How to use Bouncycastle's CMac

Question

I'm trying to use BouncyCastle's CMac implementation but apparently I'm doing it wrong. At least the following unit test (based on RFC 5297 test vectors) fails:

@Test
public void testCMacOfZeros() {
    byte[] key = {(byte) 0xff, (byte) 0xfe, (byte) 0xfd, (byte) 0xfc, //
            (byte) 0xfb, (byte) 0xfa, (byte) 0xf9, (byte) 0xf8, //
            (byte) 0xf7, (byte) 0xf6, (byte) 0xf5, (byte) 0xf4, //
            (byte) 0xf3, (byte) 0xf2, (byte) 0xf1, (byte) 0xf0, //
            (byte) 0xf0, (byte) 0xf1, (byte) 0xf2, (byte) 0xf3, //
            (byte) 0xf4, (byte) 0xf5, (byte) 0xf6, (byte) 0xf7, //
            (byte) 0xf8, (byte) 0xf9, (byte) 0xfa, (byte) 0xfb, //
            (byte) 0xfc, (byte) 0xfd, (byte) 0xfe, (byte) 0xff};

    byte[] zeros = new byte[16];

    byte[] result = new byte[16];

    CipherParameters params = new KeyParameter(key);
    BlockCipher aes = new AESEngine();
    CMac mac = new CMac(aes);
    mac.init(params);
    mac.update(zeros, 0, 16);
    mac.doFinal(result, 0);

    byte[] expected = {(byte) 0x0e, (byte) 0x04, (byte) 0xdf, (byte) 0xaf, //
            (byte) 0xc1, (byte) 0xef, (byte) 0xbf, (byte) 0x04, //
            (byte) 0x01, (byte) 0x40, (byte) 0x58, (byte) 0x28, //
            (byte) 0x59, (byte) 0xbf, (byte) 0x07, (byte) 0x3a};

    Assert.assertArrayEquals(expected, result);
}

I assume, that the CMac implementation itself is well tested, so I must miss something.

Kind of obvious maybe, but you can test the CMAC implementation using the CMAC test vectors instead of the SIV specific ones... — Maarten Bodewes, Apr 20 '15 at 18:17

score 4 · Accepted Answer · answered Feb 14 '15 at 15:21

I found my mistake:

SIV-AES uses AES in CMAC mode (S2V) and in counter mode (CTR). SIV- AES takes either a 256-, 384-, or 512-bit key (which is broken up into two equal-sized keys, one for S2V and the other for CTR)

I should have used only the first 16 bytes from the given key.

As expected, BouncyCastle works just fine.

How to use Bouncycastle's CMac

1 Answers1

Linked