fix: gmpToBytes no longer falls into the PHP "0" Elvis-falsy trap

For FF1 values where gmp_strval($val, 16) produced exactly "30", the
prior `hex2bin($hex) ?: "\x00"` would round-trip the bytes through
ASCII text. The result, the single byte 0x30, equals the string "0" —
one of PHP's handful of falsy strings — so the Elvis operator silently
swapped it for NUL.

The bug fired any time a half-block's integer value happened to equal
48 and gmpToBytes was asked for a fixed-width byte string. In FF1 that
shows up in the Q block, corrupting one round's PRF input and every
round after. The output stayed self-consistent (encrypt/decrypt are
still inverses) so a roundtrip test passed; only a cross-language
comparison surfaced it.

The cyphera adversarial FF1 b-matrix (radix 10, length 6, plaintext
'307418', case 22) hit it on round 5 of encryption: B = [0,4,8] →
num(B) = 48 → numB silently became 0x0000 instead of 0x0030, and
the ciphertext diverged from every other Cyphera SDK.

Fixed by switching to gmp_export with explicit byte-order flags, so
the path never goes through PHP's loose-typing.

Author: lgutschow

src/FF1.php

@@ -170,11 +170,14 @@ private function buildQ(string $T, int $i, string $numBytes, int $b): string
 
     private function gmpToBytes(\GMP $val, int $len): string
     {
-        $hex = gmp_strval($val, 16);
-        if (strlen($hex) % 2 !== 0) {
-            $hex = '0' . $hex;
+        // Use gmp_export rather than gmp_strval(…,16)+hex2bin: the latter
+        // round-trips through ASCII text, and when the result happens to be the
+        // single byte 0x30 ('0'), PHP's Elvis operator treated it as falsy and
+        // replaced it with NUL — silently corrupting the FF1 Q block.
+        if (gmp_cmp($val, 0) === 0) {
+            return str_repeat("\x00", $len > 0 ? $len : 1);
         }
-        $bytes = hex2bin($hex) ?: "\x00";
+        $bytes = gmp_export($val, 1, GMP_MSW_FIRST | GMP_BIG_ENDIAN);
         if (strlen($bytes) < $len) {
             $bytes = str_repeat("\x00", $len - strlen($bytes)) . $bytes;
         }