"key" argument of BcPublicKeyDataDecryptorFactory.createDataDecryptor()

Hi folks!

As I found no clear information on my propblem, neither in the API docs
nor in the source code itself, I have to ask here:

I don't understand the argument "key" of
BcPublicKeyDataDecryptorFactory.createDataDecryptor()
https://www.bouncycastle.org/docs/pgdocs1.5on/org/bouncycastle/openpgp/operator/bc/BcPublicKeyDataDecryptorFactory.html#createDataDecryptor-boolean-int-byte:A-

public PGPDataDecryptor
<https://www.bouncycastle.org/docs/pgdocs1.5on/org/bouncycastle/openpgp/operator/PGPDataDecryptor.html> createDataDecryptor(boolean withIntegrityPacket,
                                            int encAlgorithm,
                                            byte[] key)

|key| - the bytes of the key for the cipher.

Is "key" the public key, private key, fingerprint or key-id or even
something else?
I tried all the combinations but cannot decrypt a message.

I hope someone can help me there!

Best regards
Florian