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+/* -*-c-*-
+ *
+ * $Id$
+ *
+ * Protocol definition for TrIPE
+ *
+ * (c) 2003 Straylight/Edgeware
+ */
+
+/*----- Licensing notice --------------------------------------------------* 
+ *
+ * This file is part of Trivial IP Encryption (TrIPE).
+ *
+ * TrIPE 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 of the License, or
+ * (at your option) any later version.
+ * 
+ * TrIPE 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 TrIPE; if not, write to the Free Software Foundation,
+ * Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ */
+
+#ifndef TRIPE_PROTOCOL_H
+#define TRIPE_PROTOCOL_H
+
+/*----- TrIPE protocol ----------------------------------------------------*/
+
+/* --- TrIPE message format --- *
+ *
+ * A packet begins with a single-byte message type.  The top four bits are a
+ * category code used to send the message to the right general place in the
+ * code; the bottom bits identify the actual message type.
+ */
+
+#define MSG_CATMASK 0xf0
+#define MSG_TYPEMASK 0x0f
+
+/* --- Encrypted message packets --- *
+ *
+ * Messages of category @MSG_PACKET@ contain encrypted network packets.  The
+ * message content is a symmetric-encrypted block (see below).  Reception of
+ * a packet encrypted under a new key implicitly permits that key to be used
+ * to send further packets.
+ *
+ * The only packet type accepted is zero.
+ *
+ * Packets may be encrypted under any live keyset, but should use the most
+ * recent one.
+ */
+
+#define MSG_PACKET 0x00
+
+/* --- Key exchange packets --- */
+
+#define MSG_KEYEXCH 0x10
+
+#define KX_PRECHAL 0u
+#define KX_CHAL 1u
+#define KX_REPLY 2u
+#define KX_SWITCH 3u
+#define KX_SWITCHOK 4u
+#define KX_NMSG 5u
+
+/* --- Miscellaneous packets --- */
+
+#define MSG_MISC 0x20
+
+#define MISC_NOP 0u			/* Do nothing; ignore me */
+#define MISC_PING 1u			/* Transport-level ping */
+#define MISC_PONG 2u			/* Transport-level ping response */
+#define MISC_EPING 3u			/* Encrypted ping */
+#define MISC_EPONG 4u			/* Encrypted ping response */
+#define MISC_GREET 5u			/* A greeting from a NATed peer */
+
+/* --- Symmetric encryption and keysets --- *
+ *
+ * Packets consist of an 80-bit MAC, a 32-bit sequence number, and the
+ * encrypted payload.
+ *
+ * The plaintext is encrypted using Blowfish in CBC mode with ciphertext
+ * stealing (as described in [Schneier]).  The initialization vector is
+ * selected randomly, and prepended to the actual ciphertext.
+ *
+ * The MAC is computed using the HMAC construction with RIPEMD160 over the
+ * sequence number and the ciphertext (with IV); the first 80 bits of the
+ * output are used.  (This is the minimum allowed by the draft FIPS for HMAC,
+ * and the recommended truncation.)
+ *
+ * A keyset consists of
+ *
+ *   * an integrity (MAC) key;
+ *   * a confidentiality (encryption) key; and
+ *   * a sequence numbering space
+ *
+ * in each direction.  The packets sent by a host encrypted under a
+ * particular keyset are assigned consecutive sequence numbers starting from
+ * zero.  The receiving host must ensure that it only accepts each packet at
+ * most once.  It should maintain a window of sequence numbers: packets with
+ * numbers beyond the end of the window are accepted and cause the window to
+ * be advanced; packets with numbers before the start of the window are
+ * rejected; packets with numbers which appear within the window are accepted
+ * only if the number has not been seen before.
+ *
+ * When a host sends a @KX_SWITCH@ or @KX_SWITCHOK@ message, it installs the
+ * newly-negotiated keyset in a `listen-only' state: it may not send a packet
+ * encrypted under the keyset until either it has received a @KX_SWITCH@ or
+ * @KX_SWITCHOK@ message, or a @MSG_PACKET@ encrypted under the keyset, from
+ * its peer.
+ */
+
+/*----- That's all, folks -------------------------------------------------*/
+
+#endif