Fix equality predicates.

The count we passed to memcmp() in mpi_eq() and mpi_eq_abs() was actually the number of significant words in the MPI, rather than the number of bytes we wanted to compare.  Multiply by 4 to get the correct value.

To make the intent of the code more apparent, introduce a private MPI_MSW() macro which evaluates to the number of significant words (or 1-based index of the most significant word).  This also comes in handy in mpi_{add,sub,mul}_abs().

Add a couple of test cases which not only demonstrate the bug we fixed here but also demonstrate why we must compare whole words: on a big-endian machine, we would be comparing the unused upper bytes of the first and only word instead of the lower bytes which actually hold a value...

lib/mpi/cryb_mpi_add_abs.c

@@ -92,7 +92,7 @@ mpi_add_abs(cryb_mpi *X, const cryb_mpi *A, const cryb_mpi *B)
 	 */
 
 	/* add B into X word by word until we run out of B */
-	for (c = i = 0; i < (B->msb + 31) / 32; ++i) {
+	for (c = i = 0; i < MPI_MSW(B); ++i) {
 		X->words[i] += c;
 		c = (X->words[i] < c);
 		X->words[i] += B->words[i];

lib/mpi/cryb_mpi_eq.c

@@ -47,5 +47,5 @@ mpi_eq(const cryb_mpi *A, const cryb_mpi *B)
 		return (1);
 	if (A->neg != B->neg || A->msb != B->msb)
 		return (0);
-	return (memcmp(A->words, B->words, (A->msb + 31) / 32) == 0);
+	return (memcmp(A->words, B->words, MPI_MSW(A) * 4) == 0);
 }

lib/mpi/cryb_mpi_eq_abs.c

@@ -47,5 +47,5 @@ mpi_eq_abs(const cryb_mpi *A, const cryb_mpi *B)
 		return (1);
 	if (A->msb != B->msb)
 		return (0);
-	return (memcmp(A->words, B->words, (A->msb + 31) / 32) == 0);
+	return (memcmp(A->words, B->words, MPI_MSW(A) * 4) == 0);
 }

lib/mpi/cryb_mpi_impl.h

@@ -30,6 +30,9 @@
 /* n rounded up to nearest multiple of p */
 #define RUP(n, p) ((((n) + (p) - 1) / (p)) * (p))
 
+/* one-based index of the most significant word of an mpi */
+#define MPI_MSW(n) (((n)->msb + 31) / 32)
+
 /*
  * Initialize an all-zeroes mpi.
  */

lib/mpi/cryb_mpi_mul_abs.c

@@ -72,8 +72,8 @@ mpi_mul_abs(cryb_mpi *X, const cryb_mpi *A, const cryb_mpi *B)
 	 */
 	P = (X == A || X == B) ? &T : X;
 	mpi_zero(P);
-	a = (A->msb + 31) / 32;
-	b = (B->msb + 31) / 32;
+	a = MPI_MSW(A);
+	b = MPI_MSW(B);
 	if (mpi_grow(P, (a + b) * 32) != 0)
 		return (-1);
 

lib/mpi/cryb_mpi_sub_abs.c

@@ -81,7 +81,7 @@ mpi_sub_abs(cryb_mpi *X, const cryb_mpi *A, const cryb_mpi *B)
 		return (-1);
 
 	/* subtract L from G word by word until we run out of L */
-	for (c = i = 0; i < (L->msb + 31) / 32; ++i) {
+	for (c = i = 0; i < MPI_MSW(L); ++i) {
 		cn = G->words[i] < c ||
 		    G->words[i] - c < L->words[i];
 		X->words[i] = G->words[i] - L->words[i] - c;

t/t_mpi_compar.c

@@ -107,6 +107,10 @@ static struct t_cmp_case {
 	{ "3 > 1",	 3,	 1,	 1 },
 	{ "3 > 2",	 3,	 2,	 1 },
 	{ "3 == 3",	 3,	 3,	 0 },
+
+	/* 0x200 and 0x300: same msb, identical lowest byte */
+	{ "512 < 768", 512,    768,     -1 },
+	{ "768 > 512", 768,    512,      1 },
 };
 
 /*