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;;; utf7.el --- UTF-7 encoding/decoding for Emacs -*- lexical-binding:t -*-
;; Copyright (C) 1999-2025 Free Software Foundation, Inc.
;; Author: Jon K Hellan <hellan@acm.org>
;; Maintainer: emacs-devel@gnu.org
;; Keywords: mail
;; This file is part of GNU Emacs.
;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.
;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
;; GNU General Public License for more details.
;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs. If not, see <https://www.gnu.org/licenses/>.
;;; Commentary:
;; UTF-7 - A Mail-Safe Transformation Format of Unicode - RFC 2152
;; This is a transformation format of Unicode that contains only 7-bit
;; ASCII octets and is intended to be readable by humans in the limiting
;; case that the document consists of characters from the US-ASCII
;; repertoire.
;; In short, runs of characters outside US-ASCII are encoded as base64
;; inside delimiters.
;; A variation of UTF-7 is specified in IMAP 4rev1 (RFC 2060) as the way
;; to represent characters outside US-ASCII in mailbox names in IMAP.
;; This library supports both variants, but the IMAP variation was the
;; reason I wrote it.
;; The routines convert UTF-7 -> UTF-16 (16 bit encoding of Unicode)
;; -> current character set, and vice versa.
;; However, until Emacs supports Unicode, the only Emacs character set
;; supported here is ISO-8859.1, which can trivially be converted to/from
;; Unicode.
;; When decoding results in a character outside the Emacs character set,
;; an error is thrown. It is up to the application to recover.
;; UTF-7 should be done by providing a coding system. Mule-UCS does
;; already, but I don't know if it does the IMAP version and it's not
;; clear whether that should really be a coding system. The UTF-16
;; part of the conversion can be done with coding systems available
;; with Mule-UCS or some versions of Emacs. Unfortunately these were
;; done wrongly (regarding handling of byte-order marks and how the
;; variants were named), so we don't have a consistent name for the
;; necessary coding system. The code below doesn't seem to DTRT
;; generally. E.g.:
;;
;; (utf7-encode "a+£")
;; => "a+ACsAow-"
;;
;; $ echo "a+£"|iconv -f utf-8 -t utf-7
;; a+-+AKM
;;
;; -- fx
;;; Code:
(require 'base64)
(require 'mm-util)
(defconst utf7-direct-encoding-chars " -%'-*,-[]-}"
"Character ranges which do not need escaping in UTF-7.")
(defconst utf7-imap-direct-encoding-chars
(concat utf7-direct-encoding-chars "+\\~")
"Character ranges which do not need escaping in the IMAP variant of UTF-7.")
(defconst utf7-utf-16-coding-system
(cond ((mm-coding-system-p 'utf-16-be-no-signature) ; Mule-UCS
'utf-16-be-no-signature)
((and (mm-coding-system-p 'utf-16-be) ; Emacs
;; Avoid versions with BOM.
(= 2 (length (encode-coding-string "a" 'utf-16-be))))
'utf-16-be)
((mm-coding-system-p 'utf-16-be-nosig) ; ?
'utf-16-be-nosig))
"Coding system which encodes big endian UTF-16 without a BOM signature.")
(defsubst utf7-imap-get-pad-length (len modulus)
"Return required length of padding for IMAP modified base64 fragment."
(mod (- len) modulus))
(defun utf7-encode-internal (&optional for-imap)
"Encode text in (temporary) buffer as UTF-7.
Use IMAP modification if FOR-IMAP is non-nil."
(let ((start (point-min))
(end (point-max)))
(narrow-to-region start end)
(goto-char start)
(let* ((esc-char (if for-imap ?& ?+))
(direct-encoding-chars
(if for-imap utf7-imap-direct-encoding-chars
utf7-direct-encoding-chars))
(not-direct-encoding-chars (concat "^" direct-encoding-chars)))
(while (not (eobp))
(skip-chars-forward direct-encoding-chars)
(unless (eobp)
(insert esc-char)
(let ((p (point))
(fc (following-char))
(run-length
(skip-chars-forward not-direct-encoding-chars)))
(if (and (= fc esc-char)
(= run-length 1)) ; Lone esc-char?
(delete-char -1) ; Now there's one too many
(utf7-fragment-encode p (point) for-imap))
(insert "-")))))))
(defun utf7-fragment-encode (start end &optional for-imap)
"Encode text from START to END in buffer as UTF-7 escape fragment.
Use IMAP modification if FOR-IMAP is non-nil."
