Use Base64Url in WebEncoders (#56959)

Author: stephentoub

src/Shared/WebEncoders/WebEncoders.cs

@@ -5,6 +5,7 @@
 using System;
 #if NETCOREAPP
 using System.Buffers;
+using System.Buffers.Text;
 #endif
 using System.Diagnostics;
 using System.Globalization;
@@ -66,6 +67,23 @@ public static byte[] Base64UrlDecode(string input, int offset, int count)
             return Array.Empty<byte>();
         }
 
+#if NET9_0_OR_GREATER
+        // Legacy behavior of Base64UrlDecode supports either Base64 or Base64Url input.
+        // If it doesn't have + or /, it can be treated as Base64Url.
+        ReadOnlySpan<char> inputSpan = input.AsSpan(offset, count);
+        if (!inputSpan.ContainsAny('+', '/'))
+        {
+            return Base64Url.DecodeFromChars(inputSpan);
+        }
+
+        // Otherwise, maintain the legacy behavior of accepting Base64 input. Input that
+        // contained both +/ and -_ is neither Base64 nor Base64Url and is considered invalid.
+        if (offset == 0 && count == input.Length)
+        {
+            return Convert.FromBase64String(input);
+        }
+#endif
+
         // Create array large enough for the Base64 characters, not just shorter Base64-URL-encoded form.
         var buffer = new char[GetArraySizeRequiredToDecode(count)];
 
@@ -104,6 +122,23 @@ public static byte[] Base64UrlDecode(string input, int offset, char[] buffer, in
             return Array.Empty<byte>();
         }
 
+#if NET9_0_OR_GREATER
+        // Legacy behavior of Base64UrlDecode supports either Base64 or Base64Url input.
+        // If it doesn't have + or /, it can be treated as Base64Url.
+        ReadOnlySpan<char> inputSpan = input.AsSpan(offset, count);
+        if (!inputSpan.ContainsAny('+', '/'))
+        {
+            return Base64Url.DecodeFromChars(inputSpan);
+        }
+
+        // Otherwise, maintain the legacy behavior of accepting Base64 input. Input that
+        // contained both +/ and -_ is neither Base64 nor Base64Url and is considered invalid.
+        if (offset == 0 && count == input.Length)
+        {
+            return Convert.FromBase64String(input);
+        }
+#endif
+
         // Assumption: input is base64url encoded without padding and contains no whitespace.
 
         var paddingCharsToAdd = GetNumBase64PaddingCharsToAddForDecode(count);
@@ -124,6 +159,13 @@ public static byte[] Base64UrlDecode(string input, int offset, char[] buffer, in
 
         // Copy input into buffer, fixing up '-' -> '+' and '_' -> '/'.
         var i = bufferOffset;
+#if NET8_0_OR_GREATER
+        Span<char> bufferSpan = buffer.AsSpan(i, count);
+        inputSpan.CopyTo(bufferSpan);
+        bufferSpan.Replace('-', '+');
+        bufferSpan.Replace('_', '/');
+        i += count;
+#else
         for (var j = offset; i - bufferOffset < count; i++, j++)
         {
             var ch = input[j];
@@ -140,6 +182,7 @@ public static byte[] Base64UrlDecode(string input, int offset, char[] buffer, in
                 buffer[i] = ch;
             }
         }
+#endif
 
         // Add the padding characters back.
         for (; paddingCharsToAdd > 0; i++, paddingCharsToAdd--)
@@ -314,6 +357,9 @@ public static int GetArraySizeRequiredToEncode(int count)
     [SkipLocalsInit]
     public static string Base64UrlEncode(ReadOnlySpan<byte> input)
     {
+#if NET9_0_OR_GREATER
+        return Base64Url.EncodeToString(input);
+#else
         const int StackAllocThreshold = 128;
 
         if (input.IsEmpty)
@@ -337,6 +383,7 @@ public static string Base64UrlEncode(ReadOnlySpan<byte> input)
         }
 
         return base64Url;
+#endif
     }
 
 #if NET9_0_OR_GREATER
@@ -347,9 +394,11 @@ public static string Base64UrlEncode(ReadOnlySpan<byte> input)
     /// <param name="output">The buffer to place the result in.</param>
     /// <returns></returns>
     public static int Base64UrlEncode(ReadOnlySpan<byte> input, Span<char> output)
+    {
+        return Base64Url.EncodeToChars(input, output);
+    }
 #else
     private static int Base64UrlEncode(ReadOnlySpan<byte> input, Span<char> output)
-#endif
     {
         Debug.Assert(output.Length >= GetArraySizeRequiredToEncode(input.Length));
 
@@ -383,6 +432,7 @@ private static int Base64UrlEncode(ReadOnlySpan<byte> input, Span<char> output)
 
         return charsWritten;
     }
+#endif
 #endif
 
     private static int GetNumBase64PaddingCharsToAddForDecode(int inputLength)