From b86ad76bf4bef0999583f70a58844031fe1f828b Mon Sep 17 00:00:00 2001
From: James Jackson-South <james_south@hotmail.com>
Date: Tue, 21 Apr 2020 18:15:00 +0100
Subject: [PATCH] No need to calculate CRC for non-critical chunks.

---
 src/ImageSharp/Formats/Png/PngDecoderCore.cs | 23 +++++++++++---------
 1 file changed, 13 insertions(+), 10 deletions(-)

diff --git a/src/ImageSharp/Formats/Png/PngDecoderCore.cs b/src/ImageSharp/Formats/Png/PngDecoderCore.cs
index e2cd806314..272f93d108 100644
--- a/src/ImageSharp/Formats/Png/PngDecoderCore.cs
+++ b/src/ImageSharp/Formats/Png/PngDecoderCore.cs
@@ -1141,19 +1141,22 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// <param name="chunk">The <see cref="PngChunk"/>.</param>
         private void ValidateChunk(in PngChunk chunk)
         {
-            Span<byte> chunkType = stackalloc byte[4];
+            uint crc = this.ReadChunkCrc();
 
-            BinaryPrimitives.WriteUInt32BigEndian(chunkType, (uint)chunk.Type);
+            if (chunk.IsCritical)
+            {
+                Span<byte> chunkType = stackalloc byte[4];
+                BinaryPrimitives.WriteUInt32BigEndian(chunkType, (uint)chunk.Type);
 
-            this.crc.Reset();
-            this.crc.Update(chunkType);
-            this.crc.Update(chunk.Data.GetSpan());
+                this.crc.Reset();
+                this.crc.Update(chunkType);
+                this.crc.Update(chunk.Data.GetSpan());
 
-            uint crc = this.ReadChunkCrc();
-            if (this.crc.Value != crc && chunk.IsCritical)
-            {
-                string chunkTypeName = Encoding.ASCII.GetString(chunkType);
-                PngThrowHelper.ThrowInvalidChunkCrc(chunkTypeName);
+                if (this.crc.Value != crc)
+                {
+                    string chunkTypeName = Encoding.ASCII.GetString(chunkType);
+                    PngThrowHelper.ThrowInvalidChunkCrc(chunkTypeName);
+                }
             }
         }