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

Michael StJohns mstjohns at
Sun Aug 21 03:52:01 UTC 2022

On 8/20/2022 2:08 PM, David Hook wrote:
> Hi Michael,
> I don't know anything about bureaucrats, I am an engineer. You may 
> need to consult someone else on bureaucrats.
> I apologize for my apparent deficiencies in this area, but would you 
> mind explaining how Cipher.wrap() is either supposed to take a public 
> key and create an encapsulation based on it and return a secret key 
> implicitly in one clean move, or why it even makes sense to do so. The 
> method was never conceived as providing the functionality for what a 
> KEM actually does, and when I did the initial PKCS11 implementation at 
> Eracom in the late 90's and the team at Sun added the wrap/unwrap 
> functions to support it, this is definitely not was intended either - 
> it was for explicit key wrapping based on the key that was passed to 
> Cipher.init().
First - PKCS11 is a 40 year old API that probably needs to be retired.  
I spent the better part of 2 years working with the PKCS11 Oasis group 
trying to get them to properly support master secrets and KDFs and 
failed utterly.   You should not use PKCS11 as an example that the JCE 
should use as a goal.

At the base, a java class is a collection of objects.  A Cipher object

Let's build a non-parameterized ECIES-KEM which implicitly uses AES256 
bit keys to key a GCM cipher, and a KDF based on SP800-108 counter mode 
with SHA256 as the underlying hash, and with a well known label and 
context for the KDF since there is a new key for every wrap.

1) Implement CipherSpi -

public class EciesKemCipher extends CipherSpi {

     private KeyAgreement ka;
     private Cipher gcm;
     private KeyPair kp;
     private KeyPairGenerator kg;

     EciesKemCipher() {
         ka = KeyAgreement.getInstance("ECDH");
         kpg = KeyPairGenerator.getInstance ("EC");
         gcm = Cipher.getInstance ("AES/GCM/NoPadding");


    // implement a single example

     protected void engineInit (int opMode, Key key, SecureRandom 
dontcare) {

        switch (opMode) {
                initWrap((ECPublicKey) key);
            // unimpl

     private void initWrap (ECPublicKey k) {

            ECParameterSpec spec = k.getParams();
           kp = kpg.genKeyPair();
           ka.init (kp.getPrivate());
           ka.doPhase (k, true);
           byte[] sharedSecret = ka.generateSecret();

           byte[] keyStream = kdf(sharedSecret, 32 + 12); // output 44 
bytes for Key and IV
           SecretKeySpec gcmKey = new SecretKeySpec (keyStream, 0, 32, 
           IvParameterSpec gcmIv = new IVParameterSpec(keyStream, 32, 12);

           gcm.init (Cipher.MODE_ENCRYPT, gcmKey, gcmIv);
           // all ready to go

      protected byte[] engineWrap (Key k) {

            ByteBuffer outData = ByteBuffer.allocate 
(k.getEncoded().length + kp.getPublic().getEncoded().length) + 16;

            // Place a copy of the ephemeral public key I generated in 
init here for the use of the receiver.
            outData.put (kp.getEncoded());
            // One s
            outData.put (gcm.doFinal(k.getEncoded());

             byte[] result = outdata.remaining();

             // kp = null; clear cipher if it hasn't already been 
cleared, clear ka if necessary (e.g. un-init)

             return result;

   ... and unwrap and kdf function

2) Implement a provider and add the above.

> On BC's part, we've already implemented RFC 5990/SP 800-56B in Java 
> and the experience has, at best, been awkward. The new algorithms have 
> moved awkward to inappropriate. With the new algorithms, there's no 
> longer only one case of this, it's not an outlier, there should be a 
> general way of supporting KEMs in the API that doesn't involve over 
> engineering KeyGenerator and Cipher.
There's a big difference between the API and your underlying 
implementation.  Everything you want to do can be done using the current 
APIs.  As I said before, Cipher.wrap/unwrap are the appropriate APIs for 
this as they meet the contract requirements you need.   Most Ciphers 
require some extra data -e.g. IVs - that have to either be carried or 
implicitly derived.  In this case, what needs to be carried in addition 
to the encrypted key material is at least the ephemeral public key the 
wrapper creates.   I used a very simple encoding scheme above and this 
assumes that both ends know exactly what "ECIES-KEM" means.   Obviously, 
there are 100s of possible combinations of parameters and KDFs and key 
wrap algorithms.   What I would suggest is heading over to LAMPS at the 
IETF and proposing a data encoding scheme for carrying the parameters.  
Once you have that done, then come here and map JCE names against 
parameter sets to close the loop.  It won't require an API change.

> I work with a team that has had to implement all of them and had to 
> make them fit into the JCA. We have done so. Like John, I am simply 
> relaying our experience. In about 18 months these algorithms are going 
> to become mandatory, what all of us think is irrelevant. We, for our 
> part, already have a solution, but we both realize it's not "the 
> solution" - we recognize that the JVM is uniquely positioned to 
> provide leadership on this and provide a universal way of doing it.
Then suggest an API and we'll start knocking it around.  I personally 
don't think its necessary at this time and will add to API bloat.

