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Since X may point to the same MPI as either G or L (or even both), we

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can't set the carry flag based on the contents of G or L after having
modified X.  Instead, compute and stash the new carry value first.
This commit is contained in:
Dag-Erling Smørgrav 2014-12-30 19:14:59 +00:00 committed by des
parent 8d0e24dee7
commit 7a90ca7b42
1 changed files with 9 additions and 7 deletions
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16
lib/mpi/mpi_sub.c
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@ -42,7 +42,7 @@ mpi_sub_abs(cryb_mpi *X, cryb_mpi *A, cryb_mpi *B)
{
cryb_mpi *L, *G;
unsigned int i;
uint32_t c;
uint32_t c, cn;
/*
* Trivial cases: A and B are the same or equal or at least one of
@ -67,16 +67,18 @@ mpi_sub_abs(cryb_mpi *X, cryb_mpi *A, cryb_mpi *B)
if (mpi_grow(X, G->msb) != 0)
return (-1);
/* subtract B from A word by word until we run out of B */
for (c = i = 0; i < (G->msb + 31) / 32; ++i) {
X->words[i] = G->words[i] - c;
c = (G->words[i] < c) + (X->words[i] < L->words[i]);
X->words[i] -= L->words[i];
/* subtract L from G word by word until we run out of L */
for (c = i = 0; i < (L->msb + 31) / 32; ++i) {
cn = G->words[i] < c ||
G->words[i] - c < L->words[i];
X->words[i] = G->words[i] - L->words[i] - c;
c = cn;
}
/* keep propagating carry */
while (c) {
cn = (G->words[i] < c);
X->words[i] = G->words[i] - c;
c = (G->words[i] > c);
c = cn;
++i;
}
while (i > 0 && X->words[i] == 0)