11 +63,17 @@
 
 (defvar mml1991-cache-passphrase mml-secure-cache-passphrase
   "If t, cache passphrase.")
+(make-obsolete-variable 'mml1991-cache-passphrase
+                        'mml-secure-cache-passphrase
+                        "25.1")
 
 (defvar mml1991-passphrase-cache-expiry mml-secure-passphrase-cache-expiry
   "How many seconds the passphrase is cached.
 Whether the passphrase is cached at all is controlled by
 `mml1991-cache-passphrase'.")
+(make-obsolete-variable 'mml1991-passphrase-cache-expiry
+                        'mml-secure-passphrase-cache-expiry
+                        "25.1")
 
 (defvar mml1991-signers nil
   "A list of your own key ID which will be used to sign a message.")
@@ -75,6 +81,7 @@ Whether the passphrase is cached at all is controlled by
 (defvar mml1991-encrypt-to-self nil
   "If t, add your own key ID to recipient list when encryption.")
 
+
 ;;; mailcrypt wrapper
 
 (autoload 'mc-sign-generic "mc-toplev")
@@ -255,91 +262,9 @@ Whether the passphrase is cached at all is controlled by
 (autoload 'epg-configuration "epg-config")
 (autoload 'epg-expand-group "epg-config")
 
-(defvar mml1991-epg-secret-key-id-list nil)
-
-(defun mml1991-epg-passphrase-callback (context key-id ignore)
-  (if (eq key-id 'SYM)
-      (epg-passphrase-callback-function context key-id nil)
-    (let* ((entry (assoc key-id epg-user-id-alist))
-           (passphrase
-            (password-read
-             (format "GnuPG passphrase for %s: "
-                     (if entry
-                         (cdr entry)
-                       key-id))
-             (if (eq key-id 'PIN)
-                 "PIN"
-               key-id))))
-      (when passphrase
-        (let ((password-cache-expiry mml1991-passphrase-cache-expiry))
-          (password-cache-add key-id passphrase))
-        (setq mml1991-epg-secret-key-id-list
-              (cons key-id mml1991-epg-secret-key-id-list))
-        (copy-sequence passphrase)))))
-
-(defun mml1991-epg-find-usable-key (keys usage)
-  (catch 'found
-    (while keys
-      (let ((pointer (epg-key-sub-key-list (car keys))))
-        ;; The primary key will be marked as disabled, when the entire
-        ;; key is disabled (see 12 Field, Format of colon listings, in
-        ;; gnupg/doc/DETAILS)
-        (unless (memq 'disabled (epg-sub-key-capability (car pointer)))
-          (while pointer
-            (if (and (memq usage (epg-sub-key-capability (car pointer)))
-                     (not (memq (epg-sub-key-validity (car pointer))
-                                '(revoked expired))))
-                (throw 'found (car keys)))
-            (setq pointer (cdr pointer)))))
-      (setq keys (cdr keys)))))
-
-;; XXX: since gpg --list-secret-keys does not return validity of each
-;; key, `mml1991-epg-find-usable-key' defined above is not enough for
-;; secret keys.  The function `mml1991-epg-find-usable-secret-key'
-;; below looks at appropriate public keys to check usability.
-(defun mml1991-epg-find-usable-secret-key (context name usage)
-  (let ((secret-keys (epg-list-keys context name t))
-        secret-key)
-    (while (and (not secret-key) secret-keys)
-      (if (mml1991-epg-find-usable-key
-           (epg-list-keys context (epg-sub-key-fingerprint
-                                   (car (epg-key-sub-key-list
-                                         (car secret-keys)))))
-           usage)
-          (setq secret-key (car secret-keys)
-                secret-keys nil)
-        (setq secret-keys (cdr secret-keys))))
-    secret-key))
-
 (defun mml1991-epg-sign (cont)
-  (let ((context (epg-make-context))
-        headers cte signer-key signers signature)
-    (if (eq mm-sign-option 'guided)
-        (setq signers (epa-select-keys context "Select keys for signing.
-If no one is selected, default secret key is used.  "
-                                       mml1991-signers t))
-      (if mml1991-signers
-          (setq signers (delq nil
-                              (mapcar
-                               (lambda (name)
-                                 (setq signer-key
-                                       (mml1991-epg-find-usable-secret-key
-                                        context name 'sign))
-                                 (unless (or signer-key
-                                             (y-or-n-p
-                                              (format
-                                               "No secret key for %s; skip it? "
-                                               name)))
-                                   (error "No secret key for %s" name))
-                                 signer-key)
-                               mml1991-signers)))))
-    (epg-context-set-armor context t)
-    (epg-context-set-textmode context t)
-    (epg-context-set-signers context signers)
-    (if mml1991-cache-passphrase
-        (epg-context-set-passphrase-callback
-         context
-         #'mml1991-epg-passphrase-callback))
+  (let ((inhibit-redisplay t)
+        headers cte)
     ;; Don't sign headers.
     (goto-char (point-min))
     (when (re-search-forward "^$" nil t)
@@ -352,28 +277,21 @@ If no one is selected, default secret key is used.  "
       (when cte
         (setq cte (intern (downcase cte)))
         (mm-decode-content-transfer-encoding cte)))
-    (condition-case error
-        (setq signature (epg-sign-string context (buffer-string) 'clear)
-              mml1991-epg-secret-key-id-list nil)
-      (error
-       (while mml1991-epg-secret-key-id-list
-         (password-cache-remove (car mml1991-epg-secret-key-id-list))
-         (setq mml1991-epg-secret-key-id-list
-               (cdr mml1991-epg-secret-key-id-list)))
-       (signal (car error) (cdr error))))
-    (delete-region (point-min) (point-max))
-    (mm-with-unibyte-current-buffer
-      (insert signature)
-      (goto-char (point-min))
-      (while (re-search-forward "\r+$" nil t)
-        (replace-match "" t t))
-      (when cte
-        (mm-encode-content-transfer-encoding cte))
-      (goto-char (point-min))
-      (when headers
-        (insert headers))
-      (insert "\n"))
-    t))
+    (let* ((pair (mml-secure-epg-sign 'OpenPGP 'clear))
+           (signature (car pair)))
+      (delete-region (point-min) (point-max))
+      (mm-with-unibyte-current-buffer
+        (insert signature)
+        (goto-char (point-min))
+        (while (re-search-forward "\r+$" nil t)
+          (replace-match "" t t))
+        (when cte
+          (mm-encode-content-transfer-encoding cte))
+        (goto-char (point-min))
+        (when headers
+          (insert headers))
+        (insert "\n"))
+      t)))
 
