diff --git a/crypto/src/math/ec/custom/sec/SecP521R1Field.cs b/crypto/src/math/ec/custom/sec/SecP521R1Field.cs
new file mode 100644
index 000000000..43f012b8b
--- /dev/null
+++ b/crypto/src/math/ec/custom/sec/SecP521R1Field.cs
@@ -0,0 +1,131 @@
+using System;
+using System.Diagnostics;
+
+namespace Org.BouncyCastle.Math.EC.Custom.Sec
+{
+ internal class SecP521R1Field
+ {
+ // 2^521 - 1
+ internal static readonly uint[] P = new uint[]{ 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF,
+ 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0x1FF };
+ private const int P16 = 0x1FF;
+
+ public static void Add(uint[] x, uint[] y, uint[] z)
+ {
+ uint c = Nat.Add(16, x, y, z) + x[16] + y[16];
+ if (c > P16 || (c == P16 && Nat.Eq(16, z, P)))
+ {
+ c += Nat.Inc(16, z, 0);
+ c &= P16;
+ }
+ z[16] = c;
+ }
+
+ public static void AddOne(uint[] x, uint[] z)
+ {
+ Array.Copy(x, 0, z, 0, 16);
+ uint c = Nat.Inc(16, z, 0) + z[16];
+ if (c > P16 || (c == P16 && Nat.Eq(16, z, P)))
+ {
+ c += Nat.Inc(16, z, 0);
+ c &= P16;
+ }
+ z[16] = c;
+ }
+
+ public static uint[] FromBigInteger(BigInteger x)
+ {
+ uint[] z = Nat.FromBigInteger(521, x);
+ if (Nat.Eq(17, z, P))
+ {
+ Nat.Zero(17, z);
+ }
+ return z;
+ }
+
+ public static void Half(uint[] x, uint[] z)
+ {
+ uint c0 = x[0] & 1, x16 = x[16], c512 = x16 & 1;
+ Nat.ShiftDownBit(16, x, c512, z);
+ z[16] = (x16 >> 1) | (c0 << 8);
+ }
+
+ public static void Multiply(uint[] x, uint[] y, uint[] z)
+ {
+ uint[] tt = Nat.Create(34);
+ Nat.Mul(17, x, y, tt);
+ Reduce(tt, z);
+ }
+
+ public static void Negate(uint[] x, uint[] z)
+ {
+ if (Nat.IsZero(17, x))
+ {
+ Nat.Zero(17, z);
+ }
+ else
+ {
+ Nat.Sub(17, P, x, z);
+ }
+ }
+
+ public static void Reduce(uint[] xx, uint[] z)
+ {
+ Debug.Assert(xx[33] == 0);
+ Debug.Assert(xx[32] >> 18 == 0);
+ uint xx32 = xx[32];
+ uint c = Nat.ShiftDownBitsExt(16, xx, 16, 9, xx32, z) >> 23;
+ c += xx32 >> 9;
+ c += Nat.Add(16, z, xx, z);
+ if (c > P16 || (c == P16 && Nat.Eq(16, z, P)))
+ {
+ c += Nat.Inc(16, z, 0);
+ c &= P16;
+ }
+ z[16] = c;
+ }
+
+ public static void Square(uint[] x, uint[] z)
+ {
+ uint[] tt = Nat.Create(34);
+ Nat.Square(17, x, tt);
+ Reduce(tt, z);
+ }
+
+ public static void SquareN(uint[] x, int n, uint[] z)
+ {
+ Debug.Assert(n > 0);
+ uint[] tt = Nat.Create(34);
+ Nat.Square(17, x, tt);
+ Reduce(tt, z);
+
+ while (--n > 0)
+ {
+ Nat.Square(17, z, tt);
+ Reduce(tt, z);
+ }
+ }
+
+ public static void Subtract(uint[] x, uint[] y, uint[] z)
+ {
+ int c = Nat.Sub(16, x, y, z) + (int)(x[16] - y[16]);
+ if (c < 0)
+ {
+ c += Nat.Dec(16, z, 0);
+ c &= P16;
+ }
+ z[16] = (uint)c;
+ }
+
+ public static void Twice(uint[] x, uint[] z)
+ {
+ uint c = Nat.ShiftUpBit(16, x, 0, z) | (x[16] << 1);
+ if (c > P16 || (c == P16 && Nat.Eq(16, z, P)))
+ {
+ c += Nat.Inc(16, z, 0);
+ c &= P16;
+ }
+ z[16] = c;
+ }
+ }
+}
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