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+;;; hmac-def.el --- A macro for defining HMAC functions.
+
+;; Copyright (C) 1999 Shuhei KOBAYASHI
+
+;; Author: Shuhei KOBAYASHI <shuhei@aqua.ocn.ne.jp>
+;; Keywords: HMAC, RFC 2104
+
+;; This file is part of FLIM (Faithful Library about Internet Message).
+
+;; This program 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 2, or
+;; (at your option) any later version.
+
+;; This program 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 this program; see the file COPYING.  If not, write to
+;; the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+;; Boston, MA 02111-1307, USA.
+
+;;; Commentary:
+
+;; This program is implemented from RFC 2104,
+;; "HMAC: Keyed-Hashing for Message Authentication".
+
+;;; Code:
+
+(defmacro define-hmac-function (name H B L &optional bit)
+  "Define a function NAME(TEXT KEY) which computes HMAC with function H.
+
+HMAC function is H(KEY XOR opad, H(KEY XOR ipad, TEXT)):
+
+H is a cryptographic hash function, such as SHA1 and MD5, which takes
+a string and return a digest of it (in binary form).
+B is a byte-length of a block size of H. (B=64 for both SHA1 and MD5.)
+L is a byte-length of hash outputs. (L=16 for MD5, L=20 for SHA1.)
+If BIT is non-nil, truncate output to specified bits."
+  (` (defun (, name) (text key)
+       (, (concat "Compute "
+		  (upcase (symbol-name name))
+		  " over TEXT with KEY."))
+       (let ((key-xor-ipad (make-string (, B) ?\x36))
+	     (key-xor-opad (make-string (, B) ?\x5C))
+	     (len (length key))
+	     (pos 0))
+	 (unwind-protect
+	     (progn
+	       ;; if `key' is longer than the block size, apply hash function
+	       ;; to `key' and use the result as a real `key'.
+	       (if (> len (, B))
+		   (setq key ((, H) key)
+			 len (, L)))
+	       (while (< pos len)
+		 (aset key-xor-ipad pos (logxor (aref key pos) ?\x36))
+		 (aset key-xor-opad pos (logxor (aref key pos) ?\x5C))
+		 (setq pos (1+ pos)))
+	       (setq key-xor-ipad (unwind-protect
+				      (concat key-xor-ipad text)
+				    (fillarray key-xor-ipad 0))
+		     key-xor-ipad (unwind-protect
+				      ((, H) key-xor-ipad)
+				    (fillarray key-xor-ipad 0))
+		     key-xor-opad (unwind-protect
+				      (concat key-xor-opad key-xor-ipad)
+				    (fillarray key-xor-opad 0))
+		     key-xor-opad (unwind-protect
+				      ((, H) key-xor-opad)
+				    (fillarray key-xor-opad 0)))
+	       ;; now `key-xor-opad' contains
+	       ;; H(KEY XOR opad, H(KEY XOR ipad, TEXT)).
+	       (, (if (and bit (< (/ bit 8) L))
+		      (` (substring key-xor-opad 0 (, (/ bit 8))))
+		    ;; return a copy of `key-xor-opad'.
+		    (` (concat key-xor-opad)))))
+	   ;; cleanup.
+	   (fillarray key-xor-ipad 0)
+	   (fillarray key-xor-opad 0))))))
+
+(provide 'hmac-def)
+
+;;; hmac-def.el ends here