[fix] respect @unused_bits in BitString (#323)

Author: kares

src/main/java/org/jruby/ext/openssl/ASN1.java

@@ -1678,13 +1678,10 @@ private ASN1Encodable toASN1Primitive(final ThreadContext context) {
             }
             if ( type == DERBitString.class ) {
                 final byte[] data = val.asString().getBytes();
-                int padBits = 0; // padBits < 8 && padBits >= 0
-                for (int i = (data.length - 1); i > -1; i--) {
-                    int b = Byte.toUnsignedInt(data[i]);
-                    if (b != 0) {
-                        padBits = Integer.numberOfTrailingZeros(b);
-                        break;
-                    }
+                int padBits = 0;
+                IRubyObject unused_bits = getInstanceVariable("@unused_bits");
+                if (unused_bits != null) {
+                    padBits = unused_bits.convertToInteger("to_i").getIntValue();
                 }
                 return new DERBitString(data, padBits);
             }

src/test/ruby/test_asn1.rb

@@ -1189,6 +1189,16 @@ def test_bitstring
     #}
   end
 
+  def test_bit_string_unused_length
+    asn = "\x00\x04".b
+    asn.prepend "\x03#{asn.bytesize.chr}"
+    bs = OpenSSL::ASN1.decode(asn)
+    assert_equal asn, bs.to_der
+
+    bs.unused_bits = 6
+    assert_equal "\x03\x02\x06\x00", bs.to_der
+  end
+
   def test_bit_string_infinite_length
     begin
       content = [ OpenSSL::ASN1::BitString.new("\x01"), OpenSSL::ASN1::EndOfContent.new() ]