A decompiled class from the Twitter 9.88 APK -- Not original source

SessionKeyEncrypter.java

package p000;

import java.security.InvalidKeyException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.SecureRandom;

import org.bouncycastle.crypto.InvalidCipherTextException;
import org.bouncycastle.jcajce.provider.asymmetric.p445ec.KeyAgreementSpi;

import javax.crypto.Cipher;

/**
 * This is decompiled from the Twitter 9.88.0-release.0 APK. This is NOT original source code.
 * The variable names were manually filled in.
 *
 * The code uses Bouncy Castle and most of those classes are from here: https://github.com/bcgit/bc-java
 */
public final class SessionKeyEncrypter {

    public static ECNamedCurveParameterSpec secp256r1Spec() {
        X9ECParameters x9ecParams = CustomNamedCurves.getByNameLazy("secp256r1").getParameters();
        return new ECNamedCurveParameterSpec(
                "secp256r1", x9ecParams.curve, x9ecParams.getG(),
                x9ecParams.n, x9ecParams.h, x9ecParams.getSeed()
        );
    }

    public static byte[] generateSecret(PublicKey publicKey, PrivateKey privateKey) throws InvalidKeyException {
        KeyAgreementSpi dh = new KeyAgreementSpi("ECDH", new ECDHBasicAgreement(), (DerivationFunction) null);
        SecureRandom secureRandom = new SecureRandom();
        dh.engineInit(privateKey, secureRandom);
        dh.engineDoPhase(publicKey, true);
        return dh.engineGenerateSecret();
    }

    public static byte[] encrypt(int opmode, byte[] key, byte[] nonce, byte[] message)
            throws InvalidCipherTextException {
        KeyParameter keyParam = new KeyParameter(key, 0, key.length);
        GCMBlockCipher gcmCipher = new GCMBlockCipher(new AESEngine());
        AEADParameters aeadParams = new AEADParameters(keyParam, 128, nonce, null);
        gcmCipher.init(opmode == Cipher.ENCRYPT_MODE, aeadParams);
        byte[] output = new byte[gcmCipher.getOutputSize(message.length)];
        gcmCipher.doFinal(output, gcmCipher.processBytes(message, 0, message.length, output, 0));
        return output;
    }

    public static byte[] deriveKeyAndEncrypt(int opmode, byte[] sharedSecret, byte[] iv, byte[] message)
            throws InvalidCipherTextException {
        /*
         * Source is:
         * https://github.com/bcgit/bc-java/blob/master/prov/src/main/java/org/bouncycastle/jce/provider/BrokenKDF2BytesGenerator.java
         * "Broken" means it was using an "implementation ... based on draft 9 of IEEE P1363a". The comments say
         * "don't use it for anything that might be subject to long term storage."
         */
        BrokenKDF2BytesGenerator kdf = new BrokenKDF2BytesGenerator(new SHA256Digest());
        kdf.init(new KDFParameters(sharedSecret, iv));
        byte[] keyBytes = new byte[32];
        kdf.generateBytes(keyBytes, 0, 32);
        byte[] key = ArrayUtils.sliceCopy(0, keyBytes, 16);
        byte[] nonce = ArrayUtils.sliceCopy(16, keyBytes, 32);
        return encrypt(opmode, key, nonce, message);
    }
}