Issue 623 : Make BaseFont subset prefix generation deterministic

openpdf/src/main/java/com/lowagie/text/pdf/BaseFont.java

@@ -54,6 +54,7 @@
 
 import java.io.IOException;
 import java.io.InputStream;
+import java.security.SecureRandom;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.StringTokenizer;
@@ -336,6 +337,11 @@ public abstract class BaseFont {
      */
     protected IntHashtable specialMap;
 
+    /**
+     * Used to build a randomized prefix for a subset name
+     */
+    protected SecureRandom secureRandom;
+
     static {
         BuiltinFonts14.put(COURIER, PdfName.COURIER);
         BuiltinFonts14.put(COURIER_BOLD, PdfName.COURIER_BOLD);
@@ -1353,14 +1359,37 @@ public boolean isFontSpecific() {
      * 
      * @return the subset prefix
      */
-    public static String createSubsetPrefix() {
+    protected String createSubsetPrefix() {
         String s = "";
+        SecureRandom secureRandom = getSecureRandom();
         for (int k = 0; k < 6; ++k) {
-            s += (char) (Math.random() * 26 + 'A');
+            s += (char) (secureRandom.nextDouble() * 26 + 'A');
         }
         return s + "+";
     }
 
+    /**
+     * Returns a defined SecureRandom implementation.
+     * If nothing is set, returns a new
+     *
+     * @return {@link SecureRandom}
+     */
+    protected SecureRandom getSecureRandom() {
+        if (secureRandom != null) {
+            return secureRandom;
+        }
+        return new SecureRandom();
+    }
+
+    /**
+     * Sets the SecureRandom instance to be used for a subset's prefix generation
+     *
+     * @param secureRandom {@link SecureRandom}
+     */
+    public void setSecureRandom(SecureRandom secureRandom) {
+        this.secureRandom = secureRandom;
+    }
+
     /**
      * Gets the Unicode character corresponding to the byte output to the pdf
      * stream.

openpdf/src/main/java/com/lowagie/text/pdf/PdfReader.java

@@ -2650,7 +2650,7 @@ public int shuffleSubsetNames() {
         String s = getSubsetPrefix(dic);
         if (s == null)
           continue;
-        String ns = BaseFont.createSubsetPrefix() + s.substring(7);
+        String ns = createRandomSubsetPrefix() + s.substring(7);
         PdfName newName = new PdfName(ns);
         dic.put(PdfName.BASEFONT, newName);
         setXrefPartialObject(k, dic);
@@ -2670,7 +2670,7 @@ public int shuffleSubsetNames() {
         String sde = getSubsetPrefix(desc);
         if (sde == null)
           continue;
-        String ns = BaseFont.createSubsetPrefix();
+        String ns = createRandomSubsetPrefix();
         if (s != null)
           dic.put(PdfName.BASEFONT, new PdfName(ns + s.substring(7)));
         setXrefPartialObject(k, dic);
@@ -2686,6 +2686,20 @@ public int shuffleSubsetNames() {
     return total;
   }
 
+  /**
+   * Creates a unique subset prefix to be added to the font name when the font
+   * is embedded and subset.
+   *
+   * @return the subset prefix
+   */
+  private String createRandomSubsetPrefix() {
+    String s = "";
+    for (int k = 0; k < 6; ++k) {
+      s += (char) (Math.random() * 26 + 'A');
+    }
+    return s + "+";
+  }
+
   /**
    * Finds all the fonts not subset but embedded and marks them as subset.
    * 
@@ -2709,7 +2723,7 @@ public int createFakeFontSubsets() {
         s = getFontName(dic);
         if (s == null)
           continue;
-        String ns = BaseFont.createSubsetPrefix() + s;
+        String ns = createRandomSubsetPrefix() + s;
         PdfDictionary fd = (PdfDictionary) getPdfObjectRelease(dic
             .get(PdfName.FONTDESCRIPTOR));
         if (fd == null)

openpdf/src/test/java/com/lowagie/text/pdf/SubsetPrefixCreationTest.java

@@ -0,0 +1,50 @@
+package com.lowagie.text.pdf;
+
+import org.apache.commons.io.IOUtils;
+import org.bouncycastle.crypto.prng.FixedSecureRandom;
+import org.junit.jupiter.api.Test;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.security.SecureRandom;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+
+/**
+ * See : https://github.com/LibrePDF/OpenPDF/issues/623
+ */
+public class SubsetPrefixCreationTest {
+
+    @Test
+    public void createSubsetPrefixTest() throws Exception {
+        BaseFont font = BaseFont.createFont("LiberationSerif-Regular.ttf", BaseFont.IDENTITY_H,
+                BaseFont.EMBEDDED,true, getFontByte("LiberationSerif-Regular.ttf"), null);
+        assertNotEquals(font.createSubsetPrefix(), font.createSubsetPrefix());
+
+        byte[] baseSeed = new SecureRandom().generateSeed(512);
+        // init deterministic SecureRandom with a custom base seed
+        SecureRandom secureRandom = new FixedSecureRandom(baseSeed);
+        font.setSecureRandom(secureRandom);
+        assertNotEquals(font.createSubsetPrefix(), font.createSubsetPrefix()); // still different, as FixedSecureRandom generates a new random on each step
+
+        SecureRandom secureRandomOne = new FixedSecureRandom(baseSeed);
+        font.setSecureRandom(secureRandomOne);
+
+        String subsetPrefixOne = font.createSubsetPrefix();
+
+        // re-init FixedSecureRandom for deterministic generation
+        SecureRandom secureRandomTwo = new FixedSecureRandom(baseSeed);
+        font.setSecureRandom(secureRandomTwo);
+
+        String subsetPrefixTwo = font.createSubsetPrefix();
+        assertEquals(subsetPrefixOne, subsetPrefixTwo); // the desired deterministic behavior
+    }
+
+    private byte[] getFontByte(String fileName) throws IOException {
+        try (InputStream stream = BaseFont.getResourceStream(fileName, null)) {
+            return IOUtils.toByteArray(stream);
+        }
+    }
+
+}