(save-restriction
(let* ((buf (current-buffer))
(base (with-temp-buffer
(set-buffer-multibyte nil)
(insert-buffer-substring buf start end)
(funcall (utf7-get-u16char-converter 'to-utf-16))
(base64-encode-region (point-min) (point-max))
(buffer-string))))
(narrow-to-region start end)
(delete-region (point-min) (point-max))
(insert base))
(goto-char (point-min))
(let ((pm (point-max)))
(when for-imap
(while (search-forward "/" nil t)
(replace-match ",")))
(skip-chars-forward "^= \t\n" pm)
(delete-region (point) pm))))
(defun utf7-decode-internal (&optional for-imap)
"Decode UTF-7 text in (temporary) buffer.
Use IMAP modification if FOR-IMAP is non-nil."
(let ((start (point-min)))
(goto-char start)
(let* ((esc-pattern (concat "^" (char-to-string (if for-imap ?& ?+))))
(base64-chars (concat "A-Za-z0-9+"
(char-to-string (if for-imap ?, ?/)))))
(while (not (eobp))
(skip-chars-forward esc-pattern)
(unless (eobp)
(forward-char)
(let ((p (point))
(run-length (skip-chars-forward base64-chars)))
(when (and (not (eobp)) (= (following-char) ?-))
(delete-char 1))
(unless (= run-length 0) ; Encoded lone esc-char?
(save-excursion
(utf7-fragment-decode p (point) for-imap)
(goto-char p)
(delete-char -1)))))))))
(defun utf7-fragment-decode (start end &optional for-imap)
"Decode base64 encoded fragment from START to END of UTF-7 text in buffer.
Use IMAP modification if FOR-IMAP is non-nil."
(save-restriction
(narrow-to-region start end)
(when for-imap
(goto-char start)
(while (search-forward "," nil 'move-to-end) (replace-match "/")))
(let ((pl (utf7-imap-get-pad-length (- end start) 4)))
(insert (make-string pl ?=))
(base64-decode-region start (+ end pl)))
(funcall (utf7-get-u16char-converter 'from-utf-16))))
(defun utf7-get-u16char-converter (which-way)
"Return a function to convert between UTF-16 and current character set."
(if utf7-utf-16-coding-system
(if (eq which-way 'to-utf-16)
(lambda ()
(encode-coding-region (point-min) (point-max)
utf7-utf-16-coding-system))
(lambda ()
(decode-coding-region (point-min) (point-max)
utf7-utf-16-coding-system)))
;; Add test to check if we are really Latin-1.
(if (eq which-way 'to-utf-16)
'utf7-latin1-u16-char-converter
'utf7-u16-latin1-char-converter)))
(defun utf7-latin1-u16-char-converter ()
"Convert latin 1 (ISO-8859.1) characters to 16 bit Unicode.
Characters are converted to raw byte pairs in narrowed buffer."
(encode-coding-region (point-min) (point-max) 'iso-8859-1)
(goto-char (point-min))
(while (not (eobp))
(insert 0)
(forward-char)))
(defun utf7-u16-latin1-char-converter ()
"Convert 16 bit Unicode characters to latin 1 (ISO-8859.1).
Characters are in raw byte pairs in narrowed buffer."
(goto-char (point-min))
(while (not (eobp))
(if (= 0 (following-char))
(delete-char 1)
(error "Unable to convert from Unicode"))
(forward-char))
(decode-coding-region (point-min) (point-max) 'iso-8859-1)
(mm-enable-multibyte))
;;;###autoload
(defun utf7-encode (string &optional for-imap)
"Encode UTF-7 STRING. Use IMAP modification if FOR-IMAP is non-nil."
(if (and (coding-system-p 'utf-7) (coding-system-p 'utf-7-imap))
;; Emacs 23 with proper support for IMAP
(encode-coding-string string (if for-imap 'utf-7-imap 'utf-7))
(mm-with-multibyte-buffer
(insert string)
(utf7-encode-internal for-imap)
(buffer-string))))
(defun utf7-decode (string &optional for-imap)
"Decode UTF-7 STRING. Use IMAP modification if FOR-IMAP is non-nil."
(if (and (coding-system-p 'utf-7) (coding-system-p 'utf-7-imap))
;; Emacs 23 with proper support for IMAP
(decode-coding-string string (if for-imap 'utf-7-imap 'utf-7))
(mm-with-unibyte-buffer
(insert string)
(utf7-decode-internal for-imap)
(mm-enable-multibyte)
(buffer-string))))
(provide 'utf7)
;;; utf7.el ends here
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