performance tweak: remove hex string conversion

Author: bschoening

src/main/java/com/privacylogistics/FF3Cipher.java

@@ -249,7 +249,7 @@ public String encrypt(String plaintext, byte[] tweak) throws BadPaddingException
             reverseBytes(S);
             logger.trace("\tS: {}", () -> byteArrayToHexString(S));
 
-            BigInteger y = new BigInteger(byteArrayToHexString(S), 16);
+            BigInteger y = new BigInteger(1, S);
 
             // Calculate c
             c = decode_int_r(A, alphabet);
@@ -373,7 +373,7 @@ public String decrypt(String ciphertext, byte[] tweak) throws BadPaddingExceptio
             reverseBytes(S);
             logger.trace("\tS: {}", () -> byteArrayToHexString(S));
 
-            BigInteger y = new BigInteger(byteArrayToHexString(S), 16);
+            BigInteger y = new BigInteger(1, S);
 
             // Calculate c
             c = decode_int_r(B, alphabet);