Use ASCII Encoding when known to be ASCII

src/ImageSharp/Formats/Gif/GifConstants.cs

@@ -24,7 +24,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// <summary>
         /// The ASCII encoded bytes used to identify the GIF file.
         /// </summary>
-        internal static readonly byte[] MagicNumber = Encoding.UTF8.GetBytes(FileType + FileVersion);
+        internal static readonly byte[] MagicNumber = Encoding.ASCII.GetBytes(FileType + FileVersion);
 
         /// <summary>
         /// The extension block introducer <value>!</value>.
@@ -54,7 +54,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// <summary>
         /// The ASCII encoded application identification bytes.
         /// </summary>
-        internal static readonly byte[] NetscapeApplicationIdentificationBytes = Encoding.UTF8.GetBytes(NetscapeApplicationIdentification);
+        internal static readonly byte[] NetscapeApplicationIdentificationBytes = Encoding.ASCII.GetBytes(NetscapeApplicationIdentification);
 
         /// <summary>
         /// The Netscape looping application sub block size.

src/ImageSharp/Formats/Jpeg/Components/Decoder/ProfileResolver.cs

@@ -14,22 +14,22 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         /// <summary>
         /// Describes the EXIF specific markers
         /// </summary>
-        public static readonly byte[] JFifMarker = Encoding.UTF8.GetBytes("JFIF\0");
+        public static readonly byte[] JFifMarker = Encoding.ASCII.GetBytes("JFIF\0");
 
         /// <summary>
         /// Describes the EXIF specific markers
         /// </summary>
-        public static readonly byte[] IccMarker = Encoding.UTF8.GetBytes("ICC_PROFILE\0");
+        public static readonly byte[] IccMarker = Encoding.ASCII.GetBytes("ICC_PROFILE\0");
 
         /// <summary>
         /// Describes the ICC specific markers
         /// </summary>
-        public static readonly byte[] ExifMarker = Encoding.UTF8.GetBytes("Exif\0\0");
+        public static readonly byte[] ExifMarker = Encoding.ASCII.GetBytes("Exif\0\0");
 
         /// <summary>
         /// Describes Adobe specific markers <see href="http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe"/>
         /// </summary>
-        public static readonly byte[] AdobeMarker = Encoding.UTF8.GetBytes("Adobe");
+        public static readonly byte[] AdobeMarker = Encoding.ASCII.GetBytes("Adobe");
 
         /// <summary>
         /// Returns a value indicating whether the passed bytes are a match to the profile identifier

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -1005,7 +1005,7 @@ namespace SixLabors.ImageSharp.Formats.Png
 
             if (this.crc.Value != chunk.Crc)
             {
-                string chunkTypeName = Encoding.UTF8.GetString(chunkType);
+                string chunkTypeName = Encoding.ASCII.GetString(chunkType);
 
                 throw new ImageFormatException($"CRC Error. PNG {chunkTypeName} chunk is corrupt!");
             }

tests/ImageSharp.Tests/Formats/Png/PngChunkTypeTests.cs

@@ -24,7 +24,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
 
         private static PngChunkType GetType(string text)
         {
-            return (PngChunkType)BinaryPrimitives.ReadInt32BigEndian(Encoding.UTF8.GetBytes(text));
+            return (PngChunkType)BinaryPrimitives.ReadInt32BigEndian(Encoding.ASCII.GetBytes(text));
         }
     }
 }