Rewrite NXDN Dibit PN95 Code and allow custom seed value; #318

examples/cygwin_bat/complete_usage_options.txt

@@ -1,4 +1,4 @@
-Build Version: AW 2025-182-g24c1dd9 
+Build Version: AW 2025-197-gf3b89f1 
 MBElib Version: 1.3.4
 CODEC2 Support Enabled
 
@@ -137,6 +137,8 @@ Decoder options:
 Advanced Decoder options:
   -X <hex>      Manually Set P2 Parameters (WACN, SYSID, CC/NAC)
                  (-X BEE00ABC123)
+  -* <dec>      Manually Set NXDN PN95 Dibit Scrambler Seed (1-511)
+                 (-* 261)
   -D <dec>      Manually Set TIII DMR Location Area n bit len (0-10)(10 max)
                  (Value defaults to max n bit value for site model size)
                  (Setting 0 will show full Site ID, no area/subarea)

include/dsd.h

@@ -898,6 +898,7 @@ typedef struct
   uint8_t nxdn_sacch_frame_segcrc[4];
   uint8_t nxdn_alias_block_number;
   char nxdn_alias_block_segment[4][4][8];
+  uint16_t nxdn_pn95_seed;
 
   //site/srv/cch info
   char nxdn_location_category[14];
@@ -1281,7 +1282,7 @@ void ncursesClose ();
 
 //new NXDN Functions start here!
 void nxdn_frame (dsd_opts * opts, dsd_state * state);
-void nxdn_descramble (uint8_t dibits[], int len);
+void nxdn_pn95_dibit_scrambler(dsd_state * state, uint8_t * dibits, int len);
 //nxdn deinterleaving/depuncturing functions
 void nxdn_deperm_facch (dsd_opts * opts, dsd_state * state, uint8_t bits[144]);
 void nxdn_deperm_sacch (dsd_opts * opts, dsd_state * state, uint8_t bits[60]);

src/dsd_main.c

@@ -1214,6 +1214,7 @@
    memset (state->nxdn_sacch_frame_segment, 1, sizeof(state->nxdn_sacch_frame_segment));
    state->nxdn_alias_block_number = 0;
    memset (state->nxdn_alias_block_segment, 0, sizeof(state->nxdn_alias_block_segment));
+   state->nxdn_pn95_seed = 228; //default value
  
    //site/srv/cch info
    state->nxdn_location_site_code = 0;
@@ -1526,6 +1527,8 @@
    printf ("Advanced Decoder options:\n");
    printf ("  -X <hex>      Manually Set P2 Parameters (WACN, SYSID, CC/NAC)\n");
    printf ("                 (-X BEE00ABC123)\n");
+   printf ("  -* <dec>      Manually Set NXDN PN95 Dibit Scrambler Seed (1-511)\n");
+   printf ("                 (-* 261)\n");
    printf ("  -D <dec>      Manually Set TIII DMR Location Area n bit len (0-10)(10 max)\n");
    printf ("                 (Value defaults to max n bit value for site model size)\n");
    printf ("                 (Setting 0 will show full Site ID, no area/subarea)\n");
@@ -1906,7 +1909,7 @@
  
    exitflag = 0;
  
-   while ((c = getopt (argc, argv, "~yhaepPqs:t:v:z:i:o:d:c:g:n:w:B:C:R:f:m:u:x:A:S:M:G:D:L:V:U:YK:b:H:X:NQ:WrlZTF@:!:01:2:345:6:7:89:Ek:I:J:O")) != -1)
+   while ((c = getopt (argc, argv, "~yhaepPqs:t:v:z:i:o:d:c:g:n:w:B:C:R:f:m:u:x:A:S:M:G:D:L:V:U:YK:b:H:X:NQ:WrlZTF@:!:01:2:345:6:7:8*:9:Ek:I:J:O")) != -1)
      {
  
        switch (c)
@@ -2217,7 +2220,15 @@
            if (state.R > 0x7FFF) state.R = 0x7FFF;
            //disable keyloader in case user tries to use this and it at the same time
            state.keyloader = 0;
+           fprintf (stderr, "NXDN Scrambler Key set to: %05d;", state.R);
            break;
+
+         case '*':
+          sscanf (optarg, "%hu", &state.nxdn_pn95_seed);
+          if (state.nxdn_pn95_seed > 0x1FF) state.nxdn_pn95_seed = 0x1FF;
+          else if (state.nxdn_pn95_seed == 0) state.nxdn_pn95_seed = 228;
+          fprintf (stderr, "NXDN PN9 Seed Value set to: %03d;", state.nxdn_pn95_seed);
+          break;
  
          case 'H':
            //new handling for 10/32/64 Char Key

src/nxdn_deperm.c

@@ -25,26 +25,48 @@
 #include "dsd.h"
 #include "nxdn_const.h"
 
