/* Magic numbers used within secnet */
+/*
+ * This file is part of secnet.
+ * See README for full list of copyright holders.
+ *
+ * secnet 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.
+ *
+ * secnet 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
+ * version 3 along with secnet; if not, see
+ * https://www.gnu.org/licenses/gpl.html.
+ */
#ifndef magic_h
#define magic_h
-#define LABEL_NAK 0x00000000
-#define LABEL_MSG0 0x00020200
-#define LABEL_MSG1 0x01010101
-#define LABEL_MSG2 0x02020202
-#define LABEL_MSG3 0x03030303
-#define LABEL_MSG3BIS 0x13030313
-#define LABEL_MSG4 0x04040404
-#define LABEL_MSG5 0x05050505
-#define LABEL_MSG6 0x06060606
-#define LABEL_MSG7 0x07070707
-#define LABEL_MSG8 0x08080808
-#define LABEL_MSG9 0x09090909
+/* Encode a pair of 16 bit major and minor codes as a single 32-bit label.
+ * The encoding is strange for historical reasons. Suppose that the nibbles
+ * of the major number are (from high to low) a, b, c, d, and the minor
+ * number has nibbles w, x, y, z. (Here, a, b, c, d are variables, not hex
+ * digits.) We scramble them to form a message label as follows.
+ *
+ * 0 d 0 d 0 d 0 d
+ * 0 0 0 a b c 0 0
+ * z 0 0 0 0 0 z 0
+ * w x y 0 0 0 0 0
+ * ---------------
+ * f g h i j k l m
+ *
+ * and calculate the nibbles f, g, ..., m of the message label (higher
+ * significance on the left) by XORing the columns. It can be shown that
+ * this is invertible using linear algebra in GF(16), but but it's easier to
+ * notice that d = m, z = l, c = k XOR d, b = j, a = i XOR d, y = h,
+ * x = g XOR d, and w = f XOR z.
+ *
+ * Encoding in the forward direction, from a major/minor pair to a label, is
+ * (almost?) always done on constants, so its performance is fairly
+ * unimportant. There is a compatibility constraint on the patterns produced
+ * with a = b = c = w = x = y = 0. Subject to that, I wanted to find an
+ * invertible GF(16)-linear transformation which would let me recover the
+ * major and minor numbers with relatively little calculation.
+ */
+
+#define MSGCODE(major, minor) \
+ ((((uint32_t)(major)&0x0000000fu) << 0) ^ \
+ (((uint32_t)(major)&0x0000000fu) << 8) ^ \
+ (((uint32_t)(major)&0x0000000fu) << 16) ^ \
+ (((uint32_t)(major)&0x0000000fu) << 24) ^ \
+ (((uint32_t)(major)&0x0000fff0u) << 4) ^ \
+ (((uint32_t)(minor)&0x0000000fu) << 4) ^ \
+ (((uint32_t)(minor)&0x0000000fu) << 28) ^ \
+ (((uint32_t)(minor)&0x0000fff0u) << 16))
-/* uses of the 32-bit capability bitmap */
-#define CAPAB_EARLY 0x00000000 /* no Early flags yet (see NOTES) */
-#define CAPAB_TRANSFORM_MASK 0x0000ffff
-/* remaining 16 bits are unused */
+/* Extract major and minor codes from a 32-bit message label. */
+#define MSGMAJOR(label) \
+ ((((uint32_t)(label)&0x0000000fu) << 0) ^ \
+ (((uint32_t)(label)&0x0000000fu) << 4) ^ \
+ (((uint32_t)(label)&0x0000000fu) << 12) ^ \
+ (((uint32_t)(label)&0x000fff00u) >> 4))
+#define MSGMINOR(label) \
+ ((((uint32_t)(label)&0x000000ffu) << 8) ^ \
+ (((uint32_t)(label)&0x000000f0u) >> 4) ^ \
+ (((uint32_t)(label)&0xfff00000u) >> 16))
+
+#define LABEL_NAK MSGCODE( 0, 0)
+#define LABEL_MSG0 MSGCODE(0x2020, 0) /* ! */
+#define LABEL_MSG1 MSGCODE( 1, 0)
+#define LABEL_MSG2 MSGCODE( 2, 0)
+#define LABEL_MSG3 MSGCODE( 3, 0)
+#define LABEL_MSG3BIS MSGCODE( 3, 1)
+#define LABEL_MSG4 MSGCODE( 4, 0)
+#define LABEL_MSG5 MSGCODE( 5, 0)
+#define LABEL_MSG6 MSGCODE( 6, 0)
+#define LABEL_MSG7 MSGCODE( 7, 0)
+#define LABEL_MSG8 MSGCODE( 8, 0)
+#define LABEL_MSG9 MSGCODE( 9, 0)
+#define LABEL_PROD MSGCODE( 10, 0)
/*
- * The transform capability mask is a set of bits, one for each
- * transform supported. The transform capability numbers are set in
- * the configuration (and should correspond between the two sites),
- * although there are sensible defaults.
+ * The capability mask is a set of bits, one for each optional feature
+ * supported. The capability numbers for transforms are set in the
+ * configuration (and should correspond between the two sites), although
+ * there are sensible defaults.
*
- * Advertising a nonzero transform capability mask promises that
- * the receiver understands LABEL_MSG3BIS messages, which
- * contain an additional byte specifying the transform capability
- * number actually chosen by the MSG3 sender.
+ * Advertising a nonzero capability mask promises that the receiver
+ * understands LABEL_MSG3BIS messages, which contain an additional byte
+ * specifying the transform capability number actually chosen by the MSG3
+ * sender.
*
* Aside from that, an empty bitmask is treated the same as
- * 1u<<CAPAB_TRANSFORMNUM_ANCIENT
+ * 1u<<CAPAB_BIT_ANCIENTTRANSFORM
*/
+/* uses of the 32-bit capability bitmap */
+#define CAPAB_TRANSFORM_MASK 0x0000ffff
+#define CAPAB_PRIORITY_MOBILE 0x80000000 /* mobile site has MSG1 priority */
+/* remaining bits are unused */
+
/* bit indices, 0 is ls bit */
-#define CAPAB_TRANSFORMNUM_USER_MIN 0
-#define CAPAB_TRANSFORMNUM_USER_MAX 7
-#define CAPAB_TRANSFORMNUM_SERPENT256CBC 8
-#define CAPAB_TRANSFORMNUM_EAXSERPENT 9
-#define CAPAB_TRANSFORMNUM_MAX 15
+#define CAPAB_BIT_USER_MIN 0
+#define CAPAB_BIT_USER_MAX 7
+#define CAPAB_BIT_SERPENT256CBC 8
+#define CAPAB_BIT_EAXSERPENT 9
+#define CAPAB_BIT_MAX 15
-#define CAPAB_TRANSFORMNUM_ANCIENT CAPAB_TRANSFORMNUM_SERPENT256CBC
+#define CAPAB_BIT_ANCIENTTRANSFORM CAPAB_BIT_SERPENT256CBC
#endif /* magic_h */
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