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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.5 to JDK 1.8.

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package org.bouncycastle.pqc.crypto.cmce;

import java.security.SecureRandom;

import org.bouncycastle.crypto.EncapsulatedSecretGenerator;
import org.bouncycastle.crypto.SecretWithEncapsulation;
import org.bouncycastle.crypto.params.AsymmetricKeyParameter;
import org.bouncycastle.pqc.crypto.util.SecretWithEncapsulationImpl;

public class CMCEKEMGenerator
    implements EncapsulatedSecretGenerator
{
    // the source of randomness
    private final SecureRandom sr;

    public CMCEKEMGenerator(SecureRandom random)
    {
        this.sr = random;
    }

    public SecretWithEncapsulation generateEncapsulated(AsymmetricKeyParameter recipientKey)
    {
        CMCEPublicKeyParameters key = (CMCEPublicKeyParameters)recipientKey;
        CMCEEngine engine = key.getParameters().getEngine();

        return generateEncapsulated(recipientKey, engine.getDefaultSessionKeySize());
    }

    public SecretWithEncapsulation generateEncapsulated(AsymmetricKeyParameter recipientKey, int sessionKeySizeInBits)
    {
        CMCEPublicKeyParameters key = (CMCEPublicKeyParameters)recipientKey;
        CMCEEngine engine = key.getParameters().getEngine();
        byte[] cipher_text = new byte[engine.getCipherTextSize()];
        byte[] sessionKey = new byte[sessionKeySizeInBits / 8];     // document as 32 - l/8  - Section 2.5.2
        engine.kem_enc(cipher_text, sessionKey, key.getPublicKey(), sr);
        return new SecretWithEncapsulationImpl(sessionKey, cipher_text);
    }
}