 (defun mml1991-epg-encrypt (cont &optional sign)
   (goto-char (point-min))
@@ -386,78 +304,7 @@ If no one is selected, default secret key is used.  "
       (delete-region (point-min) (point))
       (when cte
         (mm-decode-content-transfer-encoding (intern (downcase cte))))))
-  (let ((context (epg-make-context))
-        (recipients
-         (if (message-options-get 'message-recipients)
-             (split-string
-              (message-options-get 'message-recipients)
-              "[ \f\t\n\r\v,]+")))
-        recipient-key signer-key cipher signers config)
-    (when mml1991-encrypt-to-self
-      (unless mml1991-signers
-        (error "mml1991-signers is not set"))
-      (setq recipients (nconc recipients mml1991-signers)))
-    ;; We should remove this check if epg-0.0.6 is released.
-    (if (and (condition-case nil
-                 (require 'epg-config)
-               (error))
-             (functionp #'epg-expand-group))
-        (setq config (epg-configuration)
-              recipients
-              (apply #'nconc
-                     (mapcar (lambda (recipient)
-                               (or (epg-expand-group config recipient)
-                                   (list recipient)))
-                             recipients))))
-    (if (eq mm-encrypt-option 'guided)
-        (setq recipients
-              (epa-select-keys context "Select recipients for encryption.
-If no one is selected, symmetric encryption will be performed.  "
-                               recipients))
-      (setq recipients
-            (delq nil (mapcar
-                       (lambda (name)
-                         (setq recipient-key (mml1991-epg-find-usable-key
-                                              (epg-list-keys context name)
-                                              'encrypt))
-                         (unless (or recipient-key
-                                   (y-or-n-p
-                                    (format "No public key for %s; skip it? "
-                                            name)))
-                           (error "No public key for %s" name))
-                         recipient-key)
-                       recipients)))
-      (unless recipients
-        (error "No recipient specified")))
-    (when sign
-      (if (eq mm-sign-option 'guided)
-          (setq signers (epa-select-keys context "Select keys for signing.
-If no one is selected, default secret key is used.  "
-                                         mml1991-signers t))
-        (if mml1991-signers
-            (setq signers (delq nil
-                                (mapcar
-                                 (lambda (name)
-                                   (mml1991-epg-find-usable-secret-key
-                                    context name 'sign))
-                                 mml1991-signers)))))
-      (epg-context-set-signers context signers))
-    (epg-context-set-armor context t)
-    (epg-context-set-textmode context t)
-    (if mml1991-cache-passphrase
-        (epg-context-set-passphrase-callback
-         context
-         #'mml1991-epg-passphrase-callback))
-    (condition-case error
-        (setq cipher
-              (epg-encrypt-string context (buffer-string) recipients sign)
-              mml1991-epg-secret-key-id-list nil)
-      (error
-       (while mml1991-epg-secret-key-id-list
-         (password-cache-remove (car mml1991-epg-secret-key-id-list))
-         (setq mml1991-epg-secret-key-id-list
-               (cdr mml1991-epg-secret-key-id-list)))
-       (signal (car error) (cdr error))))
+  (let ((cipher (mml-secure-epg-encrypt 'OpenPGP cont sign)))
     (delete-region (point-min) (point-max))
     (insert "\n" cipher))
   t)
diff --git a/lisp/gnus/mml2015.el b/lisp/gnus/mml2015.el
index 10ba126ae2b..e2e99771801 100644
--- a/lisp/gnus/mml2015.el
+++ b/lisp/gnus/mml2015.el
@@ -111,6 +111,9 @@ Valid packages include `epg', `pgg' and `mailcrypt'.")
   "If t, cache passphrase."
   :group 'mime-security
   :type 'boolean)
+(make-obsolete-variable 'mml2015-cache-passphrase
+                        'mml-secure-cache-passphrase
+                        "25.1")
 
