Simplify calculation of n, and add unit-tests.

The current implementation uses floating-points, which is unnecessary.

PiperOrigin-RevId: 646353995
Change-Id: I2b32cdd12597662efc1630969cef7a347fab94d3

src/main/java/com/google/crypto/tink/subtle/PrfAesCmac.java

@@ -78,6 +78,14 @@ public static Prf create(AesCmacPrfKey key) throws GeneralSecurityException {
     return new PrfAesCmac(key.getKeyBytes().toByteArray(InsecureSecretKeyAccess.get()));
   }
 
+  // Only visible for testing.
+  static int calcN(int dataLength) {
+    if (dataLength == 0) {
+      return 1;
+    }
+    return (dataLength - 1) / AesUtil.BLOCK_SIZE + 1;
+  }
+
   // https://tools.ietf.org/html/rfc4493#section-2.4
   @Override
   public byte[] compute(final byte[] data, int outputLength) throws GeneralSecurityException {
@@ -92,7 +100,7 @@ public byte[] compute(final byte[] data, int outputLength) throws GeneralSecurit
     // is divided. Empty data is divided into 1 empty block.
     // Step 2: n = ceil(length / blocksize)
     // TODO(b/68969256): Adding a test that computes a CMAC of length 2**31-1.
-    int n = Math.max(1, (int) Math.ceil((double) data.length / AesUtil.BLOCK_SIZE));
+    int n = calcN(data.length);
 
     // Step 3
     boolean flag = (n * AesUtil.BLOCK_SIZE == data.length);

src/test/java/com/google/crypto/tink/subtle/PrfAesCmacTest.java

@@ -83,6 +83,22 @@ public void testFipsCompatibility() throws Exception {
         () -> new PrfMac(new PrfAesCmac(CMAC_TEST_VECTORS[0].key), 16));
   }
 
+  @Test
+  public void calcN() throws Exception {
+    // AesUtil.BLOCK_SIZE == 16
+    assertThat(PrfAesCmac.calcN(0)).isEqualTo(1);
+    assertThat(PrfAesCmac.calcN(1)).isEqualTo(1);
+    assertThat(PrfAesCmac.calcN(16)).isEqualTo(1);
+    assertThat(PrfAesCmac.calcN(17)).isEqualTo(2);
+    assertThat(PrfAesCmac.calcN(32)).isEqualTo(2);
+    assertThat(PrfAesCmac.calcN(33)).isEqualTo(3);
+    assertThat(PrfAesCmac.calcN(48)).isEqualTo(3);
+    assertThat(PrfAesCmac.calcN(49)).isEqualTo(4);
+    assertThat(PrfAesCmac.calcN(0x7FFFFFF0)).isEqualTo(0x07FFFFFF);
+    assertThat(PrfAesCmac.calcN(0x7FFFFFF1)).isEqualTo(0x08000000);
+    assertThat(PrfAesCmac.calcN(Integer.MAX_VALUE)).isEqualTo(0x08000000);
+  }
+
   @Test
   public void testMacTestVectors() throws Exception {
     Assume.assumeFalse(TinkFips.useOnlyFips());