From d18909ff1b1c4e671faeaf1ed1c174ad5401f7c9 Mon Sep 17 00:00:00 2001
From: Peter Dettman <peter.dettman@bouncycastle.org>
Date: Sun, 2 Oct 2022 01:09:24 +0700
Subject: Support encoding without allocation

---
 crypto/src/math/ec/ECFieldElement.cs | 19 +++++++++-
 crypto/src/math/ec/ECPoint.cs        | 71 ++++++++++++++++++++++++++++++++++++
 2 files changed, 89 insertions(+), 1 deletion(-)

diff --git a/crypto/src/math/ec/ECFieldElement.cs b/crypto/src/math/ec/ECFieldElement.cs
index a96556482..330708088 100644
--- a/crypto/src/math/ec/ECFieldElement.cs
+++ b/crypto/src/math/ec/ECFieldElement.cs
@@ -96,8 +96,25 @@ namespace Org.BouncyCastle.Math.EC
 
         public virtual byte[] GetEncoded()
         {
-            return BigIntegers.AsUnsignedByteArray((FieldSize + 7) / 8, ToBigInteger());
+            return BigIntegers.AsUnsignedByteArray(GetEncodedLength(), ToBigInteger());
         }
+
+        public virtual int GetEncodedLength()
+        {
+            return (FieldSize + 7) / 8;
+        }
+
+        public virtual void EncodeTo(byte[] buf, int off)
+        {
+            BigIntegers.AsUnsignedByteArray(ToBigInteger(), buf, off, GetEncodedLength());
+        }
+
+#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1_OR_GREATER
+        public virtual void EncodeTo(Span<byte> buf)
+        {
+            BigIntegers.AsUnsignedByteArray(ToBigInteger(), buf[..GetEncodedLength()]);
+        }
+#endif
     }
 
     public abstract class AbstractFpFieldElement
diff --git a/crypto/src/math/ec/ECPoint.cs b/crypto/src/math/ec/ECPoint.cs
index dcda5abfc..fc0ddf035 100644
--- a/crypto/src/math/ec/ECPoint.cs
+++ b/crypto/src/math/ec/ECPoint.cs
@@ -437,6 +437,14 @@ namespace Org.BouncyCastle.Math.EC
 
         public abstract byte[] GetEncoded(bool compressed);
 
+        public abstract int GetEncodedLength(bool compressed);
+
+        public abstract void EncodeTo(bool compressed, byte[] buf, int off);
+
+#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1_OR_GREATER
+        public abstract void EncodeTo(bool compressed, Span<byte> buf);
+#endif
+
         protected internal abstract bool CompressionYTilde { get; }
 
         public abstract ECPoint Add(ECPoint b);
@@ -560,6 +568,69 @@ namespace Org.BouncyCastle.Math.EC
             }
         }
 
+        public override int GetEncodedLength(bool compressed)
+        {
+            if (IsInfinity)
+                return 1;
+
+            if (compressed)
+                return 1 + XCoord.GetEncodedLength();
+
+            return 1 + XCoord.GetEncodedLength() + YCoord.GetEncodedLength();
+        }
+
+        public override void EncodeTo(bool compressed, byte[] buf, int off)
+        {
+#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1_OR_GREATER
+            EncodeTo(compressed, buf.AsSpan(off));
+#else
+            if (IsInfinity)
+            {
+                buf[off] = 0x00;
+                return;
+            }
+
+            ECPoint normed = Normalize();
+            ECFieldElement X = normed.XCoord, Y = normed.YCoord;
+
+            if (compressed)
+            {
+                buf[off] = (byte)(normed.CompressionYTilde ? 0x03 : 0x02);
+                X.EncodeTo(buf, off + 1);
+                return;
+            }
+
+            buf[off] = 0x04;
+            X.EncodeTo(buf, off + 1);
+            Y.EncodeTo(buf, off + 1 + X.GetEncodedLength());
+#endif
+        }
+
+#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1_OR_GREATER
+        public override void EncodeTo(bool compressed, Span<byte> buf)
+        {
+            if (IsInfinity)
+            {
+                buf[0] = 0x00;
+                return;
+            }
+
+            ECPoint normed = Normalize();
+            ECFieldElement X = normed.XCoord, Y = normed.YCoord;
+
+            if (compressed)
+            {
+                buf[0] = (byte)(normed.CompressionYTilde ? 0x03 : 0x02);
+                X.EncodeTo(buf[1..]);
+                return;
+            }
+
+            buf[0] = 0x04;
+            X.EncodeTo(buf[1..]);
+            Y.EncodeTo(buf[(1 + X.GetEncodedLength())..]);
+        }
+#endif
+
         /**
          * Multiplies this <code>ECPoint</code> by the given number.
          * @param k The multiplicator.
-- 
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