Is there a KEM (Key Encapsulation Mechanism) architecture being proposed for the JCA?

[David Hook]

I'd like to just add a supporting comment on this by showing what we've 
currently done.

At the moment KEM usage in the JCA is provided in the BC API by doing 
the following:

// key pair generation
         KeyPairGenerator kpg = KeyPairGenerator.getInstance("Kyber", 
"BCPQC");
         kpg.initialize(KyberParameterSpec.kyber1024, new SecureRandom());

         KeyPair kp = kpg.generateKeyPair();

//kem usage
         KeyGenerator keyGen = KeyGenerator.getInstance("Kyber", "BCPQC");

//encapsulation step
         keyGen.init(new KEMGenerateSpec(kp.getPublic(), "AES"), new 
SecureRandom());

         SecretKeyWithEncapsulation secEnc1 = 
(SecretKeyWithEncapsulation)keyGen.generateKey();

//decapsulation step
         keyGen.init(new KEMExtractSpec(kp.getPrivate(), 
secEnc1.getEncapsulation(), "AES"));

         SecretKeyWithEncapsulation secEnc2 = 
(SecretKeyWithEncapsulation)keyGen.generateKey();

The above code will return an extension to SecretKey that has a 
getEncapsulation() method on it for "AES", with SecretKey.getEncoded() 
returning the actual shared secret. The result can either be used 
directly or getEncoded() might be called to provide a T value for a 
hybrid key agreement using ECCDH and then the destroy() method invoked. 
In the decapsulation step just SecretKey as a return would have been 
enough, but it just looked downright weird given how we were trying to 
do it and it seemed less confusing for developers if both ends produced 
the same result.

While this does mean we've got it to "fit" (it's basically the same 
pattern we used for the RSA-KEM in BCFIPS), it's clearly not portable 
and it does feel like we've stretched the API a bit much - the other 
init() methods now have to throw UnsupportedOperationException which 
does suggest this isn't really what the class is for. Some people have 
also shoehorned KEMs into KeyAgreement instead which, while it can be 
made to work, does also involve ignoring how the class is used normally 
and doing things like faking a public key. That it is even possible to 
do this multiple ways in the existing API, with neither working cleanly, 
does suggest that KEMs are a bit different from what we've seen before.

All that said, we are probably about 2 years out from when the first 
standards for these algorithms will appear and in some cases use of 
these algorithms will become mandatory. Now would be a really good time 
to have a universal solution that all of us can start supporting developed.

Thanks,
David


On 19/8/22 06:37, John Gray wrote:
>   We are starting to make use of the new PQ algorithms adopted by NIST for prototyping and development of standards.   In particular we are working on a composite KEM standard:
> See:  https://datatracker.ietf.org/doc/draft-ounsworth-pq-composite-kem/
>
> However, there is no KEM interface in the JCA (which make sense because these are new algorithms, although RSA-KEM has been out since 2010).
>
> I can add one into our toolkit (and I think David may have already added on into BC),  but I assume at some point there will be an official one added in Java and likely it won't be identical to what we do even if it is very close, which would cause backwards compatibility pain...   Perhaps we could collaborate on extending the JCA to support KEM?      Essentially it requires methods.
>
> ss, ct := encapsulate(PublicKey)
> ss := decapsulate(PrivateKey, ct)
>
> -ss is a shared secret (could come back as a Java SecretKey if you wanted as it would usually be used to derive something like an AES afterwards)
> -ct is a Cipher Text (a byte array would make sense)
> -Public and Private Keys would use the regular public and private key interface.
> -An object holding the ss and ct from the encapsulate() method could be returned, with accessor methods to get the ss and ct.   It could be called 'EncapsulatedKEMData' for example.
>
> Likely you would want a new type of KEM crypto object (like you have for Signature, MessageDigest, Cipher, Mac, SecureRandom, KeyAgreement.. etc).   Calling it KEM would seem to make sense.    😊    It could also use similar calling patterns and have a KEM.initKEM(keypair.getPublic()) or KEM.initKEM(keypair.getPrivate()), and then you would just call KEM.encapsulate() or KEM.decapsulate(ct).
>
> Then algorithms could be registered in providers as usual:
>
>      put("KEM.Kyber","com.blah.Kyber")
>      put("KEM.compositeKEM","com.entrust.toolkit.crypto.kem.compositeKEM")
>
> Then the above methods (encapsulate and decapsulate) could be defined in that new object type.   Then we would be able to make use of it and not have to worry about incompatibility issues down the road...
>
> Cheers,
>
> John Gray
>
>
>
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