 (defcustom mml2015-passphrase-cache-expiry mml-secure-passphrase-cache-expiry
   "How many seconds the passphrase is cached.
@@ -118,6 +121,9 @@ Whether the passphrase is cached at all is controlled by
 `mml2015-cache-passphrase'."
   :group 'mime-security
   :type 'integer)
+(make-obsolete-variable 'mml2015-passphrase-cache-expiry
+                        'mml-secure-passphrase-cache-expiry
+                        "25.1")
 
 (defcustom mml2015-signers nil
   "A list of your own key ID(s) which will be used to sign a message.
@@ -774,99 +780,6 @@ If set, it overrides the setting of `mml2015-sign-with-sender'."
 (autoload 'epg-expand-group "epg-config")
 (autoload 'epa-select-keys "epa")
 
-(defvar mml2015-epg-secret-key-id-list nil)
-
-(defun mml2015-epg-passphrase-callback (context key-id ignore)
-  (if (eq key-id 'SYM)
-      (epg-passphrase-callback-function context key-id nil)
-    (let* ((password-cache-key-id
-            (if (eq key-id 'PIN)
-                "PIN"
-               key-id))
-           entry
-           (passphrase
-            (password-read
-             (if (eq key-id 'PIN)
-                 "Passphrase for PIN: "
-               (if (setq entry (assoc key-id epg-user-id-alist))
-                   (format "Passphrase for %s %s: " key-id (cdr entry))
-                 (format "Passphrase for %s: " key-id)))
-             password-cache-key-id)))
-      (when passphrase
-        (let ((password-cache-expiry mml2015-passphrase-cache-expiry))
-          (password-cache-add password-cache-key-id passphrase))
-        (setq mml2015-epg-secret-key-id-list
-              (cons password-cache-key-id mml2015-epg-secret-key-id-list))
-        (copy-sequence passphrase)))))
-
-(defun mml2015-epg-check-user-id (key recipient)
-  (let ((pointer (epg-key-user-id-list key))
-        result)
-    (while pointer
-      (if (and (equal (car (mail-header-parse-address
-                            (epg-user-id-string (car pointer))))
-                      (car (mail-header-parse-address
-                            recipient)))
-               (not (memq (epg-user-id-validity (car pointer))
-                          '(revoked expired))))
-          (setq result t
-                pointer nil)
-        (setq pointer (cdr pointer))))
-    result))
-
-(defun mml2015-epg-check-sub-key (key usage)
-  (let ((pointer (epg-key-sub-key-list key))
-        result)
-    ;; The primary key will be marked as disabled, when the entire
-    ;; key is disabled (see 12 Field, Format of colon listings, in
-    ;; gnupg/doc/DETAILS)
-    (unless (memq 'disabled (epg-sub-key-capability (car pointer)))
-      (while pointer
-        (if (and (memq usage (epg-sub-key-capability (car pointer)))
-                 (not (memq (epg-sub-key-validity (car pointer))
-                            '(revoked expired))))
-            (setq result t
-                  pointer nil)
-          (setq pointer (cdr pointer)))))
-    result))
-
-(defun mml2015-epg-find-usable-key (context name usage
-                                            &optional name-is-key-id)
-  (let ((keys (epg-list-keys context name))
-        key)
-    (while keys
-      (if (and (or name-is-key-id
-                   ;; Non email user-id can be supplied through
-                   ;; mml2015-signers if mml2015-encrypt-to-self is set.
-                   ;; Treat it as valid, as it is user's intention.
-                   (not (string-match "\\`<" name))
