Avoid a stackalloc of 512 bytes

src/JsonWebToken/Cryptography/EcdsaSigner.cs

@@ -3,6 +3,7 @@
 
 #if SUPPORT_ELLIPTIC_CURVE_SIGNATURE
 using System;
+using System.Buffers;
 using System.Diagnostics;
 using System.Security.Cryptography;
 
@@ -45,9 +46,22 @@ public override bool TrySign(ReadOnlySpan<byte> data, Span<byte> destination, ou
 
             var ecdsa = _ecdsaPool.Get();
 #if SUPPORT_SPAN_CRYPTO
-            Span<byte> hash = stackalloc byte[Sha2.BlockSizeStackallocThreshold].Slice(0, _sha.HashSize);
-            _sha.ComputeHash(data, hash);
-            return ecdsa.TrySignHash(hash, destination, out bytesWritten);
+            byte[]? array = null;
+            try
+            {
+                Span<byte> hash = _sha.HashSize < Sha2.BlockSizeStackallocThreshold ?
+                    stackalloc byte[Sha2.BlockSizeStackallocThreshold].Slice(0, _sha.HashSize)
+                    : (array = ArrayPool<byte>.Shared.Rent(_sha.HashSize)).AsSpan(0, _sha.HashSize);
+                _sha.ComputeHash(data, hash);
+                return ecdsa.TrySignHash(hash, destination, out bytesWritten);
+            }
+            finally
+            {
+                if (array != null)
+                {
+                    ArrayPool<byte>.Shared.Return(array);
+                }
+            }
 #else
             byte[] hash = new byte[_sha.HashSize];
             _sha.ComputeHash(data, hash);
@@ -72,6 +86,6 @@ protected override void Dispose(bool disposing)
             }
         }
 
-    }   
+    }
 }
 #endif

src/JsonWebToken/Cryptography/Sha2.cs

@@ -30,7 +30,7 @@ public abstract class Sha2
         public abstract int BlockSize { get; }
 
         internal const int HashSizeStackallocThreshold = Sha512.Sha512HashSize;
-        internal const int BlockSizeStackallocThreshold = Sha512.Sha512BlockSize;
+        internal const int BlockSizeStackallocThreshold = Sha256.Sha512BlockSize;
 
 #if SUPPORT_SIMD
         private static ReadOnlySpan<byte> LittleEndianUInt64 => new byte[32]