Decompiled snippet from ObtainConversationKey.invokeSuspend from Twitter 9.88 APK release -- Not original source

ObtainConversationKey-snippet.java

// The IV seems to be the ephemeral public key of the sender and is the header of the encrypted 
// conversation key
byte[] ivMaybeEphemeralPublicKey = ArrayUtils.sliceCopy(0, encryptedConversationKey, 65);

int length = encryptedConversationKey.length - 65;
byte[] ciphertext = ArrayUtils.sliceCopy(65, encryptedConversationKey, length);

ECNamedCurveParameterSpec secp256r1Spec = SessionKeyEncrypter.secp256r1Spec();
ECPublicKeySpec spec = 
  new ECPublicKeySpec(secp256r1Spec.curve.decodePoint(ivMaybeEphemeralPublicKey), secp256r1Spec);
BCECPublicKey bCECPublicKey = new BCECPublicKey("ECDSA", spec, BouncyCastleProvider.CONFIGURATION);

PrivateKey privateKey = getDMKeys.PrivKey.privateKey;
byte[] sharedSecret = SessionKeyEncrypter.generateSecret(bCECPublicKey, privateKey);
byte[] decryptedBytes = SessionKeyEncrypter.deriveKeyAndEncrypt(Cipher.DECRYPT_MODE, sharedSecret, iv, ciphertext);
String decryptedConversationKey = new String(decryptedBytes, Charsets.utf8);