-                   (mml2015-epg-check-user-id (car keys) name))
-               (mml2015-epg-check-sub-key (car keys) usage))
-          (setq key (car keys)
-                keys nil)
-        (setq keys (cdr keys))))
-    key))
-
-;; XXX: since gpg --list-secret-keys does not return validity of each
-;; key, `mml2015-epg-find-usable-key' defined above is not enough for
-;; secret keys.  The function `mml2015-epg-find-usable-secret-key'
-;; below looks at appropriate public keys to check usability.
-(defun mml2015-epg-find-usable-secret-key (context name usage)
-  (let ((secret-keys (epg-list-keys context name t))
-        secret-key)
-    (while (and (not secret-key) secret-keys)
-      (if (mml2015-epg-find-usable-key
-           context
-           (epg-sub-key-fingerprint
-            (car (epg-key-sub-key-list
-                  (car secret-keys))))
-           usage
-           t)
-          (setq secret-key (car secret-keys)
-                secret-keys nil)
-        (setq secret-keys (cdr secret-keys))))
-    secret-key))
-
 (autoload 'gnus-create-image "gnus-ems")
 
 (defun mml2015-epg-key-image (key-id)
@@ -921,18 +834,15 @@ If set, it overrides the setting of `mml2015-sign-with-sender'."
          mm-security-handle 'gnus-info "Corrupted")
         (throw 'error handle))
       (setq context (epg-make-context))
-      (if mml2015-cache-passphrase
+      (if (or mml2015-cache-passphrase mml-secure-cache-passphrase)
           (epg-context-set-passphrase-callback
            context
-           #'mml2015-epg-passphrase-callback))
+           (cons 'mml-secure-passphrase-callback 'OpenPGP)))
       (condition-case error
           (setq plain (epg-decrypt-string context (mm-get-part child))
-                mml2015-epg-secret-key-id-list nil)
+                mml-secure-secret-key-id-list nil)
         (error
-         (while mml2015-epg-secret-key-id-list
-           (password-cache-remove (car mml2015-epg-secret-key-id-list))
-           (setq mml2015-epg-secret-key-id-list
-                 (cdr mml2015-epg-secret-key-id-list)))
+         (mml-secure-clear-secret-key-id-list)
          (mm-set-handle-multipart-parameter
           mm-security-handle 'gnus-info "Failed")
          (if (eq (car error) 'quit)
@@ -968,18 +878,15 @@ If set, it overrides the setting of `mml2015-sign-with-sender'."
   (let ((inhibit-redisplay t)
         (context (epg-make-context))
         plain)
-    (if mml2015-cache-passphrase
+    (if (or mml2015-cache-passphrase mml-secure-cache-passphrase)
         (epg-context-set-passphrase-callback
          context
-         #'mml2015-epg-passphrase-callback))
+         (cons 'mml-secure-passphrase-callback 'OpenPGP)))
     (condition-case error
         (setq plain (epg-decrypt-string context (buffer-string))
-              mml2015-epg-secret-key-id-list nil)
+              mml-secure-secret-key-id-list nil)
       (error
-       (while mml2015-epg-secret-key-id-list
-         (password-cache-remove (car mml2015-epg-secret-key-id-list))
-         (setq mml2015-epg-secret-key-id-list
-               (cdr mml2015-epg-secret-key-id-list)))
+       (mml-secure-clear-secret-key-id-list)
        (mm-set-handle-multipart-parameter
         mm-security-handle 'gnus-info "Failed")
        (if (eq (car error) 'quit)
@@ -1065,176 +972,37 @@ If set, it overrides the setting of `mml2015-sign-with-sender'."
       (mml2015-extract-cleartext-signature))))
 
