feat: implement bit and decodepoint

src/Ed25519.php

@@ -212,6 +212,14 @@ public function encodePoint(Point $P): string
         return bin2hex($result);
     }
 
+    /**
+     * Returns the bit at position i of the hash h.
+     */
+    public function bit(string $h, BigInteger $i): int
+    {
+        return (ord($h[(int) $i->div(8)]) >> $i->mod(8)->toDec()) & 1;
+    }
+
     /**
      * Returns the decimal representation of the input's hash (SHA-512).
      */
@@ -242,6 +250,30 @@ public function decodeint(string $s): BigInteger
         $result = new BigInteger(bin2hex($s), 16);
         return new BigInteger($result->toDec());
     }
+
+    /**
+     * Decodes a point on the Edwards25519 curve from a hexadecimal string.
+     */
+    public function decodepoint(string $encoded): Point
+    {
+        // Convert to little-endian
+        $encoded = strrev(hex2bin($encoded));
+
+        $y = $this->decodeint($encoded);
+        $x = $this->xrecover($y);
+
+        if ($x->binaryAnd(new BigInteger("1"))->cmp($this->bit($encoded, $this->b->sub(1))) != 0) {
+            $x = $this->q->sub($x);
+        }
+
+        $P = new Point($x, $y);
+
+        if (!$this->isoncurve($P)) {
+            throw new Exception("Decoding point that is not on curve");
+        }
+
+        return $P;
+    }
     // The code below is by the Monero-Integrations team
 
     /**

tests/unit/Ed25519Test.php

@@ -22,7 +22,8 @@ class Ed25519Test extends TestCase
 	private $testHintInput = "100";
 	private $testHintOutput = "5249739319241077611146023738646316455244634195485061850472191081926985295157468499557719456094638265400023549952640721037286035956983479601959945106531843";
 
-	private $testEncodedB = "005866666666666666666666666666666666666666666666666666666666666666";
+	private $testEncodedB = "5866666666666666666666666666666666666666666666666666666666666666";
+	private $testBitInput = "fffffffffffffffffffffffffffffffX";
 
 	protected function setUp(): void
 	{
@@ -85,6 +86,15 @@ public function testEncodePoint(): void
 		$this->assertEquals($this->testEncodedB, $result);
 	}
 
+	public function testBit(): void
+	{
+		$result = $this->ed25519->bit($this->testBitInput, new BigInteger(255));
+		$this->assertEquals(0, $result);
+
+		$result = $this->ed25519->bit($this->testBitInput, new BigInteger(254));
+		$this->assertEquals(1, $result);
+	}
+
 	public function testHint(): void
 	{
 		$result = $this->ed25519->Hint(new BigInteger($this->testHintInput));
@@ -107,4 +117,12 @@ public function testDecodeInt(): void
 		$this->assertEquals('100', $point->toString());
 	}
 
+	public function testDecodePoint(): void
+	{
+		$point = $this->ed25519->decodepoint($this->testEncodedB);
+		$this->assertEquals($this->ed25519->B, $point);
+
+		$this->expectException(TypeError::class);
+		$point = $this->ed25519->decodepoint("invalid");
+	}
 }