Restructure base64URL handling (#46)

The conversion is only really done in the context of a BinaryString, so
having the logic split out into a different class didn't make a lot of
sense. As a result, it's being consolidated. In addition to
organization, this should make completion of #41 easier, specifically
for emitting formats that match
`PublicKeyCredential.parse{Creation|Request}OptionsFromJSON()` methods.

src/BinaryString.php

@@ -41,6 +41,12 @@ public static function fromBase64(string $base64): BinaryString
         return new BinaryString($binary);
     }
 
+    public function toBase64Url(): string
+    {
+        $base64 = base64_encode($this->unwrap());
+        return rtrim(strtr($base64, ['+' => '-', '/' => '_']), '=');
+    }
+
     /**
      * Turns a list of 8-bit integers into a BinaryString
      *

src/CreateResponse.php

@@ -58,7 +58,7 @@ public function verify(
             $this->fail('7.1.8', 'C.challenge');
         }
 
-        $b64u = Codecs\Base64Url::encode($challenge->getBinary()->unwrap());
+        $b64u = $challenge->getBinary()->toBase64Url();
         if (!hash_equals($b64u, $cdjChallenge)) {
             $this->fail('7.1.8', 'C.challenge');
         }

src/GetResponse.php

@@ -109,7 +109,7 @@ public function verify(
             $this->fail('7.2.12', 'C.challenge');
         }
 
-        $b64u = Codecs\Base64Url::encode($challenge->getBinary()->unwrap());
+        $b64u = $challenge->getBinary()->toBase64Url();
         if (!hash_equals($b64u, $cdjChallenge)) {
             $this->fail('7.2.12', 'C.challenge');
         }

tests/BinaryStringTest.php

@@ -90,6 +90,21 @@ public function testReadUint32(): void
         self::assertSame('plaintext', $this->default->getRemaining());
     }
 
+    public function testBase64UrlIdentity(): void
+    {
+        $data = random_bytes(64);
+        $wrapped = new BinaryString($data);
+        $b64u = $wrapped->toBase64Url();
+        $decoded = BinaryString::fromBase64Url($b64u);
+        self::assertSame($data, $decoded->unwrap());
+    }
+
+    public function testBase64UrlDecode(): void
+    {
+        $decoded = BinaryString::fromBase64Url('PDw_Pz8-Pg');
+        self::assertSame('<<???>>', $decoded->unwrap());
+    }
+
     /**
      * @return array{BinaryString, BinaryString, bool}[]
      */

tests/ChallengeManagerTestTrait.php

@@ -21,7 +21,7 @@ public function testMostRecentChallengeCanBeRetrieved(): void
     {
         $cm = $this->getChallengeManager();
         $c = $cm->createChallenge();
-        $cdjValue = Codecs\Base64Url::encode($c->getBinary()->unwrap());
+        $cdjValue = $c->getBinary()->toBase64Url();
 
         $found = $cm->useFromClientDataJSON($cdjValue);
         self::assertInstanceOf(ChallengeInterface::class, $found);
@@ -32,7 +32,7 @@ public function testMostRecentChallengeCanBeRetrievedOnlyOnce(): void
     {
         $cm = $this->getChallengeManager();
         $c = $cm->createChallenge();
-        $cdjValue = Codecs\Base64Url::encode($c->getBinary()->unwrap());
+        $cdjValue = $c->getBinary()->toBase64Url();
 
         $found = $cm->useFromClientDataJSON($cdjValue);
         $again = $cm->useFromClientDataJSON($cdjValue);
@@ -46,7 +46,7 @@ public function testNoChallengeIsReturnedIfManagerIsEmpty(): void
         $cm = $this->getChallengeManager();
 
         $c = Challenge::random();
-        $cdjValue = Codecs\Base64Url::encode($c->getBinary()->unwrap());
+        $cdjValue = $c->getBinary()->toBase64Url();
 
         $found = $cm->useFromClientDataJSON($cdjValue);
 
@@ -59,7 +59,7 @@ public function testRetrievalDoesNotCreateChallengeFromUserData(): void
         $c = $cm->createChallenge();
 
         $userChallenge = Challenge::random();
-        $cdjValue = Codecs\Base64Url::encode($userChallenge->getBinary()->unwrap());
+        $cdjValue = $userChallenge->getBinary()->toBase64Url();
 
         $retrieved = $cm->useFromClientDataJSON($cdjValue);
         // The implmentation may return the previously-stored value or null,