diff --git a/crypto/src/math/ec/Mod.cs b/crypto/src/math/ec/Mod.cs
index 57f75ed12..465aa5e41 100644
--- a/crypto/src/math/ec/Mod.cs
+++ b/crypto/src/math/ec/Mod.cs
@@ -21,7 +21,7 @@ namespace Org.BouncyCastle.Math.EC
if ((u[0] & 1) == 0)
{
- InversionStep(p, u, a);
+ InversionStep(p, u, len, a);
}
if (Nat.IsOne(len, u))
{
@@ -32,15 +32,22 @@ namespace Org.BouncyCastle.Math.EC
uint[] v = Nat.Copy(len, p);
uint[] b = Nat.Create(len);
+ int uvLen = len;
+
for (;;)
{
+ while (u[uvLen - 1] == 0 && v[uvLen - 1] == 0)
+ {
+ --uvLen;
+ }
+
if (Nat.Gte(len, u, v))
{
Subtract(p, a, b, a);
Nat.Sub(len, u, v, u);
if ((u[0] & 1) == 0)
{
- InversionStep(p, u, a);
+ InversionStep(p, u, uvLen, a);
}
if (Nat.IsOne(len, u))
{
@@ -54,7 +61,7 @@ namespace Org.BouncyCastle.Math.EC
Nat.Sub(len, v, u, v);
if ((v[0] & 1) == 0)
{
- InversionStep(p, v, b);
+ InversionStep(p, v, uvLen, b);
}
if (Nat.IsOne(len, v))
{
@@ -75,21 +82,23 @@ namespace Org.BouncyCastle.Math.EC
}
}
- private static void InversionStep(uint[] p, uint[] u, uint[] x)
+ private static void InversionStep(uint[] p, uint[] u, int uLen, uint[] x)
{
int len = p.Length;
int count = 0;
while (u[0] == 0)
{
- Nat.ShiftDownWord(u, len, 0);
+ Nat.ShiftDownWord(u, uLen, 0);
count += 32;
}
- int zeroes = GetTrailingZeroes(u[0]);
- if (zeroes > 0)
{
- Nat.ShiftDownBits(u, len, zeroes, 0);
- count += zeroes;
+ int zeroes = GetTrailingZeroes(u[0]);
+ if (zeroes > 0)
+ {
+ Nat.ShiftDownBits(u, uLen, zeroes, 0);
+ count += zeroes;
+ }
}
for (int i = 0; i < count; ++i)
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