in pan-domain-auth-verification/src/main/scala/com/gu/pandomainauth/service/Crypto.scala [20:28]
private def signatureInstance() = Signature.getInstance("SHA256withRSA", "BC")
def signData(data: Array[Byte], prvKey: PrivateKey): Array[Byte] = {
val rsa = signatureInstance()
rsa.initSign(prvKey)
rsa.update(data)
rsa.sign()
}