feat: implement PublicKey#fromMultiSignatureAsset (#109)

composer.json

@@ -20,7 +20,8 @@
         "php": "^7.2",
         "bitwasp/bitcoin": "^1.0",
         "kodekeep/binary": "^1.0",
-        "kodekeep/bytebuffer": "^1.0"
+        "kodekeep/bytebuffer": "^1.0",
+        "simplito/elliptic-php": "^1.0"
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^2.16",

src/Identities/PublicKey.php

@@ -17,6 +17,7 @@
 use BitWasp\Bitcoin\Crypto\EcAdapter\EcAdapterFactory;
 use BitWasp\Bitcoin\Crypto\EcAdapter\Impl\PhpEcc\Key\PublicKey as EcPublicKey;
 use BitWasp\Bitcoin\Key\Factory\PublicKeyFactory;
+use Elliptic\EC;
 
 /**
  * This is the public key class.
@@ -37,6 +38,29 @@ public static function fromPassphrase(string $passphrase): EcPublicKey
         return PrivateKey::fromPassphrase($passphrase)->getPublicKey();
     }
 
+    /**
+     * Create a public key instance from a multi-signature asset.
+     *
+     * @param int   $min
+     * @param array $publicKeys
+     *
+     * @return \BitWasp\Bitcoin\Crypto\EcAdapter\Impl\PhpEcc\Key\PublicKey
+     */
+    public static function fromMultiSignatureAsset(int $min, array $publicKeys): EcPublicKey
+    {
+        $minKey = static::fromPassphrase('0'.dechex($min));
+        $keys   = [$minKey->getHex(), ...$publicKeys];
+
+        $curve = (new EC('secp256k1'))->curve;
+        $P     = $curve->jpoint(null, null, null);
+
+        foreach ($keys as $publicKey) {
+            $P = $P->add($curve->decodePoint($publicKey, 'hex'));
+        }
+
+        return static::fromHex(bin2hex(implode(array_map('chr', $P->encodeCompressed(true)))));
+    }
+
     /**
      * Create a public key instance from a hex string.
      *

tests/Unit/Identities/PublicKeyTest.php

@@ -34,6 +34,21 @@ public function it_should_get_the_public_key_from_passphrase()
         $this->assertSame($fixture['data']['publicKey'], $actual->getHex());
     }
 
+    /** @test */
+    public function it_should_get_the_public_key_from_a_multi_signature_asset()
+    {
+        $actual = TestClass::fromMultiSignatureAsset(3, [
+            TestClass::fromPassphrase('secret 1')->getHex(),
+            TestClass::fromPassphrase('secret 2')->getHex(),
+            TestClass::fromPassphrase('secret 3')->getHex(),
+        ]);
+
+        $this->assertSame(
+            '0279f05076556da7173610a7676399c3620276ebbf8c67552ad3b1f26ec7627794',
+            $actual->getHex()
+        );
+    }
+
     /** @test */
     public function it_should_get_the_public_key_from_hex()
     {