 (defun mml2015-epg-sign (cont)
-  (let* ((inhibit-redisplay t)
-         (context (epg-make-context))
-         (boundary (mml-compute-boundary cont))
-         (sender (message-options-get 'message-sender))
-         (signer-names (or mml2015-signers
-                           (if (and mml2015-sign-with-sender sender)
-                               (list (concat "<" sender ">")))))
-         signer-key
-         (signers
-          (or (message-options-get 'mml2015-epg-signers)
-              (message-options-set
-               'mml2015-epg-signers
-               (if (eq mm-sign-option 'guided)
-                   (epa-select-keys context "\
-Select keys for signing.
-If no one is selected, default secret key is used.  "
-                                    signer-names
-                                    t)
-                 (if (or sender mml2015-signers)
-                     (delq nil
-                           (mapcar
-                            (lambda (signer)
-                              (setq signer-key
-                                    (mml2015-epg-find-usable-secret-key
-                                     context signer 'sign))
-                              (unless (or signer-key
-                                          (y-or-n-p
-                                           (format
-                                            "No secret key for %s; skip it? "
-                                            signer)))
-                                (error "No secret key for %s" signer))
-                              signer-key)
-                            signer-names)))))))
-         signature micalg)
-    (epg-context-set-armor context t)
-    (epg-context-set-textmode context t)
-    (epg-context-set-signers context signers)
-    (if mml2015-cache-passphrase
-        (epg-context-set-passphrase-callback
-         context
-         #'mml2015-epg-passphrase-callback))
+  (let ((inhibit-redisplay t)
+        (boundary (mml-compute-boundary cont)))
     ;; Signed data must end with a newline (RFC 3156, 5).
     (goto-char (point-max))
     (unless (bolp)
       (insert "\n"))
-    (condition-case error
-        (setq signature (epg-sign-string context (buffer-string) t)
-              mml2015-epg-secret-key-id-list nil)
-      (error
-       (while mml2015-epg-secret-key-id-list
-         (password-cache-remove (car mml2015-epg-secret-key-id-list))
-         (setq mml2015-epg-secret-key-id-list
-               (cdr mml2015-epg-secret-key-id-list)))
-       (signal (car error) (cdr error))))
-    (if (epg-context-result-for context 'sign)
-        (setq micalg (epg-new-signature-digest-algorithm
-                      (car (epg-context-result-for context 'sign)))))
-    (goto-char (point-min))
-    (insert (format "Content-Type: multipart/signed; boundary=\"%s\";\n"
-                    boundary))
-    (if micalg
-        (insert (format "\tmicalg=pgp-%s; "
-                        (downcase
-                         (cdr (assq micalg
-                                    epg-digest-algorithm-alist))))))
-    (insert "protocol=\"application/pgp-signature\"\n")
-    (insert (format "\n--%s\n" boundary))
-    (goto-char (point-max))
-    (insert (format "\n--%s\n" boundary))
-    (insert "Content-Type: application/pgp-signature; name=\"signature.asc\"\n\n")
-    (insert signature)
-    (goto-char (point-max))
-    (insert (format "--%s--\n" boundary))
-    (goto-char (point-max))))
+    (let* ((pair (mml-secure-epg-sign 'OpenPGP t))
+           (signature (car pair))
+           (micalg (cdr pair)))
+      (goto-char (point-min))
+      (insert (format "Content-Type: multipart/signed; boundary=\"%s\";\n"
+                      boundary))
+      (if micalg
+          (insert (format "\tmicalg=pgp-%s; "
+                          (downcase
+                           (cdr (assq micalg
+                                      epg-digest-algorithm-alist))))))
+      (insert "protocol=\"application/pgp-signature\"\n")
+      (insert (format "\n--%s\n" boundary))
+      (goto-char (point-max))
+      (insert (format "\n--%s\n" boundary))
+      (insert "Content-Type: application/pgp-signature; name=\"signature.asc\"\n\n")
+      (insert signature)
+      (goto-char (point-max))
+      (insert (format "--%s--\n" boundary))
+      (goto-char (point-max)))))
 
