What key and input should I provide when using SipHash class?

[chlin501]

I use Bouncy Castle Java 1.8.2 SipHash class. When checking how to use SipHash, I notice the SipHashTest provides the procedure, which is nice. Thanks for detailing the test case. So my understanding the basic flow of using SipHash should be

1. Create SipHash instance
2. Initialize the Key
3. Specify input value
4. Generate result

final SipHash mac = SipHash() // step 1, create siphash 2 4 instance
mac.init(new KeyParameter(key)) // step 2, init key
mac.update(input) // step 3, update/ specify input value
final long result = mac.doFinal() // step 4, produce the hash result

Now my problems

1. What key should I generate? In the test case the key is a hard coded value. So it is not clear how I can create or derive a new one for my application? Any examples?
2. I assume input should be the value I want to hash in my application. Is it correct? The SipHash test case produces input by a loop, which is not clear to me.

Many thanks.

[chlin501]

Ok. It seems that I need to provide private key. So I create an ed25519 private key through following code

... gen = Ed25519KeyPairGenerator()
... gen.init(new KeyGenerationParameters(new SecureRandom, 128))
final ... keypair = gen.generateKeyPair()
final ... privateKey = (...) keypair.getPrivate()
final ... privKeyBytes = privateKey.getEncoded()

But when passing that privateKeyBytes variable to SipHash.init(new KeyParameter(privKeyBytes)) function, an error java.lang.IllegalArgumentException: 'params' must be a 128-bit key is thrown. This error comes from SipHash.init() will check the length of key passed in

   if (key.length != 16)
        {
            throw new IllegalArgumentException("'params' must be a 128-bit key");
        }

I specify KeyGenerationParameters's key size 128. This does not change the key length. The key length remains 32. How to fix this so that the key passed in can be 128 bit? Thanks.

I appreciate any suggestions.

[mouse07410]

Doesn't SipHash require a symmetric secret key of 128 bits?

[chlin501]

Doesn't SipHash require a symmetric secret key of 128 bits?

Use AES works for my case. Thanks.

[peterdettman]

The BC provider uses org.bouncycastle.crypto.util.CipherKeyGenerator to generate the key bytes for SipHash (and for MACs in general AFAIK), although the substance of the generation is just to choose 16 random bytes (i.e. 128 bits).

[chlin501]

Just notice that the Ed25519's key size is always 32. So using Ed25519KeyPairGenerator will always produce length 32 key.

I checked the source code. It looks like it is possible to use CipherKeyGenerator creating length 16 key (as private key), and then generating public key by the logic used in Ed25519.generatePublicKey.

I did not actually test to see if that works. Thus, I could be completely wrong. It is nice to gain some insight to how bc works, and related classes. I learn a new lesson. Thanks for the insight!