Use variable for computing buffer length. (#60)

* Use variable for computing buffer length.

Base offset of truncate method on buffer size, which is directly related to
the hash algorithm output size. Add test coverage using test vectors from
RFC 6238, which covers SHA1, SHA256, and SHA512.

See #59

* Update src/test/java/org/cryptacular/generator/TOTPGeneratorTest.java

Co-authored-by: Daniel Fisher <dfisher@vt.edu>

src/main/java/org/cryptacular/generator/AbstractOTPGenerator.java

@@ -78,15 +78,17 @@ protected int generateInternal(final byte[] key, final long count)
 
 
   /**
-   * Truncates HMAC output onto an unsigned (i.e. 31-bit) integer.
+   * Truncates HMAC output onto an unsigned (i.e. 31-bit) integer using the strategy discussed in RFC 4226,
+   * section 5.3.
    *
    * @param  hmac  HMAC output.
    *
    * @return  Truncated output.
    */
   private int truncate(final byte[] hmac)
   {
-    final int offset = hmac[19] & 0xf;
+    // Offset is the lowest 4 bits of the computed hash
+    final int offset = hmac[hmac.length - 1] & 0xf;
     return
       (hmac[offset] & 0x7f) << 24 |
       (hmac[offset + 1] & 0xff) << 16 |

src/main/java/org/cryptacular/generator/TOTPGenerator.java

@@ -17,6 +17,12 @@ public class TOTPGenerator extends AbstractOTPGenerator
   /** Digest algorithm specification. */
   private Spec<Digest> digestSpecification = new DigestSpec("SHA1");
 
+  /**
+   * Current system time in seconds since the start of the epoch, 1970-01-01T00:00:00.
+   * This value is used if and only if it is a non-negative value; otherwise the current system time is used.
+   */
+  private long currentTime = -1;
+
   /** Reference start time, T0. Default 0, i.e. 1970-01-01T00:00:00. */
   private int startTime;
 
@@ -97,8 +103,7 @@ public void setTimeStep(final int seconds)
    */
   public int generate(final byte[] key)
   {
-    final int unixTime = (int) (System.currentTimeMillis() / 1000);
-    final int t = (unixTime - startTime) / timeStep;
+    final long t = (currentTime() - startTime) / timeStep;
     return generateInternal(key, t);
   }
 
@@ -108,4 +113,28 @@ protected Digest getDigest()
   {
     return digestSpecification.newInstance();
   }
+
+
+  /**
+   * Sets the current time (supports testing). This value is used if and only if it is a non-negative value; otherwise
+   * the current system time is used.
+   *
+   * @param epochSeconds Seconds since the start of the epoch, 1970-01-01T00:00:00.
+   */
+  protected void setCurrentTime(final long epochSeconds)
+  {
+    currentTime = epochSeconds;
+  }
+
+
+  /**
+   * @return Current system time in seconds since the start of epoch, 1970-01-01T00:00:00.
+   */
+  protected long currentTime()
+  {
+    if (currentTime >= 0) {
+      return currentTime;
+    }
+    return System.currentTimeMillis() / 1000;
+  }
 }

src/test/java/org/cryptacular/generator/TOTPGeneratorTest.java

@@ -0,0 +1,77 @@
+/* See LICENSE for licensing and NOTICE for copyright. */
+package org.cryptacular.generator;
+
+import java.nio.charset.StandardCharsets;
+import org.cryptacular.FailListener;
+import org.cryptacular.spec.DigestSpec;
+import org.testng.annotations.DataProvider;
+import org.testng.annotations.Listeners;
+import org.testng.annotations.Test;
+import static org.testng.Assert.assertEquals;
+import static org.testng.Assert.assertTrue;
+
+/**
+ * Unit test for {@link HOTPGenerator}.
+ *
+ * @author  Middleware Services
+ */
+@Listeners(FailListener.class)
+public class TOTPGeneratorTest
+{
+  /** Test vectors from RFC 6238, appendix B. */
+  @DataProvider(name = "test-data-rfc6238")
+  public Object[][] getTestDataRfc6238()
+  {
+    // Key size is equal to hash length for test vectors in RFC-6238
+    // (via careful review of the main method in the reference implementation under Appendix A)
+    final String sha1Key = "12345678901234567890";
+    final String sha256Key = "12345678901234567890123456789012";
+    final String sha512Key = "1234567890123456789012345678901234567890123456789012345678901234";
+    return
+      new Object[][] {
+        {new DigestSpec("SHA1"), sha1Key, 59, 8, 94287082},
+        {new DigestSpec("SHA256"), sha256Key, 59, 8, 46119246},
+        {new DigestSpec("SHA512"), sha512Key, 59, 8, 90693936},
+        {new DigestSpec("SHA1"), sha1Key, 1111111109, 8, 7081804},
+        {new DigestSpec("SHA256"), sha256Key, 1111111109, 8, 68084774},
+        {new DigestSpec("SHA512"), sha512Key, 1111111109, 8, 25091201},
+        {new DigestSpec("SHA1"), sha1Key, 1111111111, 8, 14050471},
+        {new DigestSpec("SHA256"), sha256Key, 1111111111, 8, 67062674},
+        {new DigestSpec("SHA512"), sha512Key, 1111111111, 8, 99943326},
+        {new DigestSpec("SHA1"), sha1Key, 1234567890, 8, 89005924},
+        {new DigestSpec("SHA256"), sha256Key, 1234567890, 8, 91819424},
+        {new DigestSpec("SHA512"), sha512Key, 1234567890, 8, 93441116},
+        {new DigestSpec("SHA1"), sha1Key, 2000000000, 8, 69279037},
+        {new DigestSpec("SHA256"), sha256Key, 2000000000, 8, 90698825},
+        {new DigestSpec("SHA512"), sha512Key, 2000000000, 8, 38618901},
+        {new DigestSpec("SHA1"), sha1Key, 20000000000L, 8, 65353130},
+        {new DigestSpec("SHA256"), sha256Key, 20000000000L, 8, 77737706},
+        {new DigestSpec("SHA512"), sha512Key, 20000000000L, 8, 47863826},
+      };
+  }
+
+
+  @Test(dataProvider = "test-data-rfc6238")
+  public void testGenerate(
+      final DigestSpec digestSpec, final String asciiKey, final long currentTime, final int otpSize, final int expected)
+  {
+    final TOTPGenerator generator = new TOTPGenerator();
+    generator.setDigestSpecification(digestSpec);
+    generator.setStartTime(0);
+    generator.setTimeStep(30);
+    generator.setCurrentTime(currentTime);
+    generator.setNumberOfDigits(otpSize);
+    assertEquals(generator.generate(asciiKey.getBytes(StandardCharsets.US_ASCII)), expected);
+  }
+
+  /** Ensure the system time is used by default. */
+  @Test
+  public void testTimeBehavior() throws Exception
+  {
+    final TOTPGenerator generator = new TOTPGenerator();
+    final long t1 = generator.currentTime();
+    Thread.sleep(1001);
+    final long t2 = generator.currentTime();
+    assertTrue(t2 > t1);
+  }
+}