#86: Fix getDataString behaviour

src/main/java/com/riscure/trs/Trace.java

@@ -3,7 +3,6 @@
 import com.riscure.trs.enums.Encoding;
 import com.riscure.trs.parameter.trace.TraceParameterMap;
 
-import java.math.BigInteger;
 import java.nio.FloatBuffer;
 
 /**
@@ -149,11 +148,12 @@ public TraceParameterMap getParameters() {
 
     /**
      * Get the supplementary (crypto) data of this trace as a hexadecimal string.
+     * Each byte is represented by exactly two uppercase hexadecimal characters.
      *
      * @return the supplementary (crypto) data of this trace as a hexadecimal string
      */
     public String getDataString() {
-        return new BigInteger(getData()).toString(16);
+        return HexUtils.toHexString(getData());
     }
 
     /**

src/test/java/com/riscure/trs/TraceTest.java

@@ -0,0 +1,34 @@
+package com.riscure.trs;
+
+import com.riscure.trs.parameter.trace.TraceParameterMap;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class TraceTest {
+
+    @Test
+    void getDataStringReturnsUnsignedHexadecimal() {
+        assertDataString(new byte[] {(byte) 0xFF, (byte) 0x80, 0x01}, "FF8001");
+    }
+
+    @Test
+    void getDataStringPreservesLeadingZeroBytes() {
+        assertDataString(new byte[] {0x00, 0x00, 0x16}, "000016");
+    }
+
+    @Test
+    void getDataStringDoesNotPrefixHighBitValues() {
+        assertDataString(new byte[] {(byte) 0xFF, (byte) 0xFF, 0x55}, "FFFF55");
+    }
+
+    private static void assertDataString(byte[] data, String expected) {
+        TraceParameterMap parameters = new TraceParameterMap();
+        parameters.put("data", data);
+        Trace trace = Trace.create("title", new float[] {0}, parameters);
+
+        String actual = trace.getDataString();
+        assertEquals(expected, actual);
+        assertEquals(data.length * 2, actual.length());
+    }
+}