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DMR: RAS MD5 Hash Fringe Case Correction; #359

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lwvmobile
2026-09-07 15:19:30 -04:00
parent cd5f0f4a8e
commit 4139b19bfe
1 changed files with 18 additions and 28 deletions
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46
src/crypt-md5.c
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@@ -409,27 +409,6 @@ void ras_effective_key_creation(dsd_state * state, char * input)
fprintf (stderr, "%02X ", ctx.digest[i]);
fprintf (stderr, "\n");
//The following rules are still not entirely clear, however,
//the last taken digest byte has been observed to be true
//Furthermore, it is still unclear how any rollover is handled,
//if adding 2 is byte inclusive (effectively resetting the byte)
//or if the value is flat 56-bit value and rollover occurs
//against the flat value, using best guess on this
//until further information comes along
//Below patent also suggest all bytes cannot equal zero
//and if so, the next portion of MD5 may be used,
//or some other method may be utilized to prevent fringe cases
//https://patents.google.com/patent/US20130288643A1
//Last Collected Byte 5-bit LSB not allowed to be 0x1F
if ((ctx.digest[6] & 0x1F) == 0x1F)
{
fprintf (stderr, " Byte %d: %02X -> %02X; ", 7, ctx.digest[6], (ctx.digest[6]+2) & 0xFF);
ctx.digest[6] += 2;
}
//collect 7 bytes of the MD5 Hash to create the flat key value
unsigned long long int flat_key = 0;
for (int i = 0; i < 7; i++)
@@ -438,13 +417,24 @@ void ras_effective_key_creation(dsd_state * state, char * input)
flat_key |= ctx.digest[i];
}
//May switch to this correction method instead if the
//observed rollover is on a flat value and not a byte value
// if ((flat_key & 0x1F) == 0x1F)
// {
// fprintf (stderr, "Flat Correction: %014llX -> %014llX;\n", flat_key, flat_key+2);
// flat_key += 2;
// }
//On real world testing, it was discovered that by selecting a password (VluByDZhkPenguinkeeper) which has
//an MD5 partial hash of FFFFFFFFFEDFFF, the value actually placed into the radio was FFFFFFFFFEE001
//this indicates the entire value will roll-over, and not just a byte rollvoer (mod)
//The known rule is that the last 5 bits of the flat key value (prior to reverse)
//is that it cannot equal 0x1F (all 1 bits), if so, a correction of +2 is
//performaed onto a flat value, which increments the entire value appropriately
//Below patent also suggest all bytes cannot equal zero
//and if so, the next portion of MD5 may be used,
//or some other method may be utilized to prevent fringe cases
//https://patents.google.com/patent/US20130288643A1
if ((flat_key & 0x1F) == 0x1F)
{
fprintf (stderr, "Flat Correction: %014llX -> %014llX;\n", flat_key, flat_key+2);
flat_key += 2;
}
//byte-reverse the flat_key into the effective_key
unsigned long long int effective_key = 0;