> Of course, if it's felt that these algorithms should be ignored, it's 
> not my place to revolt, although I do feel obliged to argue. I will 
> simply try and do the best by my users, as I have no doubt will John. 
> Both of us have simply offered our comments in good faith and to alert 
> the community that things have changed and that with these new 
> algorithms there is room for a new approach. The ambiguity about how 
> these algorithms can be implemented and the excessive need to fallback 
> on propritary classes for them does suggest that there are some 
> additions to the JCA which would help. I appreciate to understand this 
> statement does involve actually understanding what these algorithms do 
> and may require some additional reading.
> As I said, I'm an engineer, my users will be able to use these 
> algorithms properly, my team will ensure that, as I have no doubt will 
> John's. What John and myself, apparently mistakenly, care about is 
> that our users should also be able to use these algorithms portably.
> Are you saying portability is no longer a consideration?
I have no idea where you got that idea.

> Regards,
> David
> On 21/8/22 02:23, Michael StJohns wrote:
>> Hi David/John -
>> I would submit that you're trying too hard to make your life simple! :-)
>> Cipher.wrap/unwrap are the correct methods.
>> For example:
>> Cipher  kem = Cipher.getInstance 
>> ("ECIES/GCM-128-64/KDF-SP800-108-COUNTER-SHA256");
>> kem.init (Cipher.WRAP_MODE, pubkey);
>> byte[] opaqueEncapsulatedKey = kem.wrap (someOtherKey);
>> The "opaqueEncapsulatedKey" would contain the data needed by the 
>> unwrap function - specifically a) the ecies ephemeral public key, b) 
>> the fact that the derived key is a GCM key of length 128 and that the 
>> GCM tag is 64 bytes long, c) the KDF, d) (optional) any mixins other 
>> than defaults required by the KDF - which would be passed in a 
>> parameter blob during init.  Cipher would NOT return the underlying 
>> generated secret used to wrap the key.  Just the public part of the 
>> key pair used to do the ECDH operation against the passed in public 
>> key.   In the RSA case, the wrapped encrypting secret would be an 
>> opaque data blob and would be part of the data passed to the unwrap 
>> function.
>> If you want a key generated for other purposes, then the right thing 
>> is using a KDF and a Key agreement function in tandem.   Strangely 
>> the KDF appears in the javacard API for 3.1, but not in the JCE/JDK API.
>> "What's the difference between a bureaucrat and an engineer?  A 
>> bureaucrat takes small solvable pieces and combines them into one 
>> insoluble mass."
>> In this case, Java provides a number of flexible primitives that can 
>> be combined as needed. In this case, the underlying Cipher 
>> implementation would wrap key agreement and kdf and cipher (GCM) 
>> instances.  It should return UnsupportedOperationException for all 
>> operations execept wrap/unwrap and the appropriate init methods.
>> Later, Mike
>> On 8/19/2022 6:38 PM, David Hook wrote:
>>> Hi Mike,
>>> KEMs can be used for key wrapping - we've actually implemented 
>>> support for this too. But they are not actually key wrapping ciphers.
>>> Here's a simple example of using Kyber for key wrapping in BC:
>>> SecretKey key =new SecretKeySpec(keyBytes,"AES");
>>> w1.init(Cipher.WRAP_MODE, kp.getPublic(),new KEMParameterSpec("AES-KWP"));
>>> byte[]data =w1.wrap(key);
>>> Cipher w2 =Cipher.getInstance(algorithm,"BCPQC");
>>> w2.init(Cipher.UNWRAP_MODE, kp.getPrivate(),new KEMParameterSpec("AES-KWP"));
>>> Key k =w2.unwrap(data,"AES",Cipher.SECRET_KEY);
>>> The behavior in this case is in line with what is given in RFC 5990 for the RSA KEM. How it works is by using the key generated
>>> by the KEM to create an AES-KWP key, which is then used to wrap keyBytes. The shortcoming is it means you have to generate the
>>> secret key separately.
>>> This is the problem though - a KEM can actually be used to generate a secret key for other purposes. For example, where
>>> someone is trying to implement a hybrid KAS scheme. But there is currently no mechanism in the Java APIs for being able to
>>> take advantage of this directly, hence our use of the KeyGenerator class and other people's attempts to make use of the KeyAgreement
>>> class. The Cipher.wrap() returns a byte[] - to be used with a KEM for secret generation it would also have to return the
>>> generated secret (I would probably also argue that passing a public key to wrap in order to generate an encapsulation of a
>>> generated encrypted secret was not the correct use of the API either, but the fact remains a byte[] is not really going to cut it).
>>> If you have any further questions, please feel free to ask. For what it is worth, I have been developing providers for the JCE/JCA since
>>> the late 90's and am actually one of the people responsible for the introduction of the existing wrap/unwrap API in the Cipher class.
>>> Thanks,
>>> David
>>> On 20/8/22 07:53, Mike StJohns wrote:
>>>> Hi This implemented as part of Javax.crypto.Cipher.  See the Java doc for the wrap and unwrap methods.
>>>> Mike
>>>> Sent from my iPad
>>>>> On Aug 19, 2022, at 12:56, John Gray<John.Gray at>  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:
>>>>> 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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