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common: Substitute modified CRC8 with CRC-5-USB

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The modified CRC8 implementation didn't detect some errors. For
example, using the modified CRC8: CC5QQLALU and DC5QQLALU calculates
to the same value.

BUG=b:37952913
BRANCH=none
TEST=make buildall
Used online CRC-5-USB calculator to test several values against
this implementation.
Signed-off-by: Sam Hurst <shurst@chromium.org>

Change-Id: I5a17941e25691872a25b41525f65f36e2ed1d4fa
Reviewed-on: https://chromium-review.googlesource.com/660812
Commit-Ready: Sam Hurst <shurst@google.com>
Tested-by: Sam Hurst <shurst@google.com>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Reviewed-by: Michael Tang <ntang@chromium.org>
This commit is contained in:
Sam Hurst
2017-09-11 11:39:55 -07:00
committed by chrome-bot
parent cfe418923d
commit cdcc09096a
3 changed files with 34 additions and 25 deletions
Download Patch File Download Diff File Expand all files Collapse all files

28
common/base32.c
View File

@@ -9,22 +9,20 @@
#include "base32.h"
#include "util.h"
uint8_t crc5_sym(int sym, uint8_t previous_crc)
{
unsigned crc = previous_crc << 8;
int i;
static const unsigned char crc5_table1[] = {
0x00, 0x0E, 0x1C, 0x12, 0x11, 0x1F, 0x0D, 0x03,
0x0B, 0x05, 0x17, 0x19, 0x1A, 0x14, 0x06, 0x08
};
/*
* This is a modified CRC-8 which only folds in a 5-bit
* symbol, and it only keeps the bottom 5 bits of the CRC.
*/
crc ^= (sym << 11);
for (i = 5; i; i--) {
if (crc & 0x8000)
crc ^= (0x1070 << 3);
crc <<= 1;
}
return (uint8_t)((crc >> 8) & 0x1f);
static const unsigned char crc5_table0[] = {
0x00, 0x16, 0x05, 0x13, 0x0A, 0x1C, 0x0F, 0x19,
0x14, 0x02, 0x11, 0x07, 0x1E, 0x08, 0x1B, 0x0D
};
uint8_t crc5_sym(uint8_t sym, uint8_t previous_crc)
{
uint8_t tmp = sym ^ previous_crc;
return crc5_table1[tmp & 0x0F] ^ crc5_table0[(tmp >> 4) & 0x0F];
}
/* A-Z0-9 with I,O,0,1 removed */

5
include/base32.h
View File

@@ -12,13 +12,14 @@
extern const char base32_map[33];
/**
* 5-bit CRC
* CRC-5-USB Initially created for USB Token Packets. It uses
* the generator polynomial X^5 + X^2 + X^0 and is 5-bits.
*
* @param sym New symbol to update CRC with
* @param previous_crc Existing CRC value
* @return The updated CRC.
*/
uint8_t crc5_sym(int sym, uint8_t previous_crc);
uint8_t crc5_sym(uint8_t sym, uint8_t previous_crc);
/**
* base32-encode data into a null-terminated string

26
test/base32.c
View File

@@ -28,6 +28,17 @@ static int test_crc5(void)
TEST_ASSERT(seen == 0xffffffff);
}
/*
* Do the same in the opposite order, to make sure a subsequent
* character doesn't obscure a previous typo.
*/
for (i = 0; i < 32; i++) {
seen = 0;
for (j = 0; j < 32; j++)
seen |= 1 << crc5_sym(i, j);
TEST_ASSERT(seen == 0xffffffff);
}
/* Transposing different symbols generates distinct CRCs */
for (c = 0; c < 32; c++) {
for (i = 0; i < 32; i++) {
@@ -91,13 +102,13 @@ static int test_encode(void)
ENCTEST("\xff\x00\xff\x00\xff", 40, 0, "96AR8AH9");
/* CRC requires exact multiple of symbol count */
ENCTEST("\xff\x00\xff\x00\xff", 40, 4, "96ARL8AH9V");
ENCTEST("\xff\x00\xff\x00\xff", 40, 8, "96AR8AH9V");
ENCTEST("\xff\x00\xff\x00\xff", 40, 4, "96ARU8AH9D");
ENCTEST("\xff\x00\xff\x00\xff", 40, 8, "96AR8AH9L");
TEST_ASSERT(
base32_encode(enc, 16, (uint8_t *)"\xff\x00\xff\x00\xff", 40, 6)
== EC_ERROR_INVAL);
/* But what matters is symbol count, not bit count */
ENCTEST("\xff\x00\xff\x00\xfe", 39, 4, "96ARL8AH8W");
ENCTEST("\xff\x00\xff\x00\xfe", 39, 4, "96ARU8AH8P");
return EC_SUCCESS;
}
@@ -148,7 +159,7 @@ static int test_decode(void)
DECTEST("\xff", 8, 0, "96");
DECTEST("\x08\x87", 16, 0, "BCDS");
DECTEST("\xff\x00\xff\x00\xff", 40, 0, "96AR8AH9");
DECTEST("\xff\x00\xff\x00\xfe", 39, 4, "96ARL8AH8W");
DECTEST("\xff\x00\xff\x00\xfe", 39, 4, "96ARU8AH8P");
/* Decode ignores whitespace and dashes */
DECTEST("\xff\x00\xff\x00\xff", 40, 0, " 96\tA-R\r8A H9\n");
@@ -171,14 +182,13 @@ static int test_decode(void)
DECTEST("\xff\xff\xff\xff\xff", 0, 0, "99999999");
/* Good CRCs */
DECTEST("\xff\x00\xff\x00\xff", 40, 4, "96ARL8AH9V");
DECTEST("\xff\x00\xff\x00\xff", 40, 8, "96AR8AH9V");
/* Detect errors in data, CRC, and transposition */
DECTEST("\xff\x00\xff\x00\xff", 40, 4, "96ARU8AH9D");
DECTEST("\xff\x00\xff\x00\xff", 40, 8, "96AR8AH9L");
/* CRC requires exact multiple of symbol count */
TEST_ASSERT(base32_decode(dec, 40, "96ARL8AH9", 4) == -1);
/* But what matters is symbol count, not bit count */
DECTEST("\xff\x00\xff\x00\xfe", 39, 4, "96ARL8AH8W");
DECTEST("\xff\x00\xff\x00\xfe", 39, 4, "96ARU8AH8P");
/* Detect errors in data, CRC, and transposition */
TEST_ASSERT(base32_decode(dec, 40, "96AQL", 4) == -1);