164 lines
4.1 KiB
C
164 lines
4.1 KiB
C
/* V.27(bis) Scrambler / Descrambler
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*
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* (C) 2020 by Andreas Eversberg <jolly@eversberg.eu>
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* All Rights Reserved
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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/* Based on orignal Scrambler code from SIT-Rom:
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r6 already has input bit at position 0.
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r6 (low) and r7 (high) are the shift register.
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The register is shifed during process, so that compare at 00BE and
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00C5 refers to the already shifted register and not to the original
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position.
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00A4 L00A4:
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00A4 : FE mov a,r6
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00A5 : ED AF djnz r5,L00AF
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00A7 : D3 01 xrl a,#001H
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00A9 : BD 22 mov r5,#022H
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00AB L00AB:
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00AB : D2 B3 jb6 L00B3
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00AD : 04 B5 jmp L00B5
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;
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00AF L00AF:
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00AF : 00 nop
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00B0 : 00 nop
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00B1 : 04 AB jmp L00AB
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;
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00B3 L00B3:
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00B3 : D3 01 xrl a,#001H
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00B5 L00B5:
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00B5 : F2 B9 jb7 L00B9
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00B7 : 04 BB jmp L00BB
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;
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00B9 L00B9:
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00B9 : D3 01 xrl a,#001H
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00BB L00BB:
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00BB : 97 clr c
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00BC : F7 rlc a
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00BD : AE mov r6,a
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00BE : 32 CB jb1 L00CB
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00C0 : FF mov a,r7
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00C1 : F7 rlc a
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00C2 : AF mov r7,a
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00C3 : 37 cpl a
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00C4 : 00 nop
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00C5 L00C5:
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00C5 : 53 26 anl a,#026H
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00C7 : C6 D0 jz L00D0
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00C9 : 04 D2 jmp L00D2
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;
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00CB L00CB:
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00CB : FF mov a,r7
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00CC : F7 rlc a
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00CD : AF mov r7,a
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00CE : 04 C5 jmp L00C5
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;
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00D0 00D0:
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00D0 : BD 22 mov r5,#022H
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00D2 00D2:
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00D2 : 83 ret
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*/
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#include <stdio.h>
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#include <stdint.h>
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#include <string.h>
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#include "scrambler.h"
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#define GUARD_COUNT 34
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/* init scrambler */
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void v27_scrambler_init(v27scrambler_t *scram, int bis, int descramble)
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{
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memset(scram, 0, sizeof(*scram));
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scram->descramble = descramble;
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/* set bits 9 and 12 (and 8 for V.27bis) */
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if (bis)
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scram->resetmask = 0x1300;
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else
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scram->resetmask = 0x1200;
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/* guard counter */
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scram->counter = GUARD_COUNT;
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}
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/* scramble/descramble one bit */
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uint8_t v27_scrambler_bit(v27scrambler_t *scram, uint8_t in)
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{
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uint8_t bit0 = in & 1;
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uint16_t shift = scram->shift;
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/* the descrambler stores the input bit */
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if (scram->descramble) {
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/* put bit 0 into shift register and shift */
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shift |= bit0;
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scram->shift = shift << 1;
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}
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/* process guaard counter */
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if (--scram->counter == 0) {
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/* restart counter */
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scram->counter = GUARD_COUNT;
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/* invert this time */
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bit0 ^= 1;
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}
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/* xor bit 0 with bits 6 and 7: polynome 1 + x^-6 + x^-7 */
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bit0 ^= ((shift >> 6) & 1);
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bit0 ^= ((shift >> 7) & 1);
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/* the scrambler stores the output bit */
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if (!scram->descramble) {
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/* put bit 0 into shift register and shift */
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shift |= bit0;
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scram->shift = shift << 1;
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}
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/* check if bits (8),9,12 are repitions of bit 0 in shift register (prior shift) */
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if (!(shift & 1))
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shift ^= ~0;
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if (!(shift & scram->resetmask)) {
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/* any repetition is not true, reset counter */
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scram->counter = GUARD_COUNT;
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}
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return bit0;
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}
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/* scramble/descramble block of bytes (LSB first) */
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void v27_scrambler_block(v27scrambler_t *scram, uint8_t *data, int len)
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{
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int i, j;
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uint8_t in, out = 0;
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for (i = 0; i < len; i++) {
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in = data[i];
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for (j = 0; j < 8; j++) {
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out >>= 1;
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// Note: 'in' will be masked to bit 0 only
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out |= v27_scrambler_bit(scram, in) << 7;
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in >>= 1;
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}
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data[i] = out;
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}
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}
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