Replace manual concatenation with actual ASN.1 encoder (#78)

While on its own the current implementation is perfectly stable due to
fixed lengths, adding support for other formats will become highly
problematic. Instead, replace the ASN.1 handling with a suitable
library.

composer.json

@@ -32,7 +32,8 @@
         "ext-gmp": "*",
         "ext-hash": "*",
         "ext-openssl": "*",
-        "firehed/cbor": "^0.1.0"
+        "firehed/cbor": "^0.1.0",
+        "sop/asn1": "^4.1"
     },
     "require-dev": {
         "maglnet/composer-require-checker": "^4.1",

src/COSE/Curve.php

@@ -36,6 +36,8 @@ enum Curve: int
 
     case ED448 = 7; // OKP
 
+    // RFC 5480
+    // §2.1.1.1 OIDs for named curves
     public function getOid(): string
     {
         return match ($this) {

src/PublicKey/EllipticCurve.php

@@ -9,6 +9,7 @@
 use Firehed\WebAuthn\COSE;
 use Firehed\WebAuthn\COSEKey;
 use Firehed\WebAuthn\Errors\VerificationError;
+use Sop\ASN1\Type as ASN;
 use UnexpectedValueException;
 
 use function gmp_pow;
@@ -31,6 +32,8 @@
  */
 class EllipticCurve implements PublicKeyInterface
 {
+    private const OID = '1.2.840.10045.2.1';
+
     // CBOR decoding: RFC 9053 §7.1.1
     private const INDEX_CURVE = -1; // ECC, OKP
     private const INDEX_X_COORDINATE = -2; // ECC, OKP
@@ -121,22 +124,19 @@ public function getPemFormatted(): string
         if ($this->curve !== COSE\Curve::P256) {
             throw new DomainException('Only P256 curves can be PEM-formatted so far');
         }
-        // Described in RFC 5480
-        // §2.1.1.1
-        // Just use an OID calculator to figure out *that* encoding
-        $der = hex2bin(
-            '3059' // SEQUENCE, length 89
-                . '3013' // SEQUENCE, length 19
-                    . '0607' // OID, length 7
-                        . '2a8648ce3d0201' // 1.2.840.10045.2.1 = EC Public Key
-                    . '0608' // OID, length 8
-                        . '2a8648ce3d030107' // 1.2.840.10045.3.1.7 = P-256 Curve
-                . '0342' // BIT STRING, length 66
-                    . '00' // prepend with NUL
-                    . '04' // uncompressed format
+
+        $asn = new ASN\Constructed\Sequence(
+            new ASN\Constructed\Sequence(
+                new ASN\Primitive\ObjectIdentifier(self::OID),
+                new ASN\Primitive\ObjectIdentifier($this->curve->getOid()),
+            ),
+            new ASN\Primitive\BitString(
+                "\x04" // Uncompressed
+                . $this->x->unwrap()
+                . $this->y->unwrap(),
+            ),
         );
-        $der .= $this->x->unwrap();
-        $der .= $this->y->unwrap();
+        $der = $asn->toDER();
 
         $pem  = "-----BEGIN PUBLIC KEY-----\n";
         $pem .= chunk_split(base64_encode($der), 64, "\n");