[TASK] Add base64url encode/decode functionality

Add static library functions that allow using base64url-compliant
values (according to https://tools.ietf.org/html/rfc4648#section-5).

The difference to classic base64 is, that the result alphabet is
adjusted like shown below, padding (`=`) is stripped completely:

 + position #62: `+` -> `-` (minus)
 + position #63: `/` -> `_` (underscore)

Resolves: #92509
Releases: main, 11.5
Change-Id: I8804824f3a7fb4e08e18ef6dee6826744dbd41bc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75868
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>

typo3/sysext/core/Classes/Utility/StringUtility.php

@@ -209,4 +209,40 @@ public static function multibyteStringPad(string $string, int $length, string $p
         }
         return $string;
     }
+
+    /**
+     * Returns base64 encoded value with a URL and filename safe alphabet
+     * according to https://tools.ietf.org/html/rfc4648#section-5
+     *
+     * The difference to classic base64 is, that the result
+     * alphabet is adjusted like shown below, padding (`=`)
+     * is stripped completely:
+     *  + position #62: `+` -> `-` (minus)
+     *  + position #63: `/` -> `_` (underscore)
+     *
+     * @param string $value raw value
+     * @return string base64url encoded string
+     */
+    public static function base64urlEncode(string $value): string
+    {
+        return strtr(base64_encode($value), ['+' => '-', '/' => '_', '=' => '']);
+    }
+
+    /**
+     * Returns base64 decoded value with a URL and filename safe alphabet
+     * according to https://tools.ietf.org/html/rfc4648#section-5
+     *
+     * The difference to classic base64 is, that the result
+     * alphabet is adjusted like shown below, padding (`=`)
+     * is stripped completely:
+     *  + position #62: `-` (minus)      -> `+`
+     *  + position #63: `_` (underscore) -> `/`
+     *
+     * @param string $value base64url decoded string
+     * @return string raw value
+     */
+    public static function base64urlDecode(string $value): string
+    {
+        return base64_decode(strtr($value, ['-' => '+', '_' => '/']));
+    }
 }

typo3/sysext/core/Tests/Unit/Utility/StringUtilityTest.php

@@ -258,4 +258,55 @@ public function multibyteStringPadReturnsCorrectResultsMultibyte(string $expecte
             StringUtility::multibyteStringPad($string, $length, $pad_string, $pad_type)
         );
     }
+
+    public static function base64urlRoundTripWorksDataProvider(): \Generator
+    {
+        yield ['a'];
+        yield ['aa'];
+        yield ['aaa'];
+        yield ['aaaa'];
+        yield [random_bytes(31)];
+        yield [random_bytes(32)];
+        yield [random_bytes(33)];
+    }
+
+    /**
+     * @test
+     * @dataProvider base64urlRoundTripWorksDataProvider
+     */
+    public function base64urlRoundTripWorks(string $rawValue): void
+    {
+        $encoded = StringUtility::base64urlEncode($rawValue);
+        $decoded = StringUtility::base64urlDecode($encoded);
+        self::assertSame($rawValue, $decoded);
+    }
+
+    public static function base64urlDataProvider(): \Generator
+    {
+        yield ['', ''];
+        yield ['a', 'YQ'];
+        yield ['aa', 'YWE'];
+        yield ['aa>', 'YWE-'];
+        yield ['aa?', 'YWE_'];
+        yield ['aaa', 'YWFh'];
+        yield ['aaaa', 'YWFhYQ'];
+    }
+
+    /**
+     * @test
+     * @dataProvider base64urlDataProvider
+     */
+    public function base64urlEncodeWorks(string $rawValue, string $encodedValue): void
+    {
+        self::assertSame($encodedValue, StringUtility::base64urlEncode($rawValue));
+    }
+
+    /**
+     * @test
+     * @dataProvider base64urlDataProvider
+     */
+    public function base64urlDecodeWorks(string $rawValue, string $encodedValue): void
+    {
+        self::assertSame($rawValue, StringUtility::base64urlDecode($encodedValue));
+    }
 }