-//NOTE: Descrambling was having an issue without a value inside the brackets, but only when
-//the PARITY table was disabled, was this due to a memory issue or overflow?
-static const uint8_t scramble_t[182] = { //values are the position values we need to invert in the descramble
-	2, 5, 6, 7, 10, 12, 14, 16, 17, 22, 23, 25, 26, 27, 28, 30, 33, 34, 36, 37, 38, 41, 45, 47,
-	52, 54, 56, 57, 59, 62, 63, 64, 65, 66, 67, 69, 70, 73, 76, 79, 81, 82, 84, 85, 86, 87, 88,
-	89, 92, 95, 96, 98, 100, 103, 104, 107, 108, 116, 117, 121, 122, 125, 127, 131, 132, 134,
-	137, 139, 140, 141, 142, 143, 144, 145, 147, 151, 153, 154, 158, 159, 160, 162, 164, 165,
-	168, 170, 171, 174, 175, 176, 177, 181
-};
-
-
-//decoding functions here
-void nxdn_descramble(uint8_t dibits[], int len)
+void nxdn_pn95_dibit_scrambler(dsd_state * state, uint8_t * dibits, int len)
 {
-	for (int i=0; i<len; i++)
+
+	uint16_t lfsr = state->nxdn_pn95_seed; //default value is 228 / 0xE4
+
+	//debug hard set
+	// lfsr = 261;
+
+	uint16_t  bit = 0;
+	uint8_t pN95[182]; memset (pN95, 0, sizeof(pN95));
+
+	for (int i = 0; i < len; i++)
 	{
-		if (scramble_t[i] >= len)
-			break;
-		dibits[scramble_t[i]] ^= 0x2;	// invert sign of scrambled dibits
+		//before feedback, take the bit
+		pN95[i] = lfsr & 1;
+
+		//since this is right shift, the taps are 0 and 4, and not 8 and 4 (9,5)
+		bit = ((lfsr >> 4) ^ (lfsr >> 0)) & 1;
+		lfsr >>= 1;
+		lfsr |= (bit << 8);
+
 	}
+
+	//convert the pN sequence to a scramble table
+	uint8_t scramble_table[182]; memset(scramble_table, 0, sizeof(scramble_table));
+	int k = 0;
+	for (int i = 0; i < len; i++)
+	{
+		if (pN95[i] == 1)
+			scramble_table[k++] = i;
+	}
+
+	//the entries into the scramble_table are the dibits (symbols) that are inverted
+	for (int i = 0; i < k; i++)
+	{
+		dibits[scramble_table[i]] ^= 0x2;
+		dibits[scramble_table[i]] &= 0x3;
+	}
+
+	//debug, make sure we don't overflow dibits array
+	// fprintf (stderr, " PN95: K: %d; Len: %d; ", k, len);
+
 }
 
 void nxdn_deperm_facch(dsd_opts * opts, dsd_state * state, uint8_t bits[144])

src/nxdn_frame.c

@@ -90,7 +90,7 @@ void nxdn_frame (dsd_opts * opts, dsd_state * state)
 	//collect lich bits first, if they are good, then we can collect the rest of them
 	for (int i = 0; i < 8; i++) lich_dibits[i] = dbuf[i] = getDibit(opts, state);
 
-	nxdn_descramble (lich_dibits, 8);
+	nxdn_pn95_dibit_scrambler (state, lich_dibits, 8);
 
 	lich = 0;
 	for (int i=0; i<8; i++) lich |= (lich_dibits[i] >> 1) << (7-i);
@@ -343,7 +343,7 @@ void nxdn_frame (dsd_opts * opts, dsd_state * state)
 	 	dbuf[i+8] = getDibit(opts, state);
 	}
 
-	nxdn_descramble (dbuf, 182); //sizeof(dbuf)
+	nxdn_pn95_dibit_scrambler (state, dbuf, 182);
 
 	//seperate our dbuf (dibit_buffer) into individual bit array
 	for (int i = 0; i < 182; i++)