 (defun mml2015-epg-encrypt (cont &optional sign)
   (let* ((inhibit-redisplay t)
-         (context (epg-make-context))
          (boundary (mml-compute-boundary cont))
-         (config (epg-configuration))
-         (recipients (message-options-get 'mml2015-epg-recipients))
-         cipher
-         (sender (message-options-get 'message-sender))
-         (signer-names (or mml2015-signers
-                           (if (and mml2015-sign-with-sender sender)
-                               (list (concat "<" sender ">")))))
-         signers
-         recipient-key signer-key)
-    (unless recipients
-      (setq recipients
-            (apply #'nconc
-                   (mapcar
-                    (lambda (recipient)
-                      (or (epg-expand-group config recipient)
-                          (list (concat "<" recipient ">"))))
-                    (split-string
-                     (or (message-options-get 'message-recipients)
-                         (message-options-set 'message-recipients
-                                              (read-string "Recipients: ")))
-                     "[ \f\t\n\r\v,]+"))))
-      (when mml2015-encrypt-to-self
-        (unless signer-names
-          (error "Neither message sender nor mml2015-signers are set"))
-        (setq recipients (nconc recipients signer-names)))
-      (if (eq mm-encrypt-option 'guided)
-          (setq recipients
-                (epa-select-keys context "\
-Select recipients for encryption.
-If no one is selected, symmetric encryption will be performed.  "
-                                 recipients))
-        (setq recipients
-              (delq nil
-                    (mapcar
-                     (lambda (recipient)
-                       (setq recipient-key (mml2015-epg-find-usable-key
-                                            context recipient 'encrypt))
-                       (unless (or recipient-key
-                                   (y-or-n-p
-                                    (format "No public key for %s; skip it? "
-                                            recipient)))
-                         (error "No public key for %s" recipient))
-                       recipient-key)
-                     recipients)))
-        (unless recipients
-          (error "No recipient specified")))
-      (message-options-set 'mml2015-epg-recipients recipients))
-    (when sign
-      (setq signers
-            (or (message-options-get 'mml2015-epg-signers)
-                (message-options-set
-                 'mml2015-epg-signers
-                 (if (eq mm-sign-option 'guided)
-                     (epa-select-keys context "\
-Select keys for signing.
-If no one is selected, default secret key is used.  "
-                                      signer-names
-                                      t)
-                   (if (or sender mml2015-signers)
-                       (delq nil
-                             (mapcar
-                              (lambda (signer)
-                                (setq signer-key
-                                      (mml2015-epg-find-usable-secret-key
-                                       context signer 'sign))
-                                (unless (or signer-key
-                                            (y-or-n-p
-                                             (format
-                                              "No secret key for %s; skip it? "
-                                              signer)))
-                                  (error "No secret key for %s" signer))
-                                signer-key)
-                              signer-names)))))))
-      (epg-context-set-signers context signers))
-    (epg-context-set-armor context t)
-    (epg-context-set-textmode context t)
-    (if mml2015-cache-passphrase
-        (epg-context-set-passphrase-callback
-         context
-         #'mml2015-epg-passphrase-callback))
-    (condition-case error
-        (setq cipher
-              (epg-encrypt-string context (buffer-string) recipients sign
-                                  mml2015-always-trust)
-              mml2015-epg-secret-key-id-list nil)
-      (error
-       (while mml2015-epg-secret-key-id-list
-         (password-cache-remove (car mml2015-epg-secret-key-id-list))
-         (setq mml2015-epg-secret-key-id-list
-               (cdr mml2015-epg-secret-key-id-list)))
-       (signal (car error) (cdr error))))
+         (cipher (mml-secure-epg-encrypt 'OpenPGP cont sign)))
     (delete-region (point-min) (point-max))
     (goto-char (point-min))
     (insert (format "Content-Type: multipart/encrypted; boundary=\"%s\";\n"