Ensure public key is on the prime order subgroup

Author: paragonie-security

src/Core/Ed25519.php

@@ -115,6 +115,20 @@ public static function publickey_from_secretkey(
         return self::sk_to_pk($sk);
     }
 
+    /**
+     * Returns TRUE if $A represents a point on the order of the Edwards25519 prime order subgroup.
+     * Returns FALSE if $A is on a different subgroup.
+     *
+     * @param ParagonIE_Sodium_Core_Curve25519_Ge_P3 $A
+     * @return bool
+     */
+    public static function is_on_main_subgroup(ParagonIE_Sodium_Core_Curve25519_Ge_P3 $A): bool
+    {
+        $p1 = self::ge_mul_l($A);
+        $t = self::fe_sub($p1->Y, $p1->Z);
+        return self::fe_isnonzero($p1->X) && self::fe_isnonzero($t);
+    }
+
     /**
      * @param string $pk
      * @return string
@@ -131,9 +145,9 @@ public static function pk_to_curve25519(
             throw new SodiumException('Public key is on a small order');
         }
         $A = self::ge_frombytes_negate_vartime(self::substr($pk, 0, 32));
-        $p1 = self::ge_mul_l($A);
-        if (!self::fe_isnonzero($p1->X)) {
-            throw new SodiumException('Unexpected zero result');
+        // check that A * L == identity point
+        if (!self::is_on_main_subgroup($A)) {
+            throw new SodiumException('Public key is not on a member of the main subgroup');
         }
         $one_minux_y = self::fe_invert(
             self::fe_sub(
@@ -283,7 +297,7 @@ public static function verify_detached(
             throw new SodiumException('Argument 3 must be CRYPTO_SIGN_PUBLICKEYBYTES long');
         }
         if ((self::chrToInt($sig[63]) & 240) && self::check_S_lt_L(self::substr($sig, 32, 32))) {
-            throw new SodiumException('S < L - Invalid signature');
+            throw new SodiumException('S >= L - Invalid signature');
         }
         if (self::small_order($sig)) {
             throw new SodiumException('Signature is on too small of an order');
@@ -306,6 +320,9 @@ public static function verify_detached(
         ParagonIE_Sodium_Compat::$fastMult = true;
 
         $A = self::ge_frombytes_negate_vartime($pk);
+        if (!self::is_on_main_subgroup($A)) {
+            throw new SodiumException('Public key is not on main subgroup');
+        }
 
         $hDigest = hash(
             'sha512',

src/File.php

@@ -630,6 +630,12 @@ public static function verify(string $sig, string $filePath, string $publicKey):
         ParagonIE_Sodium_Compat::$fastMult = true;
 
         $A = ParagonIE_Sodium_Core_Ed25519::ge_frombytes_negate_vartime($publicKey);
+        if (ParagonIE_Sodium_Core_Ed25519::small_order($publicKey)) {
+            throw new SodiumException('Public key has small order');
+        }
+        if (!ParagonIE_Sodium_Core_Ed25519::is_on_main_subgroup($A)) {
+            throw new SodiumException('Public key is not on main subgroup');
+        }
 
         $hs = hash_init('sha512');
         hash_update($hs, self::substr($sig, 0, 32));